Black Rock Seawall - Marine and coasts€¦ · Black Rock Seawall Protection – Detailed Coastal Modelling We are pleased to present our report on the results of our coastal modelling

Black Rock Seawall

Coastal Modelling

Department of Environment, Land, Water & Planning

29 June 2018

Department of Environment, Land, Water & Planning | 29 June 2018 Coastal Modelling Page 1

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Document Status

Version Doc type Reviewed by Approved by Date issued

V01 DRAFT Peter Riedel Peter Riedel 06/04/2018

V02 DRAFT FINAL Christine Arrowsmith Christine Arrowsmith 31/05/2018

V03 FINAL Christine Arrowsmith Christine Arrowsmith 29/06/2018

Project Details

Project Name Coastal Modelling

Client Department of Environment, Land, Water & Planning

Client Project Manager Cassandra Philippou

Water Technology Project Manager Joanna Garcia-Webb

Water Technology Project Director Peter Riedel

Authors Joanna Garcia-Webb, Parvin Zavarei, Oliver Nickson

Document Number 5607-01_R01v03.docx

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29 June 2018

Cassandra Philippou A/g Program Manager - Land and Built Environment |Port Phillip Region Department of Environment, Land, Water & Planning 609 Burwood Highway KNOXFIELD VIC 3180 Dear Cassandra

Black Rock Seawall Protection – Detailed Coastal Modelling

We are pleased to present our report on the results of our coastal modelling assessing the potential impact of

the proposed revetment.

Yours sincerely

Joanna Garcia-Webb Assistant Group Manager – Coastal | Senior Coastal Engineer

[email protected]

WATER TECHNOLOGY PTY LTD


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CONTENTS

1 INTRODUCTION 6

1.1 Study Approach 6

2 SITE VISIT 10

2.1 Table Rock Point to Existing Bluestone Seawall 10

2.2 Existing Bluestone Seawall to Half Moon Bay 11

3 OCEANOGRAPHIC CONDITIONS 15

3.1 Wind Climate 15

3.2 Waves 17

3.3 Water Levels 18

3.3.1 Astronomical Tide 18

3.3.2 Mean Sea Level 18

3.4 Currents 18

4 COASTAL PROCESSES 19

4.1 Sediment Supply 19

4.2 Sediment Analysis 19

4.3 Sediment Transport 21

4.3.1 Longshore Sediment Transport 22

4.3.2 Cross-shore Sediment Transport / Storm Erosion 22

5 REVETMENT IMPACT REVIEW 23

5.1 Assessment Overview 23

5.2 Limitations and Assumptions 23

5.3 Simulation Scenarios 24

5.4 Potential Impacts 25

5.4.1 Overview 25

5.4.2 Bed Level Change – 1-Month Simulations 25

5.4.3 Bed Level Change – 6-Month Simulations 35

5.5 Empirical Transport Rates 41

5.6 Limitations and Assumptions 44

5.7 Mitigation Solutions 44

6 SUMMARY & CONCLUSIONS 45

7 REFERENCES 46

APPENDICES Appendix A Numerical Modelling

Appendix B Particle Size Distribution

Appendix C Sediment Transport Model Results


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LIST OF FIGURES Figure 1-1 Site Location 7

Figure 1-2 Proposed rock revetment extents 8

Figure 1-3 Design Rock Revetment (AW Maritime, 2017) 9

Figure 2-1 (a) Rickett’s Point Intertidal flats. (b) Rickett’s Point north Outfall 10

Figure 2-2 (a) Beach immediately north of Rickett’s POint (b) Sediment seaward of Beaumaris Yacht Club 11

Figure 2-3 (a) Southern Section of the Existing Bluestone Seawall (b) Intertidal flats at foot of bluestone seawall in the vicinity of quiet corner 12

Figure 2-4 (a) Coarse sediment at the foot of the bluestone seawall (b) Rubble at the foot of the bluestone seawall 12

Figure 2-5 (a) Northern section of the Seawall (b) Start of BEach and associated rock substrate 13

Figure 2-6 (a) Outfall Located at the northern end of the seawall (b) Wide beach observed north of the seawall Black Rock Beach 13

Figure 2-7 (a) Northern Section of Black Rock Beach (b) Readily erodible cliffs immediately south of Half moon bay 14

Figure 3-1 Wind station locations 15

Figure 3-2 Comparison of Wind Speed and Direction at South Channel Island and Fawkner Beacon 16

Figure 3-3 Wind Rose for South Channel Island for 1999-2017 16

Figure 3-4 Wind Rose for South Channel Island for the representative Summer (left) and Winter (right) conditions 16

Figure 3-5 Predicted Black Rock Beach Wave Climate, Summer (left) and Winter (right) 17

Figure 4-1 Sediment Size and Layer Thickness Sampling Locations 20

Figure 4-2 Sediment Transport Processes – Long Shore Processes (Top), And Cross-Shore Processes (Bottom) 21

Figure 5-1 Sediment Transport Modelling – Timeseries Locations 26

Figure 5-2 Present Day Bed Level Change at P1 Summer (Top) & Winter (Bottom) Scenarios 29






Figure 5-8 2040 Bed Level Change at P1 Summer (Top) & Winter (Bottom) Scenarios 32






Figure 5-14 Present Day (6 Months) Bed Level Change at P1 Summer (Top) & Winter (Bottom) 37






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Figure 5-22 Longshore Transport Assessment Points 43

Figure A-1 Model Mesh 49

Figure A-2 Design Rock Revetment (AW Maritime, 2017) 50

Figure A-3 Pre-Development (Top) and Post-Development (Bottom) Model Mesh 51

Figure A-4 Wind Station Locations 53

Figure A-5 Comparison of Wind Speed and Direction At South Channel Island and Fawkner Beacon 53

Figure A-6 Wind Rose for South Channel Island for the Representative Summer (Left) and Winter (Right) Conditions 54

Figure A-7 Sediment Size and Layer Thickness Sampling Locations 56

Figure C-8 Present Day Summer Sedimentation/Erosion Patterns: No Revetment (top) & With Revetment (Bottom) 66

Figure C-9 Present Day Winter Sedimentation/erosion Patterns: No revetment (top) & With revetment (bottom) 67

Figure C-10 2040 Summer Sedimentation/erosion Patterns: No Revetment (top) & With Revetment (bottom) 68

Figure C-11 2040 Winter Sedimentation/erosion Patterns: No Revetment (top) & With Revetment (bottom) 69

Figure C-12 Present Day Summer (6 Months) Sedimentation/erosion Patterns: “No revetment” (top) & “With revetment” (bottom) 71

Figure C-13 Present Day Winter (6 Months) Sedimentation/erosion Patterns: “No revetment” (top) & “With revetment” (bottom) 72

Figure C-14 Present Day Summer Sedimentation/erosion Patterns: “No revetment” (top) & “With revetment” (bottom) – Speed-up Factor of 3 74

Figure C-15 Present Day Winter Sedimentation/erosion Patterns: “No revetment” (top) & “With revetment” (bottom) – Speed-up Factor of 3 75

LIST OF TABLES Table 3-1 Williamstown Tidal Planes (Victorian Regional Channels Authority, 2018) 18

Table 3-2 Water Level 18

Table 4-1 D50 of the Captured Sediment Samples 19

Table 5-1 Modelled Scenarios for Impact Assessment (Modelling Technique) 24

Table 5-2 Modelled Scenarios for Impact Assessment (For Use in Empirical Sediment Tranport Calculations) 24

Table 5-3 Present-Day Timeseries Output Summary – 1-Month Simulation 27

Table 5-4 2040 Timeseries Output Summary – 1-Month Simulation 28

Table 5-5 Present-day Timeseries Output Summary – 6-Month Simulation 36

Table 5-6 Indicative Longshore Sediment Transport 42

Table 5-7 Indicative Net Annual Sediment Transport 42

Table A-1 Tidal Constituents at Lorne and Flinders 52

Table A-2 SW Wave Model Parameters 55


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1 INTRODUCTION Black Rock is located in the northeast of Port Phillip Bay, refer Figure 1-1 for locality plan. Presently, a 1 km

section of the foreshore is protected by a bluestone masonry seawall constructed circa 1930’s, offering

protection to the public coastal path immediately behind the seawall. This path is demarcated by the red

polygon in Figure 1-1. The seawall has sustained significant damage in the last 10 years. Previous

investigations have identified a rock revetment to be the most appropriate mitigation solution to prevent further

damage to the bluestone seawall. The Victorian State Government Department of Environment, Land, Water

and Planning (DELWP), is proposing to construct the identified rock revetment. The extent of the proposed

rock revetment is shown by the yellow line in Figure 1-2 and the conceptual cross-sectional design (AW

Maritime, 2017) is presented in Figure 1-3.

