Wallaga Lake Causeway Options Assessment - Bega...

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O C E A N I C S A U S T R A L I A

Wallaga Lake Causeway Options

Assessment

Prepared For: Bega Valley Shire Council

Prepared By: WBM Oceanics Australia

Offices

Brisbane Denver

Karratha Melbourne

Morwell Newcastle

Sydney Vancouver



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WBM Oceanics Australia Newcastle Office: 126 Belford Street BROADMEADOW NSW 2292 Australia PO Box 266 Broadmeadow NSW 2292 Telephone (02) 4940 8882 Facsimile (02) 4940 8887 www.wbmpl.com.au ACN 010 830 421

Document: R.N0525.002.00.doc

Title: Wallaga Lake Causeway Options Assessment

Project Manager: Philip Haines

Author: Philip Haines

Client: Bega Valley Shire Council

Client Contact: Doug Mein

Client Reference: 320.3.1

Synopsis: This report investigates options for the Wallaga Lake causeway in order to improve tidal flushing of the lake. The assessment was carried out using the RMA software suite.

REVISION/CHECKING HISTORY

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21/2/01

28/2/01

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Bega Valley Shire Council

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http://www.wbmpl.com.au/

CONTENTS I



CONTENTS

Contents i List of Figures ii List of Tables iii

1 INTRODUCTION 1-1

1.1 Background 1-1 1.2 Study Objectives 1-2 1.3 Review of Background Data 1-2

2 ASSESSMENT OF EXISTING CONDITIONS 2-1

2.1 Tidal Flushing Times 2-1 2.2 Flood Tide Excursion 2-1 2.3 Physical Assimilation Capacity 2-3 2.4 Sediment Transport 2-4

2.4.1 Flood Scour Effects 2-4 2.4.2 Post Flood Infilling 2-6 2.4.3 Tidal Action 2-6 2.4.4 Overview 2-6

3 ASSESSMENT OF PRE-CAUSEWAY CONDITION 3-1

3.1 Tidal Flushing Times 3-1 3.2 Flood Tide Excursion 3-3 3.3 Physical Assimilation Capacity 3-3 3.4 Sediment Transport 3-4

3.4.1 Flood Induced Sediment Transport 3-4 3.4.2 Tide Induced Sediment Transport 3-5

4 ASSESSMENT OF POSSIBLE CAUSEWAY OPTIONS 4-1

4.1 Description 4-1 4.2 Assumptions 4-1 4.3 Results 4-3

4.3.1 Tidal Range 4-3

LIST OF FIGURES II



4.3.2 Tidal Velocities 4-3 4.3.3 Tidal Flushing Times 4-6 4.3.4 Flood Velocities and Levels 4-6 4.3.5 Sediment Transport Potential 4-11

5 SELECTION AND ASSESSMENT OF PREFERRED OPTION 5-1

5.1 Indicative Costs of Options 5-1 5.2 The Preferred Option 5-1 5.3 Wider Impacts of Preferred Option 5-3

5.3.1 Shoaling Potential 5-3 5.3.2 Entrance Stability 5-3 5.3.3 Aquatic Habitats 5-3

LIST OF FIGURES

Figure 1.1 Wallaga Lake Entrance Delta and Causeway 1-1 Figure 1.2 Different States of Wallaga Lake Entrance Delta 1-4 Figure 1.3 Hydrosurvey of Wallaga Lake Entrance Delta 1-5 Figure 2.1 Tidal Flushing Times – Existing Scenario 2-2 Figure 2.2 Flood Tide Excursion – Existing Scenario 2-3 Figure 2.3 Physical Assimilation Capacity – Existing Scenario 2-4 Figure 2.4 Conceptual Sediment Transport Model 2-5 Figure 3.1 Tidal Flushing Times – Pre-Causeway Scenario 3-2 Figure 3.2 Flood Tide Excursion – Pre-Causeway Scenario 3-3 Figure 3.3 Physical Assimilation Capacity – Pre-Causeway Scenario 3-4 Figure 3.4 Flood Induced Sediment Transport Potential – Pre-Causeway Scenario3-5 Figure 3.5 Tide Induced Sediment Transport Potential – Pre-Causeway Scenario 3-6 Figure 4.1 Model Schematisation and Bathymetry Details – All Scenarios 4-2 Figure 4.2 Peak Ebb Tide Velocities Through Entrance – All Scenarios 4-4 Figure 4.3 Peak Flood Tide Velocities Through Entrance – All Scenarios 4-5 Figure 4.4 Tidal Flushing Times – Option 1 4-7 Figure 4.5 Tidal Flushing Times – Option 2 4-8 Figure 4.6 Tidal Flushing Times – Option 3 4-9 Figure 4.7 Peak Velocities for a Typical Flood Event – All Scenarios 4-10 Figure 4.8 Sediment Transport Potential under Tidal Action – Option 1 4-11 Figure 4.9 Sediment Transport Potential under Tidal Action – Option 2 4-12 Figure 4.10 Sediment Transport Potential under Flood Conditions for Options 1 and 2 4-13 Figure 5.1 Conceptual Design of Preferred Option (Option 1) 5-2

LIST OF TABLES III



LIST OF TABLES

Table 3.1 Tidal Flushing Times Comparison for Pre-Causeway Condition 3-1 Table 4.1 Tidal ranges for all Scenarios 4-3 Table 4.2 Tidal Flushing Times for all Scenarios 4-6 Table 4.3 Comparison of Levels for Typical Flood – All Scenarios 4-11

INTRODUCTION 1-1



1 INTRODUCTION

1.1 Background

Wallaga Lake is situated on the far south coast of NSW. The lake has a surface area of approximately 7.8 km2, and drains a catchment of over 280 km2. In the late 1800’s, a causeway and bridge were constructed across the entrance delta of the lake in order to provide vehicular access along the south coast. The causeway constricted tidal flows over the entrance delta to one location only (at the northern end), reducing the tidal conveyance width from over 600 metres to less than 100 metres.

