Report Westhaven: Pile Berth Redevelopment · 2 Hydraulic Model Hydrodynamic modelling has been...

Report

Westhaven: Pile Berth Redevelopment

Hydraulic Flushing Report (Appendix D)

Prepared for Panuku Development Auckland

Prepared by (Beca)

27 April 2018

Hydraulic Modelling Report - Westhaven: Pile Berth Redevelopment

Beca // 27 April 2018 3121348 // NZ1-15308211-5 0.5 // i

Revision History Revision Nº Prepared By Description Date

1 Reza Shafiei Supplement for Resource Consent 26/04/2018

2

3

4

5

Document Acceptance Action Name Signed Date

Prepared by Reza Shafiei

27/04/2018

Reviewed by Connon Andrews

27/04/2018

Approved by Stephen Priestley

27/04/2018

on behalf of

© Beca 2018 (unless Beca has expressly agreed otherwise with the Client in writing).

This report has been prepared by Beca on the specific instructions of our Client. It is solely for our Client’s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which Beca has not given its prior written consent, is at that person's own risk.

Hydraulic Modelling Report - Westhaven: Pile Berth Redevelopment

Beca // 27 April 2018 3121348 // NZ1-15308211-5 0.5 // i

Contents 1 Introduction 1

1.1 Existing Environment 1 1.2 Proposed Configuration 1 1.3 Tidal Data 2 1.4 Tidal Flow 2 1.5 Wind 3

2 Hydraulic Model 5 2.1 Model Bathymetry and Parameters 5 2.2 Model Tidal Data 6 2.3 Modelled Scenarios 7 2.4 Calibration and Validation 7 2.5 Results 10

3 Tidal Flushing 12 3.1 Methodology and model parameters 12 3.2 Results 13

4 Conclusion 15

5 References 16

Appendices

Appendix A Westhaven Marina Time Series Concentration Plots during Neap Tides

Appendix B Westhaven Marina Daily Concentration Plots during Neap Tides

Beca // 27 April 2018 3121348 // NZ1-15308211-5 0.5 // page 1

1 Introduction

1.1 Existing Environment Development Auckland - Panuku has engaged Beca to carry out hydraulic modelling investigations of Westhaven Marina. The focus is on assessing the changes in the flushing capability of the marina.

The Westhaven Marina currently comprises two entrance/exist points open to the Waitemata Harbour, with reclamation and rock breakwater structures protecting the Marina. The reclamation on the western side is approximately 45m wide (at mean sea level) with vehicle access and parking. The rock breakwater is approximately 14m wide. The western entrance between these the reclamation and breakwater structure is approximately 60m wide. The eastern entrance is approximately 300m wide and is bound by the Silo Park/Hamer Street reclamation. Westhaven Marina covers a surface water area of approximately 67ha. Figure 1 shows the name of the existing facilities which are referred to in this report.

Figure 1.1 - Site Location

1.2 Proposed Configuration The proposed project, Westhaven: Pile Berth Redevelopment, will incorporate closing off the western entrance. It will include a new reclamation area (approximately 45m wide and 60m long), extending from the western reclamation to the breakwater (refer to Figure 2). This feature has the potential to increase flushing times for the marina.


Figure 1.2 – Proposed configuration (new reclamation area shown in black)

1.3 Tidal Data Tides within the Waitemata Harbour are semi diurnal with a maximum astronomical tidal range of approximately 3.7m. Mean and maximum astronomical tidal levels for the Ports of Auckland are presented in Table 1.1

Table 1.1 - Tidal Levels for Auckland Waitemata Harbour (source: LINZ website, 2017)

Tide Level (m) above Chart Datum

Highest Astronomical Tide (HAT) 3.70

Mean High Water Springs (MHWS) 3.39

Mean High Water Neaps (MHWN) 2.86

Mean Sea Level (MSL) 1.90

Mean Low Water Neaps (MLWN) 0.92

Mean Low Water Springs (MLWS) 0.41

Lowest Astronomical Tide (LAT) 0.05

1.4 Tidal Flow Within Westhaven Marina (in the vicinity of St Mary’s Bay) tidal flow measurements were undertaken by DHI during a full spring tidal cycle from 7pm 14 November 2017 to 8am 15 November 2017. Flow transects were


utilised at the Eastern (T2) and Western (T3) Entrances to the marina, and in the middle of the marina extending from monitoring location W8 to W4 (T1) (refer to Figure 1.3 for the location of transects). The flow data was collected with a downward facing ADCP with an accuracy of ±20%. Peak flows at each transect is presented in Table 1.2.

