a management plan for the dredging and...

A MANAGEMENT PLAN FOR THE DREDGING AND DISPOSAL OF

MATERIAL FOR THE PROPOSED TASSAL SPRING BAY JETTY, TASMANIA

An Appendix To

Marine Environmental Impact Assessment for a Proposed Jetty Including Dredging at

Spring Bay, Triabunna, March 2017

www.marinesolutions.net.au

Tassal/SBSF Jetty DMP 2

© Marine Solutions 2017. This document should only be used for the specific project and

purposes for which it was commissioned. 1

Version Author Date reviewed Reviewed by

1 of 2 Coco Cullen-Knox 24/3/16 Sam Ibbott

2 of 2 Coco Cullen-Knox 22/03/2017 Laura Smith

Note: Location maps throughout this report are representative only; for precise GPS coordinates, see the appendices. All satellite imagery used throughout is sourced from The Land Information System Tasmania (LIST) unless otherwise indicated.

1 Cover photo, Paddy’s Point, Spring Bay (photo by Marine Solutions, 2015).


TABLE OF CONTENTS

Table of Contents .......................................................................................................................................... 3

Table of Figures ............................................................................................................................................. 4

1 Executive Summary ............................................................................................................................... 5

2 Introduction .......................................................................................................................................... 8

2.1 Purpose ......................................................................................................................................... 8

2.2 Legislation ..................................................................................................................................... 8

2.3 Overview of the dredge site .......................................................................................................... 8

3 Management Plan ............................................................................................................................... 11

3.1 Odour .......................................................................................................................................... 11

3.2 Dredging methodology ............................................................................................................... 11

3.2.1 Barge-based excavation dredging ....................................................................................... 11

3.2.2 Dewatering .......................................................................................................................... 11

3.3 Disposal of the material .............................................................................................................. 13

3.4 Water quality .............................................................................................................................. 13

3.5 Toxic Dinoflagellate Cysts ........................................................................................................... 16

3.6 Waste and Spill Management ..................................................................................................... 16

3.7 Release of toxic contaminants from sediment ........................................................................... 16

3.8 Acid Sulphate Soils (ASS) ............................................................................................................. 17

3.9 Noise Disturbance ....................................................................................................................... 17

3.10 Dredge Operator Commitments ................................................................................................. 18

4 References .......................................................................................................................................... 19


TABLE OF FIGURES

Figure 1 The location of the proposed dredge works at Paddy’s Point, Spring Bay, Tasmania. ................... 9

Figure 2 Proposed design of the jetty (most recent update as of March 2017; source: Burbury

Consulting). ................................................................................................................................................. 10

Figure 3 approximate location of settlement pond proposed as the site for dewatering and dredge

disposal and dredging site .......................................................................................................................... 13

Figure 4 Suggested water quality sampling sites. ....................................................................................... 15


1 EXECUTIVE SUMMARY

This document outlines the proposed dredge process for removal of sediment around the proposed

jetty at Paddy’s Point, Spring Bay, Tasmania and highlights the commitments to be undertaken during

the works phase to minimise environmental risk.

Spring Bay Seafood’s and Tassal’s plans to dredge by excavation and commits to the following

environmental mitigation actions when doing so:

Commitment #1. Notify the EPA one week before the commencement of first dredging and provide

EPA with an outline of the proposed dredging program.

Commitment #2. Dredge spoil is to be deposited and dewatered in a depression (pond) onsite. Spoil

will be transported by truck to the disposal/dewatering site.

Commitment #3. Dredge operators will comply with the directions of Spring Bay Seafood and Tassal

personnel regarding deposition of dredge spoil.

Commitment #4. Water sampling will be undertaken at four locations during dredging practices.

Commitment #5. If at the time of excavation material becomes odorous it will be capped with non-

odorous material as soon as practicable within a maximum of 12 hours from the

odorous material becoming exposed.

Commitment #6. Waste management and spill response capability to be on site.

Commitment #7. Dredging is only to occur during daylight hours so as not to cause noise disturbance

to local residents.

