appendix l - new hope coal...new hope coal parsons brinckerhoff | 2172847g-civ-rep-001 revb i horse...

Document Name i Insert Month/Year

Appendix L Horse Creek Diversion Functional Design Report

New Hope Coal

Horse Creek Diversion Functional Design Report 27 March 2014

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB i

New Hope Coal Horse Creek Diversion Functional Design Report

Contents Page number

Glossary v

Executive summary vii

1. Introduction 1

1.1 General 1

1.2 Project client 2

1.3 Report structure 3

2. Creek diversion objectives, constraints and alternatives 4

2.1 Objectives 4

2.2 Constraints 4

2.3 Alternatives considered 5

3. Existing watercourse physical characteristics 7

3.1 Site setting 7

3.2 Geomorphology 7

3.2.1 Geomorphic characteristics 7 3.2.2 Planform and cross and longitudinal section geometry 8 3.2.3 Fluvial processes in the creek system 9

3.3 Riparian vegetation 10

3.4 In-stream habitat quality 11

4. Surface and groundwater characteristics 12

4.1 Creek hydrology 12

4.1.1 Regional setting 12 4.1.2 Hydrological model development and calibration 14 4.1.3 Design discharge estimates 14

4.2 Groundwater characteristics 16

4.2.1 Physiographic and surface water setting 16 4.2.2 Geological and hydrogeological setting 16 4.2.3 Extractive users 17 4.2.4 Groundwater dependent ecosystems (GDEs) 19 4.2.5 Groundwater impact assessment 19 4.2.6 Conclusions 20

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB ii


5. Diversion design 21

5.1 Design methodology 21

5.2 Average and design channel cross sectional geometry 21

5.3 Planform 23

5.3.1 Stage 1 diversion 24 5.3.2 Stage 2 diversion 26 5.3.3 Stage 3 and 4 diversions (final permanent diversion) 29

5.4 Optimisation of channel design 31

5.4.1 Floodplain width 31 5.4.2 Diversion bed slope 33

6. Hydraulic conditions of watercourse diversion 34

6.1 Two-dimensional flood modelling 34

6.1.1 Terrain data 34 6.1.2 Grid cell size 34 6.1.3 Boundary conditions 35 6.1.4 Roughness 35 6.1.5 Validation 35

6.2 Two-dimensional modelling results 35

6.2.1 General discussion 36 6.2.2 Velocity comparison (Appendix E2) 37 6.2.3 Bed shear stress (tractive force) (Appendix E3) 37 6.2.4 Stream power (Appendix E4) 38 6.2.5 Summary of hydraulic characteristics 38 6.2.6 Afflux 39

7. Geotechnical stability 41

7.1 Regional setting 41

7.2 Field investigations 41

7.3 Mine spoil assessment 42

7.4 Engineering properties 43

7.5 Geotechnical stability 43

7.5.1 Geometric stability 44 7.5.2 Seepage 44 7.5.3 Settlement 44

7.6 Rock armouring 47

8. Sediment transfer regime 49

8.1 General considerations 49

8.2 Existing project reach watercourse 49

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB iii


8.3 Watercourse diversions 51

8.3.1 Stage 1 diversion 51 8.3.2 Stage 2 diversion 51 8.3.3 Stage 3 diversion 52 8.3.4 Stage 4 diversion 52

9. Revegetation 55

9.1 Revegetation considerations 55

9.2 Soil 55

9.3 Existing landscape and vegetation 56

9.4 Revegetation strategy 57

9.5 Temporary diversions 58

9.5.1 Objectives 58 9.5.2 Strategy 58

9.6 Permanent diversions 58

9.6.1 Objectives 58 9.6.2 Strategy 58

10. Outcome requirements 63

10.1 Outcome 1 63

10.2 Outcome 2 64

10.3 Outcome 3 64

10.4 Outcome 4 65

10.5 Outcome 5 65

11. References 67

List of tables Page number

Table 4.1 FFA design discharges at the Windamere gauge 14 Table 4.2 Calculated design discharges in Horse Creek project reach 15 Table 4.3 DNRM registered bores within 2 km of mine lease 17 Table 6.1 Roughness values for the XP-SWMM 2D model 35 Table 6.2 ACARP guideline values 36 Table 6.3 Afflux levels for modelled scenarios 39 Table 7.1 Investigation borehole data 41 Table 7.2 Geotechnical parameters of the coal mine spoil 43 Table 9.1 Target structural diversity for revegetation zones 60 Table 9.2 Native species for revegetation of permanent diversions 61

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB iv


List of figures Page number

Figure 1.1 Elimatta project location (extract from EIS documentation) 1 Figure 1.2 Elimatta tenements (extract from EIS documentation) 2 Figure 2.1 Eastern alternative 5 Figure 3.1 Bed slope of Horse Creek 9 Figure 4.1 Regional hydrology 12 Figure 4.2 Groundwater users surrounding Horse Creek diversion 18 Figure 5.1 Location of sections through existing creek channel 22 Figure 5.2 Sections through creek channel 23 Figure 5.3 Planform components (Richardson et al , 2006) 23 Figure 5.4 Stage 1 diversion 25 Figure 5.5 Stage 2 diversion 27 Figure 5.6 Stage 3 and 4 diversions 29 Figure 5.7 Impact of floodplain width on shear stress 32 Figure 5.8 Impact of floodplain width on stream power 32 Figure 6.1 Existing floodplain pinch points 36 Figure 6.2 Western MLA boundary afflux 40 Figure 7.1 Exploratory borehole locations 42 Figure 7.2 Likely spoil backfill settlement timeline 45 Figure 8.1 Horse Creek Hjulstrom curve 50 Figure 8.2 Sediment transport regime along typical creek profile (Copeland et al 2001) 50 Figure 9.1 Soil Management Units within the southern portion of Elimatta Project MLA (AARC,

2012) 56 Figure 9.2 Indicative revegetation zones 60

List of photographs Page number

Photo 3.1 Typical Horse Creek main channel with woody debris 11 Photo 4.1 Typical Horse Creek main channel view 13 Photo 4.2 Main channel overbanks (floodplain) 13

List of appendices Appendix A Functional Design Drawings Appendix B Geomorphology Report Appendix C Riparian Vegetation Assessment Appendix D Flood Study Report Appendix E Hydraulic Characteristic Profiles Appendix F Geotechnical Investigation Report

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB v


Glossary Aggradation Progressive build-up of the longitudinal profile of a channel bed due to

sediment deposition

Anabranch Stream whose flow is divided at normal and lower stages by large islands or bars forming distinctly separate channels

AR&R Australian Rainfall and Runoff

Armour Surfacing of channel bed, bans or embankment slope to resist erosion and scour

Avulsion Sudden change in channel course that usually occurs when a stream breaks through its banks

ARI Average recurrence interval

Bankfull discharge Discharge that fills a channel to a point of overflowing

Bar Elongated deposit of alluvium within a channel not permanently vegetated

Bed Unvegetated bottom of a channel bounded by banks

Bed load Sediment that is transported in a stream by rolling, sliding, or skipping along the bed or very close to it

Bed shear The force per unit area exerted by a fluid flowing past a stationary boundary

Bed slope Inclination of channel bottom

Channel Bed and banks that confine the surface flow of a stream

Channel diversion The removal of flows by natural or artificial means from a natural length of channel

Degradation Progressive (long-term) lowering of the channel bed due to erosion

Design flow (flood) The discharge that is selected as the basis for the design or evaluation of a hydraulic structure

Discharge Volume of water passing through a channel during a given time

DNRM Department of Natural Resources and Mining

FFA Flood frequency analysis

Floodplain Nearly flat, alluvial lowland bordering a stream that is subject to frequent inundation

Fluvial geomorphology Science dealing with the morphology (form) and dynamics of watercourses

GAB Great Artesian Basin

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB vi


Head cut Channel degradation associated with abrupt change in bed elevation that generally migrates in an upstream direction

Knick point Head cut in non-cohesive alluvial material

Meander belt The distance between lines drawn tangent to the extreme limits of successive fully developed meanders

Migration Change in a position of a channel by lateral erosion of one bank and simultaneous accretion of the opposite bank

MLA Mine Lease Application

NEC Northern Energy Corporation

Oxbow Abandoned former meander loop that remains after a creek cuts a new, shorter channel across the narrow neck of a meander

Planform River characteristics as viewed from above

Point bar An alluvium deposit of sand or gravel lacking permanent vegetal cover occurring in a channel at the inside of a meander loop, usually downstream of the apex of the loop

PMF Probable maximum flood – very rare flood discharge

Reach A segment of a creek length that is arbitrarily bounded for the purposes of the study

RL Reduced level

ROM Run of mine

Sinuosity Ratio between thalweg length and the valley length of a creek

SMU Soil management unit

Stable channel A condition that exists when creek has a bed slope and cross section which allows its channel to transport the water and sediment delivered from the upstream catchment without aggradation, degradation or bank erosion

Thalweg The line extending down a channel that follows the lowest elevation of the bed

US /DS Upstream MLA boundary / downstream MLA boundary on project reach

(Generic glossary definitions from Lagasse et al (2012))

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB vii


Executive summary New Hope Coal is in the process of planning and gaining approvals for the development of the Elimatta Coal Mine in Central Queensland, proximate to Wandoan Township. The planning process has identified that a critical constraint to mining activities is the location of the coal reserve relative to Horse Creek, which runs diagonally through the coal reserve within the proposed mine lease boundary (MLA). New Hope Coal has appointed Parsons Brinckerhoff to prepare concept designs for a series of staged creek diversions of Horse Creek to facilitate access to the coal resource. The phases of implementing a creek diversion from concept to relinquishment may be described as follows:

preliminary investigations and design (functional design)

environmental approvals and conditioning – authorised under the Environmental Protection Act 1994 (EPA) as part of a resource activity

preparation of a detailed design and construction specifications

construction

monitoring, evaluation and maintenance during the operational phase of the mine

relinquishment of the mine lease on mine closure.

This report focuses on the preliminary investigation and design phase culminating in the production of a functional design report for submission with EIS documentation for the purposes of environmental authorisation of the project.

The report presents the findings of the preliminary design phase, including:

characterisation of the Horse Creek project reach

preliminary design of the diversion geometry, including planform and cross section

assessment of hydraulic and geomorphologic performance of the preliminary design

recommendations regarding optimisation of the preliminary design.

Supporting documents have been included in the Appendix, including the Functional Design Drawings (Appendix A), Geomorphology Report (Appendix B), Riparian Vegetation Assessment (Appendix C), Flood Study Report (Appendix D), Hydraulic Characteristic Profiles (Appendix E), and Geotechnical Investigation Report (Appendix F).

Parsons Brinckerhoff | 2172847G-CIV-REP-001 RevB 1


1. Introduction 1.1 General The Elimatta Coal mine consists of the development of an estimated 250 Million tonne (Mt) thermal coal resource of the Juandah formation in the Surat Basin, south-east Queensland, Australia. The Elimatta resource is located on Mine Lease Application area (MLA) 50254. Once operational approximately 8.2 Mt of run of mine (ROM) coal will be mined per annum (pa), to produce on average 5 Mtpa of product coal for export. At this estimated production rate the mine will have an operating life of 32 years.

The project is located within the Western Downs Regional Council area, approximately 40 km west of the Wandoan Township on the Darling Downs in southern Queensland, as indicated in Figure 1.1. The area around the Elimatta mine comprises predominantly agriculturally based communities and includes the towns of Wandoan to the east, Taroom to the north and Miles to the south.

The project will involve open cut mining using truck and excavator methods. A coal handling and preparation plant (CHPP) and associated mine infrastructure will be required, which is to be located on MLA 50270. A transportation corridor, contained in MLA 50271, links the resource MLA to the mine infrastructure MLA. The various tenements comprising the project are indicated on Figure 1.2. Product coal will be transported approximately 420 km by rail to the Wiggins Island Coal Terminal at Gladstone for export.

Figure 1.1 Elimatta project location (extract from EIS documentation)



Horse Creek, a tributary of Juandah Creek and ultimately the Dawson River, flows centrally through MLA 50254 in a north-easterly direction, and has been identified as a significant constraint to mining activities. A staged diversion of the creek is proposed to allow for the recovery of coal from beneath the creek bed. The diversion will be constructed in four stages, all of which will be constructed in the first six years of mining operations. The final diversion will be in operation for in excess of 25 years prior to mine closure and relinquishment, during which time the performance of the diversion may be monitored, evaluated and remedial measures undertaken where necessary.

Horse Creek is classified as a watercourse in terms of the Water Act.

Figure 1.2 Elimatta tenements (extract from EIS documentation)

1.2 Project client The project proponent is Taroom Coal Proprietary Limited (Taroom Coal), ACN 079 251 443, a wholly owned subsidiary of Northern Energy Corporation Limited (NEC), ABN 90 081 244 395. The project’s exploration tenements are held in the name of Taroom Coal but all business with the tenements is managed by NEC on Taroom Coal’s behalf.

As of 21 October 2011, Arkdale Proprietary Limited (Arkdale), a wholly owned subsidiary of New Hope Corporation Limited (New Hope), acquired 100% stake in NEC. New Hope (ASX Code: NHC) is an Australian publicly listed company with a long history, dating to the early 1950’s, of development and operation of coal mines in Queensland and overseas.



1.3 Report structure This report is structured to align with the requirements of a creek diversion functional design as contained in the Department of Natural Resources and Mines (DNRM) Draft Manual – Works that interfere with water in a watercourse: Watercourse diversions, and contains the following sections:

Section 2 – describes the project objectives, constraints and alternatives

Section 3 – describes the natural features in the existing and local watercourses including geomorphic and vegetation characteristics

Section 4 – describes the existing hydrologic characteristics of surface water and groundwater systems

Section 5 – describes the design development of the creek diversions

Section 6 – describes the hydraulic performance of the watercourse diversions

Section 7 – describes how the watercourse diversion and associated structures maintains stability and functionality for all substrate conditions

Section 8 – describes the a sediment transport regime of the existing creek and how the diversion maintains this and allows it to be self-sustaining and not negatively impact upstream and downstream reaches

Section 9 – describes the revegetation strategy for the proposed creek diversions

Section 10 – describes how the functional design of the diversions complies with the DNRM Draft Manual objectives.

The relevant sections of this report should be read in conjunction with supporting information provided in the Appendices, including Functional Design Drawings (Appendix A), Geomorphology Report (Appendix B), Riparian Vegetation Assessment (Appendix C), Flood Study Report (Appendix D), Hydraulic Characteristic Profiles (Appendix E), and Geotechnical Investigation Report (Appendix F).



2. Creek diversion objectives, constraints and alternatives

2.1 Objectives As described in Section 1 above, the project entails the mining of a coal resource on MLA 50254, through which Horse Creek passes. The current location of Horse Creek relative to the coal reserve has been identified as a critical constraint to the viability of the project, necessitating the staged diversion of approximately 11.4 km of the creek (hereinafter referred to as the “project reach”). The key objectives of diverting the project reach are as follows:

to provide access to the coal resource in a staged manner to suit mine planning timeframe requirements

to provide flood-control to prevent inundation of pits and coal processing or storage areas during prescribed flood events

to construct the diversion in such a manner as to be self-sustaining and include existing natural geomorphic, hydraulic, and ecological functions of the creek and surrounding landscape as far as is practically possible given the project constraints – it is necessary to realise that it is unlikely that returning the system to its pre-disturbance condition will be feasible

to enhance channel stability using natural methods and thus reduce long term channel maintenance requirements

to not impose liability on the State, the proponent or the community to maintain the diversion and its associated components.

2.2 Constraints Identifying project constraints is as important as identifying the project objectives, as there is no point setting up objectives that can only be partially, or not satisfied at all due to project constraints. The design of the project reach diversion is subject to the following constraints:

the project site contained within MLA 50254 is spatially constrained by the requirement to contain the diversion works within the MLA boundary, the location of a significant volume of the coal resource directly below the creek alignment, deposition areas required for out of pit mine spoil that cannot be disposed of within disused pits, and tailings storage disposal areas

the constraints described above limit the number of viable diversion alignment options, as well as the corridor width available for the diversion footprint

preferably diversions removing water from a watercourse should return it to the same watercourse at a downstream location, which gives rise to grade constraints, meaning any change in valley length arising from proposed diversion alignments will result in changed valley slope conditions

ideally the diversion design would incorporate a channel that is free to migrate laterally or longitudinally down the valley through natural geomorphic processes, however in practice should uncontrolled migration be allowed to occur there is the risk that meander bends may break through into adjacent active pits during the operational phase of the mine, or into the final void after mine closure and hence these constraints may make it necessary to stabilize banks in an engineered manner, preventing natural migration.



