waiwhetu outfall dilution assessment
TRANSCRIPT
waiwhetu outfall dilution assessment / bjt / 2017-10-30
This report has been prepared under the DHI Business Management System
certified by Bureau Veritas to comply with ISO 9001 (Quality Management)
DHI Water and Environment Ltd• ecentre, Gate 5, Oaklands Road• Albany 0752 Auckland• New Zealand• Telephone: +64 9 912 9638 • Telefax: • [email protected]• www.dhigroup.com
Waiwhetu Outfall Options - Dilution
Assessment
Numerical Modelling
Prepared for Wellington Water
Represented by Anna Bridgman (Stantec) Hutt River Mouth
Project manager Benjamin Tuckey
Project number 44801116
Approval date 30/10/2017
Revision Final 1.1
Classification Open
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CONTENTS
1 Executive Summary .................................................................................................... 1
2 Introduction ................................................................................................................. 2 2.1 Project Appreciation ....................................................................................................................... 2 2.2 Objectives of Proposed Study ........................................................................................................ 4 2.3 Co-ordinate System and Vertical Datum ....................................................................................... 4
3 Overview of Data ......................................................................................................... 5 3.1 Current, Water Level and CTD Data from NIWA Data Collection Campaign ................................ 5 3.1.1 Measurement schedule .................................................................................................................. 5 3.1.2 Mooring Deployments .................................................................................................................... 7 3.1.3 CTD Casts ...................................................................................................................................... 9 3.2 Flow and Water Level Data .......................................................................................................... 12 3.3 Bathymetric Data.......................................................................................................................... 13 3.4 Wind Data .................................................................................................................................... 15
4 Study Approach and Scenario Selection ................................................................. 17 4.1 Overview of Mixing Behaviour ..................................................................................................... 17 4.2 Near-Field and Far-Field Models ................................................................................................. 17 4.3 Scenario Selection ....................................................................................................................... 18
5 Far-Field Model Set Up and Calibration ................................................................... 20 5.1 Set Up .......................................................................................................................................... 20 5.1.1 Bathymetry and Mesh .................................................................................................................. 20 5.1.2 Open Ocean Boundaries ............................................................................................................. 21 5.1.3 Freshwater Inflows ....................................................................................................................... 21 5.1.4 Wind ............................................................................................................................................. 21 5.1.5 Wastewater Representation ......................................................................................................... 22 5.2 Calibration .................................................................................................................................... 22 5.2.1 Water Levels and Currents .......................................................................................................... 22 5.2.2 Salt Wedge and River Plume ....................................................................................................... 28
6 Near-Field Assessment ............................................................................................. 31 6.1 Wastewater Properties and Outfall Arrangement ........................................................................ 31 6.2 Receiving Water Conditions ......................................................................................................... 31 6.2.1 Receiving Water Level, Flow and Salinity .................................................................................... 31 6.2.2 Receiving Water Bathymetry ....................................................................................................... 32 6.2.3 Situations where Modifications of Assumptions were Required to Obtain Dilution
Predictions ................................................................................................................................... 34 6.3 Model Results .............................................................................................................................. 34
7 Far-Field Assessment ............................................................................................... 39 7.1 Model Results .............................................................................................................................. 39
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8 Summary ................................................................................................................... 62
9 References ................................................................................................................ 64
Executive Summary
1
1 Executive Summary
A wastewater plume dispersion study was undertaken for the existing wastewater overflow from
the Seawater Wastewater Treatment Plant into Waiwhetu Stream and for two potential
alternative wastewater overflows. The three locations for which the assessment was undertaken
were:
Option A - existing location in Waiwhetu Stream;
Option B - Hutt River (near Waiwhetu Stream mouth) close to the water surface; and
Option C - Hutt River (100 m off Barnes Street) on the sea bed.
The performance of each outfall location was assessed by determining the plume dilution that
will occur for a range of selected discharge, hydrodynamic and climate scenarios (a total of 25
scenarios) at sensitive sites within the river mouth and Wellington harbour. Both neap and
spring tidal conditions were considered, along with calm, southerly and northerly wind
conditions.
Predicted 1st, 5th, 25th and 50th percentile dilutions at the sensitive sites were calculated for each
scenario and outfall option. Percentile plots at the sensitive sites were also generated for a 30
day simulation including range of representative wind conditions within the harbour, to provide
an indication of the visitation frequency of the plume from each outfall at the sites.
Generally Outfall C produced the most dilution of the wastewater plume compared with other
potential outfall locations, before visitation of the plume occurred at the sensitive sites, while
Outfall B generally performed slightly better than Outfall A, however not for all conditions or
sites.
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2 Introduction
Stantec New Zealand (Stantec) on behalf of Wellington Water (WW) have commissioned DHI
Water and Environment Ltd (DHI) to undertake a plume dispersion study for the existing
wastewater overflow from the Seawater Wastewater Treatment Plant into Waiwhetu Stream and
for two alternative wastewater overflow, to form part of an Assessment of Environmental Effects
(AEE).
NIWA originally undertook a plume dispersion study for Hutt City Council for the existing
wastewater overflow and potential alternative discharge sites (NIWA, 2015). However, the
selected modelling approach resulted in a model of Wellington Harbour which did not cover the
Hutt River mouth in sufficient detail to provide a comparative assessment of the dilutions for
each option considered.
DHI have previously undertaken a preliminary dilution assessment for a number of outfall
options (DHI, 2011) and a near-field dilution assessment based on receiving environment
information provided by NIWA (DHI, 2016).
2.1 Project Appreciation
For the majority of the time, treated wastewater effluent from the Seaview Wastewater
Treatment Plant discharges through a short outfall south of Pencarrow Head into the open
ocean outside Wellington Harbour. On occasions, when maintenance is carried out on the
outfall or when the capacity of the outfall is exceeded, treated wastewater effluent is discharged
into the Waiwhetu Stream, which flows into the Hutt River.
A plume dispersion study was required to investigate the potential for discharging from the
existing overflow location and two other possible future alternatives in the Hutt River, as shown
in Figure 2-1 and outlined below:
Option A - existing location in Waiwhetu Stream;
Option B - Hutt River (near Waiwhetu Stream mouth) close to the water surface; and
Option C - Hutt River (100 m off Barnes Street) on the sea bed.
The salt wedge which propagates up the lower Hutt River on the incoming tide will have an
influence on the mixing of the discharged wastewater and must be considered for a wastewater
dilution assessment.
The potential discharge considered were as follows:
unplanned pipe repair that would result in a typical dry weather discharge rate of
0.55 m3/s (1.10 m3/s for Option B on outgoing tide) for a duration of 5 days;
planned maintenance works that would result in a typical dry weather discharge rate of
0.55 m3/s (1.10 m3/s for Option B on outgoing tide) for a duration of 30 days;
wet weather overflows typically discharging at a rate of 0.8 m3/s for a duration of 1 day;
wet weather occurring while the main outfall pipeline is out of operation that would result
in a peak flow up to 3 m3/s for duration of 1 day.
3
Figure 2-1 Existing wastewater outfall (Option A) and potential wastewater outfalls Options B and C near the mouth of the Hutt River.
The performance of each outfall location was assessed by determining the plume dilution that
will occur for selected discharge, hydrodynamic and climate scenarios at the following sensitive
sites (presented in Figure 2-2):
Petone Beach west;
Petone Beach east;
Waione Street Bridge;
100m downstream of Hutt/Waiwhetu confluence;
Lowry Bay;
Days Bay;
Port Road corner beach; and
Seaview Marina.
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Figure 2-2 Location of sensitive sites for plume dilution assessment.
2.2 Objectives of Proposed Study
The specific objectives of the study were:
1. Calibrate and validate a three dimensional hydrodynamic model capable of replicating
saline intrusion behaviour within the Waiwhetu Stream and Hutt River and predict
currents and water levels within vicinity of the potential outfalls;
2. Identify appropriate hydrodynamic scenarios for assessing wastewater plume dilutions
from the potential outfalls;
3. For selected discharge and hydrodynamic scenarios, undertake a near-field assessment
to determine wastewater plume dilutions from the potential outfalls within the zone of
near-field mixing; and
4. For selected discharge, hydrodynamic and climate scenarios, undertake a far-field
assessment to determine wastewater plume dilutions at sensitive sites for each of the
outfalls.
2.3 Co-ordinate System and Vertical Datum
For this study, all data is presented using the New Zealand Transverse Mercator projection and
the vertical datum is referenced to Wellington Mean Sea Level datum.
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3 Overview of Data
This section provides an overview of the data that was collected by NIWA specifically for this
study (NIWA, 2013) and describes additional data DHI have utilised for this study.