Commitment #8. Sediment transport is to be minimised, via physical barriers/filters (e.g. bunding,

baffles, geotextile cloth), and controlling the rate of dredge material being

deposited ashore.

Commitment #9. Abort dredge if a plume is observed from the dewatering drain.

Commitment #10. Periodic visual and pH monitoring to ensure ASS is not posing environmental threat,

or cap spoil with clean fill at the completion of dredging.

Commitment #11. Dredging operations are to occur during winter months, to reduce the likelihood of

causing a toxic algal bloom.


The document entitled “Marine Environmental Impact Assessment for a Proposed Jetty Including

Dredging at Spring Bay, Triabunna” (Marine Solutions, 2017) supplements this document. It provides

details of the site assessments undertaken, the results of which have provided the basis for many of the

recommendations made throughout this management plan. Open discussions with relevant

government authorities including DPIPWE and the EPA have helped to frame this DMP.

Table 1 below summarises the potential issues and associated mitigations that form the management

plan strategies.


Table 1 Potential issues associated with excavation dredging at Paddy’s Point, associated recommended mitigations and commitments of dredging

Impact Recommended Mitigation Priority* Refer to section:

Unnecessary mobilization of sediment plume from disturbance of seabed

Dredge using a barge-based excavator. Constructing a sediment fence during dredge operations will reduce the sediment plume.

1 3.2

Mobilisation of toxic dinoflagellate cysts causing at toxic algal bloom. Contamination of Spring Bay Seafood hatchery’s water intake.

Dredging operations are to occur during winter months to reduce the likelihood of causing a toxic algae bloom. Water intake is to be shut off during dredging activities and/or moved. 1 3.5

Mobilisation of contaminants impacting water quality

Water sampling will be undertaken at four locations while dredging operations are underway. (NATA lab, qualified samplers, data retention for 3 years). Preliminary works indicates this is low risk.

5 3.4

Release of unpleasant odour during dredge

Odours to be managed by submerging odorous material, or by capping odorous material with non-odorous material.

1 3.1

Potential contaminant spill from dredge operations

Waste management and spill response capability to be on site 1 3.6

Noise disturbance The proposed development site is in a rural setting with no residence in the near vicinity and therefore noise disturbance does not pose a high risk.

3 3.9

Release of toxic contaminants from sediments

Minimisation of sediment transport from the disposal/dewatering site, via

- physical barriers/filters (e.g. bunding, baffles, geotextile cloth)

- controlling the rate of dredge material being deposited ashore

Abort dredging if a significant plume is observed.

3 3.7

Release of acid from dredge material

Intermittent lime dosing or other equivalent solution to neutralize potential acidity. Periodic visual and pH monitoring to ensure ASS is not posing environmental threat, or cap spoil with clean fill at the completion of dredging.

5 3.8

* Based on perceived risk. Rated on a scale of 1 (highest priority) to 5 (lowest priority).


2 INTRODUCTION

2.1 PURPOSE

This document outlines the proposed methods for dredging a section of seabed at the proposed jetty at

Paddy’s Point, Spring Bay, Tasmania. Dredging is required to allow vessel access and reduce the risk of

damage to infrastructure and vessels.

This document also outlines the management methods of the excavated marine sediments, and

provides a table of commitments (mitigations) for the dredge operator.

These activities will be conducted in accordance with the National Assessment Guidelines for Dredging

(DEWHA 2009).

2.2 LEGISLATION

All planning of development within Tasmania is governed by Tasmania’s Resource Management and

Planning System (RMPS). There are 5 main statutes that lend legislative effect to the RMPS (DEP 2009).

These are:

- Land Use Planning and Approvals Act (1993) (LUPAA)

- The Environmental Management and Pollution Control Act (1994) (EMPCA)

- State Policies and Projects Act (1993)

- Resource Planning and Development Commission Act (1997)

- Resource Management and Planning Appeal Tribunal Act (1993)

2.3 OVERVIEW OF THE DREDGE SITE

The site of the proposed jetty site is to the southeast of Paddy’s Point approximately perpendicular to

the shoreline (Figure 1). The proposed jetty may be up to 200 m in length and run from the shore in a

southerly direction, and is required to provide in excess of 4m Chart Datum depth for vessels berthing

alongside. The jetty will be used to support current and proposed aquaculture activities in the area.

