The Influence of the Garden Island Causeway on the Environmental Values of the Southern End of Cockburn Sound

p:\csmc\manglesbay\reports\mangles_bay_study.doc 16/09/2002; 3:26 PM

THE INFLUENCE OF THE GARDEN ISLAND CAUSEWAY ON THE ENVIRONMENTAL VALUES OF THE SOUTHERN END OF COCKBURN SOUND

Prepared for:

COCKBURN SOUND MANAGEMENT COUNCIL

Prepared by:

DAL SCIENCE AND ENGINEERING PTY LTD

AUGUST 2002

REPORT NO. 02/247/1

CONTENTS

EXECUTIVE SUMMARY ______iv

1. INTRODUCTION______9 1.1 GENERAL______9 1.2 BACKGROUND TO THIS STUDY ______10 1.3 STUDY SCOPE AND APPROACH ______10 1.3.1 Scope ______10 1.3.2 Approach______11

2. REVIEW OF COCKBURN SOUND CIRCULATION AND MODELLING ______14 2.1 CIRCULATION IN COCKBURN SOUND ______14 2.2 DETERMINATION OF EXCHANGE (FLUSHING) ______15 2.3 INFLUENCE OF THE CAUSEWAY ______16

3. CAUSEWAY CIRCULATION MODELLING ______18 3.1 APPROACH ______18 3.2 MODEL FORCING______18 3.3 MODEL GRID______18 3.4 CAUSEWAY CONFIGURATIONS MODELLED ______19 3.5 THE INFLUENCE OF THE CAUSEWAY ON COCKBURN SOUND ______20 3.5.1 The effect of the Causeway on the flushing of Cockburn Sound ______20 3.6 VALIDATION______25 3.6.1 Low Frequency Forcing ______26 3.7 THE INFLUENCE OF THE CAUSEWAY ON THE MANGLES BAY REGION ______28 3.7.1 Flushing______28 3.7.2 Bottom Shear Stress ______30

4. FIELD SURVEY ______32 4.1 INTRODUCTION ______32 4.2 HYDODYNAMIC INTERPRETATION ______32

5. THE EFFECTS OF VARIOUS CAUSEWAY CONFIGURATIONS ON ECOLOGY, COASTAL PROCESSES AND HUMAN USES IN COCKBURN SOUND______37 5.1 ECOLOGY ______37 5.1.1 Water quality______37 5.1.2 Seagrass ______41 5.2 COASTAL PROCESSES ______49 5.2.1 Effects due to changes in wave energy ______49 5.2.2 Longshore sediment transport and shoreline change ______50 5.3 HUMAN USES ______53 5.3.1 Current uses ______53 5.3.2 Future uses ______55

6. CONCLUSIONS AND RECOMMENDATIONS ______57 6.1 CONCLUSIONS ______57 6.1.1 Flushing and circulation characteristics______57 6.1.2 Ecology ______57 6.1.3 Coastal processes ______58 6.1.4 Human uses ______59 6.1.5 Overview ______60 6.2 RECOMMENDATIONS ______61

7. ACKNOWLEDGMENTS ______62

8. REFERENCES ______63

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TABLES

Table 2.1 Flushing times for Cockburn Sound ______15 Table 3.1 Summary of simulations undertaken using the EFDC model ______20 Table 3.2 The effect of the Causeway on the flushing of Cockburn Sound (summer and autumn) ______21 Table 3.3 The effect of the Causeway on the flushing of various areas of Cockburn Sound in summer______23 Table 3.4 The effect of the Causeway on the flushing of various areas of Cockburn Sound in autumn ______24 Table 5.1 Median chlorophyll values in various areas of Cockburn Sound, summer 2001/2002 ______39 Table 5.2 Relative impact of various Causeway configurations on summer water quality at the southern end of Cockburn Sound ______41 Table 5.3 Relative impact of various Causeway configurations on seagrasses. Predictions on shear stress based on peak current velocities during a spring tide in summer ______48

FIGURES

Figure 3.1 Bathymetry (water depth in metres) of the southern region of Cockburn Sound with each of the four Causeway configurations, with model grid overlay ______19 Figure 3.2 Changes in dye concentration in Cockburn Sound in summer and autumn, with and without the Causeway______21 Figure 3.3 Areas of Cockburn Sound examined for flushing characteristics ______22 Figure 3.4 Changes in dye concentration in various areas of Cockburn Sound in summer, with and without the Causeway ______23 Figure 3.5 Changes in dye concentration in various areas of Cockburn Sound in autumn, with and without the Causeway______24 Figure 3.6 Location of current meters used in the validation study ______25 Figure 3.7 Comparison of modelled and field data in the southern entrance of Cockburn Sound ______27 Figure 3.8 More detailed comparison of modelled and field data in the southern entrance of Cockburn Sound ______27 Figure 3.9 Spectral analysis of modelled and field data for the current meter in the southern entrance of Cockburn Sound______28 Figure 3.10 Initial dye distribution used to investigate the circulation within Mangles Bay. Dye concentration varies between 0 (blue) and 10 (red) ______29 Figure 3.11 Instantaneous dye concentration distribution generated by circulation transporting and mixing the initial dye distribution on the 26th of January 17:30, 1998 ______30