3.1 Current, Water Level and CTD Data from NIWA Data Collection Campaign
3.1.1 Measurement schedule
During May to July 2013, NIWA deployed an Acoustic Doppler Current Profilers (ADCP) and
bed-level sensors at three mooring locations and undertook Conductivity, Temperature and
Depth (CTD) casts at 12 station location on five days. An overview of the data collection
locations is presented in Figure 3-1, while Table 3-1 presents an overview of the schedule of the
data collection.
Figure 3-1 NIWA moorings and CTD cast station locations (1-12), in the vicinity of the Hutt River mouth and Somes Island, where data was collected between May-July 2013.
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Table 3-1 Schedule of the NIWA data collection campaign (only data used in this study).
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3.1.2 Mooring Deployments
During May, June and July 2013, NIWA deployed ADCP and bed-level temperature, salinity and
pressure (MicroCat) sensors at three locations (see Figure 3-1):
2 km south of Somes Island (referred to as the Harbour mooring);
one 150 m offshore from 77 Port Road (referred to as the River mouth mooring); and
one 130 m downstream of the Waione Street bridge across the Hutt River (referred to
as the River mooring).
The ADCP at the River mouth mooring was lost and so provided no data, however the bed-level
measurements were recovered. Bed-level salinity measurements at the three NIWA mooring
locations are presented in Figure 3-2. Measurements indicate that the salinity in the harbour
remains approximately constant over time and that Hutt River flows, with a salinity of
approximately 0.0 PSU, are capable of flushing saltwater from the lower river right to the bed for
extended periods of time. NIWA note that River mouth mooring measurements (the green line in
Figure 3-2) from 21st June onwards are dubious because of possible clogging of the
measurement device. Further details concerning the data collection can be found in NIWA
(2013).
Figure 3-3 presents a comparison of the instantaneous salinity at the sea bed in the river and
the river mouth compared with the CTD cast data at the sea bed (see Section 3.1.3). CTD cast 5
was located close to the River salinity mooring, while CTD cast 8 was located close to the River
mouth mooring. It has to be assumed that CTD casts data is more accurate. There is a large
discrepancy for both sets of data. On 7th June and 9th July, with CTD cast 5 recording a salinity
of 34 PSU at seabed, while the river mooring recorded a salinity of 0 PSU. On 9th July the CTD
cast 8 recorded a salinity of 34 PSU at seabed, while the river mouth mooring recorded a salinity
of 0 PSU. It would appear that measurements from both deployed instruments in the river are
dubious and therefore it wasn’t possible to use data for this study.
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Figure 3-2 Bed-level salinity measurements at the three NIWA mooring locations.
Figure 3-3 Bed-level salinity measurements at the NIWA river mooring locations compared with CTD cast data.
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3.1.3 CTD Casts
NIWA undertook nine sets of CTD casts at 12 station locations (see Figure 3-1), on the following
five days:
9th May;
7th June;
12th June;
14th June; and
9th July 2013.
The data for all but the first set of casts has been provided to DHI. The timings of the casts
compared with water levels at Hutt River Estuary Bridge are presented in Figure 3-4. The casts
covered most parts of the tide apart from mid to low tide for the outgoing tide. The approximate
wind condition (see Section 3.4) and Hutt River flow (see Section 3.2) at the time of the CTD
casts is presented in Table 3-2.
In addition, NIWA undertook a longitudinal “spatial” survey, at a depth of 0.5 m, from the rail
bridge to 300 m downstream of the River mouth mooring and back again over a period of 1.75
hours. DHI has not made use of this data.
Figure 3-4 Water levels at Hutt River at Estuary Bridge with times of CTD casts.
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Table 3-2 Approximate wind condition and Hutt River flow at the time of the CTD casts.
Date of CTD Casts Approximate Wind Approximate Hutt River Flow
7th June Gentle northerly breeze 15 m3/s
12th June Moderate to fresh southerly breeze 11 m3/s
14th June Moderate to fresh southerly breeze 13 m3/s
9th July Moderate to fresh northerly breeze 23 m3/s
Transects plots are presented in Figure 3-5, produced by interpolating between the CTD casts
at stations 1, 3, 4, 5, 6, 8, 9, 10, 11 and 12. These provide an instantaneous overview of the salt
wedge behaviour when the set of CTD casts were carried out. There are anomalies in the
collected data at Station 2. Station 7 is located only 66 m from Station 6, close to the Waiwhetu
confluence. These CTD casts were not used.
Outfall B is located at approximately 1,700 m while Outfall C is located at approximately
2,150 m. The salt wedge is always present at both potential outfall locations, for periods when
CTD casts were undertaken.
It is also interesting to note that even during a moderate to fresh southerly breeze, where it is
observed that the river plume is ultimately transported anti clockwise around the harbour1, the
CTD casts on 12th and 14th June indicate that the river plume initially travels southwards along
the direction of the CTD transects. This observation maybe due to a combination of the
momentum of the river plume as it exits the river and the river mouth being sheltered from
southerly wind events and beyond the CTD casts, wind forcing would start to drive the behaviour
of the river plume. The field notes from NIWA provided with the CTD casts (NIWA, 2013),
support this assumption. NIWA noted that CTD cast station 11 was much more exposed to wind
than upstream casts.
Further details concerning the data collection can be found in NIWA (2013).
1 Plume is transported south down eastern side of harbour during northerly wind (Heath, 1977).
11
Figure 3-5 Measured salinity profiles between Station 1 and Station 12. Collection period (from left to right and top to bottom): from 7/06/2013 14:25 over 76 min; from 12/06/2013 08:03 over 72 min; from 12/06/2013 10:05 over 52 min; from 14/06/2013 07:47 over 54 min; from 14/06/2013 09:29 over 45 min; from 14/06/2013 10:55 over 53 min; from 9/07/2013 14:08 over 67 min; and from 9/07/2013 16:06 over 59 min.
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3.2 Flow and Water Level Data
The following flow and water level data was obtained from the Greater Wellington Regional
Council live monitoring site for the period 1st May to 2nd August 2013 to cover the period of
NIWA data collection:
1. Hutt River at Taita Gorge flow;
2. Hutt River at Estuary Bridge water level;
3. Waiwhetu Stream at White Line East flow; and
4. Wellington Harbour at Queens Wharf water level.
Time series for data in the vicinity of Hutt River are presented in Figure 3-6.
Figure 3-6 GWRC discharge and water level gauging data for the lower Hutt River and Waiwhetu Stream.
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3.3 Bathymetric Data
Data that informs the bathymetry of the models has been obtained from a number of sources.
Multi-beam data for the majority of Wellington Harbour was obtained from NIWA. Figure 3-7
presents the extent of the data, which is in a grid format, and the extent used for defining model
bed.
Figure 3-7 Extent of NIWA multi-beam grid data and portion used to inform model bathymetry.
Three sets of single-beam survey points collected in November 2013 were supplied by Stantec.
Figure 3-8 presents the locations of the three sets, coloured orange, blue and yellow, as well as
the extent for which a grid was interpolated for use in the model mesh generation.
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Figure 3-8 Single-beam bed survey points collected in November 2013 in the vicinity of the Hutt River mouth.
Cross section survey points for the Hutt River and Waiwhetu Stream were provided by Stantec.
Figure 3-9 presents the locations of the Hutt River survey points, coloured red, and Waiwhetu
Stream survey points, coloured yellow, as well as the extent for which a grid was interpolated for
use in the model mesh generation.
Figure 3-9 Hutt River and Waiwhetu Stream cross section survey points.
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In the vicinity of the Hutt River and Waiwhetu Stream confluence the bathymetry has been
modified manually in order to connect the bed levels of the stream and river in a realistic way.
The survey of spot heights undertaken on 17th February 2015, provided by Stantec, did not
agree sufficiently with the river and stream surveys and so has not been used for this study.
3.4 Wind Data
The following sets of wind data were obtained from NIWA’s climate database (cliflo.niwa.co.nz):
Hourly wind observations from Kaukau weather station (north of Wellington Harbour) for
the period January 2000 to January 2012. There was no data freely available after this
date.
Hourly wind observations from Baring Head weather station (south of Wellington
Harbour) for the period April 2007 to April 2017.
The wind station at the Kaukau is located 425 m above Mean Sea Level (MSL) and the wind
station at Baring Head is 79 m above MSL. The wind data was scaled to 10 m above MSL using
the following formula (Ahrens, 2003):
𝑊𝑆2 = 𝑊𝑆1 × 𝑙𝑛(
𝑍2𝑍𝑜
⁄ )
𝑙𝑛(𝑍1
𝑍𝑜⁄ )
Where WS2 = wind speed at height Z2 (m/s);
WS1 = wind speed at height Z1 (m/s); and
Zo = aerodynamic roughness length (m).
For the ocean Zo = 0.0002 m, therefore to scale wind speeds from 425 m to 10 m the scaling
factor is 0.83 and from 79 m to 10 m, the scaling factor is 0.88.