The requirements of the dredge zone include:

• A declared depth of -5.0 m Australian Height Datum (AHD);


• Nominal dredge footprint area of 5, 500 square metres; and

• Nominal bulk volume of 4, 700m

Figure 1 The location of the proposed dredge works at Paddy’s Point, Spring Bay, Tasmania.

Sediments at the site are characterised by spatially heterogeneous thick mud with areas of course sand

and shell grit throughout. Some trend of stratification was evident with a thin layer of mud overlaying a

layer course sand/mud and shell grit followed by thick black mud from a depth of ~50cm onward. A mild

odour was present at some sites during sediment sampling, however it is not foreseen to pose a

problem while dredging.

The proposed berthing jetty (Figure 2) is proposed to run through the proposed dredge area (hereafter

referred to as "the jetty"), dividing it into an east section (approximately two thirds of the total dredge

area) and a west section (approximately one third of the total dredge area). Located within to the

proposed dredging site is the Spring Bay Seafood’s water intake pipe.


Figure 2 Proposed design of the jetty (most recent update as of March 2017; source: Burbury Consulting).


3 MANAGEMENT PLAN

3.1 ODOUR

Field studies indicated that there was some mild odour from some core samples taken throughout the

dredge site, however it is not expected pose a problem during the dredging and dewatering phase.

If odour does pose a problem during dredging operations control may be undertaken by ensuring the

material remains submerged or is quickly capped, thus preventing the release of odours. At the

completion of the dredge works any non-submerged odorous material should be immediately covered

by non-odorous material. To facilitate this, a ready and adequate supply of non-odorous material

should be identified, and potentially present as a stockpile on site. For more detail regarding odour

control, refer to Dredging Methodology in Section 3.2 below.

3.2 DREDGING METHODOLOGY

A seafloor area of approximately dredge footprint area of 5, 500 square metres will be dredged to

provide 1m additional depth of water. This equates to a total volume of dredge material of

approximately 4,700m3.

Shore based, or barge based excavation dredging is deemed an appropriate methodology for this site.

This option is individually described in further detail below (section 3.2.1). Agitation or injection

dredging techniques is not appropriate for the initial dredge at this site, as the spoil would be disturbed,

but not removed from site, however it may be appropriate for any maintenance works.

3.2.1 Barge-based excavation dredging

Dredging will be conducted via mobilisation of a barge with excavator. The barge will contain baffle

walls for containment of the dredge material. This material will be transferred by mooring at causeway

and onto trucks. Disposal will occur on the sediment storage zone.

3.2.2 Dewatering

A dewatering process is proposed whereby dredge spoil will be deposited from the site into the

settlement pond (Figure 3). While it is unlikely, if odour presents as an issue, priority should be given

to moving the material into the settlement pond and quickly capping with non-odorous material.


Spoil may wholly or partially dewater at the intermediate zone, where run-off would return back to

the dredge-site or to the settlement pond.

A barrier within the settlement pond may also be constructed, to divide the pond into two “sub-

ponds” and allow for a two-stage dewatering process, whereby spoil is deposited into the first sub-

pond, and the second sub-pond will receive any overflow from the first. At both sub-ponds (both

stages), settlement of sediment will occur.

The settlement pond will be lined with geotextile fabric to minimise the return of fines to the bay

during the dewatering process, thus minimizing any increases in turbidity.

Adequate control measures will be implemented to slow the surface runoff of water, thus minimise

sediment transport from the dewatering site back into the port via the dewatering drain. These may

include but not be limited to;

• Bunding of the site,

• Baffles within the site to cause the dredged material to travel slowly and allow adequate time

for sediment to settle out,

• Silt curtains located across the dewatering drain prior to water re-entering the port

• Hay bale ‘filters’ placed in the drain water course

• Control of the rate of dredged material being deposited ashore

A combination of these measures is intended to enable effective dewatering, and thus avoid an

observable plume being present at the point of water draining from the spoil and re-entering the port.