The southern end of the proposed rock revetment abuts the Rickett’s Point Marine Sanctuary. To the north is

Black Rock Beach. This beach has varying widths seaward of the existing bluestone seawall, before widening

somewhat towards Half Moon Bay. Because of the sensitive nature of the surrounding coastline, DELWP

considered it prudent to assess any potential impacts the construction of the rock revetment may have on the

adjacent coastline. Water Technology has been commissioned by DELWP to undertake an assessment of the

impact of the proposed Black Rock protection project. Potential impacts could include:

◼ Scour of sediment adjacent to the northern and southern ends of the rock revetment.

◼ The seaward extent is only small, approximately 7m, however there is a chance its construction could

affect the longshore transport at the site.

◼ Changes to the cross-shore sediment transport regime at the seawall toe.

◼ Increase in sediment accretion leading to smothering of benthic fauna.

1.1 Study Approach

The aim of the project is to investigate the potential impacts of the proposed rock revetment and determine if

any additional mitigation measures are required. The overall study approach is as follows:

◼ Site visit to gain an understanding of the coastal processes and collect sediment samples for use in the

modelling assessment.

◼ An assessment of the oceanographic conditions, particularly water levels and waves. These are the key

drivers of coastal processes.

◼ Undertake coastal modelling of the coastal compartment from Half Moon Bay, Black Rock, to Table Rock

Point, Beaumaris.

◼ Undertake coastal modelling on the complete 440m rock revetment to identify impacts on existing coastal

processes.

◼ Determine whether additional engineering solutions are required to mitigate potential post-construction

impacts of the revetment on the Rickett’s Point Marine Sanctuary and northern end of Black Rock Beach.

◼ Compilation of a summary report and subsequent meeting with DELWP to discuss interim results.

◼ Should the modelling identify any impacts as a result of the proposed revetment:

◼ Coastal modelling of engineered designs to mitigate impacts

◼ Multi-criteria analysis of options

◼ The project outcomes will be presented at a community consultation session.

This report presents the final results of the study.


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FIGURE 1-1 SITE LOCATION


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FIGURE 1-2 PROPOSED ROCK REVETMENT EXTENTS


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FIGURE 1-3 DESIGN ROCK REVETMENT (AW MARITIME, 2017)


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2 SITE VISIT Members of the Water Technology project team conducted a site investigation of the Black Rock coastal region

on the 6th of February 2018. The site visit was undertaken over a low to mid-tidal cycle and was completed on

a day in which waves were observed to be small and low energy. The following sections describe the relevant

features of the study area. For the purpose of the site description, the study area has been split into two

sections: from Table Rock Point to the southern end of the existing bluestone seawall, and from the seawall

north to Half Moon Bay. The sediment distribution observed during the site investigation will reflect that of the

summer patterns; refer Section 4 for a description of seasonal sediment movement.

The sediment layer thickness was assessed throughout the study area for input into the sediment transport

models. Ten sediment samples were collected across the study area in order to characterise the sediment

size and fall velocity, as well as input into the sediment transport models. The sample locations and data are

presented in Section 4.2; further discussion of the application of this data into the modelling is provided in

Appendix A.

In general, the study area is characterised by extensive rocky reef with limited sediment. This indicates the

system would be sensitive to change, however magnitudes of change are likely to be small due to the available

sediment.

2.1 Table Rock Point to Existing Bluestone Seawall

At the southern end of the study area are two key geological features: the rocky headlands of Rickett’s Point

and Table Rock which bound Watkins Bay. These two features extend offshore for a considerable distance

and as such are likely to limit the sediment transport between adjacent beaches. Large intertidal flats were

observed at the foot of each of these headlands and across major parts of Watkins Bay (see Figure 2-1a).

These flats were observed to be ecologically vibrant with the presence of multiple species of seagrass and

seaweed growing in the rocky substrate. The flats also act to reduce wave energy that impacts the beach

environment, as larger waves will break on the offshore sections of the flats and not propagate in to the beach

face. This reduction in wave action will reduce the sediment transport rate observed in the nearshore zone.

Between the rocky intertidal flats were sections of sandy beach. The beach slope is quite flat, suggesting a

low energy environment (Figure 2-1b). The substrate of Rickett’s Point extends some 300-400m offshore, and

no sand was observed on its surface between the foot of the foredune and the most offshore point of the tidal

flat (see Figure 2-1a).

FIGURE 2-1 (A) RICKETT’S POINT INTERTIDAL FLATS. (B) RICKETT’S POINT NORTH OUTFALL

(a) (b)


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The beach between the northern side of Rickett’s Point and the start (southern section) of the existing

bluestone seawall was in general much wider than in Watkins Bay. Organic material was present along some

sections of the beach (Figure 2-2b). This section of the coastline was characterised by intertidal flats and

shallow beach gradients as observed within Watkins Bay. The sediment size throughout the beach was fairly

consistent as coarse sand. However, a section of beach seaward of the Beaumaris Yacht Club was finer and

darker in colour. (see Figure 2-2a).

FIGURE 2-2 (A) BEACH IMMEDIATELY NORTH OF RICKETT’S POINT (B) SEDIMENT SEAWARD OF BEAUMARIS YACHT CLUB

2.2 Existing Bluestone Seawall to Half Moon Bay

Observations made for this area are as follows:

◼ At the foot of the southern-most section of the bluestone seawall is a beach extending in the order of 30m

to the still water line (SWL) (see Figure 2-3a). This beach extends northward for around 100m and is then

replaced by intertidal rocky flats.

◼ Intertidal rocky flats with a sparse covering of sand can be seen along the majority of the headland that

encompasses Quiet Corner (see Figure 2-3b). The wave energy in this area would be somewhat

dissipated across these outcrops. However, with no beach present seaward of the wall, elevated water

levels would allow wave action to act directly on the seawall. The site visit was conducted at low water

levels; it is likely the area would be inundated at high tide.

(a) (b)


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FIGURE 2-3 (A) SOUTHERN SECTION OF THE EXISTING BLUESTONE SEAWALL (B) INTERTIDAL FLATS AT FOOT OF BLUESTONE SEAWALL IN THE VICINITY OF QUIET CORNER

◼ Further north past Quiet Corner sediment was present at the foot of the seawall. The sediment observed

here was coarse (see Figure 2-4a). Again, no sediment (i.e. no beach) was observed above the water line

in this section of seawall.

◼ Rubble, some of which is assumed to be broken parts of the bluestone seawall, was observed at the foot

of the seawall (see Figure 2-4b). Depending on the water level, this rubble may act to reduce wave action

by increasing the elevation of the seabed in that area, and inducing wave breaking preferentially on the

rubble. Conversely, at times of higher water levels such as during storms, the steeper localised seabed

slope may result in increased shoaling and increase the wave loading on the seawall.