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Figure 4.1 Transect location ______33 Figure 4.2 Salinity profiles obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon______34 Figure 4.3 Temperature (degrees Celsius) obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon ______35 Figure 4.4 Dissolved oxygen (% saturation) obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon ______36 Figure 5.1 Changes in dye concentration with different Causeway configurations at selected sites in summer (refined grid) ______41 Figure 5.2 Current velocity vectors during a spring tide in summer in the Mangles Bay region: comparison between present Causeway, and doubling of existing Causeway openings ______43 Figure 5.3 Current velocity vectors during a spring tide in summer in the Mangles Bay region: Comparison between present Causeway, and additional Causeway openings ______44 Figure 5.4 Current velocity vectors during a spring tide in summer in the Mangles Bay region: Comparison between present Causeway configuration, and with the Causeway removed ______44 Figure 5.5 Bottom shear stress during an incoming spring tide in summer (kg/cm/s2), in the absence of the Causeway ______45 Figure 5.6 Bottom shear stress during an outgoing spring tide in summer (kg/cm/s2), in the absence of the Causeway______46 Figure 5.7 Changes in bottom shear stress (relative to the ‘No Causeway’ configuration) caused by various Causeway configurations: incoming spring tide, summer simulation ______46 Figure 5.8 Changes in bottom shear stress (relative to the ‘No Causeway configuration) caused by various Causeway configurations: outgoing spring tide, summer simulation ______47 Figure 5.9 Refraction of (a) south west and (b) north west swell-waves through South Channel (Modified from FPA, 1972) ______51 Figure 5.10 Refraction of north east wind-waves through Mangles Bay (Modified from FPA, 1972).______52

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EXECUTIVE SUMMARY Cockburn Sound is a sheltered marine embayment located approximately 20 km south of the Perth-Fremantle area. It is the most intensively used marine embayment in Western Australia, being popular for fishing and recreation, and the site of a busy port; an industrial area that depends on port facilities; and a strategic naval base.

At the southern end of the Sound there is a rockfill causeway connecting Garden Island with the mainland. The Causeway was built between 1971 and 1973 to service a naval base on Garden Island. The Causeway is broken by two trestle bridges (one 305 m long, and one 610 m long), through which limited ocean exchange occurs. There is ongoing discussion about the degree to which the Causeway’s restriction of exchange between the waters of the southern end of Cockburn Sound and the open ocean has influenced—and still is influencing—the environment of the Sound.

The Cockburn Sound Management Council (CSMC) commissioned DAL Science & Engineering Pty Ltd (DALSE) to undertake a preliminary modelling exercise to investigate the effect of various Causeway configurations on circulation and exchange within Cockburn Sound, paying particular attention to the Mangles Bay region adjacent to the southern (mainland) end of the Causeway. The results of the modelling were then used to interpret the effects of the Causeway on the environmental quality of Cockburn Sound, and the potential environmental benefits of modifying its design. The Causeway configurations examined were as follows:

• Existing Causeway openings; • Double the size of the existing Causeway openings; • Additional Causeway openings, comprising removal of the portion between the southern opening and the mainland, and three additional openings each 200 m wide; and • No Causeway present.

Simulations of various Causeway configurations were undertaken using a validated, state-of the-art model, and focussed specifically on two weeks of ‘typical’ summer conditions (with a pronounced sea breeze cycle), and two weeks of ‘typical’ autumn conditions (relatively calm conditions). Field data were also collected to refine model representation of conditions in the Sound.

Modelling results were used to predict effects on the following aspects of environmental quality:

• Marine ecology (e.g. phytoplankton blooms, seagrass health); • Coastal processes (e.g. erosion and accretion of the shoreline); • Present recreational and commercial uses (e.g. boating, fishing, mussel aquaculture); and • Potential future uses (the proposed marina facility in Mangles Bay).

It is important to recognise that although the modelling of the Sound represents the most detailed undertaken to date, it has limitations (as do all modelling exercises) that must be borne in mind when interpreting environmental effects. Ecological interactions and coastal processes are also extremely complex, and while general trends can be identified it is rarely possible to identify all potential effects.

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EFFECTS ON CIRCULATION AND FLUSHING Modelling results indicated that removal of the Causeway produced a greater impact on the flushing characteristics of Cockburn Sound in summer than in autumn, and significantly affected current velocities within about 1–2 km of the Causeway. Based on e-folding times (the time taken for 63% of a water body to be flushed) calculated from summer simulations, removal of the Causeway produced:

• 2–3 fold improvements in flushing times in the Mangles Bay deep basin, on Southern Flats, and in the southern basin of the Sound; • Approximately a 50% improvements in flushing times in the shallows of Mangles Bay, the Rockingham/Kwinana area and eastern Garden Island, and the central basin of the Sound; and • Little change on Eastern Flats or in the northern basin of the Sound.