To provide a direct comparison of the wind data, wind roses for the period January 2008 to
January 2013 for both sets of wind observations are presented in Figure 3-10.
The wind pattern behaviour at both locations is very similar, with both locations experiencing
predominant northerly and southerly winds (although with a slight difference in alignment of
approximately 10°). The Kaukau wind data was selected as most representative of northern
Wellington Harbour.
To assist in defining scenarios for simulations, the wind data was analysed (see Table 3-3) to
assess the 25th, 50th and 90th percentiles for southerly (180 ± 30°) and northerly winds (360 ±
30°). Wind speeds less than 2.5 m/s were considered calm (although strictly a light breeze) and
discounted. An analysis of the frequency of the wind from these directions from the Kaukau data
was performed and is shown in Table 3-4.
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Figure 3-10 Wind data from Kaukau (left) and Baring Head (right) for period January 2008 to January 2013.
Table 3-3 25th, 50th and 90th percentiles for dominant wind directions of Kaukau wind data.
Wind Speed (m/s)
Southerly Wind (180 ± 30°) Northerly Wind (360 ± 30°)
25th Percentile 6.1 6.9
50th Percentile 8.7 9.2
90th Percentile 14.5 14.5
Table 3-4 Occurrence of dominant wind directions.
Direction Occurrence
Calm (< 2.5 m/s) 7%
Southerly (180 ± 30°) 27%
Northerly (360 ± 30°) 50%
Other 15%
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4 Study Approach and Scenario Selection
This section outlines the overall approach that was taken for this study and provides information
on the scenarios that have been investigated.
4.1 Overview of Mixing Behaviour
The discharged wastewater plumes will mix and dilute due to two types of mixing processes:
Near-field mixing. This refers to dilution of the wastewater discharge as it enters the
marine environment in a jet or plume like flow. In this area the flow and mixing is
determined by the discharge itself, i.e. the excess momentum and buoyancy in the jet-
like flow. Near-field mixing occurs due the entrainment of the surrounding ambient
water into the jet-like flow.
Far-field mixing. This refers to where the plume dynamics are governed by the
conditions in the surrounding water, here predominantly currents and turbulence
induced by density gradients.
Different types of models are required for assessing each phase of mixing.
4.2 Near-Field and Far-Field Models
The first modelling stage (near-field) involved the use of a dedicated near-field model, here the
empirically based Cornell Mixing Zone Expert System (CORMIX), to predict the dilution
characteristics of the near-field mixing zone. CORMIX is an industry standard tool which is well
suited for the prediction of plume geometry and entrainment based on positively/negatively
buoyant discharges, for single port or diffuser outfalls. Its main limitations relate to simplistic
assumptions regarding the spatial uniformity of the receiving water, and to the fact that both
discharge and ambient conditions are assumed to be stationary in time. The model is thus well
suited to describing plume behaviour immediately after release, but not its behaviour in a
dynamic environment.
The second modelling stage (far-field) involved the use of DHI’s fully dynamic 3-dimensional
flow and transport modelling system MIKE 3 FM, which is suitable for use where three
dimensional density stratified flows are important as is the case for this study where the mixing
behaviour of a buoyant plume and saline intrusion within the lower Hutt River must be generally
reproduced. Further details of MIKE 3 FM model, can be found in the MIKE 3 FM User Manual
(DHI, 2017).
The flexible mesh allows for a varying resolution computational grid so that a finer resolution can
be used for areas of interest (i.e. Hutt River and outfall locations) and a lower resolution can be
used for other areas. This allows large savings in simulation times without compromising model
resolution in areas of interest. The vertical model resolution is based on discretisation in layers
of varying thickness, so called sigma layers. The main limitation for the MIKE 3 FM model is its
inability to efficiently describe initial small-scale mixing process in the immediate proximity of the
outfall, hence why a separate near-field model required.
To simulate the behaviour of the wastewater plume the advection-dispersion (AD) module was
used. The AD module simulates the spread of dissolved and suspended substances subject to
the transport process described by the HD module. The wastewater plume was defined as a
conservative tracer (i.e. no decay processes).
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The hybrid application of the two models thus utilises the relative strengths of both systems,
while maintaining computational efficiency. The two models are indirectly coupled as the near-
field model will provide the initial conditions for the 3D model.
The CORMIX predictions were utilised to determine:
an appropriate resolution to use for the far-field model domain to obtain initial mixing
comparable with predictions from near-field model; and
the behaviour and characteristics of the plume at end of near-field mixing phase. As an
example, if near-field model predicted that wastewater plume did not fully vertically mix
throughout the water column and instead was trapped below the surface freshwater layer of
the salt wedge at end of near-field mixing phase, then this would need to be taken into
account when inputting the wastewater into the MIKE 3 FM model.
4.3 Scenario Selection
An overview of the scenarios selected for the dilution assessment for each outfall location is
presented in Table 4-1. These provide an envelope of the different, tidal, wind and outfall
discharge scenarios that may occur. For dry weather discharges, the Hutt River and Waiwhetu
Stream inflows were set to 25 m3/s and 0.3 m3/s respectively. For wet weather discharges,
constant flows of 100 m3/s and 2 m3/s for the Hutt River and Waiwhetu Stream respectively were
assumed. These river flows were selected with agreement from Stantec.
For the 30 day duration simulation (Scenario 25) observed winds from Kaukau over the period
1st April to 1st May 2010 were applied. A visual comparison of the wind rose for this period
compared with the wind rose of the full wind record (see Figure 4-1), indicted this period
contained a good representation distribution of both wind speed and direction.
Figure 4-1 Comparison of Kaukau wind rose for period 1st April to 1st May 2010 (left) and period January 2008 to January 2013 (right).
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Table 4-1 Overview of selected scenarios.
Scenario Overflow
Discharge Rate
Duration of
Discharge
Wind Condition Tidal
Condition
1 0.55 m3/s
(1.10 m3/s for
Option B on
outgoing tide)
5 days Calm Neap
2 Spring
3 90th Percentile Southerly Neap
4 Spring
5 50th Percentile Northerly Neap
6 Spring
7 90th Percentile Northerly Neap
8 Spring
9 0.8 m3/s 1 day Calm Neap
10 Spring
11 90th Percentile Southerly Neap
12 Spring
13 50th Percentile Northerly Neap
14 Spring
15 90th Percentile Northerly Neap
16 Spring
17 3 m3/s 1 day Calm Neap
18 Spring
19 90th Percentile Southerly Neap
20 Spring
21 50th Percentile Northerly Neap
22 Spring
23 90th Percentile Northerly Neap
24 Spring
25 0.55 m3/s
(1.10 m3/s for
Option B on
outgoing tide)
30 days Kaukau wind - 1st April to 1st May 2010 N/A
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5 Far-Field Model Set Up and Calibration
A three dimensional far-field model was developed that could reproduce both saline intrusion
within the lower Hutt River and the river, tidal and wind driven currents for the lower Hutt River
and northern Wellington Harbour. These are the important processes which will determine the
dilution and transport of the discharged wastewater plume.
5.1 Set Up
5.1.1 Bathymetry and Mesh
The model bathymetry was constructed using data outlined in Section 3.3. The full model extent
of the far-field model and bathymetry is shown in Figure 5-1, while the model bathymetry and
mesh for the lower Hutt River is shown in Figure 5-2.
Figure 5-1 Model bathymetry (NZTM coordinates and depths relative to Wellington MSL Datum).
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Figure 5-2 Model bathymetry and mesh for lower Hutt River (NZTM coordinates and depths relative to Wellington MSL Datum).
5.1.2 Open Ocean Boundaries
Water levels for the southern open ocean boundary condition of the model were based on
observed water levels from Wellington Harbour at Queens Wharf. For the calibration
simulations, the observed water levels were applied directly, while for design scenario
simulations, water levels with the non-tidal component removed were applied. A salinity of 35
PSU has been used for the open ocean boundary.
5.1.3 Freshwater Inflows
Freshwater inflows with 0 PSU have been applied for Hutt River and Waiwhetu stream.
Constant flow rates have been applied for design scenario simulation, while observed flow rates
have been applied for the calibration simulations.
5.1.4 Wind
DHI did not have access to high resolution spatially varying wind maps for the Wellington
Harbour region for the calibration period. Only wind data from Baring Head was freely available
for this period. The surrounding hills of Wellington Harbour will have an impact on the wind field,
with sheltered and more exposed areas, depending on the wind direction. Initially, a spatially
uniform wind was applied to the model, with no wind in the immediate vicinity of the Hutt River
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mouth. However this created inaccurate eddies within the river mouth. To ensure that the
momentum of the river plume exiting the river was accurately modelled a constant negative v-
velocity wind was applied instead of zero in the immediate vicinity of the Hutt River entrance.