◼ Water levels were rising during the site investigation. During this time, the still water level reached the foot

of the seawall. It is likely that wave reflection would occur off the seawall during periods of high tide. Wave

reflection patterns will affect how the sediment is distributed within the area.

FIGURE 2-4 (A) COARSE SEDIMENT AT THE FOOT OF THE BLUESTONE SEAWALL (B) RUBBLE AT THE FOOT OF THE BLUESTONE SEAWALL

(a) (b)

(a) (b)


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◼ The only significant volume of sediment accumulating seaward of the seawall was observed over the far

north section of the wall within the southern half of Black Rock Beach (see Figure 2-5a).

◼ The initiation of the beach seaward of the seawall is likely to be connected to a relatively high section of

intertidal reef which can be seen in the Figure 2-5b. This reef may act in a similar way to a submerged

offshore breakwater; waves refract around the reef leading to build-up of sediment in a salient / tombolo

formation in the reef’s lee.

FIGURE 2-5 (A) NORTHERN SECTION OF THE SEAWALL (B) START OF BEACH AND ASSOCIATED ROCK SUBSTRATE

◼ The outfall located at the northern end of the bluestone seawall has the potential to act as a sand trap,

similar to a small groyne. Accretion was not observed to either side at the time of the site visit. During

periods of flow, the outfall will cause localised scour to the beach face (see Figure 2-6a).

◼ Black Rock Beach north of the seawall is substantially wider than the other beaches observed to the south;

it is in the order of 30m to 50m wide along its length (see Figure 2-6b).

FIGURE 2-6 (A) OUTFALL LOCATED AT THE NORTHERN END OF THE SEAWALL (B) WIDE BEACH OBSERVED NORTH OF THE SEAWALL BLACK ROCK BEACH

(a) (b)

(a) (b)


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◼ The widest section of the Black Rock Beach is located approximately 500m south of Half Moon Bay. This

is visibly constrained by rock outcrops, which may serve to reduce the wave energy at the shoreline. This

reduction in wave energy may in turn promote the deposition of sediment as seen in Figure 2-7a.

◼ A sandy unvegetated cliff face was observed in the section of beach just south of Half Moon Bay (see

Figure 2-7b). These cliffs seem to be readily erodible, with cliff sediment extending over the beach face.

This sediment has a different characteristic to that of the beaches in the study area. This is likely to move

offshore during storms and does not contribute to the volume of sand on the beach.

◼ The headland separating Black Rock Beach from Half Moon Bay extends approximately 150m into Port

Phillip Bay, providing some protection from northerly waves.

FIGURE 2-7 (A) NORTHERN SECTION OF BLACK ROCK BEACH (B) READILY ERODIBLE CLIFFS IMMEDIATELY SOUTH OF HALF MOON BAY

(a) (b)


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3 OCEANOGRAPHIC CONDITIONS

3.1 Wind Climate

The wind climate around the study area is measured by the Bureau of Meteorology (BoM) at Fawkner Beacon

(086376) and South Channel Island (086344). At approximately 8km from the study site, Fawkner Beacon is

the preferred data set (refer Figure 3-1). However, wind data from Fawkner Beacon is not continuous, with

various gaps in data observed. Based on the review of wind data from these two stations, the duration and the

shape of storm events are found to be similar, with the wind speed slightly lower at Fawkner Beacon.

Additionally, wind direction from both sources indicate a similar pattern (Figure 3-2). Therefore, South Channel

Island was used as a forcing in the coastal modelling (further described in Section 5 and Appendix A).

FIGURE 3-1 WIND STATION LOCATIONS

FIGURE 3-2 COMPARISON OF WIND SPEED AND DIRECTION AT SOUTH CHANNEL ISLAND AND FAWKNER BEACON


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A wind rose for South Channel Island for a period of 1999-2017 is presented in Figure 3-3. The wind climate

at South Channel Island is dominated by winds from the north, followed by winds from the southwest and west.

Wind speeds are generally below 12m/s. During summer, over 50% of winds are from the south to northwest.

Whereas in winter, the winds are predominantly from southwest through to the north.

For the purpose of this assessment, wind data was statistically analysed to identify representative summer

and winter conditions. The Summer of 2016 was selected to represent typical summer conditions and the

winter of 2011 was chosen to represent typical winter conditions as demonstrated in Figure 3-4.

FIGURE 3-3 WIND ROSE FOR SOUTH CHANNEL ISLAND FOR 1999-2017

FIGURE 3-4 WIND ROSE FOR SOUTH CHANNEL ISLAND FOR THE REPRESENTATIVE SUMMER (LEFT) AND WINTER (RIGHT) CONDITIONS

The BoM data has been adjusted from 1-minute data to an hourly average and at 10m above sea level based

on the conversion factors provided in Resio et al (2003). Hourly average wind data is appropriate for use in

determining wind generated wave climates.


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3.2 Waves

The wave climate in the study area is fetch limited, generated by local winds blowing across Port Phillip Bay.

The site (and most of Port Phillip) is protected from swell waves generated in the Southern Ocean by the

narrow entrance to Port Phillip Bay. The longest fetches at Black Rock are 60km in a west-southwest direction

to Geelong, and 45km to the southwest.

Water Technology’s existing Port Phillip Bay spectral wave (SW) model was refined to capture refraction and

shoaling of waves as they approach the study area. Wind data measured at South Channel Island was used

to generate the wave climate for the study.

The seasonal differential of the wind climate can be seen in the wave climate, with winter waves dominated by

larger waves from west to northwest. Northerly winds, which dominate in winter, drive waves which refract

towards the coastline as they move inshore, resulting in a dominant west-northwesterly wave direction.

Summer wave conditions are predominantly from the south to southwest. Wave amplitudes for both seasons

are relatively similar due to the significant fetch in both west and southwest directions. Waves from the south

refract into the shore resulting in higher proportion of waves from the south-westerly directions. Larger waves

in summer occur infrequently due to the lack of westerly winds.

FIGURE 3-5 PREDICTED BLACK ROCK BEACH WAVE CLIMATE, SUMMER (LEFT) AND WINTER (RIGHT)

The spectral wave model was also run with representative wind speed, wind direction and water level

conditions to establish a matrix relating wind climate to wave characteristics within the study area. The wind

conditions measured at South Channel Island, and the measured water level at Williamstown were then used

with a wave correlation matrix to estimate a representative longshore sediment transport regime at the sites

(as further discussed in Section 5.5).


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3.3 Water Levels

3.3.1 Astronomical Tide

Tidal water levels within the northern section of Port Phillip Bay are relatively consistent, with little change in

the tidal signal observed in Port Phillip Bay between Williamstown and Rosebud. Long-term measured water

levels at Williamstown are used to generate tidal planes within the Bay and are presented in Table 3-1.

TABLE 3-1 WILLIAMSTOWN TIDAL PLANES (VICTORIAN REGIONAL CHANNELS AUTHORITY, 2018)

Tidal Plane Level (m AHD)

Highest recorded water level 1.33 (30/11/1934)

HAT 0.52

MHHW 0.42

MLHW 0.12

MHLW -0.08

MLLW -0.38

LAT -0.48

3.3.2 Mean Sea Level

Water levels play a key role in driving erosion as it influences at which location the wave energy is focused

within the beach system. For the focus of this study two differing scenarios for MSL were used which are

presented in Table 3-2. The predicted sea level rise increase is based on Hunter (2014), incorporating the

IPCC 2014 A1F1 climate change scenario.

TABLE 3-2 WATER LEVEL

Water Level Scenario Mean Sea Level

Present Day Present Day MSL

Present Day MSL

2040 2040 MSL (+0.2m)

2040 MSL (+0.2m)

3.4 Currents

The tidal range within Port Phillip Bay is low, with mean spring tidal range of 0.8m. As a result, tidal currents

are low (less than 0.1m/s) within much of the Bay away from the entrance. Currents are thus dominated by the

seasonal wind climate discussed in Section 3.1. Northerly winds dominant in winter result in a net current to

the south. Similarly, southerly winds which dominate in summer result in a net current field to the north. Wind

driven currents generally peak at less than 0.15m/s and are not considered an important driver of sediment

transport within the study area.