An Estuary Processes Study of Wallaga Lake was carried out in 1996 (Patterson Britton and Partners, 1996). This study indicated that the construction of the causeway had a significant impact on the entrance delta, and restricted its landward growth to one location only. Air photos clearly indicate that the entrance delta has continued to extend through the northern causeway opening due mostly to strong flood tide currents (refer Figure 1.1).

Figure 1.1 Wallaga Lake Entrance Delta and Causeway

Hydrodynamic modelling carried out as part of the Estuary Processes Study indicated that tidal flushing of many parts of the lake was poor, while water quality within the lake occasionally

CAUSEWAY

HONEYSUCKLE ISLAND

ESTUARINE SWAMP

OCEAN ENTRANCE

BRIDGE OPENING

MERRIMANS ISLAND

INTRODUCTION 1-2



exceeded levels recommended by the ANZECC guidelines, particularly along the eastern and southern shorelines, where tidal flushing is low.

The Estuary Processes Study hydrodynamic modelling also indicated, however, that flushing could be significantly improved with a second opening through the causeway and the re-establishment of the southern channel across the entrance delta. The Estuary Management Committee, as part of the implementation of the Wallaga Lake Estuary Management Plan, is now considering the possibility of improving tidal flushing, and hence water quality within the lake, through modifications of the Wallaga Lake causeway.

1.2 Study Objectives

The main objectives of this study were:

• To develop an understanding of the effect of the causeway constriction on: o Sediment transport patterns within the inlet channel under tidal and flood conditions, o Tidal flushing and water quality within the lake

• To prepare concept designs for a range of options to increase tidal flushing and hence improve water quality under open entrance conditions.

The study was carried out using the RMA hydraulic model of Wallaga Lake set-up by Patterson Britton and Partners (PBP) as part of the Estuary Processes Study. Various modifications to this model were necessary in order to simulate the pre-causeway condition and a number of causeway options as elected by the Estuary Management Committee.

1.3 Review of Background Data

The Wallaga Lake Estuary Processes Study (PBP, 1996) was reviewed to provide an appreciation of the background information available for Wallaga Lake. With regard to possible modifications to the entrance delta and its associated processes, the following issues were considered to be particularly important:

• Landward growth of the entrance tidal delta has been restricted over the timeframe of available air photos (1944 – 1999), which contrasts significantly to the development of fluvial deltas at the outlets to the lake tributaries (in particular Dignams Creek delta and Narira Creek delta, which are growing at a rate of 4,100 m3/yr and 5,100 m3/yr, respectively);

• There are continuous water level recorders at Regatta Point, within the lake, and in the entrance channel. The tidal range of the lake is generally about 5 – 20% of the ocean tidal range. This difference is accounted for by a more constricted entrance during the second gauging. Half tide level is generally perched by about 0.1 metres, however, significantly higher levels occur when the entrance is near closure and when closed;

• The hydraulic control of the lake entrance can be the bridge opening, channel friction, and/or the active marine shoals surrounding the actual ocean entrance opening. Therefore, increasing just the bridge opening (or equivalent) may not always increase tidal flushing within the lake;

INTRODUCTION 1-3



• Flushing times within the lake are generally less than 30 days, with poorest flushing in the upper reaches of Dignams Arm, Meads Bay and in Black Lagoon. Wind circulation can improve tidal flushing by about 5%;

• Excessive algal growth has been noted along the eastern shore of the lake, while elevated TP, TKN, ammonia and Faecal Coliforms, particularly after rainfall events, are often recorded adjacent to the urban areas. It is considered that leaching from septic systems could be a major contributor to these pollutants. Seagrass density and epiphytic growth also appeared to be affected by poorer water quality along the eastern shore of Wallaga Lake, with patchy distribution and a moderate cover of epiphytes;

• Saltmarsh communities occur around Honeysuckle Island, which is located within the middle of the entrance delta, while the estuarine wetland immediately north of the entrance and east of the main coastal road is a major breeding ground for the Little Tern (Sterna albifrons). Both of these areas are protected under SEPP 14 planning legislation (wetland nos. 115 and 116, respectively).

• Seagrass (Zostera sp.) is present within the shallower, but still permanently submerged, sections of the entrance delta. An additional review of historical air photos dating back to 1944 indicate that the extent of seagrass on the entrance delta, downstream of the causeway, is quite variable. It is likely that seagrass meadows on the entrance delta expand and contract in response to changes in hydraulic and sediment characteristics, including water levels, sediment transport patterns (ie seagrasses can be smothered by marine infilling or reworking of deltaic sediments) and scour of the sediments associated with higher energy flood events.

• The benthos of the entrance delta has not been assessed rigorously, with the only scientific study being carried out by the Australian Museum in 1980. Two of the three sites within Wallaga Lake were sampled near the entrance. A total of 61 species of invertebrates were recorded, which was typical of the six south coast estuaries studied by the Museum.

One issue that was not investigated in detail in the Estuary Processes Study was entrance dynamics. It is understood that the entrance has been manipulated in the past (ie opened artificially) to maintain tidal flows and to prevent inundation of low-lying riparian land and some sections of the main coastal road across the entrance causeway. It is expected that the construction of the causeway has had a significant impact on the dynamics of the lake’s entrance, as a reduction in the tidal prism would have reduced the size of the ‘steady-state’ entrance, and could have made it more vulnerable to closure.