Adding additional openings to the Causeway produced a lesser effect on flushing than removal of the Causeway, and doubling the size of the existing Causeway openings produced a lesser effect than additional openings.

EFFECTS ON ECOLOGY Potential impacts on summer water quality were inferred by considering changes in flushing time against the context of the most recent data on summer water quality: large changes in flushing times did not always equate to large changes in water quality if present water quality was good. It was inferred that removal of the Causeway should result in a considerable improvement in water quality in the Mangles Bay deep basin, small improvements in the shallows of Mangles Bay/Rockingham/Kwinana, and little change elsewhere. Adding additional openings to the Causeway or doubling the size of the existing Causeway openings will result in lesser improvements in the Mangles Bay deep basin, and little change in water quality elsewhere.

Potential impacts on seagrasses due to modifications of the Causeway were examined in terms of changes in water quality, and changes in ‘bottom shear stress’—a measure of the force per unit area exerted on the seabed by currents close to the seabed, and proportional to the current velocity, squared. This approach was taken because the historical loss of seagrass (and present stress on seagrasses at their depth limit) in the Mangles Bay/Rockingham/Kwinana area appears to be related to poor water quality, whereas on Southern Flats the loss appears due to altered current speeds and/or sediment movement.

Removal of the Causeway should result in improvements in water quality that improve the health of seagrass meadows at their depth limit in the Mangles Bay area, and theoretically allow that depth limit to be extended. Based on the theoretical return of seagrass areas lost in the past, the estimated area of potential seagrass gain is around 50 ha. Water quality may also improve sufficiently to theoretically allow about 50 ha seagrasses to grow in waters 3–10 m deep in the Rockingham/Kwinana area. Changes in current speeds and/or sediment movement on Southern Flats due to removal of the Causeway may potentially allow about 200 ha of seagrass to re-establish (if areas west of the Causeway are included).

DALSE:CSMC: INFLUENCE ON THE GARDEN ISLAND CAUSEWAY v achievable with modifications of the Causeway (rather than complete removal). However, the ‘doubled openings’ and additional openings’ configurations would increase current velocities in some parts of Southern Flats and decrease them in others. The range of tolerance of seagrasses to current speed is not known, and so it is not certain whether the reductions in current velocities would be sufficient to allow seagrasses to regrow, or whether the increases would be tolerated.

The above estimates are potential gains in seagrass due to the creation of suitable conditions, but there is no certainty that seagrasses will actually return. A feature of the species of seagrass found in Cockburn Sound appear to be that, once lost, they do not always readily re- establish even if conditions are suitable. It is possible that revegetation efforts would be needed to achieve the seagrass re-establishment theoretically possible due to improved conditions.

EFFECTS ON COASTAL PROCESSES The Causeway has two types of effect on coastal processes: changes in swell-wave and wind- wave energy arriving at shoreline, and direct interruption of longshore sediment transport due to the presence of a physical structures at the shoreline. Coastal processes in the region adjacent to the Causeway are extremely complex, and preliminary predictions on the effect of removing the Causeway may be summarised as follows:

• Under both south westerly and north westerly swell, removal of the Causeway would result in an increase in swell-wave energy in the Mangles Bay area and an increase in the eastward longshore current at the shoreline in this area. It is likely that the anticipated increase in swell-wave energy in areas eastward of Mangles Bay would become relatively less with distance from the Causeway. The removal of the Causeway will also result in a slight increase in the wind-wave energy that is experienced on the northern beaches of Cape Peron; • There is a net easterly sediment transport direction on the northern beaches of Cape Peron, and sediment is trapped on the western side of the breakwater of the Cape Peron boat ramp. This sediment is bypassed, as required, to the eastern side of the Causeway. The Causeway is not the main trap for this longshore sediment transport, and removal of the Causeway would not release this trapped sediment for continued movement eastward; • As littoral sediment transport east of the Causeway would still be trapped at the Cape Peron boat ramp, it is likely that the removal of the Causeway will result in some shoreline erosion in the Mangles Bay area due to the increase in wave energy that would be experienced in this area. It is possible that the increased wave energy may cause a flattening of the beach profiles in southern Cockburn Sound which may result in sediment deposition on some of the nearshore seagrass meadows. It is also likely that the slight realignment southwards of James Point that occurred after the Causeway was constructed, may be reversed; • On Garden Island, interruption of littoral sediment transport has resulted in erosion of the beach at Careening Bay and accretion of sand in Broun Bay and around Parkin Point. This effect would be reversed with removal of the Causeway; and • The changes in longshore sediment transport and shoreline profiles that would be caused by removal/modification of the Causeway) are akin to those caused by construction of the Causeway, in that they will be relatively small and readily addressed by sand bypassing and other management measures.