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4 COASTAL PROCESSES

4.1 Sediment Supply

Table Rock Point and Half Moon Bay are large headlands located at the southern and northern extents of the

study area respectively. Significant sediment movement is not expected around these headlands.

Along this section of the Port Phillip Bay shoreline, the predominant sediment source historically was eroding

cliffs, caused by waves attacking the cliff base. The sediment inflow due to eroding cliffs has likely been in

decline since human intervention with coastal protection structures, such as seawalls preventing the natural

erosive processes that provide sediment to the beaches. This is a common issue throughout Port Phillip Bay

(Bird, 2011).

Apart from cliff erosion as a direct sediment source, longshore transport between beaches within the study

area and cross-shore transport from the beaches onshore and offshore are the main mechanisms allowing for

sediment exchange and beach erosion and accretion.

4.2 Sediment Analysis

Beach material was collected at various locations along the study area at the locations indicated in Figure 4-1

during the site investigation. Sediment samples were sent to a laboratory where Particle Size Distribution

(PSD) of the samples was conducted using a hydrometer. Median grain diameter (D50) of the collected samples

are provided in Table 4-1. PSD curves and further details of the samples and tests are provided in Appendix

B. From location 1 close to Table Rock Point through to location 3 near Quiet Corner, sediments can be

characterised as fine sand. However, coarser grains are found from Quiet Corner towards Half Moon Bay,

especially close to location 4a, which has a median grain dimeter of 1.205mm. This grain size is typically

categorised as coarse sand.

TABLE 4-1 D50 OF THE CAPTURED SEDIMENT SAMPLES

Sediment Samples D50 (mm)

1a and 1b 0.233

2a and 2b 0.136

3a and 3b 0.155

4b, 5a, 5b 0.904

4a 1.205


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FIGURE 4-1 SEDIMENT SIZE AND LAYER THICKNESS SAMPLING LOCATIONS


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4.3 Sediment Transport

The sediment transport at Black Rock is driven by wave conditions. As noted in Section 3.4, currents in the

area are low and driven by wind conditions rather than tidal fluctuations. Wave dominated sediment transport

results in cross-shore transport as waves shift sediments across the shore profile; longshore transport occurs

when waves approach the shore on an oblique angle and direct sand along the shoreline. These two processes

result in the net longshore sediment transport present at Black Rock.

Cross-shore transport can be considered as destructive - large storm waves rapidly pull material from the

beach face into the nearshore zone, often causing erosion at the back of the beach or dune and a steep beach

face; and constructive - calmer conditions and smaller waves work to slowly shift material back onshore to

form a gently sloping beach face.

FIGURE 4-2 SEDIMENT TRANSPORT PROCESSES – LONG SHORE PROCESSES (TOP), AND CROSS-SHORE PROCESSES (BOTTOM)


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4.3.1 Longshore Sediment Transport

The longshore sediment transport at the site has two distinct periods each year. During the summer months,

dominant southerly winds result in waves entering the system from the south which results in the net sediment

transport in a northerly direction. In winter, the dominant wind direction is northerly which results in waves

entering the system from the north and consequently a net sediment movement in a southerly direction.

This seasonal pattern results in sand accumulation during winter along the southern end of Black Rock Beach,

seaward of the seawall to the north of Quiet Corner, where the rocky bluff acts as a physical barrier to sediment

movement.

4.3.2 Cross-shore Sediment Transport / Storm Erosion

Beach normal angles at Black Rock are generally in a range of 240 to 270° from true north. This means that

the majority of waves during winter and a portion of the waves during summer are typically perpendicular to

the beach. If these waves are caused by strong storms, cross-shore sediment transport resulting in beach

erosion would occur.

In order to assess the impact of the proposed revetment on sediment transport, a two-dimensional sediment

transport model was developed for the study area. The MIKE 21FM Sediment Transport (ST) model includes

both longshore and cross-shore sediment transport throughout the model domain. Details of the model setup

and input parameters are provided in Appendix A. The results, and their relevance to the proposed revetment,

are presented in Section 5.


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5 REVETMENT IMPACT REVIEW

5.1 Assessment Overview

The potential impact of the proposed rock revetment was assessed by applying modelling techniques which

are discussed in this section. The sand transport (ST) module of MIKE by DHI models was applied to predict

the impact of the proposed revetment on the present-day erosion and sedimentation patterns in the vicinity of

the bluestone seawall, to determine whether any erosion or build-up of sediments will occur. The two-

dimensional (2D) model includes sediment transport in the cross-shore and longshore directions throughout

the model domain. The ST model was coupled to the hydrodynamic (HD) and spectral wave (SW) models to

compute the bed and suspended load sediment transport. Details of the coupled model is described in detail

in Appendix A.

Modelling was initially undertaken for a 1-month duration for both summer and winter time periods. Lengthy

model run-times precluded the simulation of a longer timeframe for the initial assessment. To provide an

understanding of the impacts over a longer time-frame, whilst not limiting the study with model run-times, the

model was also run for one month while applying a speed up factor of 3. For these scenarios, the bed level

change is multiplied by 3 at each time step. The idea was to replicate bed level changes during 3 months of

summer and winter under present-day MSL. Upon examination of the results of both sets of simulations, it was

inconclusive as to the significance of predicted impacts in the study area.

The interim results were discussed with DELWP. To ensure a robust assessment of the proposed impacts, the

models were then used to simulate two 6-month periods, each representing a ‘summer’ and ‘winter’ period.

That is, a full year was simulated. The 1-month and 6-month simulations are presented and discussed in the

sections below and in Appendix C. The speed-up factor results are presented in Appendix C (Section C-1-3)

for completeness. However, they are not considered an appropriate representation of the predicted impacts.

This is because minor changes to the rate of bed level change are exacerbated significantly by the application

of the speed-up factor. The 6-month simulation results are to be applied in their stead.

Even with the inclusion of the 6-month simulation time, the overall timeframe duration doesn’t allow for

interannual variations. The sediment transport and the potential impacts were therefore supplemented with

empirical longshore sediment calculations for a 39-year period in order to complement and validate this

assessment. The calculations (detailed in Section 5.5) are compared against sediment transport rates as

determined through coastal modelling.

5.2 Limitations and Assumptions

The following presents a discussion of some of the assumptions and limitations of the modelling method used

in this assessment:

◼ The model was applied to review sediment transport patterns during typical winter and summer conditions.

Short term sediment transport as a result of storm events is not considered. Inter-annual variations in

wave conditions are not considered in the modelling. However, to mitigate this shortfall, empirical sediment

transport calculations were undertaken to include these effects.

◼ The model does not include wave reflection from the existing seawall or the proposed revetment structure.

It must be noted that the proposed revetment reflects wave energy to a lesser degree than the existing

vertical seawall.

◼ If nearby beaches were being supplied from the available sand in the footprint of the proposed revetment,

they will likely experience a reduction in sand supply which could lead to erosion. This reduction in

sediment supply is taken into consideration to some extent through the increase in sea bed elevation (by


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replicating the proposed revetment slope) and therefore limiting inundation durations and consequently

limiting sediment transport.

◼ The hydrodynamic model is calibrated against water levels in various locations within Port Phillip Bay,

however, limited information was available to validate sediment transport in the study area.

◼ Simplifying assumptions were made when necessary. This includes applying a constant grain size and

bed layer thickness over the modelling domain, although the sensitivity of the model results to these

parameters were verified through additional simulations.