Examination of air photos dating back to 1944 indicate that the entrance can be well scoured (1971, 1977, 1999) ‘steady-state’ open (1979, 1993, 6/1986, 1989, 1994), heavily shoaled (4/1986) or entirely closed (1944, 1972, 1982).

Flooding of the lake has also not been investigated in detail. Only one gauging station is located within the Wallaga Lake catchment, on Narira Creek at Cobargo. It is understood that elevated levels within the lake can cause inundation of low-lying properties, and can also flood the main coastal road along the entrance causeway, however, such flooding generally only occurs when the entrance is closed.

Council’s current entrance management policy indicates the entrance should be opened artificially once the lake levels reach RL 1.1 metres, as the lowest level of the causeway is RL 1.25 metres (pers comm.. D Mein, BVSC). Council is looking to review this policy within the next few years in light of possibly raising the level of the causeway associated with connection of residential areas around

INTRODUCTION 1-4



the north of Wallaga Lake to the local sewerage system, and replacement of the existing cast iron water main through the causeway.

A hydrosurvey of the Wallaga Lake entrance delta was carried out in December 1993. This hydrosurvey, which is shown overleaf as a digital terrain model (DTM) in Figure 1.3, was used as bathymetric data for modelling of the pre-causeway scenario and for modelling of causeway management options. While the northern half of the entrance delta has remained relatively constant over the past 50 years or so, the collection of air photos illustrate the dynamics of the delta in the immediate vicinity of the ocean entrance. Figure 1.2 outlines some of the different states of the Wallaga Lake entrance.

1944 1971

1982 1993

Figure 1.2 Different States of Wallaga Lake Entrance Delta

INTRODUCTION 1-5



Figure 1.3 Hydrosurvey of Wallaga Lake Entrance Delta

ASSESSMENT OF EXISTING CONDITIONS 2-1



2 ASSESSMENT OF EXISTING CONDITIONS

The assessment of the existing conditions was carried out using the RMA model developed as part of the Estuary Processes Study (PBP, 1996). As the conditions within the entrance, with respect to channel locations, depths and widths, are ever-changing, PBP set-up a simplified entrance channel with hydraulic parameters that achieved approximately the correct tidal range within the lake proper. As such, the results of the assessment carried out for the existing conditions should be interpreted with care.

The existing conditions were assessed for a number of different parameters in order to quantify ‘baseline values’ with which to compare the various causeway options. These parameters included: • Tidal flushing times within the lake; • Flood tide excursion; and • Physical assimilation capacity.

2.1 Tidal Flushing Times

Tidal flushing of the lake under existing conditions was determined using a mean spring tide with no wind influences. The downstream tidal boundary was defined using the mean spring tide defined in the Estuary Processes Study (PBP, 1996), which had a range of 1.5 metres at the ocean, and a range within the lake of approximately 0.3 metres. This scenario assumes an 80% reduction of the tidal range in the lake, however, as noted in Section 1.3, a reduction of up to 95% is often recorded (with a tidal range in the lake of somewhat less than 0.2 metres).

Tidal flushing times of the lake were defined on the basis of an e-folding expression. The lake was assigned an initial pollutant concentration of unity and the time taken for concentrations to reduce to a value of 1/e (or 0.37) defined as the flushing time. The flushing times of Wallaga Lake under existing entrance delta conditions (assuming an open entrance with a 0.3 metre tidal range in the lake) are presented in Figure 2.1.

As shown in this figure, the tidal flushing times of the lake range from 0 days (ie 1 tidal cycle) in the immediate vicinity of the bridge opening, to over 50 days in Fisherman’s Bay, Meads Bay, Black Lagoon and the tidal reaches of Dignams and Narira Creek. The tidal flushing time at Merrimans Island was approximately 20 days, at Regatta Point was approximately 30 days, and at Beauty Point was approximately 45 days.

2.2 Flood Tide Excursion

Flood tide excursion is the distance a water particle will move into the lake from the ocean during the flood (incoming) tide. Generally, a greater flood tide excursion will result in better exchange on ocean waters within the lake.

Flood tide excursion was assessed using the particle tracking model, RAPTOR. Particles were added to the model at the ocean entrance at low water slack. They were then tracked during the course of the flood tide until they reached their maximum excursion at high water slack. The results of the particle tracking is shown in Figure 2.2.




Figure 2.1 Tidal Flushing Times – Existing Scenario




Figure 2.2 Flood Tide Excursion – Existing Scenario

As the particle tracking model uses a random walk algorithm, the different colour gradation represents the density of particles within each element of the model. As shown in Figure 2.2, the existing flood tide excursion extends approximately 1.2 km inward from the causeway opening (ie the Wallaga Lake bridge) to a location approximately midway between Merrimans Island and the Couria Bay entrance.

2.3 Physical Assimilation Capacity

As outlined in Section 1.3, the urban areas along the eastern shoreline of Wallaga Lake are possibly contributing nutrients (and bacteria) to the water, leading to phytoplankton blooms and moderate epiphytic growth on seagrass fronds. To provide an appreciation for the dynamics of pollutants entering the lake from these urban areas, the RAPTOR random-walk particle tracking model was used again to predict the physical assimilation capacity of the lake by assessing the advection and dispersion of a point pollutant load under the influence of tidal motion.

Figure 2.3 shows the extent of pollutant dispersion over two tidal cycles when a point load was applied approximately 50 metres off Regatta Point. As indicated in this figure, the pollutant advected and dispersed in a northerly direction to about the southern end of the causeway, and also in a southerly direction to a location approximately mid-way to Beauty Point. The lateral migration of the pollutant was predicted to be about 1200 metres after 24 hours.