2.3 Alternatives considered Based on the constraints described in section 2.2 there are not a lot of opportunities to realign the diversion in a way that results in a final alignment that will not have to be constructed through mine spoil. An alternative was investigated during the development of concept designs that considers the diversion of the creek into high ground to the east of its existing alignment, running parallel to the southern and western mine lease boundaries. This alternative entails an inter-catchment transfer of flow from the Horse Creek catchment into the adjacent valley, which joins into Horse Creek downstream, and to the north, of the mine lease boundary. The alternative described above is indicated in Figure 2.1 below.

Figure 2.1 Eastern alternative



The alignment selected results in a negative longitudinal profile from the tie in point with Horse Creek to an outfall location in the adjacent valley, meaning the diversion would have to flow upslope, which is clearly not achievable. In order to achieve a positive diversion gradient, the upstream tie-in point on Horse Creek would have to be elevated, which could only be achieved by impounding flow in a dam structure. The dam embankment would have to operate in conjunction with a high level “spillway” diversion that would divert flow into the adjacent valley only once a certain threshold water level in the dam was exceeded. In terms of the current regulatory requirements this system would need to provide sufficient conveyance capacity to provide flood immunity to the pits for a 1000 year average recurrence interval (ARI) flood event.

This alternative was not considered feasible for the following reasons:

it requires a significant impoundment embankment across Horse Creek which would trap low flow storm event runoff and prevent it from entering the downstream creek system, which would have significant negative geomorphological and environmental impacts

it results in an inter-catchment transfer from a larger valley catchment into a smaller one, which would have similarly significant negative geomorphological and environmental impacts on the adjacent smaller creek system

it results in a significant afflux off mine lease in the event of higher magnitude storm events

the earthworks volumes associated with this alternative are significant and render it unsatisfactory from an economic perspective.



3. Existing watercourse physical characteristics

3.1 Site setting The project is located within the Western Downs Regional Council area, approximately 40 km west of the Wandoan Township on the Darling Downs in southern Queensland. Access to the project site is via the Yuleba Taroom Road west of the Leichhardt Highway. Current land use includes coal exploration and low intensity cattle / horse grazing.

The project site is located on a flat to gently sloping plain with an average elevation of approximately 250 m AHD. The undulating land is formed on argillaceous sediments of the Great Artesian Basin.

The functional design drawings for the project are included in Appendix A of this report.

3.2 Geomorphology Fluvial geomorphology techniques are used to provide insight into the responses of a creek to imposed changes, assess system stability, and identify causes of such instabilities. For diversion projects the findings are used to guide the selection of an appropriate cross-section geometry and planform for the relocated channel design.

A baseline geomorphology investigation was conducted by Parsons Brinckerhoff in 2013 and has been included in Appendix B for reference. This investigation included a review of work undertaken previously and included a site walkover to provide a snapshot of current geomorphic creek condition.

3.2.1 Geomorphic characteristics

The following is a brief summary of some of the relevant findings of this investigation regarding the geomorphic characteristics of the project reach (including sections 1 km upstream and downstream of the diversion tie in locations):

Horse Creek is an ephemeral creek flowing only after significant rainfall events, which typically occur between the months of November to February as described in the hydrology description contained in Section 4 of this report

the project reach lies in a broad, low slope valley and the creek has accumulated sediments in a wide, continuous floodplain (approximately 600 m wide), with elevation differential from valley to adjacent ridge lines separating adjacent catchments of < 30 m

the channel boundary is predominantly formed by deposited alluvial material, however in certain areas underlying sedimentary bed rock is evident in the creek bed or side slopes, restricting lateral and vertical movement

the creek planform generally consists of a low-moderate sinuosity, with a single macro-channel that exhibits both lateral and vertical stability

in-stream geomorphic units include lateral sand bars, mid-channel islands, chute channels, sand sheets, and pool and riffle structures



three large tortuous meander bends (i.e. not regular) provide evidence of laterally unconfined meandering sections of the creek, where the incised channel has cut into tertiary deposits of the intact valley fill and sediment is released and re-worked on the channel bed

two meander cut-off channels (oxbows), formed by short-circuiting cut-through of the creek bank were observed, one just upstream of the project reach and the other associated with a meander bend within the project reach

two examples of a fine-grained anabranching channel formed of three or more sub-channels were evident with multiple channels separated by stable vegetated islands / alluvial ridges with one low flow channel and multiple micro-channels during bankfull conditions

a stepped longitudinal profile was observed

sharp changes in gradient, or knick points, were observed in meander bend sections and at crossing points in straight sections, which are probably caused by the stream path crossing relatively resistant sandstone seams which are more stable than the surrounding sandy alluvium, resulting in a bedrock dominated channel in these areas, slowing the rate of horizontal retreat of the knick points.

3.2.2 Planform and cross and longitudinal section geometry

Survey undertaken for the development of the project was used to extract cross sections at regular intervals, produce a longitudinal section along the thalweg of the creek, and identify planform characteristics such as meander radius, amplitude and wavelength along the project reach. The information presented below represents a summary of the findings of this work, which will be used to guide the selection of an appropriate diversion planform and geometry:

the sinuosity index (ratio of thalweg length to valley length) of the project reach is approximately 1.2 when considering the channelling of the floodplain within the valley line due to hard sedimentary rock outcrops which do not form part of the floodplain at present – in practice the project reach comprises sections of straight, mildly sinuous, and highly sinuous or meandering channel

there are eight meander bends within the project reach which show a large variation in planform, with an amplitude range of between 62 and 1551 m, bend radius range of between 20 and 200 m, and wavelength range of between 217 and 1882 m

the channel bed (unvegetated sandy bed of the channel) and bankfull width varies considerably along the length of the creek

the creek channel section varies from trapezoidal to U or V-shaped, and based on approximately 140 cross sections over the project reach, the following geometric characteristics have been calculated:

Characteristics 10 percentile Average 90 percentile

Bed width (m) 3.2 5.6 8.6

Channel depth (bankfull) (m) 4.6 5.8 7.1

Channel top width (bankfull) (m) 30 40 53

Channel batter slopes Varying but generally steeper than 1V:3H

the average bed slope over the length of the project reach is 0.114% or 0.00114 m/m fall, however this varies significantly with some stretches exhibiting slopes steeper than 0.0015 m/m



the bed slope of Horse Creek from its source to approximately the confluence with Juandah Creek was extracted from 10 m SRTM contour data, and the profile is presented on Figure 3.1 below, which shows bed slopes in the range of 0.0011 to 0.0015 m/m for approximately 14 km upstream and 3 km downstream of the mine lease boundary respectively; this profile would suggest that the upper 10 km reach of the creek is likely to supply sediment into the system (due to higher gradient and associated increased stream power) while the lower reaches, incorporating the project reach, predominantly carry sediment and do not contribute significant additional material to the overall sediment load.

Figure 3.1 Bed slope of Horse Creek

3.2.3 Fluvial processes in the creek system

In addition to describing the geomorphological landform of the project reach, the purpose of the field investigation was also to identify processes that are occurring which may have a destabilising effect on the creek system, evidenced by factors such as lateral channel migration, the formation of oxbows through channel cut-off, erosion, sediment storage and deposition. Based on observations made during the site walkover it may be concluded that the following processes are occurring or have occurred in the recent past:

the low-moderate sinuosity sand bed reaches act as a conveyor, storing sediments during and after the receding limb of flow events and supplying them to downstream reaches during the ascending limb of high flow conditions - these sections are unlikely to provide material through in-situ bank erosion as the channel is relatively stable

the meander bends provide new material to the channel during high flow events, due to erosion on the apex of the curve facing highest water velocities, although there was not a significant amount of evidence of active bank erosion

two cut-off channels have formed in the creek in the past, cutting off large meander bends, and in the process created oxbows, which induce phases of disturbance response as the channel readjusts its slope to the reduced sinuosity, typically over timeframes in the order of 10 - 100 years; signs of such channel readjustments were evident at the upstream oxbow where active lateral channel migration is taking place through erosion on the outer channel bank and deposition of sand in downstream point bars



the oxbows perform an important function during high flow events and subsequent receding flows, providing additional storage capacity to reduce the likelihood of channel overtopping and acting as a fine sediment sink, respectively

anabranching channels located immediately downstream from the apex of the two large meander bends provide a complex geomorphological function, maximising bed sediment transport (work per unit area of the bed) under conditions where there is little opportunity to increase gradient, showing that the bends are a source of new material in the channel and that lower energy conditions downstream allowed subsequent deposition during flow recession allowing anabranch islands to form

episodic flow/flood events have produced bar formation / reworking, lateral channel migration, channel avulsion and local adjustments to vegetation associations

rock bars and outcrops occur in the creek bed and embankments, effectively restraining lateral and vertical movement of the channel in certain areas

the creek banks are stabilised with well-established woody vegetation, grasses and sedges.

These observations lead to the conclusion that the existing project reach may be considered stable, in that prevailing flow and sediment regimes are not leading to aggradation, degradation or to changes in cross-sectional geometry over the medium to long term.

They also indicate that Horse Creek has a wide natural capacity for adjustment and historically has shown a tendency to adjust both vertically and laterally. The creek responds to changes with progressive adjustments to planform and cross-sectional geometry to maintain its characteristic meandering form. Hence, Horse Creek is characterised by a stepped pathway of adjustment, with a wide range of disturbance responses of varying amplitude, wavelength and frequency.

3.3 Riparian vegetation A riparian vegetation assessment for the project reach was undertaken by AstralAsian Resource Consultants (AARC) in 2013 to determine the nature and condition of the riparian vegetation that will be removed during the development of the project. The data gathered during the assessment will be used to guide and develop targets for rehabilitation of the diversion footprints. The AARC report covering this assessment also provides recommendations regarding reinstatement of vegetation along the diversion footprint to ensure that the diversion footprint is restored to a condition comparable to that of the current environment.

The typical characteristics of the riparian vegetation in the creek system may be summarised as follows:

vegetation in a functional condition, in that it is representative of that typically encountered in surrounding watercourses

cattle grazing, weed invasion, selective clearing and vegetation dieback have reduced vegetation condition at most sites assessed

channel and banks have large scattered trees and grass cover, with a sandy creek bed, the riparian vegetation zone extends approximately 10 – 20 m either side of the bankfull channel width

a significant amount of woody debris was observed in the creek bed and banks

a floodplain comprising mostly grassland, with very limited woody vegetation.

The Riparian Vegetation Assessment report has been included in Appendix C. Both the Geomorphology Report and Riparian Vegetation Assessment Report contain photographs depicting typical geomorphological and vegetation features of the project reach, a single photograph is included below (Photo 3.1), which typifies conditions along the project reach.



-

Photo 3.1 Typical Horse Creek main channel with woody debris

3.4 In-stream habitat quality Horse Creek is a sand dominated system with banks that are prone to lateral accretion (i.e. gradual accumulation of sediments to the channel sides) and recurrent floodplain working. Horse Creek will be dry for large periods of time, with limited connection to the floodplain during most flow conditions. However, episodic flushing and bank over-topping is predicted during high rainfall events. The project reach incorporates a chain of ponds structure, with lower creek bed elevation refuge pools holding isolated pockets of stagnant water for extended periods. Habitat diversity increased in the meander bend and anabranching sections of the creek, with a greater range of morphology and particle sizes than the uniform straight sections of channel, which had homogenous deposits of sand-sized material. Large woody debris was observed in all the channels (see Photo 3.1 above), partly sourced from overhanging vegetation but also from further upstream. Bed or bank stabilisation by woody debris has been postulated in previous studies (Erskine et al., 2001). In Horse Creek, formation of semi-permanent log steps did dissipate energy, and store bed load. However, due to the non-cohesive nature of the bed sediment and the large velocities predicted during high flow events the presence of large woody debris only stabilised bed profiles on a temporary basis. In fact, scour and increased velocities (as reflected in increased particle size) was noted in the lee of logs and branches deposited within the channel due to creation of roughness.



4. Surface and groundwater characteristics

4.1 Creek hydrology 4.1.1 Regional setting

Horse Creek is a south-west headwater tributary of the Fitzroy Basin which flows in a north-easterly direction into Juandha Creek which is a tributary of the Dawson River.

Horse Creek is ephemeral and flows only after significant rainfall events, which typically occur between the months of November to February. The catchment extends approximately 33 km upstream and to the south of the MLA boundary, and comprises mainly rural farmland. The catchment areas contributing at the upstream and downstream MLA boundaries are 527 km2 and 574 km2 respectively.

The regional hydrology is indicated on Figure 4.1 below.

Figure 4.1 Regional hydrology



In its lower reaches, Horse Creek is characterised by a well-defined main channel flowing through a wide floodplain, in the order of 600 m wide, with several smaller flood channels. Photographs 4.1 and 4.2 below are representative of the Horse Creek main channel and floodplain. The channel and banks have large scattered trees and grass cover, with a sandy creek bed, while the floodplain comprises mostly grassland, with very limited woody vegetation.

Photo 4.1 Typical Horse Creek main channel view

Photo 4.2 Main channel overbanks (floodplain)



4.1.2 Hydrological model development and calibration

In order to assess the hydraulic performance and long-term stability of creek diversion projects it is necessary to evaluate a full range of flows that will be carried be the diversion. A hydrological model was developed to establish baseline hydrological and hydraulic characteristics for Horse Creek in order to be able to compare these with the proposed creek diversion characteristics for a range of flow events.

An XP-RAFTS model of the Juandah Creek catchment, including the Horse Creek catchment previously developed by Parsons Brinckerhoff for the West Surat Rail Link project was adopted for the project. This model was calibrated to six historical flood events recorded at the Windamere stream gauging station on Juandah Creek (Stn no.130344A), which has an upstream contributing catchment of approximately 1678 km2. This model was independently reviewed by WRM Water and Environment and their comments were incorporated in the development of the final model.

The development and functioning of this hydrological model are described in more detail in the Flood Study Report included in Appendix D.

4.1.3 Design discharge estimates

Design discharges for the project reach of Horse Creek have been estimated using various methods, including rainfall runoff models (XP-RAFTS) calibrated to historic flood events recorded at the Windamere gauge, scaled flood frequency analysis (FFA) based on recorded discharge data at the Windamere gauge, and the Rational Method. The options are described and compared below.

A FFA was undertaken using discharge data from the Windamere gauge, which has 39 years of available data from 1974 to 2013. The FFA undertaken was based on 38 data points between 1974 and 2012, fitted to a Log Pearson Type III distribution, as recommended in Australian Rainfall and Runoff (AR&R) guideline methodology (Pilgrim, 1998). Based on this FFA, design discharges at the Windamere gauge were calculated to be as contained in Table 4.1 below:

Table 4.1 FFA design discharges at the Windamere gauge

ARI (years) PB calculated flow (m3/s) WRM peer review (m3/s)

2 145 140

5 360 358

10 592 590

20 900 897

50 1,457 1,452

100 2,024 2,011

Design flows in the project reach of Horse Creek flows were estimated using various methods, including an XP-RAFTS hydrology model calibrated back to the Windamere gauge design flows, using a scaled/transposed calculation method to convert design flows at the Windamere gauge to equivalent flows in Horse Creek, and the Rational Method calculation for a single significant rainfall event; the results of which are tabulated in Table 4.2 below for comparative purposes.



Table 4.2 Calculated design discharges in Horse Creek project reach

ARI (years)

FFA Design Flow at Windamere (m3/s)

Design Discharge MLA Upstream (m3/s)

Design Discharge MLA Downstream (m3/s)

XP-RAFTS*

Scaled method (n=0.7)

Rational method

XP-RAFTS* Scaled method (n=0.7)

2 145 45 66 - 62 76

5 360 112 163 - 152 188

10 592 192 269 - 258 309

50 1,457 472 661 - 635 761

100 2,024 666 919 704-888 892 1,058

1000 4,820 1,597 2,188 2,140 2,519

*Adopted for the design flows for the various storm events for the Horse Creek diversion.

The scaled / transposed design method calculates the design flow for the project reach of Horse Creek using the Juandah Creek design flows and applying a scaled/transposed ratio of catchment areas using the formula provided in Grayson et al (1996), as follows:

Qc = Qg (Ac / Ag)n where: n = 0.5 to 0.9 (0.7 usually adopted if there is no calibration data)

Qg and Qc = flow in gauged and ungauged catchments respectively

Ag and Ac = area of gauged and ungauged catchments respectively

The Rational Method was used to calculate an indicative design flow at the upstream MLA boundary for a single storm event. The Rational Method using AR&R procedures (Pilgrim, 1998) is not strictly appropriate for catchments in the order of magnitude of Horse Creek, however the method provides and alternative indicative estimate for comparative purposes. The Rational Formula is represented by:

Qy = 2.8x10-3.Cy.A.tIy (m3/s)

Cy = coefficient of discharge (values of 0.55 and 0.7 were used to obtain a likely range of flows)

A = area of the catchment (ha)

tIy = Rainfall intensity (mm/hr)

Time of concentration for the catchment was calculated using the Bransby-Williams formula and rainfall intensity from BOM data for the project site.

Based on the tabulated range of design flows described above, and level of accuracy of the different methods, those values derived using the XP-RAFTS hydrology model calibrated to the Juandah Creek were adopted for the purposes of functional design.