5.3 Simulation Scenarios

The impact of the proposed revetment on sedimentation and erosion behaviour in the study area was assessed

by simulating the scenarios illustrated in Table 5-1. The “No Revetment” scenario refers to the existing

bathymetry along the bluestone seawall. The “With Revetment” scenario refers to the future bathymetry

including the proposed rock revetment. Model bathymetries are presented in Figure A-3.

For the 1-month simulations, each season was simulated for the present day and 2040 sea level rise scenario.

For the longer timeframe investigations, only the present-day MSL scenarios were modelled.

TABLE 5-1 MODELLED SCENARIOS FOR IMPACT ASSESSMENT (MODELLING TECHNIQUE)

Layout Present day MSL 2040 MSL (+0.2m)

Summer Winter Summer Winter

1-month Simulation

No Revetment ✓ ✓ ✓ ✓

With Revetment ✓ ✓ ✓ ✓

6-month Simulation

No Revetment ✓ ✓

With Revetment ✓ ✓

Table 5-2 summaries the modelling scenarios conducted to provide long term synthetic wave data in the study

area.

TABLE 5-2 MODELLED SCENARIOS FOR IMPACT ASSESSMENT (FOR USE IN EMPIRICAL SEDIMENT TRANPORT CALCULATIONS)

Layout SW Modelling: Matrix of Water Level and Wind Conditions

No Revetment ✓

With Revetment ✓


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5.4 Potential Impacts

5.4.1 Overview

In order to determine predicted changes in sedimentation and erosion patterns due to the proposed revetment,

the following steps were taken to analyse model results:

1. For the “No Revetment” scenario, the bed level change at the end of each modelled season

(summer/winter) was determined. This represents the cumulative change in bed level over the entire

simulation.

2. For the “With Revetment” scenario, the bed level change at the end of each modelled season

(summer/winter) was determined. This represents the cumulative change in bed level over the entire

simulation.

3. The overall predicted bed level change as a result of the proposed revetment is defined as the difference

between “No Revetment” and “With Revetment” scenarios.

Additionally, the bed level at each modelled time step was extracted from the model results at 6 representative

locations. These are displayed in Figure 5-1. Sites 1 and 2 are the same sites the empirical longshore sediment

transport is calculated (refer Section 5.5). Sites 5 and 6 are located in the Rickett’s Point Marine Sanctuary.

5.4.2 Bed Level Change – 1-Month Simulations

MIKE 21 FM ST model coupled to the HD and SW model was utilised to provide a qualitative indication of

whether the proposed revetment will modify the erosion and sedimentation patterns in the study area. Due to

model run-times, this was initially assessed as a 1-month period.

Figure C-8 to Figure C-11 (Appendix C) present the cumulative bed level change at the end of the 1-month

simulation for each of the No Revetment and With Revetment scenarios, for each season and sea level

condition. These figures indicate the change as a result of the revetment is very small – the figures look very

similar between the No Revetment and With Revetment scenarios. As per Section 5.4.1 above, the overall

predicted bed level change as a result of the proposed revetment is defined as the difference between “No

Revetment” and “With Revetment” scenarios.

Figure 5-2 to Figure 5-7 show the extracted bed-level time series for present-day MSL conditions. The

implication of these results is discussed in Table 5-3. Figure 5-8 to Figure 5-13 show the extracted bed-level

time series for the predicted 2040 MSL conditions. The implication of these results is discussed in Table 5-3.

In all areas of change, the trend of erosion / sedimentation has not changed between the “No

Revetment” and “Revetment” cases. The magnitude of any change between scenarios is small

compared with the overall changes in bathymetry over the 1-month simulation. In addition, much of

the change that is predicted is within the accuracy limitations of the model results.


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FIGURE 5-1 SEDIMENT TRANSPORT MODELLING – TIMESERIES LOCATIONS


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TABLE 5-3 PRESENT-DAY TIMESERIES OUTPUT SUMMARY – 1-MONTH SIMULATION

Output Point Result

P1

▪ Summer:

– There is no predicted change in bed level as a result of the revetment.

▪ Winter:

– There is no predicted change in bed level as a result of the revetment

P2

▪ Summer

– There is slightly less erosion predicted for the With Revetment case when compared to the No Revetment case (up to 0.06m difference)

▪ Winter

– On average, there is no difference in the erosion patterns at this site between the two cases

P3

▪ Summer:


▪ Winter:


P4

▪ Summer:

– Up to 0.1m less accretion is predicted to occur as a result of the revetment, however the cumulative change is minimal

▪ Winter:

– Less than 0.05m change is predicted to occur as a result of the revetment

P5

▪ Summer:

– The predicted change as a result of the revetment is less than 0.02m at all times.

▪ Winter:

– Up to 0.05m change is predicted to occur as a result of the revetment, however the cumulative change is minimal

P6

▪ Summer:


▪ Winter:



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TABLE 5-4 2040 TIMESERIES OUTPUT SUMMARY – 1-MONTH SIMULATION

Output Point Result

P1

▪ Summer:


▪ Winter:


P2

▪ Summer

– There is slightly less erosion predicted at Point 2 for the With Revetment case when compared to the No Revetment case (up to 0.08m difference)

▪ Winter

– On average, there is no difference in the erosion patterns at this site between the two cases

P3

▪ Summer:


▪ Winter:


P4

▪ Summer:

– Up to 0.15m less accretion is predicted to occur as a result of the revetment.

▪ Winter:

– Less than 0.02m change is predicted to occur as a result of the revetment

P5

▪ Summer:


▪ Winter:


P6

▪ Summer:


▪ Winter:



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FIGURE 5-2 PRESENT DAY BED LEVEL CHANGE AT P1 SUMMER (TOP) & WINTER (BOTTOM) SCENARIOS



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FIGURE 5-8 2040 BED LEVEL CHANGE AT P1 SUMMER (TOP) & WINTER (BOTTOM) SCENARIOS



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5.4.3 Bed Level Change – 6-Month Simulations

In addition to the assessment described in Section 5.4.2, longer duration (6 months) simulations were run for

the present-day conditions to better understand the seasonal impacts of the proposed revetment on

sedimentation and erosion patterns at Black Rock.

Figure C-12 to Figure C-13 (Appendix C) present the cumulative bed level change at the end of the 6-month

simulation for each of the No Revetment and With Revetment scenarios, for each season. These figures

indicate the change as a result of the revetment is very small – the figures look very similar between the No

Revetment and With Revetment scenarios. As per Section 5.4.1 above, the overall predicted bed level change

as a result of the proposed revetment is defined as the difference between “No Revetment” and “With

Revetment” scenarios.

Figure 5-14 to Figure 5-19 show the extracted bed-level change time series for present-day MSL conditions

for the 6 locations presented in Figure 5-1. Upon enquiry by Parks Victoria, additional locations (P7 and P8)

at the offshore boundary of the Rickett’s Point Marine Sanctuary were also assessed. Extracted bed-level

change timeseries for these additional locations are presented in Figure 5-20 and Figure 5-21. The implication

of these results is discussed in Table 5-5.

In all areas of change, the trend of erosion / sedimentation has not varied between the “No Revetment”

and “Revetment” cases. The magnitude of change is small when compared with the overall changes

in bathymetry over the 6-month simulation. In addition, much of the change that is predicted is within

the accuracy limitations of the model results.


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TABLE 5-5 PRESENT-DAY TIMESERIES OUTPUT SUMMARY – 6-MONTH SIMULATION

Output Point Result

P1

▪ Summer

– There is slightly more erosion predicted for the With Revetment case when compared to the No Revetment case (up to 0.02m difference). However, the cumulative change over the simulation is negligible.

▪ Winter


P2

▪ Summer

– There is up to 0.05m less erosion predicted as a result of the revetment.

▪ Winter

– On average, there is no change predicted as a result of the revetment.

P3

▪ Summer

– There is up to 0.2m less accretion predicted as a result of the revetment.

▪ Winter

– Up to 0.05m less accretion is predicted to occur as a result of the revetment, however the cumulative change is minimal.