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EFFECTS ON HUMAN USES A preliminary assessment on the potential impacts of removal of the Causeway on present human uses of the Mangles Bay region is as follows:

• Any removal/alteration of the Causeway is likely to cause considerable disruption to the operation of the naval base on Garden Island. Nor could the Causeway be removed before a new bridge was in place; • The Cape Peron boat ramp is intensively used by recreational boaters. The portion of the Causeway that protects the Cape Peron boat ramp from north-easterly, easterly and south-easterly winds could not be removed unless some other structure was put in place to provide similar protection. The need for some structure to protect the boat ramp will inevitably mean some reduction in the flushing of the shallow waters of Mangles Bay; • The activities of small boat users that presently shelter in the lee of the Causeway may be restricted; • Conditions will be less sheltered in the water-ski and jet-ski areas immediately adjacent to the Causeway, and these activities may be restricted to some extent; • Recreational vessel moorings may be affected by increased exposure to swell-waves and wind-waves; • Some coastal structures have been built ‘post Causeway’ (e.g. the second stage of the CBH jetty) and may not be designed for ‘pre-Causeway’ swell-wave and wind-wave conditions. If ‘under deck’ clearance in any jetty/wharf is too low, then waves contacting the underside of the deck may create uplift forces sufficient to damage the jetty structure. Groynes and walls can also be damaged by waves. The ability of coastal structures to withstand changed conditions would need to be investigated; • The re-establishment of a balanced sediment transport regime at James Point may affect activities at the BP Refinery (e.g. silting up of the water intake pipe); • Changes in water quality are unlikely to affect beach use, yachting, jet-skiing, water- skiing and pleasure boating; • Localised improvements in water quality will be due to decreased phytoplankton production in these areas. This may have some effect on the fisheries, particularly if there is no compensating gain in seagrass meadows; and • Predicted changes in current velocity and water quality on Southern Flats are unlikely to adversely affect mussel aquaculture in this area. Mussel aquaculture at the CBH jetty may be adversely affected by the significant decrease in phytoplankton production predicted to occur in this area, although any effects may be partly offset if the mussels can be grown at depths greater than 5 m (the depth limit under present conditions).

The main future human use of the region considered was a marina in Mangles Bay, the concept of which has long been supported by local government and some State government departments. It is, however, difficult to predict the impact of any alterations/modifications of the Causeway on any future marina without knowing the final design of the marina, and the management practices to be adopted. The marina will need groynes at the marina entrance, as well as a dredged channel leading to the entrance. Alterations in swell-wave and wind-wave conditions and longshore sediment transport with removal of the Causeway may necessitate alteration of groyne design and channel alignment, and affect maintenance dredging of the channel and management of any sand bypassing.

Due to increased residence times, water quality in the marina will be less than in adjacent waters: the extent to which marina water will affect water quality in Mangles Bay will depend on the size, shape, depth and design features of the marina, and may vary from negligible to measurable. The removal of the Causeway is not expected to greatly improve flushing in the Mangles Bay shallows and, as noted earlier, the portion of the Causeway

DALSE:CSMC: INFLUENCE ON THE GARDEN ISLAND CAUSEWAY vii protecting the Cape Peron boat ramp (or some replacement structure) will need to stay, which will further lessen any flushing of this area. The ‘doubled opening’ and ‘additional opening’ configurations are expected to result in little improvement of water quality in the Mangles Bay shallows. The marina design will need to achieve sufficient dilution of marina water to avoid adverse environmental impacts in Mangles Bay, irrespective of the presence/absence of the Causeway. Any direct seagrass losses involved in construction of a marina may, however, be viewed as more acceptable if removal/modification of the Causeway improves water quality in some areas sufficiently to increase the likely success of any mitigation efforts carried out to offset seagrass losses.

OVERVIEW Modelling results have indicated that removal of the Causeway would result in conditions that theoretically permit the re-establishment of 100 ha of seagrass in the Mangles Bay/Rockingham/Kwinana area (due to improved water quality) and 200 ha on Southern Flats (due to reduced current velocities), but there is considerable uncertainty about whether the seagrasses would return or how long this would take. Doubling the width of Causeway openings or adding additional openings would produce lesser improvements in water quality, and because the ‘threshold’ velocity tolerated by seagrasses is not known, it is uncertain whether all the predicted changes in current velocities on Southern Flats (both increases and decreases) would be beneficial to seagrasses.

Any removal/alteration of the Causeway could cause considerable disruption to the activities of the naval base on Garden Island: removal of all or part of the Causeway could not be undertaken before an alternative structure was in place. Adverse effects on some recreational uses and commercial activities in southern Cockburn Sound may also occur. The portion of the Causeway that protects the Cape Peron boat ramp from north-easterly, easterly and south- easterly winds cannot be removed—unless some other structure is put in place to provide similar protection. Even if other parts of the Causeway are removed/modified, the need for some structure to protect the boat ramp will reduce the potential for improved flushing of the shallow waters of Mangles Bay.