Table 1.2 - Measured peak flows at the transects within Westhaven Marina

Monitoring Location Flood Tide Flow (m3/s) Ebb Tide Flow (m3/s) T1 59 143 T2 112 244 T3 37 93

Figure 1.3 – Monitoring locations

1.5 Wind Wind data is available from the National Institute of Water and Atmospheric Research (NIWA) which was located at Mechanics Bay covering the period January 1955 to January 1962 (refer to Figure 1.4). The wind rose shows a bi-modal wind climate with the strongest winds from the East-North-East and from West-South-West with a predominant wind direction from the Southwest. The mean wind speed is approximately 5 m/s.


Figure 1.4 - Wind Rose for data recorded at Mechanics Bay (1955 to 1962)


2 Hydraulic Model

Hydrodynamic modelling has been completed with the Delft3D FM Suite 2017 HMWQ Version 1.2.2.36603 (Delft3D FM). Delft3D FM is the new hydrodynamic engine, developed by Deltares, that runs on unstructured grids. The numerical concept is based on the combination of 2D/3D finite volume cells with 1D flow networks. Delft3D FM includes a module to carry out the hydrodynamic computation (D-Flow FM) for 2D and 3D shallow water equations. For this study 2D shallow water equations have been adopted.

2.1 Model Bathymetry and Parameters Bathymetry data used in this model is the most recent 2017 survey data set held by Port of Auckland for the Waitemata Harbour. Where additional information was required for the intertidal areas LINZ LIDAR data flown in 2016 was used. Figure 2.1 presents the bathymetry data and the model domain. Generally seabed levels within Westhaven Marina are at -2.5m, transitioning to approximately -7.5m CD and -5.5m CD at the eastern and western entrances.

Figure 2.1 - Model Domain (in part)


The grid consists of 60388 nodes and 119441 grid cells. It has a typical grid cell size of approximately 12m in the Westhaven Marina, and 50m in the outer zone of the Westhaven Marina. The resolution is chosen such that a balance between computational time, scale of processes and level of detail is achieved.

Bed friction was varied throughout the domain with the majority of the area being assigned a roughness (Manning's n) of 0.02.

Turbulence effects were computed by means of the Reynolds stress equation model. The Reynolds stresses are modelled using the eddy viscosity concept. This concept expresses the Reynolds stress component as the product between a flow as well as grid-dependent eddy viscosity coefficient. In addition to using the turbulence model in D-Flow FM a constant background mixing coefficient was specified, which is a background value for the vertical eddy viscosity in the momentum equations. Horizontal background eddy viscosity and diffusivity were set equal to 0.4 m2/s. The default values for the horizontal eddy viscosity and diffusivity are 1 m2/s, and can be smaller, but greater than zero, depending on the cell size. The horizontal eddy viscosity and diffusivity are generally an order of magnitude larger than the vertical eddy viscosity and diffusivity. Vertical background eddy viscosity and diffusivity were set equal to 5x10-5 m2/s, which are D-Flow FM’s default/recommended values.

2.2 Model Tidal Data The model was run using constant recurrence of 1.9m mean Neap tide and 3.0m mean Spring tide representative of mean tidal conditions (refer to Figure 2.2).

(a) Time series of mean 1.9m Neap tide

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(b) Time series of mean 3.0m Spring tide

Figure 2.2 - Tidal Data used in Model

Various model scenarios have been completed for this full time period and also for various sub periods to ascertain the sensitivity to spring and neap conditions.

2.3 Modelled Scenarios The following tidal scenarios were modelled:

n Spring tides recorded during the ADCP Westhaven measurements (flow measurement) from 7pm 14th November 2017 to 8am 15th November 2017. This simulation was used to calibrate the model to the recorded measurements.

n Mean Spring tide 3.0m (refer to Figure 2.2). This simulation is used to assess the hydrodynamic changes and drive the water quality model excluding wind effects.

n Mean Neap tide 1.9m (refer to Figure 2.2). This simulation is used to assess the hydrodynamic changes and drive the water quality model excluding wind effects.

n The above mean spring and neap tides including a constant 5 m/s wind speed from NW (i.e. 75º in the model). This simulation is used to assess the hydrodynamic changes and drive the water quality model.

n The above mean spring and neap tides including a constant 5 m/s wind speed from SW (i.e. 275º in the model). This simulation is used to assess the hydrodynamic changes and drive the water quality model.