P4

▪ Summer

– There is up to 0.04m more accretion predicted as a result of the revetment

▪ Winter

– The first half of the simulation indicates up to 0.08m less accretion is predicted as a result of the revetment. The second half indicates up to 0.08m more accretion is predicted to occur as a result of the revetment.

P5

▪ Summer

– Less than 0.05m more accretion is predicted as a result of the revetment

▪ Winter

– Up to 0.02m more erosion is predicted during the simulation as a result of the revetment. However, the cumulative change over the simulation is negligible.

P6

▪ Summer


▪ Winter


P7

▪ Summer


▪ Winter


P8

▪ Summer


▪ Winter



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FIGURE 5-14 PRESENT DAY (6 MONTHS) BED LEVEL CHANGE AT P1 SUMMER (TOP) & WINTER (BOTTOM)



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5.5 Empirical Transport Rates

Constructing a revetment at the site has the potential to interrupt the local longshore sediment transport

regime, which can lead to impacts on adjacent foreshores. Potential impacts to longshore sediment transport

were investigated using the results from the hydrodynamic model discussed in Section 3.2, in conjunction with

desktop analytical techniques.

The MIKE SW model was run for both with and without revetment scenarios for a range of wind speed/direction

and water levels. The resultant matrix of modelled wave conditions was then used to generate a synthetic

wave dataset for a period of 1977 to 2016 through interpolation of these modelled wave conditions against

both the measured wind records at South Channel Island and the measured water level at Williamstown.

Wave conditions from the synthetic dataset were extracted at two locations along the proposed revetment for

simulations with and without the revetment in place (refer to Figure 5-22 for locations). The enhanced

Kamphuis formula (Kamphuis, 1991; van Rijn, 2001) was then applied to predict the potential longshore

transport at each location for both with and without revetment scenarios.

The method uses wave energy in the breaker zone to assess the potential sediment transport. The technical

approach is based on there being an unlimited supply of sand available across the inshore profile which can

be moved by the incident waves. However, being a reef system, the study foreshore does not have an unlimited

supply of sediment. Consequently, the quantities of sediment transported calculated by the Kamphuis

technique will be a substantial over-estimate of actual quantities. Nevertheless, the approach still provides a

reliable indication of the relative magnitudes and directions of longshore sediment transport in the area.

The Kamphuis technique of calculating longshore sediment transport is based on the incident wave height;

and the angle between the breaking wave and the shoreline. It accounts for the influence of bed slope and

sediment grain size. The original formula has been further enhanced to include the influence of the peak wave

period (van Rijn, 2001).

The formula for calculating the longshore sand transport is:

𝑄 = 2.33𝐻𝑠,𝑏𝑟2 𝑇𝑝

1.5𝑚0.75𝑑50−0.25𝑠𝑖𝑛0.6(2𝜃𝑏𝑟)

where:

Q = onshore sediment transport rate (immersed mass) in kg/s

Hs,br = significant wave height at breaker line (m)

Tp = wave peak period (s)

br = wave angle at breaker line

d50 = median particle size in surf zone (m)

m = beach slope

The bed slope was determined using the wave model bathymetry in the area surrounding the output locations.

The sediment is assumed to be a fine sand size having a D50 characteristic of 0.3mm. This is based on the

suite of samples collected during the site investigation.

The predicted potential sediment transport volumes (for the selected 40 years) are presented in Table 5-6 for

each of the output points for both ‘with’ and ‘without’ the revetment. Positive transport indicates sediment

transport is to the northwest; negative transport signifies sand being transported in the opposite direction.

The results indicate that net sediment transport is to the northwest and that the change in sediment transport

potential due to the construction of the proposed revetment is minimal.


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TABLE 5-6 INDICATIVE LONGSHORE SEDIMENT TRANSPORT

Scenario Gross Northwesterly Transport

(+ve tonnes)

Gross Southeasterly Transport

(-ve tonnes)

P1 P2 P1 P2

No Revetment +102,106 +30,027 -76,587 -3,139

With Revetment +102,085 +30,026 -76,593 -3,136

Difference -21 -1 -6 3

The predicted potential net annual longshore sediment transport rates for the output points are also presented

in Table 5-7 for both ‘with’ and ‘without’ the revetment. The results indicate slightly higher sediment transport

rates for the southern extent of the proposed revetment compared to the northern extent. There is minimal

change to the predicted annual net sediment transport rates for the scenario with revetment.

TABLE 5-7 INDICATIVE NET ANNUAL SEDIMENT TRANSPORT

Scenario Annual Net Sediment Transport (tonnes/yr)

P1 P2

No Revetment 637.98 660.64

With Revetment 637.30 660.67

Difference -0.68 0.03

The directionality of these results is in line with the most recent study undertaken in the area (AW Maritime,

2017).


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FIGURE 5-22 LONGSHORE TRANSPORT ASSESSMENT POINTS


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5.6 Limitations and Assumptions

The implication of the assumptions discussed in Section 5.2 is that the modelled response to the proposed

revetment can be applied as a decision-making tool to assess the potential impacts of the proposed rock

revetment. The model results should not be interpreted as a definitive volumetric ‘answer’ to any changes in

the sediment transport regime.

5.7 Mitigation Solutions

The modelling has indicated there is minimal change to the coastal processes due to the revetment. As a

result, the multi-criteria analysis of potential mitigation options was not undertaken. However, it is

recommended that the beaches be monitored upon construction for evidence of any impacts. Manual

bypassing may be required for the small amount of sediment that could potentially build-up at the ends of the

structure. Similarly, this monitoring will identify if any long-term terminal scour effects are visible, either at the

toe or seasonally at the ends of the structure.

Whilst regular beach surveys are a quantitatively robust method of comparing beach condition, beach

photographic monitoring is also a useful, although qualitative, cheaper alternative in analysing beach

behaviour. Photographic monitoring can be conducted at 6-monthly intervals at the end of the summer and

winter. Photos should also be taken immediately following severe storms. They should be taken from a set

vantage point to allow accurate comparisons between images. The images can be used to supplement

available data if undertaking adaptation option design in the future (if required). These images can also be

used to support funding applications, and in educating the community about natural fluctuations in beach

shape.

For example, photos can be taken from the northern end of the proposed revetment looking south, and from

the southern end looking north. Water Technology have proformas for these which allow DELWP or local

community foreshore managers to undertake the inspections, once initial guidance is provided. Coastal

specialists should review the data every couple of years, or if erosion is causing an issue.

An additional option is for a Fluker Post (or a few) to be installed to enable the public to assist with photo

monitoring. This is a great way of obtaining community involvement and ownership of managing the coast. It

also improves understanding of natural beach fluctuations.


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6 SUMMARY & CONCLUSIONS The existing wave climate, coastal processes and the impact of the proposed revetment on sedimentation and

erosion patterns along the existing bluestone seawall at Black Rock have been reviewed.

A site visit was conducted to provide background information on the existing sediment layer thickness and to

identify the key geomorphological features. Sediment samples were collected at various locations and sent to

laboratory to obtain Particle Size Distribution (PSD). Sediment information was used as input in the sediment

transport model.

The results of the sand transport modelling indicate a minimal change to the sedimentation and erosion

processes during both summer and winter seasons. Up to 0.2m change over small areas is predicted as a

result of the proposed revetment. The total change corresponds with a volume of approximately 160m3 of

sedimentation and 250m3 of erosion across the model domain over the 6-month summer simulation; and 6

and 100m3 over the winter simulation respectively. Longshore transport rates are predicted to change by up

to 0.7m3/yr at the sites of maximum change.

In all areas of change, the trend of erosion / sedimentation has not changed between the “No Revetment” and

“Revetment” cases. The magnitude of change is small when compared with the overall changes in bathymetry.