For design scenarios, spatial wind fields were applied in the same way.
5.1.5 Wastewater Representation
To simulate the behaviour of the wastewater plume the advection-dispersion (AD) module was
used. The AD module simulates the spread of dissolved and suspended substances subject to
the transport process described by the HD module. The wastewater plume was defined as a
conservative tracer (i.e. no decay rate).
5.2 Calibration
The model was deemed suitability calibrated to meet the objectives of the study. Calibration was
achieved with the following key specifications, with details of the calibration provided in sections
below:
5 vertical equidistant sigma layers to -2.5 m and 4 z levels layers below this with
thickness of 1 m, 3, 5 and 10 m respectively.
Density assumed as a function of salinity only.
Resistance length: 0.05 m.
Wind friction factor: 0.001255
Horizontal eddy viscosity: Smagorinsky formulation, constant 0.28.
Vertical eddy viscosity: log law formulation
Horizontal and vertical dispersion turned off.
5.2.1 Water Levels and Currents
Generally there was a good agreement between observed and predicted water levels for the
whole calibration period (15th May to 7th July 2013). This is illustrated through the comparison of
observed and predicted water levels at Estuary Bridge on Hutt River for the period 2nd to 15th
June 2013, presented in Figure 5-3.
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Figure 5-3 Comparison of observed and predicted water levels for Hutt River at Estuary Bridge for period 2nd to 15th June 2013.
Currents were available from ADCPs within the Hutt River and mid Harbour. Both data sets
have some inherent issues as discussed below.
The location of the mid harbour ADCP was selected since, for the previous NIWA assessment,
one of the potential outfall locations was 2 km south of Somes Island. This is no longer a
potential option. Ideally an ADCP would have been located within the harbour closer to river
mouth, to illustrate the model can reasonably reproduce currents where the Hutt River enters
the Harbour. However the mid harbour ADCP was the only source of current data to calibrate
the harbour model.
The Hutt River ADCP data was not useful for calibration, since as outlined in NIWA (2013), the
ADCP was located in a back eddy which resulted in net flow upstream (i.e. positive depth
averaged north-south orientated v–velocities) for typical river flow conditions. The formation of
this eddy will be very dependent on the detailed local lower river bathymetry at the time of
measurements which are not well represented in the model. The DHI model predicts a net flow
out into the harbour (i.e. negative v-velocities) as would be expected.
The issue with the ADCP location is further illustrated if you compare surface v-velocity with
wind speed at Baring Head, as shown in Figure 5-4. Even for periods with very little wind speed,
there are periods of surface water flowing upstream for every tidal cycle (apart from when
elevated flows in river). This is not representative of general flow behaviour for the river. Stantec
(2016) undertook a dye test for Hutt River, where dye was released into surface waters on the
incoming tide at Outfall B location, with moderate north-westerly winds. The dye was observed
to still flow downstream out of the harbour mouth into the harbour, indicating no surface waters
flowing upstream.
24 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 5-4 Surface v-velcoity current from River ADCP compared with Baring Head wind speed. A positive v-velcoity corresponds to upstream flow.
The mid harbour ADCP data provided by NIWA was not post processed and there are obviously
periods with spikes of erroneous data that typically occur when the observation bin (height
above the instrument) is actually out of the water. An example of this issue is provided in Figure
5-5. This figure shows v-velocities that have been passed through a low pass filter to remove
high frequency noise for top and third from top bin against water levels at Queens Wharf in
Wellington Harbour. There is an obvious peak in current velocity as water levels drop pre and
post low tide. It is our opinion that these velocities are erroneous as currents speeds up to
0.5 m/s are unlikely to occur at this location, especially approaching low water.
Figure 5-5 Example of v velocity from bins close to surface, from mid harbour ADCP, plotted with water levels at Wellington Harbour at Queens Wharf.
A comparison of predicted u and v current velocities has only been undertaken for the top
portion of the water column for the mid harbour data, since this is the only area where the
wastewater plume will be located. The model performance for the deeper parts of the harbour
25
(i.e. depth greater than -3 m) have not been assessed since any plume concentrations at depth
will be negligible. The observed and predicted u and v velocities have been compared for the
period 1st to 17th June 2013 at approximately -0.5 m Wellington Datum as presented in Figure
5-6. At times this depth will be close to be the water surface or at a water depth of 1.5 m.
The corresponding wind behaviour at Baring Head is also presented for the corresponding
period. The odd spike due to the bin being exposed above the water surface is still apparent
however. In general there is a reasonable agreement, especially for the v-velocity component,
where can be considered the dominant velocity direction. There were a number of events where
wind speeds greater than 10 m/s. As expected for a northerly event, both observed and
predicted v velocities are negative (heading in a southerly direction) and, while for a southerly
event, both observed and predicted v velocities are positive.
In order to evaluate the performance of the calibrated model, different statistical indices were
calculated to verify the accuracy of the model results. In order to evaluate the performance of
the model, different statistical indices were computed to verify the accuracy of the model results.
The following statistical parameters have been evaluated as suggested by Ji (2008):
Mean Absolute Error (MAE):
N
n nn POabsN
MAE1
)(1
Root Mean Square Error (RMS):
2
1)(1
N
n nn PON
RMS
Relative Root Mean Square Error (RRE):
100minmax
OO
RMSRRE
where:
N = number of observation - prediction pairs
On = the value of the nth observed data
Pn = the value of the nth predicted data
Omax = maximum value of observations
26 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Omin = minimum value of observations
The MAE indicates the average deviation between model predictions and the observed data but
does not give an indication of over prediction or under prediction. The RMS error is the average
of the squared differences between observed and predicted values and is more widely used to
assess model performance as it gives higher weightings to larger observation-prediction
differences. It is also useful to give the difference in observed and predicted values as a
percentage to measure performance. The RRE is often used for hydrodynamic modelling and is
the ratio of the RMS error to the observed change.
A summary of the calculated statistics of the model agreement with observed data is presented
in Table 5-1. The statistical analysis supported the visual interpretation of the model calibration.
Table 5-1 Summary of calibration statistical analysis.
Location Parameter Statistical Performance Parameter
MAE RMS RRE
Estuary Bridge Water Level (m) 0.05 0.07 3.13
NIWA Harbour
Mooring
U Velocity (m/s) 0.05 0.06 16.8
V Velocity (m/s) 0.06 0.07 13.7
27
Figure 5-6 Comparison of surface observed and predicted u (top) and v (middle) velocities mid harbour for period 1st to 17th June 2013. Corresponding wind data from Baring Head also presented (bottom).
28 waiwhetu outfall dilution assessment / bjt / 2017-10-30
5.2.2 Salt Wedge and River Plume
To assess the performance of the model in reproducing the behaviour of the saline intrusion and
river plume, a visual comparison was made between the observed and predicted salinities. The
aim of the calibration was not to obtain a perfect match between observed and predicted
salinities, but instead to replicate the general behaviour.
A comparison of the observed and predicted salinities is presented in Figure 5-7 and Figure 5-8.
Note that some liberty was taken for location of salinity extraction beyond 3,000 m (i.e.
difference of up to 500m in east-west direction). The reason for this is that the behaviour of the
plume will be very influenced by wind field for this area, which is not well resolved within the
model. It should also be noted that it is typically extremely difficult to resolve sharp freshwater
and saltwater interfaces in 3D models such as MIKE 3 FM. Any type of numerical dispersion
which is inherent in this type of model breaks down the buoyancy effect of the surface
freshwater (which maintains the freshwater layer), resulting in mixing across the vertical layers.
For this reason the focus is on re-producing overall thickness and general behaviour of river
plume layer, in this case considered to be for salinities less than 30 PSU.
Visually there is a sufficient agreement between the observed and predicted salinity distribution
for the river and river mouth. The model does appear to under predict the maximum extent
upstream of the salt wedge, however this was not the main focus of the model. The predicted
river plume is slightly thicker than suggested by the salinity data collected during southerly wind
events on 12th and 14th of June, however this is not unexpected with the wind data that was
available for Wellington Harbour.
29
Figure 5-7 Comparison of measured salinity profiles (left) with predicted salinity profiles (right) from 7/06/2013 14:25; 12/06/2013 08:03; 12/06/2013 10:05; and 14/06/2013 07:47.
30 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 5-8 Comparison of measured salinity profiles (left) with predicted salinity profiles (right) from 14/06/2013 9:29; 14/06/2013 10:55; 9/07/2013 14:08; and 9/07/2013 16:06.
31
6 Near-Field Assessment
This section describes the near-field modelling that was carried out using the CORMIX
modelling system. The near-field modelling provides predictions of the behaviour and dilution of
the wastewater plume in the near-field mixing zone.