If an obvious plume is observed at the point water is draining from the shore back into the port, then

the dredging is to be aborted until measures to quell the plume are in place. The dewatering discharge

point will be visually monitored while the dredging works are underway.


Figure 3 approximate location of settlement pond proposed as the site for dewatering and dredge disposal and dredging site

3.3 DISPOSAL OF THE MATERIAL

The dredge spoil will be retained within a 8,000m2 pen just north of the dredging site, to be used as fill

and capped. Therefore, assuming minimal post-dredge bulking, the entire amount of won material could

be accommodated within the settling pond if filled to a depth of ~0.75m.

3.4 WATER QUALITY

It is anticipated that water quality at the dewatering discharge will be satisfactory to re-enter the port

via the dewatering drain. A series of sediment tests indicate that the sediments at this site are not likely

to be acid producing when dredged, and they have low levels of contaminants, including metals.

~Proposed location for dredge spoil disposal/dewatering

~ Proposed location for dredge site

~ Location of Spring Bay Seafood’s water intake


Notwithstanding this, a series of water quality monitoring sites will be sampled daily during the first

week of dredging. Integrated samples from each site shown in Figure 4 will be tested for turbidity, pH,

hardness, total suspended solids and total and dissolved metals. These sites correlate with;

• (a) 800m upstream of the dredge site,

• (b) 800m downstream of the dredge site,

• (c) within proximity to recreational use areas on the other side of the bay

• At the dredge site / Immediately in front of the Spring Bay Seafood’s hatchery water intake

Water quality sampling sites shown in

Figure 4 below are intended as a guide only. Following the first week of dredging, a review of the

dredging process and water quality sampling frequencies should be undertaken, in conjunction with the

EPA.


Figure 4 Suggested water quality sampling sites.

Upstream

Downstream

Close proximity to recreational use

Spring Bay Seafood’s water intake

WQ 1

WQ 2

WQ 3

WQ 4


3.5 TOXIC DINOFLAGELLATE CYSTS

Toxic algal blooms in eastern Tasmania appear to be increasing. Preliminary surveys indicate resting

stages of toxic dinocysts are present, atop the sediments, which are >2m deep. Areas of limited wave

action and current, such as the jetty development site, are at risk of accumulating toxic microalgal

cysts and a large disturbance can trigger an algal bloom. Minimising sediment disturbance is the

main mitigation strategy against algal blooms as the other important factors to stimulate a bloom

such as sufficient nutrients and light are less able to be manipulated. Dredging activities should

occur during winter months, no later than 21st August, and minimizing sediment disturbance

remains an important mitigation.

Due to the close proximity of Spring Bay Seafood’s water intake a toxic algae bloom would pose

significant threats to the survival of the oyster hatchery for which the water is used. Moving the intake

location could be an appropriate mitigation measure to reduce the threat to the hatchery.

3.6 WASTE AND SPILL MANAGEMENT

Contractors on site should be aware of their environmental obligations and provide facilities for

adequate disposal of waste, which should be removed from site.

Given there is a range of machinery involved in the dredging phase, spill kits, appropriate to

hydrocarbons found in the machinery should be available on site. This may include but is not be limited

to Petrol, Diesel, Hydraulic Fluid and Oils.

3.7 RELEASE OF TOXIC CONTAMINANTS FROM SEDIMENT

Contaminant tests identified no contaminant guidelines (ANZECC) were exceeded in sediments

throughout the site. Therefore the release of contaminants does not pose a threat during dredging

operations. The installation of a silt screen at the dewatering drainage point and around the dredge site

is however recommended to minimize the bulk movement of resuspended sediments.