4.2 Groundwater characteristics 4.2.1 Physiographic and surface water setting

The project site is located over a flat to gently sloping floodplain, which has been cleared, and now covered by shrubby regrowth and isolated strands of gum trees. The average elevation is approximately 250 m AHD. The site is bisected by the northeast flowing, ephemeral Horse Creek, which is the main watercourse within the site and a tributary of Juandah Creek. The catchment area of Horse Creek forms a floodplain, which is 527 km2 at the southern (upstream) boundary of the site, increasing to 574 km2 at the northern (downstream) boundary.

Flow in the creek is ephemeral, responsive to high rainfall events which typically occur between November and February. The creek bed is incised between 5 to 8 m below the flood plain. It is proposed to move the section of Horse Creek within the MLA towards the western boundary, on the edge of the floodplain, with parts of the diversion located on the edge of the floodplain and cutting into bedrock at various depths, which are shown on the longitudinal profiles included on the Functional Design Drawings included in Appendix A.

4.2.2 Geological and hydrogeological setting

The project site is located on the eastern edge of the Surat Basin, part of the Great Artesian Basin (GAB). The geology of the GAB is described in detail in the EIS, (AGE, 2012). As described in the EIS, the geology potentially affected by the proposed creek diversion is from the following main geological units:

Quaternary sediments: Clay, sand, gravel; flood plain alluvium, residual and weathered soil

Injune Creek Group/Middle to Late Jurassic: Calcareous lithic sandstone, siltstone, mudstone, coal, conglomerate.

A geotechnical site investigation undertaken to assist with the design of the creek diversion included a series of 13 boreholes which confirmed the presence of these units, as well as the presence of residual soil over weathered bedrock. A summary of the geology encountered at the site is included in section 7 below. Groundwater was encountered only in BH01 (24 September 2013) and BH10 (26 September 2013) at about 6.0 m depth. These bores are located close to the existing creek alignment, whereas all others are located away from the present creek channel on the edge of the floodplain.

In addition to the geotechnical investigation drilling for the proposed creek diversion, a review of the existing registered groundwater users in the local area was sourced from the Department of Natural Resources and Mines (DNRM) database. Table 4.3 details known registered bores within the mine lease and in the surrounding area. Information evident from the geotechnical borehole logs suggest that along the existing creek alignment, the alluvium is thickest, up to 7 m in the centre of the mine lease increasing to 8.5 m in the northern margin of the lease. The thickness of alluvium decreases with distance away from the Creek across the floodplain, with depths of less than 5 m in the area of the proposed diversion. This assessment concurs with the comment made in the EIS that ‘the alluvium associated with Horse Creek is not widespread, and forms only a thin, patchy and partially saturated aquifer’ (AGE, 2012).

The alluvium is considered to form an unconfined aquifer, with water held within the gravels at the base of the alluvium. Within the alluvium, only that area located close to the creek has recorded water bearing zones with water levels of 6 m below ground level (bgl). It is inferred that the alluvium is unsaturated towards the edges of the flood plain (including the diversion route) and partly saturated in a restricted area close to the valley axis and current channel. The alluvial aquifer is recharged primarily from rainfall, and when the creek is flowing. Groundwater flows from the areas of elevated topography in the head waters of Horse Creek towards the north (AGE, 2012). There is no permanent baseflow in Horse Creek; groundwater flows through the gravels in the bed of the Creek, and is removed by evaporation and evapotranspiration along the creek bed.



The alluvial sediments, where present, are underlain by the Juandah Coal Measures, a subgroup of the Walloon Coal Measures, and part of the Injune Creek Group, which are comprised predominantly of sandstone with interbedded coal, siltstones and mudstones. The Juandah Coal Measures form the bedrock beneath the alluvium and are the target coal seams of the mine lease. Groundwater occurs predominantly within these coal seams (AGE, 2012). The vertical hydraulic connection between the different water-bearing zones in the coal seams is considered to be low with the interbedded sandstones, siltstones and mudstones acting as semi-confining / confining layers separating the water-bearing zones. Recharge to the bedrock is very low, and occurs where the coal seams sub-crop beneath creeks and from direct rainfall infiltration where the seams are exposed at the surface.

According to the EIS, the hydraulic connection between the alluvium and underlying Juandah Coal Measures is poorly understood (AGE, 2012). However, multilevel bores constructed as part of the EIS at several sites across the mine lease indicate the water head in the alluvium is higher than in the bedrock (Coal Measures), indicating that the alluvium associated with Horse Creek likely recharges the underlying Coal Measures predominately after periods of sustained rainfall. The volume of this recharge has not been calculated, although is likely to be small considering the limited extent of the alluvium and availability of recharge.

The groundwater levels across the site generally indicate the potentiometric surface is a subdued reflection of the surface topography with groundwater flow from south to north (AGE, 2012). Along the existing alignment of Horse Creek, groundwater levels in the Juandah Coal Measures define a gentle northward gradient of 13 m over 6.3 km (2 x 10-3). The groundwater flow is controlled by the topography and surface drainages to the north, and not the dip of the coal seams which is generally to the south. Salinity is generally lower within the alluvial deposits than within the coal measures, which typically contain saline groundwater (AGE, 2012). The higher salinity in the coal measures is most likely a result of lower recharge rates to the coal seams that concentrate the rainfall recharge and greater groundwater residence times increasing water/rock interaction and mineral dissolution.

4.2.3 Extractive users

A search of the DNRM groundwater database indicates that there is one registered bore located within the mine lease, and seven other registered bores within 2 km of the mine lease boundary. The registered bores identified as part of this study are shown in Table 4.3 and Figure 4.2. None of these registered bores are accessing groundwater from the alluvial sediments associated with Horse Creek. The majority of these registered bores intersect and extract from the underlying bedrock at depths below 18 m. The use of these bores is limited to non-intensive stock (AGE, 2012).

Table 4.3 DNRM registered bores within 2 km of mine lease

ID/RN Easting Northing Geology Borehole depth (m)

Depth to groundwater (m)

RN14631 763861 7118297 unknown n/a n/a

RN14632 762190 7115406 unknown n/a n/a

RN14633 760132 7118218 unknown n/a n/a

RN14744 758294 7113740 INJUNE CREEK GROUP n/a n/a


RN34952 758540 7120713 BIRKHEAD FM-HUTTON FM n/a n/a





Figure 4.2 Groundwater users surrounding Horse Creek diversion



4.2.4 Groundwater dependent ecosystems (GDEs)

GDEs are natural ecosystems that require access to a groundwater source to meet all or some of their water requirements on a permanent or intermittent basis, so as to maintain their communities of plants and animals, ecosystem processes and ecosystem services (Richardson et al, 2011).

The National Atlas of Groundwater Dependent Ecosystems (known as the ‘Atlas’) managed by the Bureau of Meteorology (BOM) identifies ecological and hydrogeological information on known groundwater dependent ecosystems and ecosystems that potentially could use groundwater. GDEs identified in the Atlas are mapped using remote sensing although still require field verification to positively identify their location and dependence on groundwater.

As shown in Figure 4.2, there are 15 mapped GDEs within the area of the existing and proposed diversion of Horse Creek. All GDEs mapped within the Atlas are located within the alluvium associated with the existing Horse Creek alignment. As part of the mine planning process, the EIS sought to verify the actual presence of the (Atlas) mapped GDEs. It states that ‘no permanent surface water bodies reliant on groundwater flows are known to be present within EPC 650 (the mine lease)’ (AGE, 2012). It also states that ‘all the creeks including Horse Creek are flashy ephemeral systems that flow with surface water following rainfall events, and are not fed by a permanent discharge from underlying aquifers. Some subsurface flow of groundwater downstream through Horse Creek alluvium is expected to occur, but this does not express at the surface.’ The EIS notes that ‘deep rooted remnants of the native vegetation along the creek lines are also expected to contribute to evapotranspiration of water from the creek alluvium’ (AGE, 2012). A flora and fauna study undertaken as part of the mine’s development reported that “this community exists within a drainage depression that connects with Horse Creek during high flow events. The topography of the area results in overland flow pooling on the surface rather than flowing directly into Horse Creek, creating a seasonal wetland during periods of sufficient rainfall. The temporary nature of the standing water and the topography of the site suggest that the wetland is fed by surface water sources only. No natural springs are known from or were observed in the area.” (AARC, 2013). Therefore the riparian and wetland vegetation is likely to be dependent on stream flow and the shallow alluvial aquifers associated with Horse Creek. Such aquifers are recharged by rainfall / runoff and are not thought to be connected to any permanent groundwater source.

Further to this, a number of GDEs mapped by Atlas downstream of the mine lease within the alluvium of Horse Creek and its oxbow lakes, range from ‘high’ to ‘moderate’ potential. These ecosystems, according to the EIS do not appear to hold permanent water, and may be a surface depression that collects surface water runoff (AGE, 2012).

4.2.5 Groundwater impact assessment

4.2.5.1 Groundwater resource

The diversion of Horse Creek will have negligible impact on groundwater resources due to the lack of alluvial sediments (and aquifer) and shallow connected (bedrock) aquifers in the area of the proposed creek diversion. Additionally, the underlying aquifers in the coal measures rely on very limited recharge contribution from the existing alluvial sediments within the mine lease along Horse Creek.

4.2.5.2 Extractive users

There are no known registered groundwater users extracting groundwater from the potentially affected aquifers in and around Horse Creek.



4.2.5.3 GDEs

Although a number of GDEs within the mine lease and immediately downstream of the proposed creek diversion are ‘mapped’ by DNRM, it is likely that the riparian and wetland vegetation is dependent on stream flow and the shallow alluvial aquifers associated with Horse Creek, with the aquifers being recharged by rainfall / runoff and are not thought to be connected to any permanent groundwater source.

4.2.5.4 Horse Creek

This study indicates that groundwater does not contribute baseflow to Horse Creek in its current alignment and is also unlikely to contribute any in the proposed diversion. Therefore, the proposed diversion of Horse Creek should not impact baseflow conditions downstream of the mine lease.

4.2.6 Conclusions

Shallow groundwater resources within the mine lease along the existing alignment of Horse Creek are located primarily within alluvial gravels associated with, and close to, the current Creek channel. The groundwater within these sediments is found at depths of 6 to 8 m and therefore does not contribute baseflow to the Creek, nor is it considered to support local ecosystems. The alluvium is recharged primarily from rainfall and the main mechanism for discharge is through evapotranspiration. The underlying aquifers within bedrock of the Juandah Coal Measures are not hydraulically connected to the shallow alluvial aquifer of Horse Creek or to any associated ecosystems. The proposed creek diversion is located on the edge of the Horse Creek floodplain, in an area where the alluvium is thin to not present. There are no known users of groundwater from potentially affected aquifers within the area of the proposed creek diversion.

Therefore, the impact to the groundwater resource and existing users from the proposed diversion of Horse Creek is considered to be negligible.



5. Diversion design 5.1 Design methodology One of the objectives of the diversion design is to ensure long-term stability, which is sustainable without the need for removal of sediment or grade control. A creek is considered to be stable where the sediment load supplied from upstream can be transmitted without net erosion or deposition (regime conditions). The shape of a creek, in terms of planform (sinuosity, meander wavelength, amplitude, arc length, etc.), section geometry (bankfull width, depth, sideslopes, etc.), channel/bed slope, and velocity may change through erosion and deposition in response to changes. The variables that control the stable dimensions of a creek are the discharge or flow, sediment load, bed material, bank material, bank vegetation, and valley slope. Changes to a controlling variable will result in the development of new stable regime channel geometry over time.

The following design methodology has been adopted for the proposed diversions:

an idealised existing channel cross section was developed to represent the project reach, based on the average magnitude for width, depth and channel slope as obtained from survey undertaken for the site including the project reach

as the project reach may be considered geomorphically stable the diversion channel geometry was considered as the primary source of reference for selecting diversion geometry, where high variability in real data was evident, hydraulic geometry relationships were used as a general guideline to assist in the selection of appropriate geometric form

the bankfull discharge capacity of the existing idealised channel and the diversion channel were determined using normal depth calculations (assuming a free flowing channel), and hydraulic characteristics represented by velocity, stream power and shear force were calculated for comparison and to confirm the suitability of the preliminary geometric design

a variety of diversion planform options (including bend radius, sinuosity and amplitude) were considered for the individual stages of the diversion, with the primary objective of replicating existing creek conditions, such as thalweg length and hence bed slope, as closely as is practically possible

based on the above considerations a preferred planform was selected and 3-dimensional landforms were created for the four stages of the diversion

the hydrologic and hydraulic characteristics of each stage of the diversion were evaluated using 2D flood modelling for a range of flows, including 2, 5, 10, 50, 100 and 1000 ARI events, and compared against the base case flood modelling undertaken for Horse Creek.

5.2 Average and design channel cross sectional geometry

As described above the average, or idealised, existing -channel cross-sectional geometry was determined using statistical analysis of 139 sections taken at 100 m intervals along the project reach, which yielded the following approximate average channel dimensions (these are somewhat subjective as they rely on interpretation of the base width and depth to bankfull level):

average base width – 5.6 m

average channel depth (bankfull) – 5.8 m



average bankfull width – 40 m

average bankfull width to depth ratio – 6.9.

The existing channel side slopes vary considerably from section to section, but generally incorporate a steeper grassed section up from the channel bed, which flattens as the embankment approaches the bankfull height. In order to approximate this bank profile to achieve a more reflective average channel shape, overlapping cross-sections were extracted at approximately 400 m intervals and were graphically superimposed over each other. A “best fit” average channel was then created by varying the side slopes to achieve an average top width of 40 m, to align with the statistically determined average dimensions described earlier.

Figure 5.1 indicates the locations at which representative sections were taken.

Figure 5.1 Location of sections through existing creek channel

Overlapping cross-sectional plots taken at the locations depicted above are included on Figure 5.2 below; this figure also includes a section which best represents the average (idealised) channel conditions at present (dashed black line), as well as a design cross section which has been adopted for the design of the diversions (solid black line). The average existing channel incorporates side slopes of 1V:1.9H for the initial 3.5 m of the channel depth, flattening to 1V:4.3H to bankfull height.



The design cross-section adopted differs from the average channel section in that a slightly wider base of 6 m, shallower depth of 5 m, and constantly flatter side slopes of 1V:3H from bed to bankfull depth have been adopted. Flatter side slopes are preferred to enhance stability until vegetation can become established and to facilitate revegetation efforts. Inner bank benching will be incorporated at bends to mimic conditions currently. This channel will be constructed in a 200 m wide engineered floodplain.

Figure 5.2 Sections through creek channel

5.3 Planform Planform may be described as the form of a creek when viewed in plan. For meandering channels the planform is defined by a number of physical parameters as shown in Figure 5.3, being bankfull channel width (W), meander width (wm), wavelength ( ), angle of valley crossing ( ) and bend radius (rc).

.

Figure 5.3 Planform components (Richardson et al , 2006)



For diversion projects appropriate values for planform geometry have to be selected to develop design solutions that result in equilibrium conditions between the channel, flow and sediment transport capacity. The best source of information for selection of appropriate geometry is the project reach itself, provided it exhibits stable characteristics. In the absence suitable reach data, or to validate design assumptions, hydraulic geometry relationships may be used for general guidance.

Some of the recognised hydraulic geometry relationships are embodied in the formulas developed by Leopold and Wolman (Lagasse et al, 2012 and Copeland et al, 2001) as follows:

W = a Q b (where Q = bankfull discharge a = 4.24 and b = 0.5 for sand-bed streams)

= 11 W 1.01 (m)

= 4.6 rc 0.98 (m)

rc = 2.3 W (m) (range of 2 – 3 W provided)

Am (half full amplitude) = 3 W 1.1.

For the purposes of the preliminary design, a channel width (equating to bankfull flow) of approximately 40 m has been adopted based on statistical analysis of the geometry of the existing stable creek reach. The other planform parameters vary widely and it is difficult to extract suitable design geometry from real data. For this reason the hydraulic geometry equations have been used for general guidance, yielding the geometric dimensions calculated below. It should be recognised that these relationships predict a range of values that would yield stable channel geometry.

W = 49 - 58 m (based on a 95% confidence level of sample size used to develop the relationship formula, and bankfull discharge based on average bed slope of the diversion) – a statistically derived width (40 m) from project reach information was used in preference to these values predicted by the hydraulic geometry relationships as mentioned above

= 10.9 W 1.01 or 12.34 W = 452 – 493 m (used for design – 450 m)

= 4.7 rc 0.98 (m) - to get wavelength equivalence to the above equation a bend radius (rc) of 105 m would be required

rc = 2.3 W (m) (range of 2 – 3 W provided) = 80 – 120 m (used for design – 85 m)

Am (half full amplitude) = 2.7 W 1.1 = 140 m (used for design – 100 m).