Due to the constrained channel cross section and large wastewater discharge compared with
the flow of Waiwhetu Stream (a ratio of almost 2:1 for the “dry weather” scenarios and 1:1 for
“wet weather” scenarios), realistic wastewater behaviour predictions could not be obtained with
CORMIX for outfall Option A. Very little dilution of wastewater will occur within Waiwhetu Stream
and the far-field model is sufficient to predict dilutions beyond the confluence with Hutt River.
6.1 Wastewater Properties and Outfall Arrangement
As per the previous studies (DHI, 2011 and 2016), the wastewater is assumed to be freshwater
(i.e. salinity = 0 PSU), as are the river inflows.
The previous studies (DHI 2011 and 2016) illustrated that the temperature difference between
summer and winter has a negligible effect on the initial mixing of the wastewater plume,
therefore density differences between the treated wastewater effluent and receiving water are
thus represented in terms of salinity alone. Therefore, it was assumed that the wastewater
discharge temperature will be equal to the receiving water temperature.
The following outfall arrangement is associated with each location:
Option B, a horizontally discharging 1.6 m diameter pipe at approximately -0.485 m
(Wellington Datum 1953) or Mean Low Water Spring, protruding 20 m out from the left
river bank at an angle of 40° from the river bank.
Option C, a horizontally discharging 1.6 m diameter pipe on the seabed with two duckbill
valve 900 mm diameter ports, discharging vertically, with a port height 1.4 m above the
sea bed, 100 m from the left river bank.
6.2 Receiving Water Conditions
6.2.1 Receiving Water Level, Flow and Salinity
Water level, flow and salinity conditions at the potential outfall locations B and C, for the different
tide and weather conditions were determined from the far-field model.
With agreement from Stantec, for dry weather conditions approximate mean flows were selected
as constant boundary conditions for Hutt River and Waiwhetu Stream (25 m3/s and 0.3 m3/s
respectively). For wet weather conditions a constant flow of 100 m3/s and 2 m3/s were used for
Hutt River and Waiwhetu Stream, also with agreement from Stantec.
Water level, flow and salinity predictions for the two outfall locations and scenarios are outlined
in Table 6-1. Salinity stratification was incorporated in CORMIX by having a density jump at the
depth of the indicated salt wedge interface, with 0 PSU above and 35 PSU assumed below this
depth.
32 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Table 6-1 Water level, flow and salinity predictions for the different outfall locations and scenarios.
Outfall
Location Parameter
Condition
Dry Weather
High Tide
Dry Weather
Mid Ebb
Dry Weather
Low Tide
Dry Weather
Mid Flood
Wet
Weather
Low Tide
Spring Neap Spring Neap Spring Neap Spring Neap Spring
Option B
Water
Level
(m)
1.0 0.7 0.3 0.3 -0.3 -0.2 0.3 0.3 0.3
Flow
(m3/s) 25.0 25.0 55.0 45.0 25.0 25.0 -2.0 5.0 100
Salinity
Behaviour Salt wedge interface approx. -0.5 m below water surface
Salt wedge
interface
approx. -1.4
m below
water
surface
Option C
Water
Level
(m)
1.0 0.7 0.3 0.3 -0.3 -0.2 0.3 0.3 0.3
Flow
(m3/s) 25.0 25.0 50.0 40.0 25.0 25.0 -2.0 8.0 100
Salinity
Salt wedge interface approx. -0.5 m below water surface
Salt wedge
interface
approx. -1.0
m below
water
surface
6.2.2 Receiving Water Bathymetry
For all outfall options a cross section Manning’s n of 0.03 was applied in the CORMIX models.
Surveyed cross sections at the outfall locations for Option B and C were obtained from the
GWRC set of Hutt River cross sections.
When predicting dilutions for rivers, CORMIX assumes a rectangular cross section. Therefore
the cross sections at the location of Option B and C were schematized to an appropriate
rectangular cross section, ensuring that the schematized rectangular cross section has the
same cross sectional area as the surveyed cross section. Comparisons of the schematised
cross sections with the actual cross sections are shown in Figure 6-1 to Figure 6-2.
Option B was schematised as 162 m wide and a bed level of -2.0 m (Wellington Datum 1953).
Option C was schematised as 241 m wide and a bed level of -2.0 m (Wellington Datum 1953).
33
Figure 6-1 Option B – Actual cross section and schematised cross section for CORMIX. Levels in Wellington Datum 1953.
Figure 6-2 Option C – Actual cross section and schematised cross section for CORMIX. Levels in Wellington Datum 1953.
34 waiwhetu outfall dilution assessment / bjt / 2017-10-30
6.2.3 Situations where Modifications of Assumptions were Required to Obtain Dilution Predictions
Although CORMIX is on the whole a very flexible tool, there are still some criteria which must be
met before CORMIX will provide a prediction of the behaviour of the discharged wastewater. A
good example is that for submerged outfalls, the diameter of the outfall pipe must be one third or
less of the total water depth before a prediction will be provided.
For some of the scenarios and discharge locations, different assumptions were necessary either
as a result of the assumptions not meeting the required criteria of CORMIX or where predictions
were obviously erroneous. Detailed below are the situations where the CORMIX inputs or
outputs were different from the assumptions described in the sections above.
6.2.3.1 Option B Option B is essentially a surface discharge for all states of the tide, sometimes discharging
directly into the top of water column or discharging above the water surface. To ensure that the
wastewater plume would discharge into the freshwater layer overlying the salt wedge, and that
CORMIX would acknowledge the existence of the salinity stratification (i.e. if outfall invert below
freshwater layer, CORMIX unfortunately assumes no stratification), the outfall was included for
all scenarios as a 1.6 diameter pipe above the water surface. CORMIX was able to incorporate
the fact the outfall is protruding 20 m out from the left river bank at an angle of 40° from the river
bank.
6.2.3.2 Option C It was not possible to assume a two port outfall with CORMIX and similar to Option B the outfall
centre had to discharge into freshwater layer for CORMIX to acknowledge the salinity
stratification. The outfall was schematised as a 0.55 m diameter outfall (smaller than 0.9m since
the outfall dimeter must always be less than 1/3 the water depth), with the pipe centre just within
the freshwater layer. Sensitivity testing illustrated that the plume is positively buoyant and would
rise quickly to the freshwater layer. It can be assumed that the CORMIX predictions will be
conservative without this initial mixing as the plume rises through the salt wedge. CORMIX was
able to incorporate the fact the outfall is 100 m from the left river bank.
6.3 Model Results
The CORMIX model was utilised to provide predictions of the following 100 m downstream of
the discharge point and at the edge of the near-field mixing zone (see Table 6-2 and Table 6-3)
for the specified discharge and climate scenarios:
Dilution;
Plume width;
Distance from outfall; and
Plume thickness.
CORMIX provides the following advice with regard to uncertainty in the model predictions with
every model simulation results file, “extensive comparison with field and laboratory data has
shown that the CORMIX predictions on dilutions (with associated plume geometries) are reliable
for the majority of cases and are accurate to within about +-50% (standard deviation)”.
The following is a summary of the predicted dilution behaviour of the wastewater discharged
from Option B:
35
Salinity stratification for the full tidal cycle significantly decreases the dilution of wastewater,
since the wastewater plume will be very positively buoyant compared with the receiving
water which inhibits the mixing of wastewater. The wastewater will remain within the
overlying freshwater layer where it fully mixes vertically.
For the dry weather condition with a 1.1 m3/s discharge on outgoing tide, dilutions range
from 3.6 fold to 6.0 fold at a distance of 100 m from outfall and 3.1 to 8.8 fold at end of
near-field mixing zone, with a range of 12 to 350 m from the outfall.
For the wet weather conditions with a constant 0.8 m3/s discharge, a dilution of 3.3 fold is
achieved at a distance of 100 m from the outfall location and 1.8 fold at end of near-field
mixing zone 2 m downstream of the outfall.
For the wet weather conditions with a constant 3.0 m3/s discharge, a dilution of 2.4 fold is
achieved at a distance of 100 m from the outfall location and 2.3 fold at end of near-field
mixing zone 19 m downstream of the outfall.
The following is a summary of the predicted dilution behaviour of the wastewater discharged
from Option C:
Salinity stratification for the full tidal cycle significantly decreases the dilution of wastewater,
since the wastewater plume will be very positively buoyant compared with the receiving
water which inhibits the mixing of wastewater. The wastewater will remain within the
overlying freshwater layer where it fully mixes vertically.
For the dry weather condition with a 0.55 m3/s discharge, dilutions range from 8.9 fold to
28.0 fold at a distance of 100 m from outfall and 8.9 to 23.0 fold at end of near-field mixing
zone, with a range of 79 to 534 m from the outfall.