3.8 ACID SULPHATE SOILS (ASS)

Acid sulphate soils (ASS) are sediments that occur naturally in waterlogged environments (Thornton

2010). These sediments contain iron sulphides, most commonly in the form of iron pyrite (Thornton

2010). ASS occur in two main forms: potential acid sulphate soils (PASS), where the pyrite is retained in

a reduced state (not oxidised), and actual acid sulphate soils (AASS), where the pyrite is oxidised by

exposure to air. The oxidation of ASS results in the formation of sulphuric acid (Thornton 2010). ASS are

harmless when undisturbed on the seafloor (DEP 2009), however, dredging activities that expose PASS

to air can result in destructive leaching of acid into the environment (DEP 2009; Thornton 2010).

To characterise the ASS-related risk of dredging this site, field pH (pHF) and field pH peroxide (pHFOX) ASS

Screening Tests was conducted at 10sampling sites in February. These tests assist in determining the

likelihood of soils being AASS or PASS and are simple and low-cost. PASS/AASS testing was conducted at

the core that appeared most likely to be PASS or AASS as determined by field-based stratigraphical

examination including a visual/odour assessment of sediments in each core. Refer to Marine Solutions

(2016) for details of testing.

The peroxide field tests pHF and pHFOX indicated a low probability of Potential or Actual Acid Sulphate

Soil (PASS or AASS) material being present, and therefore represents a low risk of acid generation by

dredge material (Marine Solution 2016). The results of laboratory CRS testing supported the conclusions

of the field tests that acid generation is low risk.

However, it is suggested that the proponent commits to visual and pH monitoring periodically to ensure

ASS problems are not occurring, or to covering the spoil with clean fill after the dredging is done.

3.9 NOISE DISTURBANCE

The proposed development site it is in a rural setting with no residence in the near vicinity and therefore

noise disturbance does not pose a high risk.


3.10 DREDGE OPERATOR COMMITMENTS

The commitment list below is to guide operators in their requirements during the dredging operations

to deepen the Paddy’s Point wharf area. It does not imply that permit conditions, planning approvals or

other requirements from statutory bodies have been superseded, but rather is meant to be a pragmatic

operational summary of tasks to be undertaken during the dredging process.

Spring Bay Seafood’s and Tassal plans to dredge by excavation and commits to the following

environmental mitigation actions when doing so:

Commitment #1. Notify the EPA one week before the commencement of first dredging and provide

EPA with an outline of the proposed dredging program.

Commitment #2. Dredge spoil is to be deposited and dewatered in a depression (pond) onsite. Spoil

will be transported by truck to the disposal/dewatering site.

Commitment #3. Dredge operators will comply with the directions of Spring Bay Seafood and Tassal

personnel regarding deposition of dredge spoil.

Commitment #4. Water sampling will be undertaken at four locations during dredging practices.

Commitment #5. If at the time of excavation material becomes odorous it will be capped with non-

odorous material as soon as practicable within a maximum of 12 hours from the

odorous material becoming exposed.

Commitment #6. Waste management and spill response capability to be on site.

Commitment #7. Dredging is only to occur during daylight hours so as not to cause noise disturbance

to local residents.

Commitment #8. Sediment transport is to be minimised, via physical barriers/filters (e.g. bunding,

baffles, geotextile cloth), and controlling the rate of dredge material being

deposited ashore.

Commitment #9. Abort dredge if any plume is observed from the dewatering drain.

Commitment #10. Periodic visual and pH monitoring to ensure ASS is not posing environmental threat,

or cap spoil with clean fill at the completion of dredging.

Commitment #11. Dredging operations are to occur during winter months, no later than the 21st

August, to reduce the likelihood of causing a toxic algal bloom.


4 REFERENCES

DEP (2009) Dredging and land reclamation in the Derwent: a guidance document to support Best

Practice Management. Derwent Estuary Program 2009

DEWHA (2009) National Assessment Guidelines for Dredging, Commonwealth of Australia, Canberra

Marine Solutions (2016). Environmental Risk Assessment for Dredging of material at the Site of a

Proposed Jetty at Paddy’s Point, Spring Bay, Tasmania, Report to TASSAL, February 2016.

Thornton L (2010) Acid sulphate soils management plan: Norwood-Mowbray 110kv transmission circuit,

Doc No: T9086-PLN-EMP-002

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