Merrimans Island

Couria Bay




Figure 2.3 Physical Assimilation Capacity – Existing Scenario

2.4 Sediment Transport

Sediment transport and entrance dynamics were not investigated in detail in the Estuary Processes Study (PBP, 1996). Also, the RMA model developed as part of the Estuary Processes Study contained a simplified entrance, and as such is inappropriate for assessment of bed shear and potential transport rates for the existing scenario. Due to these limitations, the existing sediment transport processes at the Wallaga Lake entrance delta have been assessed to a preliminary level, from air photo interpretation.

Sediment transport at the entrance delta is dependent on a number of different processes, which become significant during different entrance states. The key factors controlling the sediment transport processes are shown in Figure 2.4, and are described in detail in the following sections.

2.4.1 Flood Scour Effects

Flood waters enter the entrance delta area through the bridge opening at the northern end of the culvert. The primary flood flow path is through the main entrance channel, while a secondary flood flow path is located to the immediate north of Honeysuckle Island (refer Figure 2.4). These two flood flow paths rejoin before the floodwaters reach the ocean. During large floods, the entrance spit located on the northern side of the ocean entrance would be scoured away, resulting in a wide entrance condition.

Regatta Point

Beauty Point

Causeway




Figure 2.4 Conceptual Sediment Transport Model

The shape and orientation of sand waves within the secondary channel indicates that significant flows rarely utilise this flow path, resulting in a relatively small sediment transport potential. Although not obvious in the air photos, it is likely that a considerable amount of sand within the main channel would be transported downstream, towards the entrance, during flood events.

It is unlikely that any significant flows would enter the entrance delta area from overtopping of the causeway during a flood event. As such, the section of the delta to the south and west of

HIG

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EBB TIDE DELTA

ENTRANCE SPIT

DELTA SAND LOBES

EBB TIDE CHANNEL FLOOD TIDE CHANNEL

PRIMARY FLOOD FLOW PATH

SECONDARY FLOOD FLOW PATH

AREA OF MINIMAL SEDIMENT TRANSPORT

POTENTIAL

ACTIVE FLOOD TIDE DELTAS




Honeysuckle Island would not be significantly affected by flooding, and would have negligible sediment transport potential. Historical air photos indicate that this section of the delta has changed very little since 1944.

2.4.2 Post Flood Infilling

Following a major flood event, the ocean entrance to the lake is wide, while a considerable amount of sediment would be present within the local nearshore environment. Normal wave action would transport much of this sediment onto the open coast beach, or into the entrance where it would build-up in the form of sand lobes radiating around the entrance opening. The additional sand deposited on the beach could also be transported back into the entrance by longshore processes to reform the northern entrance spit. The reformation of the spit could also be enhanced by a small net southerly transport of sediment along the ocean beach, as well as aeolian transport of sand from the beach and high dunes during dominant north-easterly winds.

It is also possible that storm conditions could washover the entrance spit, infilling the former flood scoured channel behind. If considerable infilling occurs, the ebb tide channel could become constricted, which would reduce the tidal efficiency of the entrance. Under extreme conditions, complete infilling of the ebb tide channel could initiate closure of the entrance.

2.4.3 Tidal Action

Under a normal ‘regime’ entrance condition, separate ebb tide and flood tide channels would be present. The ebb tide channel would generally occupy the relict flood-scoured channel, while the flood tide channel could be located between successive delta lobes (refer Figure 2.4). Active flood tide and ebb tide deltas would be present, while the higher flood tide velocities within the main entrance channel (up to the bridge) would slowly work marine sands towards the distal edge of the entrance delta. Air photos indicate that the distal edge of the delta in the immediate vicinity of the bridge opening has significantly prograded into the lake since the construction of the causeway.

2.4.4 Overview

It is considered that much of the entrance delta is now relict as a result of the construction of the causeway. Active sections of the delta (ie those sections with a notable sediment transport potential) are generally confined to the northern and eastern channels, and in the immediate vicinity of the ocean entrance. Likely sediment transport potential in the entrance delta before the causeway was constructed is discussed in Section 3.4.

ASSESSMENT OF PRE-CAUSEWAY CONDITION 3-1



3 ASSESSMENT OF PRE-CAUSEWAY CONDITION

An assessment of the pre-causeway conditions was carried out to determine the impacts that construction of the causeway has had on Wallaga Lake. This assessment replicated the methodology adopted for the assessment of the existing condition, however, the RMA model was modified to incorporate the whole of the entrance delta area, but excluded the causeway.

The entrance delta bathymetry adopted for the pre-causeway assessment was derived mostly from the hydrosurvey provided by DLWC (refer Figure 1.3). It is considered that a large portion of the entrance delta would not have changed since pre-causeway times, as the causeway has made relict most of the delta to the west and south of Honeysuckle Island. For the sections to the north and east of the island, the bathymetry adopted in the pre-causeway model was the same as that for the existing conditions model. It is considered that a deeper channel would likely have existed within this area prior to construction of the causeway, however, the width and depth of such a channel are unknown. By adopting the same channel conditions as at present we are adopting a conservative approach (or precautionary principle), as we are maximising the potential difference between existing conditions and pre-causeway conditions.

A refined pre-causeway entrance channel condition using comparisons with other ‘natural’ coastal lagoon systems, and/or a series of sensitivity simulations could be undertaken in the future, however, the limited budget for this particular project precluded any such works from being carried out at this time.

3.1 Tidal Flushing Times

Tidal flushing times are generally a function of tidal range within the waterway. For the pre-causeway condition, the tidal range of Wallaga Lake was predicted to be approximately 0.4 metres. This is about 0.1 metres greater than the tidal range under existing conditions.

The predicted tidal flushing times within Wallaga Lake are shown in Figure 3.1. When comparing this figure to the existing scenario flushing times plot (Figure 2.1) it can be seen that the majority of the difference occurs closer to the entrance area, while the flushing times in the more extreme sections of the lake remain relatively unchanged. A comparison of tidal flushing times at a few locations around the lake is provided in Table 3.1.