The design geometry adopted is generally in accordance with hydraulic geometry relationships developed for stable sandy-bed creeks, other than amplitude which is lower than guidelines suggest, due to mine planning constraints which limit the available floodplain width for development of meander amplitude, and possibly channel width for the bankfull flows – these are discussed further under Section 5.4, dealing with optimising the diversion design in subsequent phases of the project.

5.3.1 Stage 1 diversion

The Stage 1 diversion, as indicated on Figure 5.4, will be constructed ahead of mining commencing and will be excavated through natural ground. Stage 1 comprises both temporary sections that will be mined through at a later stage, as well as a section that will be incorporated into the final permanent diversion. The diversion alignment follows a gently meandering planform, initially through the existing floodplain, then cutting through higher ground prior to dropping back into the exiting floodplain prior to re-connecting to Horse Creek. In the floodplain the diversion will entail the construction of a low flow channel only, while through higher relief an engineered floodplain, approximately 200 m wide, will be excavated. Levees will be required to protect the active pits from flood inundation as indicated below. A cutting with a maximum depth of approximately 12 m will be required to achieve the desired longitudinal profile. Based on geotechnical investigation boreholes drilled in this area, it is likely that the excavation will intersect sandstone at depths varying between 5.5 and 7 m deep, meaning the low flow channel will be excavated into sandstone.



Figure 5.4 Stage 1 diversion

The characteristics of the Stage 1 diversion may be summarised as follows:

the diversion replaces 2700 m of Horse Creek with a constructed thalweg length of 2580 m, which results in a length change factor of 0.96

the average grade of the section of the Horse Creek thalweg that is being replaced is 0.00102 m/m, while that of the diversion will be 0.00107 m/m

the diversion will be constructed partially through the existing floodplain, and partially through higher ground, where a 200 m wide engineered floodplain will be constructed in cut



a section of the diversion is likely to intersect with the sandstone layer underlying the alluvial floodplain, resulting in a low flow channel profile cut into the sandstone strata, which will constrain the ability of the channel to migrate laterally or vertically and alter the creek substrate for vegetation regrowth

the cross sectional geometry adopted for the diversion was based on the design methodology described earlier, however a more gentle meander patterns has been adopted as the diversion and existing creek reach being diverted are very similar in length

due to the close correlation between the bed slope and cross-sectional geometry of the diversion and existing creek the hydraulic characteristics of both will be very similar for all flood events up to bankfull level, and thereafter will divergent due to the dissimilarity in floodplain width – the hydraulic characteristics of the existing creek and the Stage 1 diversion are described below based on average channel geometry for the existing creek, design channel geometry for the diversion and normal depth flow conditions (flows are based on the average of the upstream and downstream MLA boundary flows predicted by the hydrologic model):

Characteristic Existing Stage 1 ACARP

Q2 – Flow (m3/s) 54 54 -

Q2 - Channel velocity (m/s) 1.1 1.0 < 1.5

Q2 - Shear force (N/m2) 21.1 20.5 < 40.0

Q2 - Stream power (N/m.s) 22.1 21.0 < 60.0

Q50 – Flow (m3/s) 550 550 -


Q50 - Shear force (N/m2) 36.3 47.4 < 80

Q50 - Stream power (N/m.s) 54.7 84.6 < 220

Bankfull discharge (m3/s) 134.2 136.2 -

Bankfull – Shear force (N/m2) 26.3 29.3 -

Bankfull – Stream power (N/m.s) 32.1 38.0 -

the calculated characteristics of the Stage 1 diversion show very close correlation with the existing creek up to bankfull conditions and thereafter are comparatively higher, due to the narrower floodplain, but are still well within ACARP guidelines

the values above represent idealised conditions and were calculated to test the preliminary geometry of the diversion design prior to undertaking detailed flood modelling; a more accurate prediction of hydraulic performance is presented in the results of the 2D hydraulic modelling of both the existing creek and the Stage 1 diversion described in Section 6 below (these comments are equally applicable to all stages described below).


The Stage 2 diversion, as indicated on Figure 5.5, will be constructed to be operational in year 2 of mining operations and will be excavated through natural ground. Stage 2 is a temporary diversion that will be mined through in year 5 of mining operations, and hence has a design life of approximately 3 years. The diversion alignment follows a gently meandering planform through the existing alluvial floodplain, cutting off a significant meander bend in the existing Horse Creek thalweg. There is little opportunity to create a more sinuous alignment better reflecting the natural planform in this area due to the location of the pit wall to the west and Perretts Road and high ground to the east.



The diversion will entail the construction of a low flow channel only, with levees to the west to protect the active pits from flood inundation. A cutting with a maximum depth of approximately 6.3 m will be required to achieve the desired longitudinal profile. Based on geotechnical investigation boreholes drilled in this area, it is likely that the excavation will intersect sandstone at depths varying between 4 – 6.5 m deep, meaning that the low flow channel bed may intersect the sandstone sub-strata for part of the alignment.

Figure 5.5 Stage 2 diversion



The characteristics of the Stage 2 diversion may be summarised as follows:

the diversion replaces 2940 m of Horse Creek with a constructed thalweg length of 989 m, which results in a length change factor of 0.34

the average grade of the section of the Horse Creek thalweg that is being replaced is 0.0013 m/m, while that of the diversion will be 0.0038 m/m, which represents a significant steepening of the longitudinal profile to grades similar to those encountered in the upper reaches of the catchment (i.e. sediment supplying)

the cross sectional geometry adopted for the diversion is based on the design methodology described above

due to the significant difference in the bed slope of the diversion and the existing creek, the hydraulic characteristics of the diversion are considerably different to those of the existing channel – the hydraulic characteristics of the existing creek and the Stage 2 diversion are described below based on average channel geometry for the existing creek, design channel geometry for the diversion and normal depth flow conditions:

Characteristic Existing Stage 2 ACARP

Q2 – Flow (m3/s) 54 54 -


Q2 - Shear force (N/m2) 26.5 57.1 < 40.0

Q2 - Stream power (N/m.s) 31.2 93.4 < 60.0

Q50 – Flow (m3/s) 550 550 -


Q50 - Shear force (N/m2) 45.0 136.9 < 80

Q50 - Stream power (N/m.s) 75.3 401.4 < 220

Bankfull discharge (m3/s) 151.5 256.9 -

Bankfull – Shear force (N/m2) 33.6 104.3 -

Bankfull – Stream power (N/m.s) 46.2 255.3 -

the anticipated hydraulic characteristics associated with the steep bed slope of the Stage 2 diversion are dissimilar to the those encountered in the existing section of creek and exceed ACARP recommendations significantly

in order to improve or manage higher than desired hydraulic characteristics of this temporary diversion, either grade control through the introduction of a drop structure/s (a drop of approximately 2.5 m would be required to achieve similar grades over the length of the diversion), or engineered stabilising measures such as lining or rock armouring would need to be incorporated into the design (for the purpose of the Functional Design it has been assumed that the channel will be rock armoured to achieve stability – with nominal design as shown of the drawings in Appendix A) .



5.3.4 Stage 3 and 4 diversions (final permanent diversion)

The Stage 3 diversion, as indicated on Figure 5.6, will be constructed to be operational in year 4 of mining operations and will be excavated partially through natural ground, and partially through mine spoil. Stage 3 forms part of the final diversion landform, and comprises an engineered floodplain through mine spoil, approximately 200 m wide, containing a meandering low flow channel, as well as a low flow channel constructed through the natural floodplain prior to re-connecting to Horse Creek at the downstream extent of the diversion. The Stage 3 alignment is constrained by the need to tie-in to the Stage 1 diversion at the upstream end, mine spoil dumps to the south, the mine lease boundary to the north, and the requirement to tie the diversion back into Horse Creek on the mine lease. Stage 4 will be constructed to be operational in year 5 of mining operations and will be constructed entirely through mine spoil. This section completes the final landform of the permanent diversion, which by this time incorporates Stage 4, part of Stage 1 and Stage 3. The Stage 4 diversion will comprise an engineered floodplain, approximately 200 m wide, containing a meandering low flow channel and incorporates a fill bund 100 m wide between the engineered floodplain and final void location. Stage 4 is constrained in much the same way as is Stage 3. The Stage 3 and 4 diversions come into operation within a year of each other.

Figure 5.6 Stage 3 and 4 diversions



The characteristics of the final diversion landform incorporating Stage 1, Stage 3 and Stage 4 may be summarised as follows:

the Horse Creek valley length is approximately 9.52 km long, with a thalweg length of approximately 11.45 km – while the constructed diversion valley length will be approximately 7.25 km and have a meandering thalweg length of approximately 8.15 km long

the full diversion replaces approximately 11.45 km of Horse Creek with a diversion thalweg length of 8.15 km, which results in an overall length change factor of 0.71

the average grade of the section of the Horse Creek thalweg that is being replaced is 0.00114 m/m, while that of the diversion will be 0.00158 m/m - comprising 4.56 km of Stage 4 at 0.00184 m/m (existing section of reach - 0.00114 m/m), 1.24 km of Stage 1 at 0.00107 m/m (existing section of reach - 0.00102 m/m) and 2.35 km of Stage 3 at 0.00137 m/m (existing section of reach - 0.00119 m/m)

the sinuosity of the existing Horse Creek thalweg is approximately 1.2, while that of the final diversion is 1.12

the cross sectional geometry adopted for the diversion is based on the design methodology described above

the hydraulic characteristics of the final stages of the diversion are tabulated below based on average channel geometry for the existing creek, design channel geometry for the diversion and normal depth flow conditions:

Characteristic Stage 3 Stage 4 ACARP

Existing

0.00119m/m

Diversion

0.00137m/m

Existing

0.00114m/m

Diversion

0.00184m/m

Q2 – Flow (m3/s) 54 54 54 54 -

Q2 - Channel velocity (m/s) 1.1 1.1 1.1 1.25 < 1.5

Q2 - Shear force (N/m2) 24.4 25.2 23.4 31.9 < 40.0

Q2 - Stream power (N/m.s) 27.5 28.4 25.8 40.0 < 60.0

Q50 – Flow (m3/s) 550 550 550 550 -

Q50 - Channel velocity (m/s) 1.6 1.9 1.6 2.2 < 2.5

Q50 - Shear force (N/m2) 41.2 58.1 40.1 74.6 < 80

Q50 - Stream power (N/m.s) 66.0 114.1 63.5 164.7 < 220

Bankfull discharge (m3/s) 145 154 142 179 -

Bankfull – Shear force (N/m2) 30.7 37.5 29.4 50.4 -

Bankfull – Stream power (N/m.s)

40.4 55.0 37.9 85.7 -

the calculated characteristics of the Stage 3 diversion show close correlation with the existing creek up to bankfull conditions and thereafter are comparatively higher, due to the narrower floodplain, but are still well within ACARP guidelines

the calculated characteristics of the Stage 4 diversion are higher than those of the existing creek for all flows due to the significantly steeper bed grade and narrower floodplain, but are still well within ACARP guidelines; the grades for this diversion could be reduced by increasing the sinuosity of the diversion in this section and/or incorporating the oxbow shown in Figure 5.5 above, the merits and demerits of which are discussed in Section 5.4 below



the values above represent idealised conditions and were calculated to test the preliminary geometry of the diversion design prior to undertaking detailed flood modelling; a more accurate prediction of hydraulic performance is presented in the results of the 2D hydraulic modelling of both the existing creek and the Stage 1 diversion described in Section 6 below (these comments are equally applicable to all stages described below).

5.4 Optimisation of channel design The calculations described above indicate that the preliminary geometry, longitudinal profile and planform selected for the various stages of diversions show results, for discharges up to a 50 year ARI event, which are similar to, or well within ACARP guidelines to the sections of Horse Creek being diverted. The exception to this generalisation is the Stage 2 temporary diversion, which will have to be stabilised using an engineering approach rather than natural processes. Flows in excess of bankfull magnitude result in more divergent comparative hydraulic characteristics due to the narrower floodplain width of the diversion. This analysis of the idealised channel geometry results suggests that the diversion designs could be optimised through iterative change of floodplain width, bed slope and possibly a wider bankfull width. Recommendations are presented below about further development, or optimisation of the design, to improve hydraulic characteristics in subsequent more detailed phases of the project.

5.4.1 Floodplain width

The existing project reach is characterised by an incised channel in a broad floodplain. Where a channel is contained in a wide floodplain the channel velocity, shear forces and stream power begin to asymptotically approach a maximum value once the depth of flow exceeds the bankfull depth. Reducing the floodplain width changes the hydraulic characteristics of the floodplain, resulting in a steady increase in velocity, shear forces and stream power for higher order of magnitude flood events.

The preliminary diversion design incorporates an engineered floodplain, approximately 200 m wide through spoil and natural ground where the alignment results in a cut profile. The selection of this width being driven largely by spatial constraints arising from mine planning requirements, such as maximising recovery of coal resources, and providing sufficient space for out of pit placement of spoil material. There would be hydraulic benefit in increasing this width; however this would come at an economic cost to the mine, in that the loss of area associated with a wider floodplain would translate into higher spoil dumps and associated haulage costs.

An assessment of the hydraulic benefit of increasing this floodplain was undertaken and is presented on Figure 5.7 and Figure 5.8 below. The figures are based on calculations utilising average bed slopes for the full extent of both the existing creek reach (0.00114 m/m) and diversion reach (0.00158 m/m), varying the floodplain width from 200 to 300 m. The figures indicate that both shear and stream power are more sensitive to bed slope than floodplain width. Increasing the floodplain width results in a minimal hydraulic performance benefit with flows up to a 50 year ARI, although the benefits become more significant when approaching higher order magnitude flood events such as 1000 year ARI.



Figure 5.7 Impact of floodplain width on shear stress

Figure 5.8 Impact of floodplain width on stream power

The evaluation of the preliminary design planform geometry relative to the hydraulic geometry relationships that have been developed for general guidance for sandy bed creeks, discussed in Section 5.3, indicated that the floodplain width would need to be in the order of approximately 300 m to satisfy guidance regarding meander amplitude, and it is recommended that a design iteration be undertaken with this floodplain width during future phases of design development.



5.4.2 Diversion bed slope

The Stage 1 and Stage 3 permanent diversions incorporate bed slopes which are within the range of 0.0010 to 0.0015 m/m, which are representative of a 30 km reach of Horse Creek in which the diversion is located, as shown on Figure 3.2. The Stage 4 diversion preliminary design incorporates a sinuous low flow channel with a bed slope of 0.00184 m/m, which is somewhat steeper than this range resulting in hydraulic characteristics (velocity, shear force and stream power) that, while within ACARP guidelines, are significantly higher than those anticipated for the section of creek being replaced. The Stage 4 diversion ties into the existing Horse Creek channel upstream and the Stage 1 channel downstream, and is hence grade constrained. There are three possible alternatives for amending the preliminary design such that the bed slope will more closely approximate the existing grade of Horse Creek, being:

incorporating the existing oxbow immediately upstream of the Stage 1 diversion into the Stage 4 diversion – with no changes to the preliminary design planform this would add approximately 840 m to the diversion length, which translates into a Stage 4 bed slope of 0.00144 m/m which is very similar to the existing bed slope of 0.00130 m/m for this reach of the creek – this could be achieved without undertaking construction off mine-lease

increasing the sinuosity of the low flow channel – the preliminary design incorporates a low flow channel which has a sinuosity index of 1.3, crosses the diversion floodplain axis at 480, a wavelength of approximately 450 m, and bend radii of approximately 90 m; in order to achieve a bed slope improvement similar to that achievable by including the oxbow the sinuosity index would have to be increased to approximately 1.5 by increasing the floodplain axis crossing angle to 60o and reducing bend radii to approximately 60 m; while achieving a closer match in bed slope this planform is quite dissimilar to the existing section of creek being replaced, and every additional bend introduced into the diversion is a source of potential instability

floodplain widening which would allow for an increase in amplitude of the sinuous low flow channel and in so doing increase its length – in order to achieve a similar magnitude of improvement offered by incorporating the oxbow, a floodplain approximately 330 m wide would be required and the floodplain axis crossing angle would need to be increased to approximately 58o - this width is more in line with that suggested by the hydraulic geometry relationships detailed in Section 5.3 above.

For the purposes of the functional design the preliminary planform described above has been retained for detailed hydraulic analysis (covered in Section 6 below), without implementing any of the options described above. Should the project progress into a detailed design phase at some stage in the future, it is recommended that the incorporation of the oxbow into the Stage 4 diversion be considered further, as well increasing sinuosity in the Stage 4 diversion to reduce bed slope.



6. Hydraulic conditions of watercourse diversion

6.1 Two-dimensional flood modelling In order to establish baseline hydraulic conditions for the project reach of Horse Creek and evaluate the performance of the proposed diversion stages individually, flood models were set up for the following landforms:

existing project reach of Horse Creek, extending past the upstream and downstream mine lease boundaries to determine baseline (undeveloped) flood conditions

Stage 1 diversion landform, including flood protection levees

Stage 1 and Stage 2 diversion landforms (i.e. both operating simultaneously), including flood protection levees

final landform of the diversion incorporating permanent elements of Stage 1, Stage 3 and Stage 4.