The above conclusions are predictions of the environmental effects likely as a result of modifying the Causeway, not an evaluation of whether the environmental benefits justify the costs. The latter evaluation would require a formal cost-benefit analysis, and would be most appropriately carried out by the CSMC. As a ‘lead in’ to such an exercise, preliminary calculations undertaken by the Department for Planning and Infrastructure (DPI) indicate that removal of Causeway rockfill material would cost approximately $5.5 million (assuming that the material could be disposed of within 10 km of the site). Construction of a bridge between the two existing trestle bridges, and to land, would cost about $120 million. Removing Causeway rockfill material and bridge construction would also affect the environment (e.g. turbidity, scouring, loss of rockfill habitat), and effects would need to be carefully appraised.

On the basis of available data it is concluded that the environmental benefits of removing the Causeway do not justify the associated costs. For the other modifications to the Causeway considered (wider openings, additional openings), there is the risk of adverse environmental effects outweighing the environmental benefits.

Should the CSMC decide that further investigation is warranted, it is recommended that the Department of Defence be invited to prepare a submission on the implications to the naval base of modifying or removing the Causeway. A number of recommendations are also provided in the main document to improve confidence in modelling results, and predictions of effects on ecology and coastal processes.

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1. INTRODUCTION

1.1 GENERAL Cockburn Sound is a sheltered marine embayment located approximately 20 km south of the Perth-Fremantle area. It is 16 km long and 9 km wide, with a 17−22 m deep central basin, and sheltered from ocean swells along almost its entire western side by Garden Island. In terms of both its depth and its degree of shelter from ocean swell, the Sound is unique along Perth’s metropolitan coast—and for several hundred kilometres to the north and south. As a result of these features, it is also the most intensively used marine embayment in Western Australia: it is popular for fishing and recreation; and the site of a busy port, an industrial area that depends on port facilities, and a strategic naval base.

Cockburn Sound has a history of extensive modifications and environmental degradation. In 1954 industrial development commenced with the building of an oil refinery at James Point, and this was followed in swift succession by iron, steel, alumina and nickel refining/processing plants; chemical and fertiliser production plants; and a bulk grain terminal. Wharves and groynes were built and channels dredged for shipping access: the Sound became the ‘outer harbour’ for the Fremantle Port Authority. At the northern end of the Sound, a wastewater treatment plant was commissioned at Woodman Point in 1966 to treat sewage from Perth’s southern suburbs. At the southern end of the Sound, a rockfill causeway connecting Garden Island with the mainland was built between 1971 and 1973. The causeway is broken by two trestle bridges (one 305 m long, and one 610 m long), through which limited ocean exchange occurs. The causeway was built to service a naval base on Garden Island, which was constructed between 1973 and 1978.

The developments that took place from 1954 onwards resulted in degradation of the environment, and in the 1970s conflict with recreational users became an additional issue. Environmental studies in the 1970s identified two major environmental problems:

• Deteriorating water quality, due to ‘blooms’ of phytoplankton (microscopic algae floating in the water); and • Widespread loss of seagrass on the eastern margin of the Sound, as a result of light starvation, due in turn to the shading caused by increased growth of epiphytes (algae that grow on seagrass leaves) and phytoplankton. This loss took place mainly between the late 1960s and early 1970s.

The decline in seagrass meadows and increase in phytoplankton levels were linked to a massive increase in nutrient loading to the Sound, mainly from two sources: an outfall shared by the Kwinana Nitrogen Company (KNC) and the CSBP fertiliser works, and the outfall of Woodman Point Wastewater Treatment Plant (WWTP). Nitrogen discharged from these two outfalls was identified as the nutrient responsible for the increased algal growth.

The construction of the Causeway during the early 1970’s was also coincident with ecological degradation of the Sound. Although the increased nitrogen input was considered the main cause of deteriorating water quality and seagrass loss (noting that the major seagrass loss occurred before construction of the Causeway), it was considered that the Causeway also contributed to the problem by reducing the flushing of Cockburn Sound. The effect of the Causeway on flushing is complex, as

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the Causeway influences the flow through both the southern and northern entrances. In addition to the reduction in flushing, the Causeway has altered the circulation patterns in the southern portion of Cockburn Sound, specifically within about 1500 m of the Causeway structure.

Management efforts by both government and commercial organisations have resulted in a large decrease in nitrogen inputs to the Sound since the 1980s, yet water quality improvements have not been commensurate with this decrease. A number of explanations have been advanced for this, and it has been postulated that one contributing factor may be the reduction in flushing and altered circulation due to the Causeway.