Table 3.1 Tidal Flushing Times Comparison for Pre-Causeway Condition

Location Existing Flushing Time Pre-causeway Flushing Time Improvement

Merrimans Island 20 days 12 days 40%

Regatta Point 30 days 25 days 17%

Beauty Point 45 days 40 days 11%

Fisherman’s Bay 50 days 47 days 6%

Meads Bay 53 days 50 days 6%




Figure 3.1 Tidal Flushing Times – Pre-Causeway Scenario




3.2 Flood Tide Excursion

Results of the flood tide excursion assessment for the pre-causeway condition are shown in Figure 3.2. This figure indicates that ocean water would reach approximately Merrimans Island during the course of one flood tide. As well as flowing within the main entrance channel, a significant amount of flow would also travel through the ‘secondary flood flow path’, as defined in Section 2.4, located to the immediate north of Honeysuckle Island.

Comparing Figure 3.2 with the flood tide excursion predicted for the existing condition (refer Figure 2.2) indicates that under pre-causeway condition, the tide would be able to penetrate into the lake much further than at present, thus improving the efficiency of tidal flushing for the pre-causeway scenario. This would be one of the main factors why tidal flushing times are lower for the pre-causeway condition closer to the entrance than at the more distant locations.

Figure 3.2 Flood Tide Excursion – Pre-Causeway Scenario

3.3 Physical Assimilation Capacity

As for the existing scenario, the physical assimilation capacity of the pre-causeway condition was assessed by applying a point pollutant load approximately 50 metres off Regatta Point. The extent of the pollutant over two tidal cycles is shown in Figure 3.3.




Figure 3.3 Physical Assimilation Capacity – Pre-Causeway Scenario

Comparing Figure 3.3 with the physical assimilation capacity plot for the existing conditions (refer Figure 2.3), it can be seen that the upstream migration of the pollutant plume was quite similar, however, for the pre-causeway condition, the pollutant was able to migrate into the entrance delta and out of the lake (note that the plot indicates an artificial accumulation of pollutant particles in the bottom pocket of the entrance delta due to the resolution of the modelling. In reality, these particles would be transported out of the lake).

Therefore, it is considered that under pre-causeway conditions the physical assimilation capacity of the lake would be much higher, as there is more opportunity for pollutants to be evacuated out of the system rather than simply dispersed throughout the lake.

3.4 Sediment Transport

As no air photos of the pre-causeway condition are available, the sediment transport potential for this scenario has been assessed using the sediment transport module of RMA. Sediment transport potential was predicted for the entrance delta area for both tidal and typical flood conditions. The results of the model simulations are discussed in the following sections.

3.4.1 Flood Induced Sediment Transport

A typical flood event was simulated in the model. This event was sized to prevent overtopping of the causeway under existing conditions, ie it reached a maximum level of approximately RL 1.25 metres.




The predicted sediment transport potential within the entrance delta area for this simulation is shown in Figure 3.4.

Figure 3.4 Flood Induced Sediment Transport Potential – Pre-Causeway Scenario

As shown in the above figure, the majority of the sediment transport potential is concentrated around the ocean entrance, while a small potential is predicted within the main entrance channel and also between Honeysuckle Island and the mainland. These potentials within the entrance delta proper are considered to be small when compared to the tide-induced sediment transport potential (refer Section 3.4.2). For larger flood events (ie events that would result in overtopping of the causeway under current conditions), it is expected that the sediment transport potential within the entrance delta would be considerably greater than that represented in Figure 3.4. However, given that such events occur infrequently, it is still considered that the majority of sediment transport on the entrance delta is associated with tidal action.

3.4.2 Tide Induced Sediment Transport

Sediment transport potential within the entrance delta area was also assessed based on tidal processes (for typical spring tide conditions). The results of this assessment are shown in Figure 3.5.




Ebb Tide Flood Tide

Figure 3.5 Tide Induced Sediment Transport Potential – Pre-Causeway Scenario

Figure 3.5 demonstrates that significant potential for sediment transport occurs at the narrow entrance to the lake at the peak of both the ebb tide and the flood tide. It is considered that the entrance dimensions have a significant impact on the overall tidal dynamics of the lake. With time, the dimensions of the entrance will adjust to the sediment transport potential (ie the potential will be realised by actual sediment transport).

Also, sediment transport potential occurs within the main entrance channel on both the flood and ebb tides, although the potential is greater on the flood tide, which would cause a net landward transport of sediment. It is considered that the predicted sediment transport potential within the main channel for the pre-causeway condition would be approximately the same (or slightly lower) than the transport potential under existing conditions, given that the channel dimensions are the same for the two scenarios.

Significant sediment transport potential also occurs between Honeysuckle Island and the mainland. The magnitude of this potential is similar to that in the main channel, however, the potential only occurs during the flood tide. Very little potential occurs during the ebb tide, and this only occurs at the commencement of the ebb run. As such, it is considered that bed sediment from between Honeysuckle Island and the mainland could be transported into the lake under pre-causeway conditions.

Air photos indicate that a relict channel exists in this vicinity, which lends support to the fact that there was a higher sediment transport potential at this location during pre-causeway times.

ASSESSMENT OF POSSIBLE CAUSEWAY OPTIONS 4-1



4 ASSESSMENT OF POSSIBLE CAUSEWAY OPTIONS

4.1 Description

Causeway options were formulated which would improve tidal flushing of Wallaga Lake. The objective was to determine an option that could return tidal flushing characteristics to those of the pre-causeway condition. A long list of options was presented to the Estuary Management Committee. The Committee then selected three (3) of these options to be assessed by numerical modelling. These options were:

1 Construction of a second opening at the southern end of the causeway;

2 Construction of a second opening at the southern end of the causeway plus creation of a deep channel connecting this new opening to the ocean entrance; and

3 Construction of a second opening at the southern end of the causeway plus placement of several smaller openings (ie culverts) along the remaining section of causeway.