For the wet weather conditions with a constant 0.8 m3/s discharge, a dilution of 6.8 fold is
achieved at a distance of 100 m from the outfall location and 6.7 fold at end of near-field
mixing zone 20 m downstream of the outfall.
For the wet weather conditions with a constant 3.0 m3/s discharge, a dilution of 16.9 fold is
achieved at a distance of 100 m from the outfall location and 24.1 fold at end of near-field
mixing zone 292 m downstream of the outfall.
The model mesh elements at Outfall B are approximately 25 m wide, while at Outfall C the mesh
elements are approximately 40 m wide. To incorporate the near-field CORMIX predictions into
the far-field model, the following logic was followed for including sources based on the plume
predictions 100 m from outfall:
Option B
For dry weather conditions – 3 sources across 65 m width, across top layer, attached to
left river bank.
For wet weather conditions and 0.8 m3/s discharge – 1 source across 7 m width, across
top layer, centred 20 m from left river bank.
For wet weather conditions and 3.0 m3/s discharge – 1 source across 19 m width,
across top layer, centred 20 m from left river bank.
36 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Option C
For dry weather conditions – 3 sources across 86 m width, across top layer, centred
100 m from left river bank.
For wet weather conditions and 0.8 m3/s discharge – 1 source across 30 m width,
across top layer, centred 100 m from left river bank.
For wet weather conditions and 3.0 m3/s discharge – 1 sources across 43 m width,
across top layer, centred 100 m from left river bank.
37
Table 6-2 Option B - Predicted plume dimensions and dilutions a horizontal distance of 100 m downstream from outfall and at end of near-field mixing zone.
Discharge Rate
(m3/s)
Tide
Plume Dimensions at 100 m End of Near Field Mixing Zone
Distance from Bank (m)
Width
(m)
Thickness
(m)
Dilution
(fold)
Distance from Bank
(m)
Distance Downstream
(m)
Width
(m)
Thickness
(m)
Dilution
(fold)
1.10
Spring
High 70.0 16.9 0.5# 4.7 108.8 349.3 52.4 0.5# 8.5
Mid Ebb Attached to bank 74.5 0.6# 6.0 26.5 12.2 3.1 2.3+ 3.7
Low Attached to bank 91.8 0.5# 3.6 26.6 23.0 24.7 0.3# 3.1
Neap
High 66.4 16.8 0.5# 4.7 90.2 281.1 42.4 0.5# 7.7
Mid Ebb Attached to bank 83.9 0.5# 5.6 26.9 11.9 3.3 2.3+ 3.5
Low Attached to bank 106.0 0.5# 3.9 27.1 24.7 27.2 0.3# 3.3
0.80 Spring Mid Ebb 20.6 7.0 1.4# 3.3 20.6 2.0 3.9 1.4# 1.8
3.00 Spring Mid Ebb 24.7 18.6 1.4# 2.4 24.7 18.6 3.9 1.4# 2.3
Note: # Attached to surface, + Fully vertically mixed
38 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Table 6-3 Option C - Predicted plume dimensions and dilutions a horizontal distance of 100 m downstream from outfall and at end of near-field mixing zone
Discharge Rate
(m3/s)
Tide
Plume Dimensions at 100 m End of Near Field Mixing Zone
Distance from Bank (m)
Width
(m)
Thickness
(m)
Dilution
(fold)
Distance from Bank
(m)
Distance Downstream
(m)
Width
(m)
Thickness
(m)
Dilution
(fold)
0.55
Spring
High 100 53.6 0.5# 13.2 100 534.2 150.8 0.5# 23.0
Mid Ebb 100 140.3 0.5# 8.9 100 78.9 22.2 0.5# 8.9
Low 100 34.9 0.5# 10.9 100 171.4 48.4 0.5# 13.1
Mid Flood 100 241.0 0.5# 28.0^ N/A N/A N/A N/A N/A
Neap
High 100 49.5 0.5# 12.7 100 432.7 122.1 0.5# 20.7
Mid Ebb 100 30.6 0.5# 10.3 100 122.4 34.6 0.5# 11.1
Low 100 36.5 0.5# 11.1 100 192.2 54.3 0.5# 13.8
Mid Flood 100 100.9 0.5# 17.6 N/A N/A N/A N/A N/A
0.80 Spring Mid Ebb 100 29.6 1.0# 6.8 100 19.5 5.9 1.0# 6.7
3.00 Spring Mid Ebb 100 42.8 1.0# 16.9 100 291.6 82.4 1.0# 24.1
Note: # Attached to surface, + Fully vertically mixed, ^ Plume flowing upstream, reasonable prediction not possible for mid flood with dry weather conditions.
39
7 Far-Field Assessment
This section presents the wastewater dilution predictions from the far-field model for the
selected scenarios.
7.1 Model Results
To provide an overview of the wastewater plume behaviour, plots are provided of 5th and 50th
percentile dilutions, at the surface, for selected scenarios. In this case spring tide, dry weather
conditions with calm, 90th percentile northerly and 90th percentile southerly wind conditions.
These are presented in Figure 7-1 to Figure 7-9. The percentile analysis of dilutions, covers the
duration of the discharge (5 days for dry weather and 1 day for wet weather) and two
subsequent days after the discharge finishes. For calm conditions, the wastewater plume tends
to migrate southwards towards the harbour entrance, with reasonable lateral spread across the
harbour. For southerly wind, the plume is transported northwards towards Petone beach, while
for northerly wind, the plume is very thin and closely follows the eastern shoreline out through
the harbour entrance.
The 1st, 5th, 25th and 50th percentiles of dilutions at the surface at the sensitive sites for all
scenarios is presented in Table 7-1 to Table 7-8. The 5th and 50th percentile dilutions are the
main focus for Stantec, therefore the minimum dilutions for these percentiles for each discharge
condition are also presented in Table 7-9 to Table 7-11.
To provide an indication of the visitation frequency of the plume from each outfall at the sites,
percentile plots (see Figure 7-10 to Figure 7-16) at the sensitive sites were also generated for
the 30 day simulation (Scenario 25), which included a range of representative wind conditions
within the harbour. This illustrates the percentage of time at each site that a dilution is predicted
to be achieved. The plot for Waione Street Bridge was not included, since it is predicted the
plume will never be transported to this location for any outfall or condition. For these plots any
dilutions greater than 10,000 fold were presented as 10,000 fold.
Generally Outfall C produced the most dilution of the wastewater plume compared with other
potential outfall locations, before visitation of the plume occurred at the sensitive sites,
especially for the eastern sites. Outfall B generally performed slightly better than Outfall A,
however not for all conditions and sites.
40 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-1 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall A with dry weather conditions and calm wind condition.
41
Figure 7-2 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall B with dry weather conditions and calm wind condition.
42 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-3 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall C with dry weather conditions and calm wind condition.
43
Figure 7-4 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall A with dry weather conditions and 90th percentile southerly wind condition.
44 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-5 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall B with dry weather conditions and 90th percentile southerly wind condition.
45
Figure 7-6 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall C with dry weather conditions and 90th percentile southerly wind condition.
46 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-7 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall A with dry weather conditions and 90th percentile northerly wind condition.
47
Figure 7-8 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall B with dry weather conditions and 90th percentile northerly wind condition.
48 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-9 5th percentile (top) and 50th percentile dilution (bottom), at surface, for Outfall C with dry weather conditions and 90th percentile northerly wind condition.
49
Table 7-1 Percentile dilutions at surface for scenarios at Petone Beach west.