For this study the effect of vessel movement has not been considered, which is conservative.

2.4 Calibration and Validation Model was validated against measured tidal flow in the Westhaven Marina. Comparison of the measured and modelled results is given in Figures 2.3 to 2.5. In general the model describes the flow variations well and matches the available measurements and is considered suitable for this study.

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Figure 2.3 - Spring tide flow at T1, Westhaven (refer to Figure 1.3)

Figure 2.4 - Spring tide tidal flow at T2, Westhaven (refer to Figure 1.3)

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Figure 2.5 - Spring tide tidal flow at T3, Westhaven (refer to Figure 1.3)

Figures 2.6 and 2.7 show the typical ebb and flood velocities vectors, respectively. Also, typical eddy formation during ebb and flood is presented in the Figures 2.6 and 2.7. As a general observation, for both ebb and flood tides, the flow tends to move from the western entrance towards the eastern entrance.

Figure 2.6 - Typical ebb velocity vectors

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Figure 2.7 - Typical flood velocity vectors

2.5 Results Tidal velocities for various locations within the Westhaven Marina as shown in Figure 1.3 for the existing and proposed situations are presented in Tables 2.1 and 2.2. Results are presented to 2 decimal places and where required to 3 decimal places for comparison purposes only.

Figure 2.1 - Spring tide velocities for Westhaven Marina

Location 0m/s Wind 5m/s Wind from NE 5m/s Wind from SW

Existing Proposed % Change Existing Proposed %

Change Existing Proposed %

Change

W1 0.01 0.00 -43% 0.009 0.008 -11% 0.006 0.005 -16% W2 0.05 0.03 -38% 0.05 0.03 -36% 0.05 0.03 -38%

W3 0.46 0.08 -83% 0.46 0.08 -82% 0.47 0.08 -83% W4 0.22 0.07 -70% 0.22 0.06 -72% 0.22 0.07 -66%

W5 0.27 0.25 -8% 0.27 0.24 -9% 0.27 0.25 -8% W6 0.06 0.07 11% 0.06 0.07 12% 0.06 0.07 9%


Location 0m/s Wind 5m/s Wind from NE 5m/s Wind from SW W7 0.006 0.007 5% 0.007 0.008 5% 0.009 0.009 -5%

W8 0.03 0.02 -22% 0.023 0.019 -17% 0.04 0.03 -12%

Figure 2.2 - Neap tide velocities for Westhaven Marina




Change

W1 0.003 0.002 -26% 0.00 0.01 13% 0.003 0.003 0%

W2 0.02 0.01 -45% 0.02 0.01 -45% 0.021 0.017 -19% W3 0.31 0.02 -94% 0.31 0.02 -92% 0.31 0.02 -95%

W4 0.12 0.04 -64% 0.12 0.04 -69% 0.12 0.05 -58% W5 0.16 0.14 -13% 0.16 0.14 -14% 0.16 0.14 -12%

W6 0.04 0.03 -18% 0.04 0.03 -16% 0.04 0.03 -21% W7 0.003 0.003 0% 0.005 0.005 -2% 0.006 0.006 -3%

W8 0.008 0.008 0% 0.013 0.013 1% 0.025 0.026 3%

The results suggest the following:

n With the existing configuration, tidal flows enter the Marina thought the western and eastern entrances. The maximum velocities at the ‘western mouth’ and the ‘eastern entrance’ reference points are 0.46m/s and 0.27m/s respectively.

n Under the proposed configuration the maximum velocity at the eastern entrance (W5) decreases slightly (0.25m/s), while the maximum velocity near the western mouth (W3) reduces by approximately 83% to 0.08m/s.

n The western area in Westhaven Marina will be affected by closing off the western entrance. The reference point (i.e. W1 to W4) velocities are significantly decreased (26% to 94%).

n Under the existing scenario an eddy is formed at the eastern entrance, which remains under the proposed configuration. A second eddy is present on the southern side of the rock breakwater, north of the reference point W6 late in a flood tide, which slightly shifts towards west in the ebb tide. This eddy is still present under the proposed configuration. A third eddy is formed on the western side of the marina, south of the western reclamation, in the ebb tide, but its presence is less dominant. Following the formation of the eddy on the ebb tide, all water within the Marina flows towards the eastern entrance and no additional eddies are formed.

n Flow velocities in the southern side of the Westhaven Marina (W8- i.e. St Marys Bay) are unlikely to be affected by closing off the western entrance.