4.2 Assumptions

For Option 1, the width of the southern opening was assumed to be 100 metres. This width is comparable to the width of the existing opening at the northern end of the causeway. The bathymetry for Option 1 was based on the existing hydrosurvey (refer Figure 1.3) plus the assumed channel configuration adopted for the existing conditions scenario. That is, the bathymetry for Option 1 was the same as the bathymetry adopted for the pre-causeway scenario.

Option 2 was assumed to be exactly the same as Option 1 except for a deep channel, which connects the causeway opening to the existing ocean entrance. This channel was assumed to be approximately 80 metres wide with a bed elevation of –1.4m AHD. These dimensions are the same as the simplified main channel.

Option 3 was assumed to be exactly the same as Option 1 except for four (4) additional small openings within the causeway. These additional openings were assumed to be 10 metres wide, with a height that exceeded normal tidal range, and were spaced at approximately 60 metre intervals. The 10 metre widths were considered to be representative of 4 x 2.4m wide box culverts placed side by side.

The adopted schematised networks and model bathymetry for the three options are presented in Figure 4.1, along with comparative plots for the existing scenario and the pre-causeway scenario.




Figure 4.1 Model Schematisation and Bathymetry Details – All Scenarios




4.3 Results

4.3.1 Tidal Range

Tidal ranges for the three causeway options, along with the existing and pre-causeway scenarios are presented in Table 4.1. As seen in this table, all options achieve the same tidal range as the pre-causeway condition, or better, and are up to 0.15 metres greater than the range under existing conditions. It is important to note, however, that the tidal range of the lake is dependent on a number of different factors, including the condition of the entrance, so the predicted increase in tidal range may not always be achieved, depending on the condition of these other factors.

Table 4.1 Tidal ranges for all Scenarios

Scenario Tidal Range

Existing conditions 0.3 m

Pre-causeway 0.4 m

Option 1 0.4 m

Option 2 0.45 m

Option 3 0.4 m

4.3.2 Tidal Velocities

Predicted peak ebb and flood tide velocities across the entrance delta for the three options are presented in Figures 4.2 and 4.3. Also shown in this figure are comparative plots for the existing conditions and pre-causeway scenarios.

As shown in Figures 4.2 and 4.3, the highest velocities within the main channel occur under existing conditions. The velocity distribution pattern associated with the pre-causeway condition are approximately the same as those for both Options 1 and 3. Option 2 on the other hand would have lower velocities within the main entrance channel, but higher velocities in the new channel located to the south-west of Honeysuckle Island. Only immediately under the new bridge opening would velocities exceed approximately 0.8 m/s for Option 2 during the peak of the flood tide, while the velocity would only reach about 0.6 m/s for Options 1 and 3.

Overall, Options 1 and 3 best replicate the pre-causeway velocity distribution across the entrance delta. Option 2 would tend to redirect a large proportion of the tidal flows through the new southern channel, which could significantly affect the flow / sediment balance within the northern channel (possibly causing siltation of the channel).




Figure 4.2 Peak Ebb Tide Velocities Through Entrance – All Scenarios




Figure 4.3 Peak Flood Tide Velocities Through Entrance – All Scenarios




4.3.3 Tidal Flushing Times

Tidal flushing times for the three causeway options are presented in Figures 4.4 to 4.6. For ease of comparison between these plots and those for the existing conditions and pre-causeway scenarios, Table 4.2 highlights tidal flushing times for a number of key locations around the lake.

Table 4.2 Tidal Flushing Times for all Scenarios

Location Existing Pre-causeway Option 1 Option 2 Option 3

Merrimans Island 20 days 12 days 15 days 10 days 15 days

Regatta Point 30 days 25 days 25 days 21 days 25 days

Beauty Point 45 days 40 days 41 days 41 days 41 days

Fisherman’s Bay 50 days 47 days 48 days 50 days 48 days

Meads Bay 53 days 50 days 51 days 52 days 51 days

As can be seen from Table 4.2, Option 2, with the larger tidal range, has lower flushing times closer to the entrance, and in fact has lower flushing times than the pre-causeway condition. However, at the more extreme locations in the lake, Options 1 and 3 better replicate the pre-causeway conditions.

4.3.4 Flood Velocities and Levels

A typical flood event was simulated for each option schematisation. The resulting peak velocities across the entrance delta are presented in Figure 4.7, along with the same plots for the existing conditions and pre-causeway scenarios. As shown in this figure, velocities within the main channel for the existing conditions scenario exceed 1.5 m/s, however, this assumes that the flood remains totally within the main channel, which is not necessarily the case. For the pre-causeway condition, peak velocities in the channel would be about 0.8 m/s, while velocities between Honeysuckle Island and the mainland would also reach a similar magnitude.

For all three causeway options, the peak flood velocity in the main entrance channel was the same, or slightly less, than that predicted for the pre-causeway condition, while the peak velocities under the new southern opening would be less than 1 m/s.

There was also a difference in the height of the flood within the lake. Table 4.3 outlines comparative flood levels for the three options and the existing conditions and pre-causeway scenarios. As highlighted by Table 4.3, the addition of a second opening in the causeway can have significant flood mitigation benefits, which would approximately replicate the flooding conditions that prevailed prior to construction of the causeway.