Scenario Outfall A Outfall B Outfall C
Percentile Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
2 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
3 196 203 253 327 193 209 282 381 183 193 249 335
4 191 206 257 358 192 215 282 390 181 192 249 368
5 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
6 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
7 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
8 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
9 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
10 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
11 258 277 423 792 231 243 404 776 199 212 364 768
12 268 292 412 966 220 247 427 894 195 221 448 864
13 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
14 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
15 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
16 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
17 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
18 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
19 69 75 112 209 63 65 106 205 54 58 96 205
20 74 80 110 255 59 67 113 234 53 60 117 231
21 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
22 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
23 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
24 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
25 186 234 >1000 >1000 152 199 >1000 >1000 184 227 >1000 >1000
50 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Table 7-2 Percentile dilutions at surface for scenarios at Petone Beach east.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
2 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
3 156 166 200 243 134 144 196 269 142 148 172 209
4 145 154 209 251 132 142 198 264 141 149 180 223
5 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
6 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
7 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
8 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
9 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
10 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
11 261 323 508 >1000 245 276 431 >1000 191 207 339 866
12 273 316 566 >1000 260 296 506 934 209 220 348 746
13 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
14 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
15 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
16 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
17 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
18 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
19 71 89 138 284 65 76 116 269 53 58 93 234
20 74 87 155 284 70 80 134 248 57 61 94 200
21 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
22 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
23 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
24 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
25 211 278 >1000 >1000 169 237 >1000 >1000 208 273 >1000 >1000
51
Table 7-3 Percentile dilutions at surface for scenarios at Waione Street Bridge.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
2 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
3 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
4 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
5 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
6 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
7 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
8 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
9 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
10 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
11 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
12 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
13 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
14 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
15 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
16 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
17 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
18 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
19 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
20 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
21 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
22 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
23 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
24 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
25 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
52 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Table 7-4 Percentile dilutions at surface for scenarios at 100m downstream of Hutt/Waiwhetu confluence.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 3 3 5 14 3 3 17 >1000 >1000 >1000 >1000 >1000
2 2 3 5 21 3 4 6 >1000 >1000 >1000 >1000 >1000
3 3 3 4 10 3 4 9 249 398 472 >1000 >1000
4 2 3 4 19 3 4 6 335 409 521 >1000 >1000
5 3 3 5 15 2 3 6 >1000 >1000 >1000 >1000 >1000
6 3 3 5 18 3 4 7 >1000 >1000 >1000 >1000 >1000
7 3 3 5 13 2 3 6 >1000 >1000 >1000 >1000 >1000
8 3 3 5 17 3 4 7 >1000 >1000 >1000 >1000 >1000
9 3 3 14 >1000 7 14 105 >1000 >1000 >1000 >1000 >1000
10 3 3 9 >1000 7 15 297 >1000 >1000 >1000 >1000 >1000
11 3 3 13 >1000 8 10 457 >1000 >1000 >1000 >1000 >1000
12 3 3 11 >1000 7 17 >1000 >1000 >1000 >1000 >1000 >1000
13 3 4 12 >1000 5 9 55 >1000 >1000 >1000 >1000 >1000
14 3 4 9 >1000 7 14 175 >1000 >1000 >1000 >1000 >1000
15 3 4 12 >1000 5 9 54 >1000 >1000 >1000 >1000 >1000
16 3 4 9 >1000 7 14 154 >1000 >1000 >1000 >1000 >1000
17 2 2 6 >1000 3 5 45 >1000 >1000 >1000 >1000 >1000
18 2 2 3 >1000 3 5 195 >1000 >1000 >1000 >1000 >1000
19 1 1 5 >1000 3 3 218 >1000 >1000 >1000 >1000 >1000
20 1 2 3 >1000 3 5 >1000 >1000 >1000 >1000 >1000 >1000
21 2 2 6 >1000 2 3 18 >1000 >1000 >1000 >1000 >1000
22 2 2 3 >1000 3 5 112 >1000 >1000 >1000 >1000 >1000
23 2 2 6 >1000 2 3 18 >1000 >1000 >1000 >1000 >1000
24 2 2 3 >1000 3 5 104 >1000 >1000 >1000 >1000 >1000
25 3 3 4 6 3 4 6 31 >1000 >1000 >1000 >1000
53
Table 7-5 Percentile dilutions at surface for scenarios at Lowry Bay.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 58 59 63 99 67 68 77 168 93 94 102 165
2 67 69 75 116 74 76 86 124 92 99 109 158
3 65 66 77 98 54 57 71 123 94 97 113 133
4 59 60 73 102 56 58 76 118 92 94 115 142
5 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
6 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
7 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
8 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
9 94 99 117 177 115 118 135 201 159 164 200 313
10 96 103 123 224 112 119 144 314 175 184 218 576
11 71 76 109 327 75 80 112 347 142 146 214 581
12 66 72 100 279 71 76 112 317 137 139 221 543
13 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
14 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
15 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
16 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
17 26 28 33 47 33 33 38 54 43 44 54 80
18 28 30 35 62 32 34 41 88 48 50 58 154
19 21 22 31 92 21 22 31 94 39 40 57 153
20 20 21 29 80 20 21 31 87 37 38 59 148
21 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
22 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
23 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
24 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
25 94 128 377 >1000 71 134 497 >1000 126 165 548 >1000
54 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Table 7-6 Percentile dilutions at surface for scenarios at Days Bay.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 74 79 101 184 80 83 103 179 113 114 127 237
2 80 86 114 163 79 84 102 152 121 129 146 283
3 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
4 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
5 147 153 200 270 131 138 189 346 257 264 306 402
6 108 116 188 306 117 124 177 342 233 246 291 460
7 141 150 193 296 105 108 151 789 237 240 257 364
8 104 114 201 369 106 109 141 655 220 231 254 416
9 100 113 155 471 114 127 157 555 135 146 192 862
10 104 109 151 325 114 119 156 416 152 157 173 655
11 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
12 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
13 124 135 182 >1000 147 158 211 >1000 223 230 324 >1000
14 122 136 198 683 147 153 216 790 215 218 319 >1000
15 146 148 293 >1000 171 176 288 >1000 248 277 347 >1000
16 138 143 292 >1000 171 184 307 >1000 242 273 390 >1000
17 29 32 43 122 33 35 43 145 37 39 48 212
18 29 31 43 83 33 34 43 109 41 43 46 163
19 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
20 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
21 34 37 50 319 41 44 59 387 60 62 86 668
22 34 38 54 188 41 43 59 216 59 60 85 370
23 40 41 81 >1000 48 49 81 >1000 69 76 96 >1000
24 39 40 82 >1000 48 52 85 >1000 68 75 107 >1000
25 100 108 166 305 87 99 145 393 153 165 211 327
55
Table 7-7 Percentile dilutions at surface for scenarios at Port Road corner beach.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 21 21 23 27 22 28 76 106 38 41 57 77
2 17 18 25 39 37 69 100 152 41 45 67 153
3 31 35 58 90 37 61 84 334 92 98 131 217
4 30 35 67 112 31 37 131 336 79 87 116 332
5 26 31 60 90 44 61 93 619 117 123 208 326
6 23 26 60 107 35 48 150 704 100 104 198 530
7 25 30 57 86 45 56 88 598 105 112 182 291
8 22 25 56 99 34 46 137 681 91 95 181 465
9 81 83 96 175 103 106 118 214 169 179 220 487
10 81 87 112 146 89 108 150 210 216 252 397 782
11 29 39 140 >1000 101 115 195 >1000 332 467 837 >1000
12 30 37 127 >1000 93 110 291 >1000 290 354 >1000 >1000
13 31 47 84 >1000 64 72 131 >1000 >1000 >1000 >1000 >1000
14 26 29 92 >1000 57 65 384 >1000 754 949 >1000 >1000
15 31 46 80 >1000 61 69 121 >1000 >1000 >1000 >1000 >1000
16 26 29 87 >1000 55 62 358 >1000 721 894 >1000 >1000
17 22 23 26 47 30 30 33 58 45 48 58 126
18 22 24 31 40 25 30 42 55 58 67 104 205
19 9 12 34 393 30 33 52 458 91 123 218 446
20 9 11 32 458 29 32 83 453 80 98 301 485
21 9 13 24 >1000 19 21 37 >1000 300 360 >1000 >1000
22 7 9 25 >1000 18 20 115 >1000 224 310 >1000 >1000
23 9 13 23 >1000 19 20 34 >1000 290 344 >1000 >1000
24 7 8 24 >1000 17 19 108 >1000 218 294 >1000 >1000
25 23 26 42 80 33 43 89 249 95 105 153 256
56 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Table 7-8 Percentile dilutions at surface for scenarios at Seaview Marina.
Scenario Outfall A Outfall B Outfall C
Dilution (Fold)
1st 5th 25th 50th 1st 5th 25th 50th 1st 5th 25th 50th
1 26 29 37 48 67 70 79 124 62 69 83 91
2 32 36 48 61 74 82 111 144 73 77 89 106
3 36 39 49 61 36 37 45 283 76 78 88 104
4 30 34 49 73 37 38 43 277 74 76 86 109
5 34 36 49 56 35 36 43 626 98 99 105 172
6 26 30 48 70 39 40 49 568 98 100 105 197
7 34 35 48 55 34 35 41 600 94 95 101 159
8 26 30 47 68 38 38 47 555 96 97 103 187
9 82 83 98 208 100 104 121 251 156 175 201 434
10 79 84 131 176 110 116 169 235 196 204 312 606
11 29 33 61 >1000 54 56 62 >1000 152 162 217 >1000
12 27 35 58 >1000 53 55 67 >1000 131 156 245 >1000
13 36 45 57 >1000 56 57 78 >1000 260 271 776 >1000
14 34 38 58 >1000 52 55 111 >1000 245 263 >1000 >1000
15 36 44 56 >1000 54 56 75 >1000 250 259 747 >1000
16 34 37 57 >1000 50 53 106 >1000 235 254 >1000 >1000
17 23 24 27 56 29 30 34 69 42 47 54 112
18 22 24 36 48 31 33 46 62 53 55 80 154
19 9 10 17 571 15 16 17 574 42 45 58 653
20 9 10 17 553 15 15 19 626 38 45 67 693
21 10 13 17 >1000 16 17 22 >1000 79 84 221 >1000
22 10 10 17 >1000 15 16 33 >1000 74 83 349 >1000
23 10 13 17 >1000 16 17 21 >1000 76 80 214 >1000
24 10 10 17 >1000 15 16 31 >1000 72 80 337 >1000
25 26 30 42 53 35 37 42 97 81 89 100 119
57
Table 7-9 Minimum 5th and 50th percentile dilutions for dry weather simulations for sensitive sites for each outfall.