3 Tidal Flushing

3.1 Methodology and model parameters Advection and diffusion modelling has been completed with DELWAQ. DELWAQ is the engine of the D-Water Quality programme of the Delft3D FM. It is based on a rich library from which relevant substances and processes can be selected to quickly put water and sediment quality models together. The finite volume approach underlying DELWAQ allows it to be coupled to hydrodynamics of the D-Flow FM engine (1D-2D-3D). DELWAQ models water quality using the outcome of D-Flow FM. It solves the advection diffusion reaction equation on unstructured grids.

Predicting water quality in a marina is complicated due to the range of pollutants which may enter the marina, interactions between them and local meteorological conditions. For this assessment water quality has been assessed by determining the flushing potential for the existing and proposed marina configurations.

Flushing times are typically calculated as the time required to reduce an initial pollutant concentration within a semi enclosed water body (PIANC, 2008). The e-folding method has been used, which calculates the time it takes for a contaminant introduced into a uniformly mixed body of water to achieve a desired dilution level of 37% (time taken for 37% of dye to remain in sample area i.e. 63% has been flushed out of the Marina). An acceptable e-folding time is four days (96 hours or 8 tidal cycles). This is consistent with the recommendation of the US Environmental Protection Agency (EPA, 1985), who recommended a complete water exchange of a basin in four days as ‘good’ and an exchange of water in ten days as ‘fair’ and poor if a longer time was required.

A fraction simulation method in DELWAQ was used for the flushing assessment. The fraction simulation is a simulation where different fractions of water are labelled and traced through the model domain. In this study two types of water were followed:

n “initial” water, which is present in the model domain at the start of the simulation n “BOUNDARY” water entering the model domain from the open boundary, and replacing the initial water

during the simulation

The entire Westhaven Marina was initially labelled at a uniform 100 percent concentration with a passive tracer at time t=0. The subsequent change in the tracer mass in different parts of the Westhaven Marina area was then tracked by solving the 3D mass transport equation. At the open boundary, the inflow tracer dye concentration was 0.

An important parameter in water quality modelling is the dispersion coefficient. DELWAQ uses constant horizontal dispersions, but uses the outcome of the hydrodynamic turbulence model for the vertical dispersions. A homogeneous horizontal dispersion coefficient of 0.15 m2/s was used in this study.

In areas of relatively high tidal flow diffusion is less affected by the assigned coefficient as tidal movement dominates. However, in areas of very low current velocity diffusion becomes the dominant process for the exchange of water. Therefore, assignment of diffusion coefficient in these environs requires careful consideration as it will directly affect the e-folding/flushing times.

To complete a relative comparison between the existing and proposed scenarios the following cases were modelled:


n Mean Spring tide 3.0m (refer to Figure 2.4) with 100% concentration within the Westhaven Marina excluding wind.

n Mean Neap tide 1.9m (refer to Figure 2.4) with 100% concentration within the Westhaven Marina excluding wind.

n The above mean spring and neap tides including a constant 5 m/s wind speed from NW (i.e. 75º in the model) with 100% concentration within the Westhaven Marina.

n The above mean spring and neap tides including a constant 5 m/s wind speed from SW (i.e. 275º in the model) with 100% concentration within the Westhaven Marina.

3.2 Results For each scenario the time taken for each reference point to reach 37% concentration has been recorded as the time taken for that point to be ‘flushed’. These results are presented in Tables 3.1 and 3.2. Refer to Figure 1.3 for result locations.

In many cases the e-folding time fluctuates with the tide and in these cases the presented e-folding time is based on the last down crossing to meet the 37% threshold. The colour coding in Tables 3.1 and 3.2 reflect “good” (green) water quality and “fair” (orange) with respect to the PIANC and USEPA criteria. Time series concentration data for spring and neap tides for each monitoring location are presented in Appendix A. Spatial plots of daily concentration for a spring tides, for the existing and proposed, are presented in Appendix B.