Figure 4.4 Tidal Flushing Times – Option 1




Figure 4.7 Peak Velocities for a Typical Flood Event – All Scenarios




Table 4.3 Comparison of Levels for Typical Flood – All Scenarios

Scenario Flood Level

Existing conditions 1.24 m

Pre-causeway 0.92 m

Option 1 0.94 m

Option 2 0.92 m

Option 3 0.94 m

4.3.5 Sediment Transport Potential

The sediment transport potential of Options 1 and 2 were also assessed using the RMA model. Option 3 was not modelled, because based on the results to date, Option 3 is virtually identical to Option 1. That is, the additional culverts through the causeway for Option 3 do little to enhance the tidal penetration into the lake, and hence do no impact on overall tidal flushing.

Predicted sediment transport potential under peak ebb tide and peak flood tide conditions for Options 1 and 2 are shown in Figures 4.8 and 4.9, respectively.

Peak Ebb Tide Potential Peak Flood Tide Potential

Figure 4.8 Sediment Transport Potential under Tidal Action – Option 1




Peak Ebb Tide Potential Peak Flood Tide Potential

Figure 4.9 Sediment Transport Potential under Tidal Action – Option 2

From Figure 4.8 it can be seen that Option 1 produced a reasonable amount of sediment transport potential across the entrance delta area. The transport potential distribution is very similar to that for the pre-causeway condition, except that sediment transport in the southern section of the delta is concentrated at the new bridge opening

Sediment transport potential across the entrance delta for Option 2 (refer Figure 4.9) is considerably less than Option 1 and the pre-causeway scenario. The only area of significant sediment transport potential is at the peak of the flood tide within the new southern channel. Even then, this sediment transport potential is no greater than what would be expected to occur within the existing entrance channel.

Sediment can also be scoured out of the entrance delta during flood conditions. Flood velocities across the entrance delta for the Options and the existing conditions and pre-causeway scenarios have been presented previously in Figure 4.7. The actual sediment transport potentials associated with these flood velocities for Options 1 and 2 are shown in Figure 4.10. This figure demonstrates that the creation of a deep southern channel (Option 2) relieves considerable pressure from the entrance delta area, particularly in the vicinity of the new southern bridge opening.

Based on the results of the tidal and flood induced sediment transport results, it is considered that a localised scour hole is likely to form under the new southern bridge opening for Option 1 (and Option 3), while for Option 2, the dredged channel would remain relatively stable. The diversion of a large proportion of the flow into the new southern channel in Option 2, however, is likely to affect the existing northern channel by possibly increasing siltation, resulting in an overall shallowing of the channel.




Option 1 Option 2

Figure 4.10 Sediment Transport Potential under Flood Conditions for Options 1 and 2

SELECTION AND ASSESSMENT OF PREFERRED OPTION 5-1



5 SELECTION AND ASSESSMENT OF PREFERRED OPTION

5.1 Indicative Costs of Options

As a final criteria to which the options were assessed, indicative costs for each of the options were estimated. The cost of constructing a second bridge at the southern end of the causeway was estimated at $4 million. This includes removal of the existing structure and contingencies associated with the significant interruption to the main coastal road traffic. The cost of dredging a southern channel would be in the order of $1 million, assuming that the material could be placed locally. The installation of culverts along the causeway would cost approximately $400,000. This also includes a high contingency for the significant interruption of the main coastal road traffic.

Therefore, the indicative costs for the three options are:

Option 1: $4,000,000

Option 2: $5,000,000

Option 3: $4,400,000

5.2 The Preferred Option

It is important to appreciate that the tidal range of Wallaga Lake, and hence the tidal flushing potential, is controlled by a number of factors, including the size of the opening at the causeway, the head loss across the entrance delta and the condition / size of the ocean entrance. While increasing the opening through the causeway and/or improving the efficiency of flow through the entrance channel can increase tidal range and improve tidal flushing, there will be times when conditions in the lake are totally dominated by the entrance condition, ie when it is heavily shoaled or closed.

Nonetheless, based on the analyses carried out as part of this investigation, it is considered that Option 1 would be the most appropriate option for potentially improving tidal flushing within Wallaga Lake. There are a number of reasons why this option is preferred over the other options, as outlined below:

• Option 1 best replicates the tidal flushing characteristics of the pre-causeway scenario;

• Option 1 is least likely to result in changes (including shallowing) to the existing entrance channel

• Option 1 involves minimal actual works to the causeway. Although the results for Option 1 and Option 3 are virtually identical, Option 3 involves the installation of several culverts through the causeway that are additional to the southern bridge opening. The results indicate that these culvert openings would have a minimal impact on tidal and flood processes of the lake;

• Option 1 would not involve major dredging or other physical changes to the entrance delta, although some local lowering of bed level under the new bridge could be carried out to prevent localised bed scour and associated deposition.

A conceptual design of Option 1 is provided as Figure 5.1.




Figure 5.1 Conceptual Design of Preferred Option (Option 1)

LOCATION OF NEW BRIDGE

POSSIBLE AREA OF DREDGING IN ORDER TO

REDUCE SCOUR

New Bridge Deck Level RL 1.25 AHD (dependant on proposed causeway works)

Dredged

Existing

Channel

Channel

Bed

Bed

RL -1.4 AHD

RL 0.0 AHD

100m




It is notable that the preferred option is similar to that initially proposed in the Estuary Processes Study (PBP, 1996), however, this previous study predicted between a 50% and 100% increase in tidal range within the lake with a substantial improvement in flushing potential. These substantial changes to tidal behaviour were generated due to the exacerbated causeway opening (250 metres) and channel dimensions (150 metres wide) adopted in the previous investigation. The much reduced increase in tidal range and improvements to tidal flushing potential predicted by this study represent a more realistic scenario given the likely physical and financial constraints of implementation.