Models were created using XP-SWMM, which is a computer program for simulating depth-averaged, two and one-dimensional free-surface flows such as occurs from floods and tides. XP-SWMM 2d is based on the computational engine TUFLOW which was originally developed for modelling two-dimensional (2D) flows. XP-SWMM 2d allows improved functionality and creation of models with multiple grid size domains.

The model was used to simulate the 2, 10, 50, 100 and 1000 year ARI flood events using inflow hydrographs from the XP-RAFTS model, described in Section 4.1 above.

6.1.1 Terrain data

A digital elevation model (DEM) covering the extent of the MLA derived from 1:10000 and 1:12000 scale aerial photography (Cottrell, Cameron and Steen, 29 September 2009) was used as the basis for the flood models. Diversions were modelled using 12d design software to create amended DEM’s for incorporation in XP-SWMM 2d models for each diversion operational scenario described in Section 6.1 above.

The baseline survey resolution is +/- 0.15–0.2 m within the top of banks of Horse Creek.

6.1.2 Grid cell size

XP-SWMM 2d is a grid-based modelling system, which converts the ground into a series of vertical and horizontal grids, or cells, of nominated dimensions. The smaller the cell size, the more accurately the model reflects the actual ground with associated model result accuracy, however the processing time is significantly increased. For larger flood events that will not be contained within the creek banks and will spill onto the floodplain, larger cell dimensions may be adopted without loss of modelling accuracy. For flows that will be contained within the stream banks a smaller cell is required to achieve modelling accuracy.

Two grid cell sizes were used for the hydraulic analysis of Horse Creek and floodplain for the 2, 10, 50, 100 and 1000 year ARI events. A 5 m x 5 m cell size grid was adopted to define the channel geometry within the banks of Horse Creek. Horse Creek has a typical bottom width of 5–10 m and a top width of 25–50 m. Beyond the tops of banks (the floodplain), the terrain is generally much flatter and shows only a gradual variation in slope. Floodplains can be defined by a larger grid cell size. The Horse Creek floodplain is defined by a 20 m x 20 m cell size grid.



6.1.3 Boundary conditions

The upstream boundary of Horse Creek model was located approximately 1 km upstream of the MLA boundary. Hydrographs from the XP-RAFTS model for the Horse Creek catchment upstream of the MLA and tributary sub-catchments to Horse Creek within the MLA were applied as inflow points at the creek bed level of Horse Creek in the XP-SWMM 2d models.

The downstream boundary of the Horse Creek model was located approximately 3.7 km downstream of the MLA boundary. The downstream boundary was set as a head-flow type with creek bed and floodplain gradient of 0.002 %. This type allows XP-SWMM 2d to determine the tailwater level based on normal depth calculations.

6.1.4 Roughness

Roughness values for the XP-SWMM 2d model area were assigned to different land use types using the aerial photography. Table 6.1 lists the Manning’s ‘n’ value for the land use types identified in the model area.

Table 6.1 Roughness values for the XP-SWMM 2D model

Land use Manning’s ‘n’ value

Water courses with no vegetation 0.03

Rural (bare/grass floodplain) 0.04

Riparian zone/light vegetation 0.06

Dense vegetation (copses of trees) on floodplain 0.08

6.1.5 Validation

Due to the lack of surveyed flood level information in the MLA during flood events it has not been possible to calibrate the XP-SWMM 2d model by comparing water levels predicted by the model with recorded water levels for a known flood event.

6.2 Two-dimensional modelling results Two-dimensional flood modelling of the existing project reach of Horse Creek as well as the individual stages of the creek diversion were undertaken to identify the existing hydraulic characteristics of the creek system, and to evaluate the performance of the various stages of the diversion. For the purpose of evaluation and comparison the hydraulic characteristics represented by channel velocity, bed shear stress, and stream power have been extracted from flood mapping along the centreline of the low flow channel, for the base case and all diversion stages, for 2, 50 and 1000 year ARI events, and are presented graphically in Appendix E. Discussions regarding the results are presented in the relevant sections below. The profiles represent the entire project reach for each scenario, and are hence inclusive of existing and diverted sections of the reach, and extend over 1 km upstream and downstream of the full extent of the final diversion for all scenarios.

It should be noted that the ground models prepared to represent the different stages of the creek diversion as they are developed reflect the preliminary nature of the design, and are hence not of a detailed design quality. This impacts particularly on transitions between natural ground and final landforms, where a lack of fine integration results in some flood modelling anomalies. For the purposes of the commentary below these anomalies have been ignored.

Afflux has been discussed under a separate heading below. A full Flood Study Report is included in Appendix D, which includes detailed flood maps for all scenarios.



6.2.1 General discussion

The base case flood modelling indicates that the flood flows are contained within the meandering low flow channel for events up to approximately 5 year ARI, thereafter breaking out into the floodplain and ultimately occupying the full floodplain for upwards of 100 year ARI events. The floodplain width within MLA 50254 varies considerably with a number of “pinch points” where sedimentary rock spurs confine the creek valley, restricting its width to 250 – 300 m, as indicated on Figure 6.1, which has been extracted from base case flood mapping for a 1000 year ARI event.

Figure 6.1 Existing floodplain pinch points

The results obtained from the flood modelling are described under the headings of velocity, bed shear and stream power below (and graphically in Appendix E), and include comments regarding the performance of the diversions compared to the base case characteristics, as well as ACARP guideline values which are replicated in Table 6.2 below. The commentary should be read in conjunction with the profile plots provided for the base case and diversion stages for flow depth (Appendix E1 – no commentary provided), velocity (E2), bed shear (E3) and stream power (E4).

Table 6.2 ACARP guideline values

Scenario Velocity (m/s) Bed Shear Stress (N/m2)

Stream Power (N/ms or W/m2)

2 year ARI flood < 1.5 < 40 < 60

50 year ARI flood < 2.5 < 80 < 220



6.2.2 Velocity comparison (Appendix E2)

Velocity profiles for the base case (existing creek) indicate a wide variation in values along the length of the project reach due to the stepped nature of the longitudinal profile. 2 year ARI velocities are predominantly below 1.5 m/s although there are a number of localised sections where velocities exceed this value. The same general comments are valid for the 50 year ARI event, with velocities well below 2.5 m/s for almost the whole project reach. 1000 year ARI values are similar to those of the 50 year event due the wide nature of the floodplain; however at pinch points the values diverge and approach 3.5 m/s.

The Stage 1 diversion does not significantly alter the velocity profile for the creek, although through the diversion itself velocities are slightly elevated for all 3 events, more noticeably for the higher order events due to the 200 m wide engineered floodplain being narrow than the existing floodplain. Velocities through the diversion are below 1.5 m/s and 2.5 m/s for 2 and 50 year events respectively. The maximum velocity for a 1000 year event is 3.5 m/s, and is at the same locations as for the base case which is not within the extent of the diversion.

The Stage 2 temporary diversion experiences high velocities over its full extent for all flood events, in the order of 1.8 m/s, 3.4 m/s and 5.4 m/s for the 2, 50 and 1000 year events respectively. This was anticipated due the significant steepening of the bed slope, and engineered floodplain pinch point created by levees. The velocity profile of the project reach upstream and downstream of the Stage 2 diversion remains unchanged. The temporary diversion velocity characteristic clearly does not approach those of the existing creek and exceeds ACARP guidelines.

The Stage 3 and 4 diversions represent the final landform and demonstrate velocities predominantly below 1.5 m/s and 2.5 m/s respectively for 2 and 50 year ARI events respectively, with 1000 year ARI event results also generally below 2.5 m/s. Due to the engineered nature of the diversion the results display far less variation in magnitude, and follow a predictable pattern governed by extended lengths of constructed constant bed slope.

6.2.3 Bed shear stress (tractive force) (Appendix E3)

Bed shear profiles for the base case mirror those of the base case velocity profile, with a wide variation over profile length. 2 year ARI values are generally below 40 N/m2, although there are a significant number of localised sections of the reach where this value is exceeded. The same general comments are valid for the 50 year ARI event, with values below 80 N/m2, but again with instances of shear in excess of this value. 1000 year ARI values are similar to those of the 50 year event due the wide nature of the floodplain; however at pinch points the values diverge with a maximum of 220 N/m2 predicted.

The Stage 1 diversion does not significantly alter the shear profile for the project reach of the creek. Values through the diversion itself are less than 40 N/m2 and 80 N/m2 for 2 year and 50 year ARI events respectively, and are predominantly below 80 N/m2 for the 1000 year ARI event as well. For a 1000 year ARI event values upstream and downstream of the diversion are unchanged with peak values predicted in the same locations and same magnitudes.

The Stage 2 temporary diversion experiences high bed shear over its full extent for all flood events, in excess of 40 N/m2 and 80 N/m2 for the 2 and 50 year events respectively, increasing up to 275 N/m2 for the 1000 year ARI event. This was anticipated due the significant steepening of the bed slope, and floodplain pinch point created by levees. The shear profile of the project reach upstream and downstream of the Stage 2 diversion remains unchanged in shape and magnitude. The temporary diversion shear characteristic clearly does not approach those of the existing creek and exceeds ACARP guidelines.



The Stage 3 and 4 diversions represent the final landform and demonstrate bed shear predominantly below 40 N/m2 and 80 N/m2 for the 2 and 50 year events respectively, with 1000 year ARI event results also predominantly below 80 N/m2. Due to the engineered nature of the diversion the results display far less variation in magnitude and follow a predictable pattern governed by extended lengths of constructed constant bed slope.

6.2.4 Stream power (Appendix E4)

Stream power profiles for the base case mirror those of the base case velocity profile, with a wide variation over profile length. 2 year ARI values are generally below 60 N/ms, although there are a significant number of localised sections of the reach where this value is exceeded. The same general comments are valid for the 50 year ARI event, with values below 220 N/ms, but again with instances (although fewer than for the 2 year ARI event) where this value is exceeded. 1000 year ARI values are similar to those of the 50 year event due the wide nature of the floodplain; however at pinch points the values diverge with a number of locations recording values in excess of 600 N/ms.

The Stage 1 diversion does not significantly alter the stream power profile for the project reach of the creek. Values through the diversion itself are less than 60 N/ms and 220 N/ms for 2 year and 50 year ARI events respectively, and are predominantly below 220 N/ms for the 1000 year ARI event as well. For a 1000 year ARI event values upstream and downstream of the diversion are unchanged with peak values predicted in the same locations and same magnitudes.

The Stage 2 temporary diversion experiences high stream power over its full extent for all flood events, in excess of 60 N/ms and 220 N/ms for the 2 and 50 year events respectively, increasing up to 1600 N/ms for the 1000 year ARI event. This was anticipated due the significant steepening of the bed slope, and floodplain pinch point created by levees. The stream power profile of the project reach upstream and downstream of the Stage 2 diversion remains unchanged in shape and magnitude. The temporary diversion stream power characteristic clearly does not approach those of the existing creek and exceeds ACARP guidelines.

The Stage 3 and 4 diversions represent the final landform and demonstrate stream power values predominantly below 60 N/ms and 220 N/ms for the 2 and 50 year events respectively, with 1000 year ARI event results also predominantly below 220 N/ms. Due to the engineered nature of the diversion the results display far less variation in magnitude and follow a predictable pattern governed by extended lengths of constructed constant bed slope.

6.2.5 Summary of hydraulic characteristics

The hydraulic characteristics, represented by velocity, bed shear stress and stream power, displayed by the permanent diversions are representative of characteristics of the existing project reach for a wide range of flows, and are generally well within ACARP guidelines. The results from the 2D-flood modelling verify the idealised channel calculations described in Section 5 above, and support the geometric dimensions selected for the purposes of functional design. As described in the Section 5 there is scope to optimise the design further during subsequent phases of the project, and certain recommendations have been made in this regard.



The hydraulic characteristics displayed by the Stage 2 temporary diversion are not comparable with existing reach characteristics, and are significantly above ACARP guidelines. It is likely that this diversion will not operate as a stable channel and will be prone to erosion and vertical degradation. This would be unacceptable for a permanent structure but can be justified for a temporary structure, provided stability is engineered into it. This engineered stability could take the form of grade control by way of a drop structure constructed in the channel, or rock armouring to prevent embankment erosion. The geotechnical investigation identified that the bed of the constructed channel will extend into sandstone, which will prevent vertical degradation for a significant portion of its length, and also provide a source of rock armouring for the embankments that would be sufficiently durable for the life of this diversion. The selection of an acceptable solution is left to subsequent phases of design development, however a nominal design for rock armouring was undertaken for 1000 year ARI conditions to verify the viability of this as an engineering solution, which has been included in the design drawings contained in Appendix A.

6.2.6 Afflux The various flood models developed for the purpose of the project were interrogated to identify the impact of the various diversion stages in terms of afflux water levels at the upstream, downstream and western boundaries of the mine lease. A summary of the results has been tabulated below, which includes the approximate water level at peak discharge as well as the variation of the scenario to the base case water level. The information has been collected and summarised in Table 6.3. Comprehensive afflux mapping has been included in the Flood Study Report.

Table 6.3 Afflux levels for modelled scenarios

ARI MLA Boundary

Peak water level for scenario (m AHD)/Change (m)

Base case

Stage 1 Stage 2 Stage 3/4

2 year Upstream 247.17 247.18 0.01 247.17 0 247.08 -0.09

Downstream 234.29 234.26 -0.03 234.14 -0.15 234.25 -0.04

5 year Upstream 248.64 248.64 0 248.54 -0.1 248.39 -0.25

Downstream 235.58 235.63 0.05 235.53 -0.05 235.38 -0.2

10 year Upstream 249.46 249.45 -0.01 249.46 0 249.38 -0.08

Downstream 236.37 236.37 0 236.33 -0.04 235.57 -0.8

50 year Upstream 250.06 250.06 0 250.07 0.01 250.06 0

Downstream 236.63 236.62 -0.01 236.63 0 235.94 -0.69

100 year Upstream 250.18 250.18 0 250.24 0.06 250.20 0.02

Downstream 236.71 236.71 0 236.70 -0.01 236.11 -0.6

1000 year Upstream 250.89 250.89 0 251.45 0.56 251.01 0.12

Downstream 236.91 236.92 0.01 236.91 0 236.75 -0.16

The results tabulated above may be summarised as follows:

there is effectively no adverse afflux impact arising from the Stage 1 diversion for any flood event either at the upstream or downstream MLA boundary



the Stage 2 diversion will result in a 0.56 m afflux at the upstream MLA boundary during a 1000 year ARI event, due to the constrained floodplain width caused by a flood protection levee to the west of the diversion and higher ground to the east – this constriction causes a backwater upstream during the flood event; during subsequent phases of the project the “opening up” of this constriction should be investigated by moving the levee further west and assessing the impact on mining operations

the Stage 3 and Stage 4 diversions (modelled as a final landform) do not cause any adverse afflux impacts at either the upstream or downstream MLA boundaries.

In addition to the upstream and downstream MLA boundaries, the afflux on the western MLA boundary was evaluated to identify any impacts on the adjacent property. An extract from the Flood Study Report is included below (Figure 6.2) for the Stage 3/4 diversion based on a 1000 year ARI, which reflects a worst case scenario for this boundary. The afflux is presented on a “Was Dry Now Wet” output (green hatching) and represents an afflux of approximately 800 mm.

Figure 6.2 Western MLA boundary afflux



7. Geotechnical stability 7.1 Regional setting The regional geology for the project reach was obtained utilising the Geological Survey of Queensland’s 1:250,000 series St Lawrence sheet, Queensland Department of Employment, Economic Development and Innovation interactive resource and tenure map (https://webgis.dme.qld.gov.au/webgis/webqmin/viewer.htm), which identified the following main geological units:

Qa - NSB/Quaternary: Clay, sand, gravel; flood plain alluvium

Ji - Injune Creek Group/Middle to Late Jurassic: Calcareous lithic sandstone, siltstone, mudstone, coal, conglomerate.

7.2 Field investigations Field investigations comprising boreholes and trial pits were undertaken to confirm the presence of these units in areas where the diversion is to be constructed through natural ground. Exploration pits were excavated to below the excavated profile of the diversion channel. Laboratory testing was undertaken on samples of representative materials taken from exploration pits to determine the engineering properties of the materials in order to assess stability of the diversion cross section for all strata encountered.

The extent of the field investigations and borehole data are included in the Table 7.1 and Figure 7.1 below.