In addition, much of the change that is predicted is within the accuracy limitations of the model result.

Longshore sediment transport was also assessed by applying empirical equations to a synthetic long-term

wave and water level data. The results indicate a minimal change in longshore sediment transport as a result

of the construction of the proposed revetment.

Due to limitations in sediment transport modelling, and the subsequent assumptions required when

undertaking such modelling, the modelled response to the proposed revetment can be applied as a decision-

making tool to assess the potential impacts of the proposed rock revetment. The model results should not be

interpreted as a definitive volumetric ‘answer’ to any changes in the sediment transport regime.

It is recommended that the beaches be monitored upon construction for evidence of any impacts. Manual

bypassing may be required for the small amount of sediment that could potentially build-up at the ends of the

structure. Similarly, this monitoring will identify if any long-term terminal scour effects are visible, either at the

toe or seasonally at the ends of the structure.

Whilst regular beach surveys are a quantitatively robust method of comparing beach condition, beach

photographic monitoring is also a useful, although qualitative, cheaper alternative in analysing beach

behaviour. Photographic monitoring can be conducted at 6-monthly intervals at the end of the summer and

winter. Photos should also be taken immediately following severe storms. They should be taken from a set

vantage point to allow accurate comparisons between images. The images can be used to supplement

available data if undertaking adaptation option design in the future (if required). These images can also be

used to support funding applications, and in educating the community about natural fluctuations in beach

shape.


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7 REFERENCES AW Maritime Pty Ltd (2017). Black Rock Seawall Protection: Design Report, report prepared for Department

of Environment, Land, Water & Environment

Bird, E., (2011). Changes on the Coastline of Port Phillip Bay, Published by the Victorian Government

Department of Sustainability and Environment, Melbourne, Victoria.

Hunter, J., (2014). Derivation of Revised Victorian Sea-Level Planning Allowances Using the Projections of

the Fifth Assessment Report of the IPCC, Research conducted for the Victorian Coastal Council.

Kamphuis, J.W. (1991). “Alongshore sediment transport rate”, Journal of Waterway, Port, Coastal, and Ocean

Engineering 117.6, 1991: pp 624-640.

Resio, D.T., Bratos S.M., Thompson, E.F. (2003). Coastal Engineering Manual: Chapter II-2 Meteorology and Wave Climate, US Army Corp of Engineers, Report No EM 1110-2-1100 (Part II)

Van Rijn, L.C. (2001). Longshore Sediment Transport, Report Z3054. Delft, The Netherlands: Delft Hydraulics.

Victorian Regional Channels Authority (2018). Vic Tides 2018: Edition 2, Geelong, Australia.


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APPENDIX A NUMERICAL MODELLING


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A-1 Hydrodynamic and Wave Modelling Package

Coastal water levels vary as a result of various physical forcing factors including: wind, waves, currents and

tides. Hydrodynamic and wave modelling is the assessment of water levels and currents in and around Port

Phillip Bay as a result of these forcing factors.

An existing Water Technology hydrodynamic model of Port Philip Bay was utilised for the assessment. A

number of modifications were made to the model in order to accurately replicate coastal processes.

DHI Water and Environment’s MIKE 21/3 Coupled Hydrodynamic and Spectral Wave model has been utilised

for this study. The MIKE 21/3 Coupled Model Flexible Mesh (FM) hydrodynamic (HD) and spectral wave (SW)

flow model is based on an unstructured flexible mesh and uses a finite volume solution technique. The mesh

is comprised of triangle and quadrilateral elements. This approach enables a variation of the horizontal

resolution of the model mesh within the model area, and therefore for a finer resolution in selected sub-areas.

The computational triangular mesh of the model is made with sufficiently small cells to resolve the detailed

conditions in the study area.

The coupled model simulates the mutual interaction between waves and currents. The hydrodynamic module

simulates water level variations and flows in response to a variety of forcing functions such as:

◼ Momentum dispersion

◼ Bottom shear stress

◼ Coriolis force

◼ Wind shear stress

◼ Wave radiation stresses

The modelling system is based on the numerical solution of the two-dimensional shallow water equations - the

depth-integrated incompressible Reynolds averaged Navier-Stokes equations. Thus, the model consists of

continuity and momentum equations.

The spectral wave module simulates the growth, decay and transformation of wind-generated waves and swell

in offshore and coastal areas.

The model includes the following phenomena:

◼ Wave growth by action of wind

◼ Non-linear wave-wave interaction

◼ Dissipation due to white-capping

◼ Dissipation due to bottom friction

◼ Dissipation due to depth-induced wave breaking

◼ Refraction and shoaling due to depth variations

◼ Wave-current interaction

◼ Effect of time-varying water depth and flooding and drying

A-2 Model Layout

The model mesh for Port Phillip Bay was developed by applying the available bathymetric data, supplemented

by the recent hydrographic survey data.


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The mesh was refined to capture sufficient details in the vicinity of the proposed revetment.

The model resolution increases towards the project area, where the mesh size is approximately 4m. The model

mesh applied for the simulation has 17,624 nodes and 32,191 elements and is presented in Figure A-1. The

location of the model open boundary is also shown here.

The following bathymetric data was applied to the model mesh:

◼ Port Phillip Bay entrance detailed bathymetric survey (6m resolution)

◼ Future Coasts LiDAR Bathymetric Survey (Victorian Government, acquired between Nov 2008 – Apr

2009)

FIGURE A-1 MODEL MESH

The post-development model mesh includes the proposed rock revetment based on the preliminary design

provided by AW Maritime (2017) presented in Figure A-2. A 1V:2H slope was adopted for the face of the

revetment. A closer view of the model mesh at Black Rock for both pre and post-development scenarios is

presented in Figure A-3.


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FIGURE A-2 DESIGN ROCK REVETMENT (AW MARITIME, 2017)


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FIGURE A-3 PRE-DEVELOPMENT (TOP) AND POST-DEVELOPMENT (BOTTOM) MODEL MESH


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A-3 Boundary Conditions

Tidal boundary conditions in the model were determined by linearly interpolating between predicted tides for

Lorne, at the western end of the model boundary and Flinders at the eastern end. The tidal prediction was

carried out by MIKE by DHI Tidal Prediction Tool and were based on the 5 main tidal constituents for Lorne

and Flinders as shown in Table A-1. These constituents were obtained by harmonic analysis of measured

water levels provided by the Bureau of Meteorology (BOM) for Lorne and Flinders.

TABLE A-1 TIDAL CONSTITUENTS AT LORNE AND FLINDERS

Constituent Lorne Flinders

Amplitude (m) Phase (°) Amplitude (m) Phase (°)

O1 0.61 319 0.81 327

K1 0.21 56 0.23 50

N2 0.20 88 0.21 88

M2 0.14 26 0.17 291

S2 0.12 273 0.15 34

A-4 Simulation Period

The summer and winter wave climate and consequently sediment transport processes typically follow these

patterns at the study area:

◼ In summer, waves are predominantly from the southwest, driven by southerly winds.

◼ In winter, waves are predominantly from the northwest, driven by northerly winds.

In order to capture the effect of seasonality on the potential impact from the proposed revetment, both summer

and winter conditions were simulated. Typical summer and winter conditions were selected as discussed in

Section A-5 and hence the following simulation periods were modelled:

◼ Summer: 1 month (including one day warmup period): January 2016

◼ Winter: 1 month (including one day warmup period): July 2011

A-5 Wind Forcing

The wind climate around the study area is measured by the Bureau of Meteorology (BoM) at Fawkner Beacon

(086376) and South Channel Island (086344). At approximately 8km from the study site, Fawkner Beacon is

the preferred data set (refer Figure A-4). However, wind data from Fawkner Beacon is not continuous, with

various gaps in data observed. Based on the review of wind data from these two stations, the duration and the

shape of storm events are found to be similar, with the wind speed slightly lower at Fawkner Beacon.