Sensitive Site Outfall A Outfall B Outfall C
5th 50th 5th 50th 5th 50th
Petone Beach west 203 327 199 381 192 335
Petone Beach east 154 243 142 264 148 209
Waione Street Bridge >1000 >1000 >1000 >1000 >1000 >1000
100m downstream of
Hutt/Waiwhetu confluence 3 6 3 31 472 >1000
Lowry Bay 59 98 57 118 94 133
Days Bay 79 163 83 152 114 237
Port Road corner beach 18 27 28 106 41 77
Seaview Marina 29 48 35 97 69 91
Table 7-10 Minimum 5th and 50th percentile dilutions for wet weather simulations (0.8 m3/s discharge) for sensitive sites for each outfall.
Sensitive Site Outfall A Outfall B Outfall C
5th 50th 5th 50th 5th 50th
Petone Beach west 277 792 243 776 212 768
Petone Beach east 316 >1000 276 934 207 746
Waione Street Bridge >1000 >1000 >1000 >1000 >1000 >1000
100m downstream of
Hutt/Waiwhetu confluence 3 >1000 9 >1000 >1000 >1000
Lowry Bay 72 47 76 201 139 313
Days Bay 109 83 119 416 146 655
Port Road corner beach 29 40 62 210 179 487
Seaview Marina 33 48 53 235 156 434
Table 7-11 Minimum 5th and 50th percentile dilutions for wet weather simulations (3.0 m3/s discharge) for sensitive sites for each outfall.
Sensitive Site Outfall A Outfall B Outfall C
5th 50th 5th 50th 5th 50th
Petone Beach west 75 209 65 205 58 205
Petone Beach east 87 284 76 248 58 200
Waione Street Bridge >1000 >1000 >1000 >1000 >1000 >1000
100m downstream of
Hutt/Waiwhetu confluence 1 >1000 3 >1000 >1000 >1000
Lowry Bay 21 47 21 54 38 80
Days Bay 31 83 34 109 39 163
Port Road corner beach 8 40 19 55 48 126
Seaview Marina 10 48 15 62 45 112
58 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-10 Percentile plot of dilutions at Petone Beach west for Scenario 25.
Figure 7-11 Percentile plot of dilutions at Petone Beach east for Scenario 25.
59
Figure 7-12 Percentile plot of dilutions at 100m downstream of Hutt/Waiwhetu confluence for Scenario 25. Note logarithmic scale.
Figure 7-13 Percentile plot of dilutions at Lowry Bay for Scenario 25. Note logarithmic scale.
60 waiwhetu outfall dilution assessment / bjt / 2017-10-30
Figure 7-14 Percentile plot of dilutions at Days Bay for Scenario 25. Note logarithmic scale.
Figure 7-15 Percentile plot of dilutions at Port Road corner beach for Scenario 25. Note logarithmic scale.
61
Figure 7-16 Percentile plot of dilutions at Seaview Marina for Scenario 25. Note logarithmic scale.
62 waiwhetu outfall dilution assessment / bjt / 2017-10-30
8 Summary
Stantec commissioned DHI to undertake a plume dispersion study for the existing wastewater
overflow from Seawater Wastewater Treatment Plant into Waiwhetu Stream and for two
potential alternative wastewater overflows, to form part of an AEE. The three locations for which
the assessment was undertaken were:
Option A - existing location in Waiwhetu Stream;
Option B - Hutt River (near Waiwhetu Stream mouth) close to the water surface;
Option C - Hutt River (100 m off Barnes Street) on the sea bed.
The potential scenarios for overflow discharge were as follows:
typical dry weather discharge rate of 0.55 m3/s (1.10 m3/s for Option B on outgoing tide)
for a duration of 5 days;
typical dry weather discharge rate of 0.55 m3/s (1.10 m3/s for Option B on outgoing tide)
for a duration of 30 days;
wet weather discharge rate of 0.8 m3/s for a duration of 1 day; and
wet weather discharge rate of 3.0 m3/s for a duration of 1 day.
The discharged wastewater plumes will mix and dilute due to two types of mixing processes;
near-field mixing (driven by buoyancy and momentum of discharged plume) and far-field mixing
(driven conditions in the surrounding water i.e. currents).
Near-field dilution predictions were undertaken with the empirically based model CORMIX, while
far-field dilution predictions were carried out with the fully dynamic 3D model, MIKE 3 FM. The
two models were indirectly coupled, with the near-field model providing the general initial
conditions for the wastewater plume within the 3D model. The salt wedge which propagates up
the lower Hutt River will have an influence on the mixing of the discharged wastewater, therefore
it was important that this behaviour was reasonably reproduced by the 3D model.
The 3D model was calibrated using data collected specifically for the study by NIWA, in
conjunction with data obtained from GWRC. Generally there was a good agreement between
observed and predicted water levels within the river mouth and there was a reasonable
agreement between observed and predicted current speeds within the harbour at the water
surface. There was also a sufficient agreement between the observed and predicted salinity
distribution for the river and river mouth.
Near-field dilution predictions were calculated for Outfall B and Outfall C for a range of tidal and
wastewater discharge conditions. Near-field dilution predictions were not possible for Outfall A,
since the discharge rates were of the same order as the stream flow. For Outfall B and C,
salinity stratification during the full tidal cycle significantly decreases the dilution of wastewater,
since the wastewater plume will be very positively buoyant compared with the receiving water
which inhibits the mixing of wastewater. The wastewater will remain within the overlying
freshwater layer (after rising from the sea bed for Outfall C), where it fully mixes vertically.
The performance of each outfall location was assessed by determining the far-field plume
dilution that will occur for a range of selected discharge, hydrodynamic and climate scenarios (a
total of 25 scenarios) at sensitive sites within the river mouth and harbour. Both neap and spring
tidal conditions were considered, along with calm, southerly and northerly wind conditions.
Predicted 1st, 5th, 25th and 50th percentile dilutions at the sensitive sites were calculated for each
scenario and outfall location. Spatial maps of the predicted 5th and 50th percentile dilutions for
63
each outfall, were also presented for selected scenarios to provide an overview of the
wastewater plume behaviour for different types of wind conditions. For calm conditions, the
wastewater plume tends to migrate southwards towards the harbour entrance, with reasonable
lateral spread across the harbour. For southerly wind the plume is transported northwards
towards Petone beach, while for northerly wind, the plume is very thin and closely follows the
eastern shoreline out through the harbour entrance.
Percentile plots at the sensitive sites were also generated for the 30 day simulation which
included a range of representative wind conditions within the harbour.
Generally Outfall C produced the most dilution of the wastewater plume compared with other
potential outfall locations, before visitation of the plume occurred at the sensitive sites,
especially for the eastern sites. Outfall B generally performed better than Outfall A, however not
for all conditions and sites.
64 waiwhetu outfall dilution assessment / bjt / 2017-10-30
9 References
Ahrens, C. (2003); Meteorology Today: An introduction to Weather, Climate and the
Environment. Brooks/Cole
DHI (2011); Hutt River / Wellington: Preliminary Wastewater Dilution Assessment. Report
prepared for HCC.
DHI (2016); Waiwhetu Outfall – CORMIX Modelling Two Additional Options. Technical memo
prepared for HCC.
DHI (2017); MIKE 3 Flow Model FM, Hydrodynamic Module, User Guide.
Doneker, R.L and G.H Jirka (2007); CORMIX User Manual: A Hydrodynamic Mixing Zone Model
and Decision Support System for Pollutant Discharges into Surface Waters. U.S Environment
Protection Agency.
Heath (1977); Circulation and hydrography of Wellington Harbour. New Zealand Oceanographic
Institute oceanographic summary 12.
NIWA (2013); HCC Plume Dispersal: Data Report. Report prepared for MWH NZ Ltd. on behalf
of Hutt City Council, September 2013.
NIWA (2015); HCC Plume Dispersal: Summary Report. Report prepared for MWH NZ Ltd. on
behalf of Hutt City Council, March 2015.
Stantec (2016); Flood tide flows from the Hutt River to the Waiwhetu Stream Memorandum.