Table 7.1 Investigation borehole data

Borehole No Top soil (mBGL*)

Alluvium (mBGL*)

Residual soil (mBGL*)

Weathered rock (mBGL*)

BH01 0.2 6.95+ - -

BH02 0.2 5.3 7.4+ -

BH03 - 5.5 8.0 9.87+

BH04 0.3 4.5 5.1 13.2+

BH05 0.2 6.0 6.8 12.5+

BH06 0.2 3.65 6.0 8.22

BH07 0.2 - 0.8 8.0+

BH08 0.3 6.4 - 7.95+

BH09 0.2 - 5.7 8.8+

BH10 0.2 8.5+ - -

BH11 0.2 2.69+

BH12 0.2 2.95+

BH13 - 2.4 - 2.76+

https://webgis.dme.qld.gov.au/webgis/webqmin/viewer.htm



Figure 7.1 Exploratory borehole locations

7.3 Mine spoil assessment A significant proportion of the diversion length will be constructed through mining spoil, which will only be placed once mining commences, hence the properties of this material could not be determined in-situ. Insite Geology (2009) reported five main groups of rock types within the overburden, coal seams, interburden and floors, including sandstone, sandstone/siltstone, carbonaceous mudstone, and coal. Waste material removed to access the coal resource will generally be placed back in the pit as spoil.



In accordance with the EIS report Groundwater Assessment), there are five coal seams of the Juandah Coal Measures, namely UG, Y, A, B and C in increasing depth below natural surface within the project. The overburden thickness is estimated to vary from between 20 to 60 m to the top of the A seam being around 40 m in the majority of the mining area. The depth of the floor of the deepest target seam, the C seam, is reported to be up to approximately 100 m. However, the majority of the coal resource is within 50m depth, which reflects the average depth of spoil below the constructed creek diversion where construction is through spoil.

It is understood that the project will involve open cut mining using truck and excavator methods. Due to the factors such as varieties of parent rock/soil materials, excavation methods, compaction and age of spoil, the resulting spoil will be generally a highly heterogeneous material composing many elements and with a wide variety of possible textures, involving particles from sub-micron to boulder size.

A literature review of mine spoil properties was undertaken to identify suitable engineering values for adoption for design purposes, which is described fully in the Geotechnical Investigation Report included in Appendix F.

7.4 Engineering properties Based on the field tests undertaken for in-situ soils and a literature review undertaken for the mine spoil, engineering properties adopted for preliminary design purposes are included in Table 7.2 below.

Table 7.2 Geotechnical parameters of the coal mine spoil

Unit weight (kN/m3)

Cohesion c’ (kPa)

Frictional angle ’ (°)

Permeability kv (m/s)

Permeability kh (m/s)

Very stiff clay 19 4 27 - -

Hard clay 20 4 28 - -

Loose sand 19 0 30 - -

Medium dense sand 20 0 33 - -

Dense sand 20 0 36 - -

Very dense sand 21 0 40 - -

Mine spoil 20 0 30 1x10-5 5x10-5

Sandy clay/clay/ clay liner/leeve

For seepage analysis 5x10-10 5x10-9

Clayey sand/sand/ sandy gravel

For seepage analysis 3x10-7 3x10-6

7.5 Geotechnical stability In order to assess the geotechnical stability of the proposed staged diversions the following calculations were undertaken to verify design assumptions:

cross sectional geometry slope stability (1V:3H channel slopes and 1V:4H engineered floodplain slopes)

seepage of water to adjacent mine pits during flood events (due to proximity of the diversion to pits during operation and the final void after closure)

settlement of mine spoil over time, both spoil settlement and differential settlement at the spoil / natural ground interface.



A full description of these investigations is contained in the diversion Geotechnical Report, which is included in Appendix F. A summary of the findings is included below.

7.5.1 Geometric stability

The stability of the channel and floodplain embankments was assessed using SlopeW, for scenarios where diversions are to be constructed through natural ground as well as through spoil, applying the engineering properties described above. Slopes were assessed for short and long term stability against factors of safety of 1.25 and 1.5 respectively, where relevant incorporating the surcharged loading of mine spoil dumps adjacent to the floodplain embankments. No embankment instability was identified in any of the scenarios modelled.

7.5.2 Seepage

Seepage analysis was undertaken for the various stages of the diversion to assess to implications of the proximity of the diversion alignment to active pits. The seepage analysis was based on 1000 year ARI water levels and flood durations as predicted by the hydrology models for the various diversion stages. The analysis for the Stage 1 diversion assumed a boundary condition of active pits immediately adjacent to the outer extent of the 200 m wide floodplain, which is a conservative assumption as the pits migrate away from floodplain over time, creating significantly longer seepage paths. The seepage calculation for Stage 1 was considered representative for conditions for the Stage 2 diversion as well so a separate seepage model was not created. A seepage model for the Stage 4 diversion adjacent to the pit/final void was created to assess subsurface flow through mine spoil; this model was considered representative of Stage 3 as well.

The seepage analysis results show that additional water inflow during the 1000 year ARI flood event towards the mine pit adjacent to the Stage 1 diversion is less than 10% of the anticipated inflow from the groundwater for the duration of the flood event. The Stage 4 model predicted higher seepage values through spoil, although still contributing less than 20% of the groundwater inflow expected to enter the adjacent pit/final void during the duration of the flood.

Given that the 1000 year ARI flood events are extremely rare and groundwater flow is continuous, the inflows into the pits due to seepage were considered manageable.

7.5.3 Settlement

Open cut mine spoil backfill is typically subject to the settlement mechanisms of creep and collapse settlement. Creep settlement generally occurs under conditions of constant stress and moisture content on completion of the placement over a long period of time. The collapse settlement is generally caused by inundation of spoil by groundwater or surface water, and occurs relatively quickly after inundation occurs.

There is little information available locally regarding settlement of mine spoil, so a literature review was undertaken to gain insight into prediction and mitigation methods used in other countries. The review is described more fully in the Geotechnical Report (which includes references) while relevant findings are described below.

7.5.3.1 Collapse settlement

Several case studies in the UK indicated that open cut mine backfills, placed without systematic compaction, were usually susceptible to collapse compression on inundation. The case histories indicate a maximum compression from 2 % to 6 % of the depth of saturated backfill can be expected. These studies reported settlement monitoring indicating that most of the collapse settlement takes place within 3 or 4 years after the commencement of inundation i.e. when mining and associated dewatering ceases and groundwater tables rise to pre-mining levels.



The collapse settlement component of overall settlement hence occurs during, and shortly after the completion of mining, and can be managed as part of ongoing mining operations.

7.5.3.2 Creep settlement

The creep settlement represents a slow consolidation of the spoil material over time, which generally follows a log-time relationship, with magnitude dependent on the compactive state of backfill. The review of technical literature yielded observations of creep settlement ranging from 0.1% to 0.4% of the depth of inundated backfill with a full compaction, up to 1.5% in areas of poor compaction, and approximately 2.5% in uncompacted backfill. The effect of the log-time relationship means that the rate of creep settlement slows with time.

7.5.3.3 Total settlement prediction and mitigation

Mine spoil backfilling is generally placed without systematic compaction, and it is unlikely that systematic compaction of spoil will be undertaken on this project for practical and economic reasons. Most spoil will experience some degree of compaction during the operation of placement over time, through the running of haul trucks over the placed spoil and also of the progressive layered placement of spoil backfill, which acts in the way of sequential pre-loading of lower layers as backfill height increases.

For the functional design stage of this project, it has been assumed that the spoil backfill is partially compacted for the reasons described above, that groundwater levels inundate the spoil backfill to approximately 50% of pit height during mining, and recover to ground surface within 3 – 4 years after mining ceases. Based on these assumptions and the design methodology described in the Geotechnical Report, the following total settlements have been estimated (these are progressively cumulative so reflect anticipated total settlement at the end of each period – see Figure 7.2 below for graphical representation):

during mining operations (partial inundation of backfill) – 0.63 m

3 - 4 years after mining ceases (complete inundation of backfill) – 0.97 m

long term settlement (100 years) (long term creep) – 1.12 m.

Figure 7.2 Likely spoil backfill settlement timeline



7.5.3.4 Differential settlement

Differential settlement will occur at:

the transition between construction of the diversion through natural ground and mine spoil

locations of transitions between significant differences in spoil backfill height.

If a mine pit depth of 65 m and a batter ration of 1:1 are assumed at transitions, the differential settlement over the transition is estimated to be:

967 mm over 65 m distance, i.e. approximately 1.5 % slope from natural to settled spoil over the length of the transition within 3 years after the mine closure if no remedial measures are taken

1121 mm over 65 m distance, i.e. approximately 1.7% slope from natural to settled spoil over the length of the transition 100 years after mine closure if no remedial measures are taken.

These equate to the calculations of settlement in Section 7.5.3.3 above, but will just occur differentially rather than as a uniform block.

7.5.3.5 Settlement mitigation

Based on the predictions described above it is evident that settlement of approximately 1.12 m will occur below the diversions constructed in spoil over a period of approximately 100 years, with the bulk occurring during and shortly after mining ceases. This settlement will be encountered in sections of the diversion constructed through spoil only, which will result in differential settlement at interface points where these sections tie-into existing sections of Horse Creek or diversions. The differential settlement will occur over a length of diversion equivalent to the depth of the pit at transition (assuming a 1V:1H high wall slope), resulting in localised steep creek bed gradients over these transition sections unless trimmed.

The following options are available to mitigate the predicted settlements described above:

pre-loading the spoil backfill (overfill the area beneath the diversion alignment) to cause an accelerated rate of early creep settlement – this measure is unlikely to be effective as the diversions areas required to be operational soon after placement of backfill, and hence the pre-loading could only be applied for a very short duration

compact the spoil backfill material below the creek diversion to an engineering standard in order to minimise settlement – this is unlikely to be practical from an operational or economic perspective

pre-saturate the floodplain during construction to accelerate collapse settlement – this is dependent on the availability of a significant amount of water on site and may not be practical or economically viable

make no allowance for settlement in the design of the diversion and manage it through the monitoring and evaluation plan that is required to be developed during the detailed design phase of the project, which would possibly identify grade correction measures (infill) for areas exhibiting threshold levels of settlement

make full allowance for settlement in the design by adopting a design surcharge approach to bed levels, equivalent to the total long term settlement anticipated i.e. constructing the diversion 1 m higher than the required design levels where settlement is not considered – this is impractical as the step in levels at interfaces would be appreciable, impeding flows in the creek and would require considerable tapering earthworks upstream and downstream of the diversion



make partial allowance for settlement by adopting a design surcharge approach to bed levels, equivalent to the short term settlement anticipated – which could be a level surcharge of approximately 0.6 m, followed by grade correction in accordance with the monitoring and evaluation plan during mining operations (up to 4 years after completion of mining activities); the small amount of settlement predicted over the long term could be accommodated through natural geomorphic responses to changes in creek geometry and planform that occur continuously in creek systems.

For the purpose of the functional design it has been assumed that a partially surcharged design level approach will be adopted, allowing for 0.6 m of surcharge elevation of design levels. Based on predicted settlement calculations this surcharge will settle to design levels by the time mining ceases. It is recommended that the settlement predicted to occur in the 3 to 4 years after mining ceases and groundwater levels re-establish, be managed by way of grade correction triggered by threshold deflections in the monitoring and evaluation plan, as measured at benchmark survey locations provided during construction of the diversions. The minimal additional long term settlement will not be managed, as this is likely to occur over a period of 100 years and can be absorbed by natural geomorphic processes continuously occurring in the creek system.

Adopting a surcharged design level approach will offer limited benefit at the transitions, where the levels will be brought back to natural ground level i.e. the surcharge level will taper back to natural ground levels, meaning after settlement localised depressions at the transitions will occur. There is no way to accommodate such deflections in design or construction and it will result in localised steeper grades over the transition length, which would be prone to erosion during flood events. Remedial works would have to be undertaken during mining activities to address this issue. It may also be possible to compensate for the localised steeper grades by stabilising them with heavier vegetation planting or selective placement of woody debris, increasing the Manning’s n value which would reduce flow velocities over these zones.

7.5.3.6 Placement and compaction of spoil backfill

While most mine spoil backfill operations are uncontrolled it is recommended that a measure of control be implemented within the zone of material that forms the diversion channel to increase density, decrease permeability and restrict variability. The measures should incorporate:

limiting the placement of layers to a maximum depth of 3 m

control backfill and compact the zone within 1 m of the design invert of the diversion channel to an engineered fill standard; based on the limited lateral migration expected in the channel, the width of this engineered compaction could be restricted to a corridor incorporating the low flow channel plus 10 m on either side of the channel (to be confirmed during detailed design).

7.6 Rock armouring Ideally creek diversions should be stabilised using natural methods such as vegetation and bed slope optimisation, which promotes normal geomorphic processes, such as lateral migration. In certain circumstances project constraints require that engineering protection methods be adopted to provide enhanced stability. In order to provide engineered stability rock armouring is proposed for the following elements of the Functional Design:

outside banks of meander bends to provide stability while vegetation is established and prevent unwanted migration of bends towards active pits or final voids, using a hard durable rock source

flexible, graded riprap facing to the full extent plus upstream and downstream, of the low flow channel in transition zones between spoil and natural ground, designed for stability in a 1000 year event at the bed grade envisaged after mine closure settlement, using a hard durable imported rock source



flexible, graded riprap facing to the full extent of the Stage 2 low flow channel, designed for stability in a 1000 year event at the bed grade of the temporary diversion, using sandstone excavated from the Stage 1 diversion which would provide acceptable durability for the limited life of the diversion

Nominal design cross sections for these areas are presented on the Functional Design drawings in Appendix A.



8. Sediment transfer regime 8.1 General considerations The sediment transfer regime of a creek is a function of the interaction between two sets of variables – one being the quantity and characteristics of the sediment at a particular location in the creek, and the other being the capacity of the creek at that location to transport the sediment. The first group is dependent on the mechanisms producing the sediment such as the geology, reach stability, land use, vegetation and topography of the watershed; and hydrology, such as rainfall intensity, duration, seasonal variation etc. The second group of variables is dependent on the hydraulic characteristics of the creek channel, such as bed slope, roughness, bed shear, stream power and velocity.

The total sediment discharge comprises a suspended load component (washload (fine evenly suspended particles) and suspended bed material load), and a bed load component. The bed load sediment remains in contact with the channel bed during movement. Bed material (sediment other than the washload) is transported at the capacity of the creek and is related to measurable hydraulic characteristics, whereas the washload is only dependent on availability, and as such is independent of the carrying capacity of the creek.

In the context of creek diversions, unless the diversion or associated works is amending the first set of variables described above, the washload component of sediment transfer will remain unchanged. The hydraulic characteristics of the diversion will however impact on the carrying capacity of the creek, hence impacting bed sediment load. The relative calculations to quantify the impact of the diversion on sediment transport capacity described in Section 8.3 below are hence limited to bed load only.

8.2 Existing project reach watercourse The sediment transfer regime of Horse Creek over the project reach was considered in the geomorphology assessment undertaken for the project and is the findings are contained in the Geomorphology Report included in Appendix B. A summary of the relevant findings is included below:

the project reach creek bed is comprised of loose, unconsolidated sediments that are predominantly medium to coarse grained sands with minor gravels and silt and clay found in selected reaches only - this was based on sampling and grain size distribution analysis at 5 sites and a further 15 observational sites along the reach

the sand bed grading distributions from samples taken along the project reach are very similar, demonstrating a similar sediment source and transport mode was in operation at these sites

bank erosion has been reported as a principal supply of new material to rivers (ACARP, 2000); however for the Horse Creek project reach evidence of fluvial erosion (i.e. direct removal of bank material by flowing water) was limited to isolated pockets on the meander bend systems, suggesting that fluvial erosion is not a major component of the sediment supply chain with the only mass movement of bank material (i.e. slumping) caused by livestock accessing the creek for water

the sediment load for Horse Creek is unknown and the flashy runoff nature of the catchment and associated limited data availability makes it difficult to calculate an accurate figure; an estimate of suspended and bed load was undertaken using the methodology described in the Geomorphology Report, with results in the order of 9,472 and 18,944 kg/year, respectively; based on a literature review it was concluded that these quantities are likely to be a significant underestimation of likely actual loads

the calculations described above, while being quantitatively indicative only, suggest that the total sediment discharge for the creek is bed load dominated.



A Hjulstrom curve was generated which illustrates the relationship between sediment size and the velocity regime predicted to erode, transport and deposit channel material in Horse Creek, and is shown on Figure 8.1 below. The diagram predicts that the particles lie within the erosion zone for all flow scenarios. This analysis indicates that the sites investigated are prone to erosion for all ARI events suggesting that the project reach of the creek is in a degradation sediment transport regime stage of its evolution, and that increases in velocity arising from bed slope steepening or floodplain narrowing as a result of the creek diversion will not increase the tendency for movement of channel sediment.

Figure 8.1 Horse Creek Hjulstrom curve

The transitioning of creeks from sediment supplying to sediment depositing regimes along their catchment length is depicted graphically on Figure 8.2. As described above it is likely that the project reach of Horse Creek is in a degradation sediment transport regime.