Figure 3.1 - e-folding times for Westhaven Marina - Spring tide




Change

W1 35.5 49.0 38% 32.3 44.0 36% 36.5 57.0 56% W2 26.0 35.8 38% 25.5 35.8 40% 26.5 35.8 35% W3 20.8 36.3 75% 21.0 36.3 73% 20.8 36.3 75% W4 19.5 33.0 69% 19.5 33.0 69% 19.5 33.0 69% W5 8.3 31.5 282% 8.3 31.5 282% 8.3 31.5 282% W6 31.0 44.3 43% 21.0 34.0 62% 20.8 34.0 64% W7 46.0 56.8 23% 46.0 56.5 23% 45.5 57.8 27% W8 46.5 70.3 51% 46.8 60.5 29% 37.0 57.5 55%

Figure 3.2 - e-folding times for Westhaven Marina - Neap tide




Change

W1 60.5 158.8 162% 52.0 180.0 246% 62.0 131.8 113% W2 46.5 117.3 152% 46.5 118.3 154% 46.5 93.5 101% W3 45.3 64.3 42% 45.3 75.8 67% 45.0 76.0 69% W4 43.8 69.3 58% 43.8 45.0 3% 31.8 81.5 157% W5 8.5 40.5 376% 8.5 40.8 379% 8.5 40.5 376% W6 33.0 69.8 111% 33.5 46.8 40% 33.3 82.3 147%


Location 0m/s Wind 5m/s Wind from NE 5m/s Wind from SW W7 67.5 93.0 38% 69.5 93.8 35% 66.8 113.5 70% W8 79.0 121.5 54% 73.5 103.3 40% 56.8 117.8 107%

The results suggest the following:

n Under the existing configuration the e-folding times at all locations are less than 96 hours which is within the ‘good’ flushing criteria (PIANC).

n In general Westhaven Marina experiences increased e-folding times with the proposed configuration. However, the increases for majority of areas within Westhaven Marina still remain within the “good” 96 hour limit.

n For a spring tide, all the areas remain within the “good” 96 hour limit. n The westernmost area (W1 and W2) and southernmost area (W7 and W8– i.e. St Marys Bay) of

Westhaven Marina experience increased e-folding times to the “fair” 240 hour limit in a neap tide. n Allowing for wind forcing from NE or SW has minor effect on e-folding times in the spring tide. n The NE wind results in generally longer e-folding times in the neap tide. n The SW wind results in generally shorter e-folding times in the neap tide.

It is noted that the e-folding estimates do not allow for vessel movement and therefore are considered to be a conservative estimate for relative comparison purposes.


4 Conclusion

For flushing of Westhaven Marina, the numerical model study has demonstrated the following:

n Closing off the western entrance to Westhaven Marina decreases the current velocities in the western area, and has minor/negligible effect on the other areas within the marina.

n Closing off the western entrance to Westhaven Marina increases e-folding times in the marina. However, the increases for majority of areas remain within the “good” 96 hour limit. This is particularly the case for spring tides.

n The westernmost and southernmost areas of Westhaven Marina experience increased e-folding times to the “fair” 240 hour limit for Neap tides.

n Considering NE or SW wind forcing caused minor effect on e-folding times in the spring tide. n Considering NE wind forcing caused longer e-folding times in the neap tide. n Considering SW wind forcing caused shorter e-folding times in the neap tide.

For spring tides, the resultant e-folding times in the marina are less than for the existing inner Viaduct Harbour. For neap tides, the resultant e-folding times in the marina are similar than for the existing inner Viaduct Harbour. (Beca & Tonkin and Taylor, 2018). Water quality effects related to flushing would therefore be similar to the conditions in the Viaduct Harbour today.

Flushing is only one parameter affecting water quality. Measures such as management and treatment of stormwater discharges, and removal of jetsam and flotsam are also important in determining water quality within a basin. These measures will need to be managed, as they are at present.


5 References

PIANC, 2008: Protecting water quality in marinas. PIANC No. 98-2008.

PIANC (2008): RecCom Working Group 98, Protecting water quality in marinas, RecCom report 98 – 2008.

Beca, Tonkin and Taylor, 2018: America’s Cup, Wynyard Hobson, Coastal processes and Dredging Technical Report.


Appendix A

Westhaven Marina Time Series Concentration Plots during Neap Tides


Figure A.1 - Monitoring Location W1 during Neap Tides


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Figure A.9 - Monitoring Location W1 during Spring Tides


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Appendix B

Westhaven Marina Daily Concentration Plots during Neap Tides


Figure B.1 - Existing situation (left) and proposed configuration (right) during Neap tides (Time 0 to 72 hours)

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