1.2 BACKGROUND TO THIS STUDY The Cockburn Sound Management Council (CSMC) is a Committee of the Board of the Water and Rivers Commission (WRC), and is a State Government initiative aimed at coordinating environmental planning and management in Cockburn Sound. The CSMC have prepared an Environmental Management Plan (EMP) for Cockburn Sound, one component of which is a Research and Investigation program intended to improve the information and knowledge base used for management. A specific item identified in the Research and Investigation program is the need to determine the influence of the Causeway on the environmental quality of Cockburn Sound, and the potential environmental benefits of modifying its design.

The CSMC required investigation of the potential effects of modifying the Causeway on flushing and circulation in the southern end of Cockburn Sound, particularly Mangles Bay. Any alterations in flushing and circulation characteristics in this region also have the potential to affect marine ecology (e.g. phytoplankton blooms, seagrass health), coastal processes (e.g. erosion and accretion of the shoreline), present recreational and commercial uses (eg mussel aquaculture), and potential future uses (e.g. the proposed marina facility in Mangles Bay). The CSMC commissioned DAL Science and Engineering Pty Ltd (DALSE) to prepare a report on ‘Determining the Influence of the Garden Island Causeway on the Environmental Values of Mangles Bay, Rockingham’. The results of DALSE’s studies are provided in this document.

1.3 STUDY SCOPE AND APPROACH

1.3.1 Scope Numerical circulation modelling was identified as the appropriate tool to assess the influence of various Causeway design options, and assist in assessing impacts upon the marine ecology, coastal processes and current and future uses within the region. The briefing document for the study identified three study aims as follows:

1. Model the water circulation and exchange in Cockburn Sound, with a specific focus on the nature of water movement in Mangles Bay. 2. Apply the model to identify how water circulation and exchange in Mangles Bay may change under different design options of the Garden Island Causeway. 3. Identify the positive and negative impacts of each scenario on the ecology, coastal processes, existing and future uses of Mangles Bay. Numerical modelling of various Causeway configurations comprised the majority of the work undertaken. Interpretations of the positive and negative impacts of various

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configurations on the ecology, coastal processes, and existing and future uses of Mangles Bay were undertaken as preliminary exercises, and were predicated on the results of the numerical modelling. It is important to acknowledge that a degree of uncertainty is involved in the output of any numerical modelling exercise, and that the complexities of ecological interactions and coastal processes add further uncertainties to any predictions. Thus, it was possible to identify general trends in effects on ecology and coastal processes, but not the exact degree and spatial extent of those effects.

As the pivotal element of this study, numerical circulation modelling comprised the following components:

• A review of previous work that has examined the effect of the Causeway; • Numerical circulation modelling to determine the effect the Causeway has had on the flushing of Cockburn Sound as a whole, and Mangles Bay in particular; • Fine-scale modelling of the region within 1500 m of the Causeway to determine the relative merits of different Causeway configurations upon the flushing of Cockburn Sound, and in particular, Mangles Bay; • A field survey of water characteristics (salinity, temperature and dissolved oxygen) within Mangles Bay; and • Deployment of two current meters; one in the Mangles Bay region, and one immediately west of the Causeway. The data collected from these current meters were used to validate the model used by DALSE.

The first three of the above components were undertaken by DALSE; the field survey was undertaken joint by DALSE and Murdoch University’s Marine and Freshwater Research Laboratory (MAFRL); and the last component was undertaken the Centre for Water Research, the University of Western Australia. The last component is also the subject of a separate report (Pattiaratchi, 2002).

1.3.2 Approach

Circulation modelling The Environmental Fluid Dynamics Code (EFDC) was used for the numerical modelling in this project. The model has, and is, being used within Cockburn Sound for projects ranging from the examination of cooling water discharges through to estimation of the exchange between Cockburn Sound and the adjacent waters. The model has already been calibrated and validated using data collected on the eastern margin, north of James Point, and against data measured to the west of the Causeway. The results from the model have been externally peer reviewed and accepted by Government agencies.

The model has two features that allow this model to provide a good representation of the circulation within the vicinity of the Causeway, as follows:

1. For modelling purposes, a study area is divided into grid cells of specific sizes. Unlike some numerical models, the EFDC model allows the insertion of thin barriers to block the flow though one face of a numerical model grid cell, and so allow a better representation of features (e.g. offshore breakwaters and the Causeway) that are smaller in width than the grid cells used to divide the modelled area.

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2. The EFDC model is able to simulate the movement of an inert tracer while simultaneously modelling the circulation patterns in the region. This benefit reduces the computational effort needed to investigate circulation and flushing patterns.

The circulation model was refined, and then used to investigate the following Causeway design configurations:

The study brief also identified investigation of the effect of dredging of the existing channels, but this was not modelled for reasons discussed in Section 3.4.