Figure 8.2 Sediment transport regime along typical creek profile (Copeland et al 2001)



8.3 Watercourse diversions A stable channel is one in which the bed slope and cross section allows it to transport the water and sediment delivered from the upstream catchment without aggradation, degradation or bank erosion over a short period of time. When this condition exists the bed slope and cross section are considered to be in equilibrium. If any of the controlling variables of the equilibrium are changed the creek will respond by altering cross section, slope or planform. These are processes that are occurring in the project reach of Horse Creek already in response to historic changes that have occurred (evidenced by active meander migration).

The geomorphic relationship describing this state of equilibrium is expressed in a qualitative expression developed by Lane (1955):

Q.S Qs.D50, where

Q = discharge or flow

S = bed slope

Qs = bed material (sediment) discharge

D50 = median sediment size by weight

This relationship indicates that an increase in bed slope (S) will lead to an increase in the sediment discharge (Qs) for equilibrium to be re-established, assuming all other parameters remain constant. In the absence of additional bed material input from the upstream catchment, the long-term response to this change will be a readjustment of bed channel slope to achieve equilibrium. This could occur through an increase in sinuosity through planform adjustment, or a combination of aggradation and degradation of the creek bed. Where armouring prevents the formation or movement of meander bends the response will be restricted to the latter. These principles of equilibrium were used to guide the design of Stage 1 - 4 diversions and predict impacts of changes to the bed slope and cross section on channel stability.


For the Stage 1 diversion the proposed length and bed slope are very similar to that of the section of creek being diverted (0.00107 m/m replacing 0.00102 m/m or 50 mm over a km), and it is expected that there will be no change in the sediment transport regime resulting from this small change. Due to the stepped nature of the longitudinal profile of the project reach there are sections of natural bed slope which significantly exceed the diversion bed slope.


The Stage 2 diversion removes a tortuous meander bend in the project reach resulting in a significant steepening of bed slope from 0.0013 to 0.0038 m/m. This diversion will only be operational for approximately 3 years, which represents an insufficient time period for a geomorphic response in the creek to this change. In order to prevent any dramatic short term responses to this change, the channel will be lined with appropriately sized rock armour based on predicted water velocities to provide an engineered stability rather than geomorphic equilibrium. This will prevent the channel from becoming a source of sediment through instability and will also prevent the onset of any meander formation.




For the Stage 3 diversion the proposed length and bed slope are very similar to that of the section of creek being diverted (0.00137 m/m replacing 0.00119 m/m or 180 mm over a km), and it is expected that there will be no change in the sediment transport regime resulting from this small change. Due to the stepped nature of the longitudinal profile of the project reach there are sections of natural bed slope which significantly exceed the diversion bed slope.


The Stage 4 diversion replaces a number of meander bends in the project reach resulting in a steepening of bed slope from 0.00114 to 0.00184 m/m (or 0.7m/km of diversion). The proposed design incorporates a meandering low flow channel in an engineered floodplain through spoil, with the meander amplitude extending across the whole floodplain width. To prevent undesirable meander migration, the meander bends will be stabilised with appropriately sized rock armour based on predicted water velocities.

A comparative evaluation of sediment transport of the creek system has been undertaken for the final permanent creek diversion (Stage 4). There are numerous methods for calculating bed load sediment transfer rate, which yield significantly variable results hence the analysis described below has been undertaken for purely relative comparison purposes, and has been based on the following criteria:

only bed load sediment transfer rates are calculated

due to the variable nature of the bed slopes the analysis has been based on average values for the existing project reach and final diversion landform (incorporating Stage 1, 3 and 4), with associated average bed shear stresses

the analysis has been performed for in-channel (2 year ARI) and bankfull flows

the bed material sand is represented by a median particle size Dm = 0.83 mm based on particle distribution laboratory analysis with an assumed density of 2650 kg/m3.

The results from the analysis using 6 different calculation methods to estimate sediment transfer capacity (Qs) are tabulated below, with the existing creek considered first (average bed slope of 0.00114 m/m), followed by the diversion (average bed slope of 0.00158 m/m).

The in-channel 2 year ARI results are presented first, followed by the bankfull flow:



The following conclusions may be drawn from this comparative analysis:

predictive methods provide significantly different results and hence should be considered for relative comparison only

the Meyer-Peter bed load equation is widely used and possibly provides the best order of magnitude estimate (Richardson et al 2001)

Formulas:

f = o / (( s - ).d) = R S / (( s - ).d) Q (m3/s) 54 Existing creek with 54 Final diversion with

B2 = bottom width channel (m) B2 (m) 6 average bed slope 6 average bed slope

R = hydraulic radius (m) R (m) 2.09 and 2 yr ARI flow 1.82 and 2 yr ARI flow

S = bed slope of channel (m/m) S (m/m) 0.00114 0.00158Dm = median sediment size (m) Dm (m) 0.000828 0.000828

s = 2650 kg.m2C 56 56

= 1000 kg.m2 Cx 6.75 6.75

a = s - ) / = 1.65 f 1.744 2.105C - Chezy coeffMethod Formula qs/(agd3)0.5 qs Qs (kg/s) qs/(agd3)0.5 qs Qs (kg/s)

Meyer-Peter (1934) qs / (agd3)0.5 = 8 (f - 0.047)1.5 17.68 0.00170 27.1 23.62 0.00228 36.2

Shields (1936) qs / (agd3)0.5 = 10. Cx (f - 0.076) f1.5 259.18 0.02497 397.0 418.00 0.04027 640.3Cx = (C / g0.5). ( / s)

Einstein-Brown (1942) qs / (agd3)0.5 = 23.6 f3125.18 0.01206 191.8 220.07 0.02120 337.1

Kalinske (1947) qs / (agd3)0.5 = 10 f2.5 40.16 0.00387 61.5 64.27 0.00619 98.5

Bonnefille (1963) qs / (agd3)0.5 = 5.5 f1.5 (4.26 f0.5 - 1)1.25 85.93 0.00828 131.6 131.26 0.01265 201.1

Hassanzadeh (2007) qs / (agd3)0.5 = 24 f2.5 96.39 0.00929 147.7 154.26 0.01486 236.3

Formulas:f = o / (( s - ).d) = R S / (( s - ).d) Q (m3/s) 142 Existing creek with 165 Final diversion with

B2 = bottom width channel (m) B2 (m) 6.0 average bed slope 6.0 average bed slope

R = hydraulic radius (m) R (m) 2.63 and bankfull flow 2.6 and bankfull flow

S = bed slope of channel (m/m) S (m/m) 0.00114 0.00158Dm = median sediment size (m) Dm (m) 0.000828 0.000828

s = 2650 kg.m2 C 56 56

= 1000 kg.m2 Cx 6.75 6.75

a = s - ) / = 1.65 f 2.195 3.007C - Chezy coeffMethod Formula qs/(agd3)0.5 qs Qs (kg/s) qs/(agd3)0.5 qs Qs (kg/s)

Meyer-Peter (1934) qs / (agd3)0.5 = 8 (f - 0.047)1.5 25.18 0.00243 38.6 40.74 0.00392 62.4

Shields (1936) qs / (agd3)0.5 = 10. Cx (f - 0.076) f1.5 464.69 0.04477 711.8 1031.05 0.09934 1579.4Cx = (C / g0.5). ( / s)

Einstein-Brown (1942) qs / (agd3)0.5 = 23.6 f3 249.43 0.02403 382.1 641.59 0.06181 982.8

Kalinske (1947) qs / (agd3)0.5 = 10 f2.5 71.35 0.00687 109.3 156.78 0.01510 240.2

Bonnefille (1963) qs / (agd3)0.5 = 5.5 f1.5 (4.26 f0.5 - 1)1.25 144.15 0.01389 220.8 291.18 0.02805 446.0

Hassanzadeh (2007) qs / (agd3)0.5 = 24 f2.5 171.23 0.01650 262.3 376.27 0.03625 576.4



the transport capacity of the diversion is not less than that of the existing watercourse and so additional deposition of material is not expected in the diversion creek

increasing bed slope increases the sediment transport capacity of the channel - this impact will be localised and a range of mitigation measures will be employed to reduce this occurring in the diverted creek by identifying ‘at risk’ reaches and protecting with bank armouring, geotextile linings and vegetation planting

incorporating the existing cut-off oxbow upstream from the Stage 1 diversion should be considered in subsequent stages of deign development, which will reduce average bed slope to closer proximate the existing creek bed slope and hence sediment transfer capacity.



9. Revegetation 9.1 Revegetation considerations Assessments completed on the geomorphology, soil and condition of vegetation within the pre-existing Horse Creek project reach have been used to guide the development of the conceptual revegetation strategy for the Horse Creek Diversion. Information has been obtained from the following assessment reports:

Horse Creek Riparian Vegetation Assessment, AustralAsian Resource Consultants, October 2013

Horse Creek Diversion Geomorphology Report, Parsons Brinckerhoff, October 2013

Elimatta Project Waterway Morphology and Aquatic Ecology Assessment Report, AustralAsian Resource Consultants, April 2012

Elimatta Project Soil and Land Suitability Assessment, Australasian Resource Consultants, April 2012.

The information presented in these assessments is synthesised in the following subsections.

9.2 Soil Soils in the catchment are predominantly vertosols (cracking clay soils) but sododsols are present along the creek corridor in the upper portion of the project reach (ASRIS, 2013). This has important implications as the sododsols are susceptible to erosion if vegetation is removed (DNRM, 2006). Conversely, vertosols are relatively more stable and resistant to erosion due to having better aggregated profiles.

Six soil management units (SMUs) have been identified within the MLA; Horse Creek Alluvium SMU, Cheshire SMU, Rolleston SMU, Downfall SMU, Kinnoul SMU and Juandah SMU (AARC, 2012).

Soils within the Kinnoul, Cheshire and Rolleston SMUs have instances of sodic soil layers. Accordingly, these soils are likely to be prone to locally severe occurrences of sheet, rill and gully erosion due to uncontrolled surface water runoff from the hard setting surface soils. Whilst also displaying high levels of exchangeable sodium, soils of the Juandah and Downfall SMUs are at less of a risk of dispersion due to the relief of the mostly flat plains and broad ridge tops on which they occur (AARC, 2012).

The Downfall, Kinnoul, Cheshire, Rolleston and Juandah units possess sodic subsoils with increasing levels of exchangeable sodium within the upper 900 mm of the profile. Salinity also increases with depth within these profiles, to levels considered moderate to highly saline by 900-1000 mm. An exception to this is the Horse Creek Alluvium SMU, with no signs of sodicity or salinity present within the profile. All soils present within the MLA are considered moderately deficient of major soil nutrients (AARC, 2012).

Useable soil resources are mainly confined to the surficial horizons and locally in the upper part of the subsurface horizons which contain seed-stock, micro-organisms and nutrients necessary for plant growth. The following list presents the soil management units in terms of the quality of their topsoil resource (from most to least suitable) and outlines their recommended stripping depths:

Horse Creek Alluvium -1000 mm

Cheshire – 300 mm

Rolleston – 200 mm

Downfall – 200 mm

Kinnoul – 100 mm



Juandah – 0 mm (AARC, 2012).

The distribution of SMUs within the portion of the Elimatta Project MLA that will be subject to diversion of Horse Creek is shown in Figure 9.1 below.

Figure 9.1 Soil Management Units within the southern portion of Elimatta Project MLA (AARC, 2012)

9.3 Existing landscape and vegetation The Elimatta Project site is located within a rural landscape that has undergone considerable change since European discovery in the 19th century. The local landscape has been significantly impacted by grazing pressure, with large expanses of remnant vegetation cleared in favour of open grasslands and improved pastures.

The topography of the area consists of very gently to moderately inclined, undulating hills, dissected by Horse Creek and its tributaries. Horse Creek and its tributaries comprise creek beds, associated banks and some small alluvial plains. Horse Creek dissects the Elimatta Project site in a north-easterly direction, whilst many of its tributaries move across the landscape in an east-west direction. The area surrounding Horse Creek is predominantly non-remnant grassland with vegetation clusters scattered sparsely (AARC, 2012).



The channel and banks of Horse Creek have large scattered trees and grass cover, with a sandy creek bed. The riparian vegetation zone is between 10-20 m on either side of the incised low flow channel. There are also a significant amount of woody debris in the creek bed and banks.

Horse Creek provides limited in-stream stable habitat, with AUSRIVAS scores ranging from 53-64 out of 135. This ‘moderate’ scoring is due to recent erosion / high sediment loads, streamside vegetation consisting mostly of grasses/sedges and evidence of cattle grazing contributing to stream erosion. No in-stream floating, rooting or trailing aquatic flora species have been recorded (AARC, 2012).

The Riparian Vegetation Assessment (AARC, 2013) assessed six sites along the project reach of Horse Creek at locations that were representative of the impact area as a whole. This included one site located upstream and one site located downstream of the proposed creek diversion. Riparian vegetation at five of the six sites was representative of Regional Ecosystem (RE) 11.3.25, which is described as ‘Eucalyptus tereticornis or E. camaldulensis woodland fringing drainage lines’. The vegetation at these sites comprised variable densities of tree species such as Forest Blue Gum (Eucalyptus tereticornis), River She-oak (Casuarina cunninghamiana), Rough-barked Apple (Angophora floribunda), River Paperbark (M. trichostachya), Sally Wattle (A. salicina) and Ironwood (A. excelsa). The shrub layer at these sites was very sparse and groundcover was generally dominated by exotic grasses and weeds. The sixth site, the downstream location, was representative of RE 11.3.2b, which is described as ‘Palustrine Wetland’. This site comprised Eucalyptus camaldulensis (and occasionally E. populnea and/or E. tereticornis) woodland, which was observed in drainage depressions with a ground layer of grasses or sedges. The dominant tree species was Forest Blue Gum (E. tereticornis). There was also a dense low shrub layer dominated by the weed Paddy’s Lucerne (S. rhombifolia) (AARC, 2013).

The vegetation on the project reach of Horse Creek is in functional condition, representative of surrounding watercourses in the region. Cattle grazing, weed invasion, selective clearing and vegetation dieback has reduced vegetation condition. An average BioCondition score of 0.72 has been recorded from sites in the stretch of the watercourse that is going to be diverted (AARC, 2013).

Removal of riparian vegetation along Horse Creek as part of diversion construction activities will disrupt habitat connectivity along the creek and remove fauna habitat. The diversion also has the potential to alter the hydrological regime of Horse Creek, which could impact on riparian vegetation communities located outside of the diversion area (AARC, 2013).

9.4 Revegetation strategy Due to the fact that diversions will be both temporary and permanent, revegetation objectives and strategies have been tailored to meet varying operational requirements for each stage of the diversion project. Temporary diversions are expected to be in place for less than three years, serving to facilitate the progressive extraction of the coal resource. Permanent diversions will be established within six years of mining activities commencing.

Separate revegetation objectives and strategies have been developed for temporary diversions, and permanent diversions. These are presented in the following subsections.



9.5 Temporary diversions 9.5.1 Objectives

Due to the short lifespan of temporary diversions and the fact that they will be excavated as mining progresses, revegetation objectives will focus on maintaining structural integrity of the diversions and minimising downstream impacts. This includes maintaining a stable landform, providing adequate groundcover to minimise erosion and sedimentation, minimising the spread of weeds and minimising impacts on water quality.

9.5.2 Strategy

Revegetation of temporary diversions will involve:

mechanically ripping the subgrade of temporary diversion banks in preparation for topsoil application

applying locally stripped topsoil at a minimum depth of 100 mm to minimise impacts of subsoil dispersion and to provide an effective growth medium for revegetation

installing erosion control devices such as jute mesh and compost blankets on the more erosion prone areas of the diversions to minimise scouring. The jute mesh will be used on banks to stabilise the batters after the reapplication of topsoil - it will be installed and pinned as per the manufacturer’s installation specifications; compost blanket will be applied over the jute mesh to provide instant soil surface protection, initiate soil micro-biological processes and help retain soil moisture, allowing for rapid vegetation establishment which is essential for stability

planting of fast growing, hardy, deep rooted shrubs (e.g. Vetiver grass) to provide bank stabilisation

direct seeding of grasses (e.g. Japanese millet, Couch), applied using with a bonded fibre matrix hydromulch to provide effective groundcover

managing weed infestations through control programs in response to annual monitoring

minimising the spread of weeds from vehicles, machinery and imported fill

establishing physical barriers around diversions to prevent livestock and vehicles from damaging revegetation areas.

9.6 Permanent diversions 9.6.1 Objectives

Generally the revegetation of permanent diversions will incorporate geomorphic and riparian vegetation features that are consistent with the pre-mining environment. A key objective for the revegetation of permanent diversions will be to ensure that self-sustaining vegetation communities are achieved. Additionally, revegetation along permanent diversions will aim to restore habitat connectivity with the remaining portions of Horse Creek.

9.6.2 Strategy

In line with the objectives for permanent diversions, revegetation will involve:

planting a diverse mix of native trees, shrubs and grasses

reinstating woody debris in the diverted landscape

avoiding the introduction of weeds



ensuring revegetated areas are protected from the impacts of livestock grazing

monitoring diversion stability and revegetation success for a period of at least 20 years to confirm revegetation objectives have been achieved prior to decommissioning of the mine.