Additionally, wind direction from both sources indicate a similar pattern (Figure A-5). Therefore, South Channel

Island was used as a forcing in the HD/SW model.


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FIGURE A-4 WIND STATION LOCATIONS

FIGURE A-5 COMPARISON OF WIND SPEED AND DIRECTION AT SOUTH CHANNEL ISLAND AND FAWKNER BEACON

South Channel Island wind data was statistically analysed to identify representative summer and winter

conditions. The summer of 2016 was selected to represent typical summer conditions and the winter of 2011

was chosen to represent typical winter conditions as demonstrated in Figure A-6.


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FIGURE A-6 WIND ROSE FOR SOUTH CHANNEL ISLAND FOR THE REPRESENTATIVE SUMMER (LEFT) AND WINTER (RIGHT) CONDITIONS

The spectral wave model was also run with representative wind speed, wind direction and water level

conditions to establish a matrix relating wind climate to wave characteristics within the study area. The wind

conditions measured at South Channel Island, and the measured water level at Williamstown were then used

with a wave correlation matrix to estimate a representative longshore sediment transport regime at the sites.

A-6 Hydrodynamic Model Parameters

A-6-1 Eddy Viscosity

The transfer of momentum through sub-grid scale turbulence is modelled through the inclusion of eddy

viscosity in the horizontal extent. The eddy viscosity is given by a “Smagorinsky-type” formulation. This

expresses the effects of sub-grid scale turbulence by an effective eddy viscosity related to a characteristic

length scale and the local spatial current variations.

A-6-2 Bed Resistance

To include bed resistance a Manning M (reciprocal of Manning’s n) number of 50 m1/3/sec in Bass Strait, 20

m1/3/sec at the bay entrance and 32 m1/3/sec is applied throughout Port Phillip Bay.


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A-7 Spectral Wave Model Parameters

Simulations were undertaken using the directionally decoupled parametric formulation assuming quasi-stationary conditions. That is, it was assumed that the time evolution of the wave spectrum within Port Philip Bay was negligible. Wave parameters applied are summarised in Table A-2.

TABLE A-2 SW WAVE MODEL PARAMETERS

Model Parameter Defined in the model

Basic Equations Spectral Formulation: Directionally decoupled parametric formulation

Time formulation: Quasi stationary formulation

Spectral Discretisation 360-degree rose with 16 directions

Solution technique Low order, Fast algorithm

Max number of iterations: 500

Tolerance (RMS-norm of residual): 1*10-5

Tolerance (Max-norm of change in Hs): 0.001

Wind generation Formula SPM73

Wave breaking Depth induced wave breaking γ1 = 0.8

Wave spilling due to overly steep waves γ2 = 1

Bottom Friction Nikuradse roughness length, kn = 0.01m

Initial Condition Spectra from empirical formula, applying JONSWAP fetch growth expression

Boundary Condition Closed boundary

A-8 Sediment Transport Model Parameters

MIKE 21 FM Sand Transport (ST) model was coupled to the hydrodynamic and wave models. The ST module

calculates the resulting transport of non-cohesive materials based on the mean horizontal flow conditions found

in the hydrodynamic calculations and wave conditions from wave calculations (DHI, 2017). For the purpose

of this assessment, sediment grab samples were collected along the shore. Additionally, sediment layer

thickness was assessed at a few key locations illustrated in Figure A-7. The results indicated that a sediment

size of 0.3mm and a sand layer thickness of 0.3m was appropriate for the model. Details of the Particle Size

Distribution (PSD) analyses are provided in Appendix B.


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FIGURE A-7 SEDIMENT SIZE AND LAYER THICKNESS SAMPLING LOCATIONS


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There was no data at the study area available to validate the ST model, this is not unusual for sediment

transport studies. Therefore, appropriate modelling parameters were selected based on experience from

similar studies. The following parameters were applied to the ST model:

◼ Combined wave and current formulation was applied. For this formulation the sand transport rates are

found by interpolation in sediment transport tables.

◼ A porosity of 0.4 was applied for the sediments. A constant grain size of 0.3mm (D50) and a grading

coefficient of 1.58 was applied.

𝑆𝑒𝑑𝑖𝑚𝑒𝑛𝑡 𝑔𝑟𝑎𝑑𝑖𝑛𝑔 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 = (𝐷84

𝐷16

)0.5

◼ A sediment layer thickness of 0.3m was applied over the domain. The thickness layer was selected based

on a review of the sediment thickness measurements across the study area. In order to ensure the

thickness layer is appropriate, a sensitivity test was conducted where a sediment thickness of 0.5m was

applied to the entire domain, for both the pre-development and post-development scenarios. The results

indicated that a sediment thickness layer of 0.3m can adequately resolve the sediment transport

processes at the study area.

◼ Morphological impact on the hydrodynamics was taken into consideration by including a bed level

feedback.

Department of Environment, Land, Water & Planning | May 2016 Coastal Modelling

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APPENDIX B PARTICLE SIZE DISTRIBUTION


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APPENDIX C SEDIMENT TRANSPORT MODEL RESULTS


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C-1 Potential Impacts

C-1-1 Cumulative Bed Level Change - 1-Month Simulations


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FIGURE C-8 PRESENT DAY SUMMER SEDIMENTATION/EROSION PATTERNS: NO REVETMENT (TOP) & WITH REVETMENT (BOTTOM)


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FIGURE C-9 PRESENT DAY WINTER SEDIMENTATION/EROSION PATTERNS: NO REVETMENT (TOP) & WITH REVETMENT (BOTTOM)


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FIGURE C-10 2040 SUMMER SEDIMENTATION/EROSION PATTERNS: NO REVETMENT (TOP) & WITH REVETMENT (BOTTOM)


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FIGURE C-11 2040 WINTER SEDIMENTATION/EROSION PATTERNS: NO REVETMENT (TOP) & WITH REVETMENT (BOTTOM)


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C-1-2 Cumulative Bed Level Change – 6-Month Simulations


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FIGURE C-12 PRESENT DAY SUMMER (6 MONTHS) SEDIMENTATION/EROSION PATTERNS: “NO REVETMENT” (TOP) & “WITH REVETMENT” (BOTTOM)


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FIGURE C-13 PRESENT DAY WINTER (6 MONTHS) SEDIMENTATION/EROSION PATTERNS: “NO REVETMENT” (TOP) & “WITH REVETMENT” (BOTTOM)


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C-1-3 Applying Speed-up Factor

Prior to simulating the sediment transport models for a long-term period (Section 5.4.3), the morphological

changes in the study area for a longer duration were modelled by running the model for a period of one month

while applying a speed up factor of 3 to replicate bed level changes during 3 months of summer and winter

under present day MSL. For these simulations, the difference in bed level as a result of the proposed revetment

is presented for summer and winter conditions in Figure C-14 and Figure C-15 respectively.

A similar sedimentation and erosion pattern is predicted with an increased magnitude leading to accumulation

of sand along the toe of the proposed revetment. It must be noted that, applying a speed-up factor might result

in an overestimation of the accretion and erosion patterns. This is evident in the results of the long-term model

simulations, where sedimentation and erosion are balanced out by opposing forces throughout the simulation.


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FIGURE C-14 PRESENT DAY SUMMER SEDIMENTATION/EROSION PATTERNS: “NO REVETMENT” (TOP) & “WITH REVETMENT” (BOTTOM) – SPEED-UP FACTOR OF 3


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FIGURE C-15 PRESENT DAY WINTER SEDIMENTATION/EROSION PATTERNS: “NO REVETMENT” (TOP) & “WITH REVETMENT” (BOTTOM) – SPEED-UP FACTOR OF 3


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Black Rock Seawall - Marine and coasts€¦ · Black Rock Seawall Protection – Detailed Coastal Modelling We are pleased to present our report on the results of our coastal modelling