Marine ecology Predicting the impact of various Causeway configurations required an understanding of the complex interactions between physical, chemical, biological processes, and human usage, in the Mangles Bay area. It was also considered important to understand the physical, chemical and biological linkages between Mangles Bay, Cockburn Sound and the Sepia Depression (the region seaward of the Causeway), to place the influence of the Causeway in a regional as well as a local perspective.

Water quality data from the intensive field sampling survey undertaken for this study, plus the flushing estimates obtained in this study, were used in conjunction historical water quality data to refine the conceptual understanding of the main biological, chemical and physical components of Cockburn Sound, and the interactions between them. Changes in water clarity, and dilution of existing nutrient inputs to the area, were examined through the use of relative changes in conservative tracer concentrations.

Potential influences on seagrasses due to both changes in water quality and changes in current velocity were examined. Constriction of the flow through the existing Causeway openings has resulted in a loss of seagrass on the Southern Flats, and this is believed to be either the direct result of increased currents per se, and/or sediment mobilisation from the increased currents. Irrespective of the reason, the openings have reduced seagrass coverage in the region immediately adjacent. Increasing the cross-sections through which exchange can occur through the Causeway will reduce current velocities in some areas and increase currents in other areas: if currents are still greater than the limits of seagrass tolerance, further losses may occur. Conversely, in very calm areas increasing current velocities can have beneficial effects on seagrass health, by reducing the loads of epiphytes and lessening accumulations of detritus within the meadows. The influence of velocities on seagrass distribution in the region was examined empirically (i.e. with respect to conditions before the Causeway was constructed) to establish likely impacts of various Causeway configurations.

Coastal processes A simple wave ray analysis (for predominant swell and sea conditions) was used to compare change in wave energy distribution at the shoreline with the existing

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Causeway and complete removal of the Causeway (two extremes). This enabled some insight into comparative longshore drift regimes. In support of the wave ray analysis, a brief review of the historical records of shoreline change following construction of the causeway was undertaken to examine what changes happened as a comparison.

Current and future uses The impacts of various Causeway configurations on existing and potential future uses of Cockburn Sound were assessed using predicted changes in current velocities, water quality and coastal processes. Existing uses in two main areas were examined:

• Mangles Bay adjacent to Causeway—this areas is not popular for swimming as it is very shallow and full of seagrass, is heavily used for boat moorings, and has limited access. The area has private boat ramps (including Mangles Bay Fishing Club, and The Cruising Yacht Club), a dog beach, a jetski and waterski area, and is popular for recreational fishing; and • Southern Flats—this area is popular for yachting and recreational fishing. Mussel aquaculture also takes place at the northern edge of Southern Flats at the ‘drop-off’ into deeper water.

The main future use of the region considered was a marina in Mangles Bay.

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2. REVIEW OF COCKBURN SOUND CIRCULATION AND MODELLING

2.1 CIRCULATION IN COCKBURN SOUND Modelling of water movement in Cockburn Sound—and of substances transported by water—has been the subject of a number of studies from about 1977 until present. The hydrodynamics of the Cockburn Sound region have been reviewed in some detail by Hearn (1991), D’Adamo (1992) and the DEP (1996). There are also numerous detailed supporting documents of field work and modelling studies carried out as part of the SMCWS that are summarised and referenced in DEP (1996). Studies since 1996 have been more ‘site-specific’ than ‘ecosystem-wide’, and have focussed on the effects of proposed developments within and adjacent to Cockburn Sound: these studies have not altered the fundamental understanding of water movement in the Sound (as established in earlier work), but have refined understanding of some aspects. The following summary is drawn largely from work done up to 1996, while areas of improved understanding due to more recent work are also discussed.

The hydrodynamics of Perth’s coastal waters is a complex combination of wind- forced waves, tides, large-scale currents (the Leeuwin and Capes Currents) and localised currents due to density differences in the water column, and long period waves (Pattiaratchi et al., 1995). The complexity is due to variations in the relative strength of each of these factors with the weather and season.

As a result of the protected nature of the Sound, the three main processes that control its hydrodynamics are (Hearn, 1991):

• Wind; • Horizontal pressure gradients due to wind, tides, waves, atmospheric pressure and continental shelf waves (which create differences in water pressure due to differences in water level); and • Horizontal pressure gradients due to buoyancy effects (differences in water density).

Waves and currents in Cockburn Sound are primarily a result of wind forcing. During summer the dominant wind direction is south to south-west, and winds are typically quite persistent: 50% of winds have speeds of 5–9 m/s. The daily sea breeze cycle is also very important. In winter the main wind direction is westerly, though northerly winds often occur: winds are more variable with occasional periods of calm and strong storm winds, and 50% of winds have speeds of 2–7 m/s.

Horizontal pressure gradients are the result of differences in water pressure between two areas. Differences in water pressure may be grouped into those driven by wind, tides, waves, seiches and atmospheric pressure (i.e. differences in water pressure due to differences in water level); and those driven by horizontal differences in water density (often called ‘buoyancy effects’).