The actions that will be undertaken to implement this strategy are outlined in the following subsections.

9.6.2.1 Ripping

Mechanical ripping of the subgrade will be performed concurrently with the final landform sculpting. Where planting is to occur over compacted clay, the top 150 mm of the clay liner shall be left roughened prior to the application of topsoil.

9.6.2.2 Topsoil amelioration and application

Stockpiled topsoil taken from the diversion corridor will be tested for key physical and chemical soil parameters including electrical conductivity, sodium content, chloride content, pH, dispersion, magnesium, calcium to magnesium ratio, oxygen content and texture. Soil analysis results will be used to identify application rates for soil ameliorants. Typically, amelioration will involve the application of gypsum and fertiliser, as required to improve the water retention, adjust soil chemistry for optimal growing conditions and improve soil fertility.

Topsoil will be applied at a depth of 150mm on diversion banks and floodplain areas. No topsoil will be applied in the bed of the creek diversion areas. The spread topsoil will be lightly compacted to reduce the risk of saturation and slumping especially on batter slopes. Highly alkaline, dispersive subsoils (e.g. Downfall, Kinnoul, Cheshire, Rolleston and Juandah) will not be used as a substitute for topsoil in any part of the diversion.

The surface of the topsoil will be left in a roughened state prior to the application of seed mix.

9.6.2.3 Erosion stabilisation

Erosion stabilisation will involve the installation of jute mesh overlaid with compost blanket. The jute mesh will be used on banks to stabilise all batters after the reapplication and amelioration of topsoil. It will be installed and pinned as per the manufacturer’s installation specifications. Compost blanket will provide instant soil surface protection, initiate soil micro-biological processes and help retain soil moisture, allowing for rapid vegetation establishment which is essential for stability.

9.6.2.4 Seeding and planting

Permanent diversions will be revegetated using a combination of direct seeding and tubestock planting of native and exotic species selected to reflect the pre-mining environment. Seed and planting will be undertaken to reflect the composition of riparian vegetation observed during the Riparian Vegetation Assessment (AARC, 2013). Areas requiring revegetation will be divided into five zones, as shown in Figure 9.2.



Figure 9.2 Indicative revegetation zones

The methodologies and targeted structural diversity will vary between zones. The methods and targeted structural diversity for each of the five zones for temporary and permanent diversions are listed below in Table 9.1.

Table 9.1 Target structural diversity for revegetation zones

Zone Location Methodology Structural diversity

1 Creek bed Self-regenerating None

2 Lower bank Direct seeding & tubestock planting Grasses, sedges

3 Middle bank Direct seeding & tubestock planting Grasses, sedges, shrubs, trees

4 Upper bank Direct seeding & tubestock planting Grasses, sedges, shrubs, trees

5 Floodplain Direct seeding Grasses

A seed mix of sterile exotic grasses (such as Japanese millet and Couch) and native grasses and sedges, will be densely applied over the roughened topsoil within four days of topsoil application. The sterile exotic grasses will provide short term bank stabilisation and help to minimise soil loss while the native grasses and sedges become established. This will be followed by tubestock planting of a range of native tree, shrub and sedge species, examples of which are shown below in Table 9.2.



Table 9.2 Native species for revegetation of permanent diversions

Common name Scientific name Zone

1 2 3 4 5

Trees

Forest Blue Gum Eucalyptus tereticornis

Rough-barked Apple Angophora floribunda

Moreton Bay Ash Corymbia tessellaris

River Paperbark Melaleuca trichostachya

Ironwood Acacia excelsa

Poplar Box Eucalyptus populnea

River She-oak Casuarina cunninghamiana

Sally Wattle Acacia salicina

Shrubs

Spiked Sida Sida subspicata

Sandalwood Santalum lanceolatum

Native Raspberry Rubus parvifolius

Grasses and sedges

Forest Bluegrass Bothriochloa bladhii

Umbrella Cane Grass Leptochloa digitata

Kangaroo Grass Themeda triandra

Twirly Windmill Grass Enteropogon ramosus

Slender Bamboo Grass Stipa verticillata

Purple Wiregrass Aristida ramose

Common Rush Juncus usitatus

Sticky Sedge Cyperus fulvus

Common Fringe-rush Fimbristylis dichotoma

Spiny-headed Mat-rush Lomandra longifolia

9.6.2.5 Installation of woody debris

The use of woody debris such as hollow logs is extremely important for the development of the site as a habitat corridor and to expedite biological processes within the watercourse to encourage macrophyte migration and establishment from natural upstream reaches. Any appropriate debris collected during the construction period will be stockpiled for installation in permanent diversions. Woody debris will be placed upon banks and within terrestrial areas. Placement of woody debris will be made in consultation with the design engineers to ensure that significantly sized debris does not pose a threat to the hydrological and physical functionality of the diversion.



9.6.2.6 Site access

Revegetated areas will be fenced off to provide protection from vehicles and livestock. Maintenance access points will be installed at regular intervals along the diversion alignments. Regular inspections of fences and repair works will be undertaken as required.

9.6.2.7 Monitoring

Annual BioCondition monitoring of revegetation areas will be undertaken to monitor rehabilitation success. Monitoring will be undertaken at a number of predetermined sites along the diversions and control and reference sites upstream and downstream of the project site. Upstream and downstream monitoring sites will be consistent with the sites used for the Riparian Vegetation Assessment (AARC, 2013) for continuity.

Targeted weed control programs will be initiated if annual monitoring detects weed infestations (>25% of cover).



10. Outcome requirements The current approval process for creek diversions on mine sites entails a certification process which is outcome based. The outcome requirements are detailed in the DNRM Draft Manual – Works that interfere with water in a watercourse: Watercourse diversions. This Section sets out how the requirements set out in this document have been complied with, and where the outcome requirements cannot be met, and what criteria have been adopted in their place.

10.1 Outcome 1 The requirement of Outcome 1 is that the watercourse diversion incorporates natural features (including geomorphic and vegetation) present in the landscape and in local watercourses.

The criteria for achieving Outcome 1, as detailed in The Guideline, are as follows:

undertaking a geomorphic and riparian vegetation study of the existing watercourses that will be influenced by the watercourse diversion and the proposed diversion route

develop a conceptual plan of the watercourse diversion incorporating the geomorphic and riparian features of the existing watercourse and/or the landscape along the proposed diversion route.

Geomorphic and riparian vegetation studies have been undertaken and are included as Appendix B and C of this report respectively. A summary of the fluvial geomorphology and riparian vegetation of the Horse Creek system over the project reach is included in Section 3 of this report.

The development of the Horse Creek Diversion design is described in Section 5 of this report, and covers the staged implementation of the diversion to suit mining activities. Each stage of the diversion has been considered as a separate stand-alone diversion, and evaluated as such. Functional design drawings for each stage have been included in Appendix A of this report.

The functional design incorporates geomorphic features that are consistent with the existing watercourse, including a meandering low flow channel within a floodplain, equivalent cross sectional geometry, meander wavelength and bend radii within the range of those in the existing creek planform, and bed slopes that fall within a range that is representative of similar reaches of Horse Creek.

A revegetation strategy to replicate existing watercourse vegetation features is outlined in Section 8 of this report.

A number of features of the existing watercourse to be diverted could not be reflected in the diversion designs, which include the following:

a wide floodplain (600 m) could not be incorporated into the design due to the constraints described in Section 2 of this report, and a 200 m corridor has been provided for in the functional designs – the impact of adopting this criteria is described in Section 5.4.1 of this report

the Stage 1 diversion will include the construction of the low flow channel partially through sandstone, which will result in a change in strata and will prevent channel lateral or vertical migration; there are sections of the existing creek where bedrock is exposed in the bed and embankments, so this is already a feature, however the continuity and length of this relevant section of the diversion is unlike existing conditions



the Stage 2 diversion cuts off a tortuous meander bend in Horse Creek resulting in a significantly steeper bed slope than the reach of creek being diverted; due to spatial constraints and mine planning it will not be possible to achieve a suitably sinuous planform to replicate bed slope in this area, and as a result of this it is proposed that the channel be stabilised using engineering methods (rock armouring) for its entire length – this is a justifiable approach given the limited life of this diversion and preliminary design calculations indicate that this would be a viable engineering solution

the Stage 3 and 4 diversions are to be constructed through mine spoil and as such represent a significant change in substrate, there is no way to prevent this other than by selecting an alternative route through natural ground.

10.2 Outcome 2 The requirement of Outcome 2 is that the watercourse diversion maintains the existing hydrologic characteristics of surface water and groundwater systems.

The criteria for achieving Outcome 2, as detailed in The Guideline, are as follows:

undertaking a preliminary investigation of surface and ground water interactions identifying connectivity between surface and groundwater flows, identifying any users that are dependent on this interconnectivity, and outline how the diversion will be designed to address dependencies identified

development of a hydrologic model for base case and diversion scenarios to demonstrate that hydrological conditions can be maintained.

A preliminary investigation of surface water and groundwater systems has been undertaken and a hydrological model for the Horse Creek system has been developed, both of which are described in Section 4 of this report. The preliminary investigation did not identify any dependency issues and concluded that the diversion would have a negligible impact on groundwater conditions in the project reach or surrounding area.

An XP-RAFTS hydrology model was developed for the Horse Creek catchment and was used to determine peak discharges for 2, 5, 10, 50, 100 and 1000 year ARI events. Section 4 describes the methodology adopted for the calculation of these flows. The hydrology model was peer reviewed by WRM Water and Environment Pty Ltd, and comments from the review were incorporated into the final model. The model could not be verified using actual flow records as the creek is ungauged and there are no surveyed flood levels in the project reach to compare with modelled results.

10.3 Outcome 3 The requirement of outcome 3 is that the hydraulic characteristics of the watercourse diversion are comparable with other regional watercourses and are suitable for the region in which the diversion is located.

The criteria for achieving outcome 3, as detailed in The Guideline, are as follows:

development of a hydraulic model to allow for the relative comparison of base case hydraulic conditions to diversion characteristics

identify potential afflux off mine lease.



A hydraulic model was developed using XP-SWMM with hydrograph flow inputs from the XP-RAFTS hydrology model, and various flood events were modelled for the base case scenario (no diversions), as well as for each stage of the diversion separately. Comparative flood maps depicting the result of flood modelling for all of the scenarios have been produced, as well as afflux mapping to identify impacts at mine lease boundaries that may impact adjacent landowners. A Flood Study Report has been compiled and is included in Appendix D; a summary of the relevant findings is contained in Section 6 of this report. The results for all diversion scenarios and flood events did not exhibit abnormal characteristics relative to the base case modelling results, and were within ACARP guideline limits, other than for the temporary Stage 2 diversion, which will need to be stabilised using engineering methods for the limited time of its operation.

10.4 Outcome 4 The requirement of outcome 4 is that the watercourse diversion maintains a sediment transport, and water quality regime that allows the diversion to be self-sustaining and not result in material or serious environmental harm to upstream and downstream reaches.


an investigation into the current watercourse sediment transportation regime to determine if the system is in an aggrading, degrading or dynamic equilibrium stage

an investigation that determines if the proposed watercourse diversion can achieve this outcome.

A geomorphic investigation was undertaken which included a classification of the bed sediment transport characteristics of the existing creek system. This study identified that the system is likely to be in a degrading stage of its evolution. The sediment characterisation and transport regime assessment are described fully in the Geomorphology Report contained in Appendix B, and a summary of the relevant points relating to sediment transport regime are included in Section 8 of this report. An investigation was undertaken to assess the sediment transport characteristics, in the form of bed load transfer capacity, of the diversion relative to the existing watercourse. The outcome of this assessment indicated that the transport capacity of the diversion would not be less than the current capacity, and that the sediment transport regime would hence be maintained.

The Stage 2 diversion incorporates significantly increased bed slopes for reasons described earlier in this report, which may result in conditions that are prone to erosion, and hence it has been recommended that this temporary diversion be stabilised using engineering methods.

10.5 Outcome 5 The requirement of outcome 5 is that the watercourse diversion and associated structures maintain stability and functionality and are appropriate for all substrate conditions they encounter.


a geotechnical investigation to identify in-situ substrate conditions, including an analysis of physical properties to a depth exceeding that of the channel bed depth

identification of areas through which the creek diversion will be constructed in spoil material, determination of the characteristics of the spoil material and a description of likely subsoil groundwater movements in the diversion footprint.



A geotechnical site investigation has been undertaken for the site to identify substrate conditions, and a desktop evaluation of spoil properties has been undertaken. Stability calculations have been undertaken based on the preliminary diversion geometry and embankments were found to be globally stable. Seepage analysis has been undertaken to identify potential groundwater movements from the diversion into adjacent voids or pits, which were found to be minimal relative to expected groundwater contributions. Settlement in spoil backfill for short and long term timeframes have been estimated, and mitigation and remedial measures have been identified. Preliminary calculations have been undertaken to assess the engineering viability of stabilising transition zones and the Stage 2 diversion with flexible riprap armour, and results indicate that this is a viable solution in both instances.

A Geotechnical Investigation report has been prepared covering these abovementioned aspects fully, and is included as Appendix F of this document. A summary of the findings is included in Section vegetation of the Horse Creek system over the project reach is included in Section 7 of this report.



11. References AARC, 2013. Horse Creek Riparian Vegetation Assessment. Elimatta project. Prepared for New Hope Coal. Australasian Resource Consultants.

AARC (2012) Waterway Morphology and Aquatic Ecology Assessment Report. Appendix S in Elimatta Project EIS.

ACARP (2000) Maintenance of Geomorphic Processes in the Bowen River Basin Diversions. Stage 1. Report # C8030.

AGE, 2012. Elimatta Project Groundwater Assessment. Prepared for Northern Energy Corporation. EIS Appendix M. Australasian Groundwater and Environmental Consultants Pty Ltd (AGE). BOM: GDE Atlas http://www.bom.gov.au/water/groundwater/gde/

Brice JC (1981) Stability of Relocated Stream Channels, Final Report No. FHWA/RD-80/158, US Department of Transportation - Federal Highway Administration

Brierley GJ and Fryirs KA (2005) Geomorphology and River Management. Application of the River Styles Framework. Blackwell Publishing.

Copeland RR, McComas DN, Thorne CR, Soar PJ, Jonas MM, and Fripp JB (2001) Hydraulic Design of Stream Restoration Projects (ERDC/CHL TR-01-28). US Army Corps of Engineers

DNRM, 2013. Manual – Consultation Draft Works that interfere with water in a watercourse: watercourse diversions. Department of Natural Resources and Mines.

DNRM (2006) Stream bank planting guidelines and hints. River Fact Sheet Series # R31. Department of Natural Resources and Mines. Queensland Government.

Earth Tech (2002) Bowen Basin River Diversions. Design and Rehabilitation Criteria. Australian Coal Association Research Programme.

Erskine WD, Saynor MJ, Evans KJ, Boggs GS (2001) Geomorphic research to determine the off-site impacts of Jabiluka Mine on Swift (Ngarradj) Creek, Northern Territory.

Grayson, et al., 1996. Hydrological Recipes, Estimation Techniques in Australian Hydrology, Report prepared by RB Grayson, RM Argent, RJ Nathan, TA McMahon and RG Mein, Cooperative Research Centre for Hydrology

Lagasse PF, Zevenbergen LW, Spitz WJ, and Arnesan LA (2012), Stream Stability at Highway Structures, 4th edition, US Department of Transportation – Federal Highway Administration

Lane EW, 1955, Design of Stable Channels, Transactions of the American Society of Civil Engineers, Vol 120, pp 1234-1279

Pilgrim (1998), Australian Rainfall and Runoff. A Guide to Flood Estimation, Volumes 1 and 2, Editor in Chief DH Pilgrim, Institution of Engineers

PSM (2012) Horse Creek Diversion Hydrologic, Morphologic and Hydraulic Design. Report number 376.02.

http://www.bom.gov.au/water/groundwater/gde/



Richardson, S., et al., 2011. Australian groundwater-dependent ecosystem toolbox part 1: assessment framework, Waterlines report, National Water Commission, Canberra.

Richardson EV, Simmons DB and Lagasse PF (2001), River Engineering for Highway Encroachments (FHWA NHI 01-004 HDS6), US Department of Transportation – National Highway Institute.

Sattler PS & Williams RD (1999) The Conservation Status of Queensland's Bioregional Ecosystems. Queensland Environmental Protection Agency, Brisbane.

US Army Corps of Engineers (1994) Channel Stability Assessment for Flood Control Projects (EM 1110-2-1418).

WJC and PSM, 2012. Horse Creek Meander Belt Diversion Design. New Hope Corporation. EIS, Appendix N. WJC Meynink and PSM Australia.

appendix l - new hope coal...new hope coal parsons brinckerhoff | 2172847g-civ-rep-001 revb i horse...

Documents