5.3 Wider Impacts of Preferred Option

5.3.1 Shoaling Potential

Based on the results of the modelling carried out to date, it is considered that the preferred option would not result in a significantly different shoaling potential within the main entrance channel from the existing scenario. However, given that flood tide currents are predicted to be stronger than ebb tide currents under the new causeway opening, it is possible that marine sands could prograde beyond the current distal edge of the delta in the vicinity of this new southern opening.

This slightly enhanced shoaling within the lake could be overcome to some degree by carrying out minor dredging, or local deepening of the channel, under and immediately adjacent to the new opening.

5.3.2 Entrance Stability

As shown by the historical air photos, the entrance to Wallaga Lake can be quite variable, ranging from flood-scoured to closed. The condition of the entrance is dependent on the prevailing ocean and lake conditions.

By reducing the constriction to tidal flow at the causeway (ie create an additional opening through the causeway), the typical tidal prism of the lake would be increased. This could result in a slightly larger ‘regime state’ entrance. However, the regime state was observed to occur less than 50% of the time (refer Section 1.3). During other times, the entrance is dominated by flood related processes, or prevailing ocean conditions.

5.3.3 Aquatic Habitats

Seagrass coverage of the entrance delta area has been highly variable over time. Case studies on other entrance delta areas indicate that seagrass extent tends to expand when the entrance is closed or near-closed as the higher mean water levels increase the extent of available habitat suitable for seagrass colonisation, and sediment mobility is reduced. When the entrance opens, however, water levels are reduced, and seagrass beds tend to die-off due to overexposure.

From the modelling results, the increase in tidal variations associated with the second causeway opening does not result in a lowering of low tide levels. Rather, high tide levels within the lake are predicted to increase by approximately 0.1 metres. It is considered that the low water levels in the lake are controlled more by the lake entrance and friction across the entrance delta than the causeway




constriction. As such, implementation of Option 1 would not unduly aerially expose existing seagrass beds beyond that which would occur by changes to the entrance condition.

While recognizing that ambient light level is also a major control on the distribution of seagrasses, any reduction in ambient light level, either due to increased turbidity or increased water depths, may result in a decline in the maximum depth inhabited by seagrass. Under Option 1, it is predicted that high tide levels within the lake will increase by 0.1 m, which could result in the loss of seagrass in deeper areas. It would be expected that seagrass occurring in deeper sections of Wallaga Lake and the entrance channel might be lost, although there is also potential for colonisation in shallower waters inundated by the elevated high tide levels. This would mean that there would be a shift in the distribution of seagrass along the depth profile.

The impacts of this shift in seagrass distribution on the overall extent (area) of seagrass in the estuary would depend, among other factors, on lake topography. It would be expected that the overall extent of seagrass would decline if broad tidal flats were inundated beyond the euphotic zone of seagrass (ie. the maximum depth at which seagrass can exist). Alternatively, any increase in the area of shallow waters (within the euphotic zone of seagrass) would result in an increase in seagrass extent. It is beyond the scope of this study to determine the predicted extent of seagrass in Wallaga Lake under Option 1. Detailed assessments would be required of the lake topography (and seagrass/depth relationships) to predict likely changes in seagrass extent with the increase in high tide levels. However, preliminary observations indicate that most of Wallaga Lake has relatively steep and uniformly sloping banks (ie. no net increase or decrease in broad tidal flats in the euphotic zone), thus major changes in extent are not anticipated in this area. Within the entrance channel area, there is a greater potential for changes in seagrass extent given the larger area the larger area of shallow sand beds. However, other factors, such as bed instability, would be expected to have a more profound influence on seagrass extent in the entrance channel area than the predicted 0.1 m increase in high tide levels.

Air photos indicate that seagrass beds are generally present within the slightly deeper sections of the entrance delta in the vicinity of the second causeway opening (refer Figure 1.2). It is considered that implementation of Option 1 would cause local disturbances to these beds either through localised dredging under the bridge opening, or by sediment transport processes, which could either scour existing beds or smother them with sand scoured from elsewhere. The disturbance to the local benthic environment in this vicinity would be for a short-time only (up to 12 months or so) until the bed form achieves a balance with the new tidal (and sometimes flood) flows.

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FIGURE 4.1

EXISTING CAUSEWAY CONFIGURATION PRE-CAUSEWAY CONFIGURATION

OPTION 1 OPTION 2 OPTION 3

NEW SOUTHERN OPENING NEW SOUTHERN OPENING + CHANNEL NEW SOUTHERN OPENING + ADDITIONAL CULVERTS

WALLAGA LAKE CAUSEWAY OPTIONS ASSESSMENT

MODEL SCHEMATISATION AND BATHYMETRY DETAILS



FIGURE 4.3





PEAK FLOOD TIDE VELOCITIES THROUGH ENTRANCE (SPRING TIDE)



FIGURE 4.2





PEAK EBB TIDE VELOCITIES THROUGH ENTRANCE (SPRING TIDE)



FIGURE 4.7





PEAK VELOCITIES FOR TYPICAL FLOOD EVENT

C:\Users\z2274090\Documents\Work\Projects\wrl2016067 Merimbula, Curalo, Wallaga CMP\Correspondence\From Council\Recieved

20161020\Studies and documents\Wallaga\A3_figures.doc


FIGURE 5.1

WALLAGA LAKE CAUSEWAY OPTIONS ASSESSMENT CONCEPTUAL DESIGN OF PREFERRED OPTION (OPTION 1)

LOCATION OF NEW BRIDGE STRUCTURE

POSSIBLE AREA OF DREDGING IN ORDER TO

REDUCE SCOUR POTENTIAL

New Bridge Deck Level RL 1.25 AHD (dependant on proposed causeway works)

Dredged

Existing

Channel

Channel

Bed

Bed

RL -1.4 AHD

RL 0.0 AHD

100m

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