Three distinct hydrodynamic regimes have been identified in Cockburn Sound based on the relative importance of wind and pressure gradients in determining circulation patterns and flushing: ‘summer’, ‘autumn’ and ‘winter-spring’ (DEP, 1996).

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2.2 DETERMINATION OF EXCHANGE (FLUSHING) Recent studies of Cockburn Sound have furthered understanding in two areas: the flushing of Cockburn Sound; and the hydrodynamics of the eastern margin, including the characteristics of the cooling water discharges north of James Point.

Flushing is a measure of the exchange and replenishment of the water in an area with surrounding water. A long flushing time reflects low exchange with adjacent waters and a short flushing time reflects higher exchange. The water quality of Cockburn Sound is due in part to its enclosed nature, which reduces exchange with the water of Owen Anchorage to the north and the open ocean to the west. Flushing times affect the dilution of nutrient and contaminant inputs as well as a variety of ecological processes, and can be used to assess the ecological implications of changes in the circulation of Cockburn Sound.

There are many ways to measure the time over which Cockburn Sound is flushed. To be consistent with previous modelling of Cockburn Sound (DEP, 1996) the ‘e-folding’ time has been used, which estimates the time taken for 63% of a water body (in this case, Cockburn Sound) to be flushed. A summary of the most recent estimates of flushing times is given in Table 2.1, and are consistent with previous estimates by the DEP (1996).

Table 2.1 Flushing times for Cockburn Sound

AUTUMN WINTER WINTER STORM SUMMER 37 days 22 days 28 days 44 days Note: Reproduced with permission from Cockburn Cement Limited (Cockburn,, 2000).

The above estimates refer to flushing of the Sound as a whole. Flushing is least in summer because the prevailing winds set up circulation gyres that tend to confine water within the Sound (DAL, 2001). As noted previously, flushing in winter is primarily due to the horizontal pressure gradients (buoyancy) that occur: flushing during winter storms is actually less because wind mixing lessens the pressure gradients that drive water movement. Flushing in autumn is also primarily due to pressure gradients, but takes longer than flushing in winter: this is mainly because the movement of denser bottom waters out of the Sound (over the barrier of Parmelia Bank and through the Causeway) in autumn requires more energy than the movement of denser waters into the Sound during winter. There is also little vertical mixing (due to wind) in autumn to aid flushing of bottom waters.

Flushing times in more ‘localised’ areas along the eastern margin of the Sound have also been estimated to examine the impact of proposed harbour developments, and include the influence of cooling water discharges from Western Power and the BP Refinery (JPPL, 2001). The shallow waters of the eastern shelf are well mixed and flushing times are approximately 1 day or less. Flushing times are far greater in the partially enclosed waters of the Jervoise Bay Northern Harbour, generally 5–14 days. The most poorly flushed area of the Sound is considered to be the bottom waters of the southern basin, but no estimates of flushing times have been made to date.

There have been no investigations of changes in flushing times from year to year due to changes in wind patterns and pressure gradients (e.g. due to differences in Swan River flow, groundwater discharge).

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2.3 INFLUENCE OF THE CAUSEWAY Several studies have used numerical circulation modelling to investigate the effect of the Causeway on the environment of Cockburn Sound. Circulation modelling is able to determine the movement of water within Cockburn Sound, and specifically the exchange of water between Cockburn Sound and the surrounding water bodies. A circulation modelling approach also allows direct comparison between pre- and post- Causeway cases. Circulation dynamics are able to show how well water exchange is enhanced or inhibited, principally by the Causeway and to a lesser extent harbour developments along the eastern margin.

A review of numerical circulation modelling of the Causeway is given in Speedy (1994). Further investigation of the Causeway since Speedy (1994) was undertaken in the Southern Metropolitan Coastal Waters Study (DEP, 1996), but the most comprehensive investigation to date has been that of Speedy (1994). This investigation comprised of determining the effects on the Causeway on the flushing of Cockburn Sound and the changes in flow as a result of the Causeway.

Speedy’s (1994) modelling study was undertaken with a two-dimensional, vertically averaged circulation model. The modelled circulation was driven by a combination of alongshore pressure gradients, tide and winds. Where possible, all forcing parameters were estimated from field data. Two limitations of the modelling were that the model did not incorporate the effects of temperature and salinity gradients, and that only vertically averaged currents were simulated.

The conclusions of Speedy’s (1994) study were:

• The Causeway has affected the rate of exchange through the southern entrance of Cockburn Sound, causing flushing times to increase, on average, by a factor of 2.6; • The Causeway affects the direction and magnitude of current flows that are within 1500 m of the Causeway; • The flows through the southern entrance have been changed due to the Causeway obstructing flow in places and facilitating flow through the High Level Bridge and the Trestle Bridge; • The openings through the Causeway act to increase the current velocities through the openings and decrease current velocities near the rock-filled portions of the Causeway; and • The Causeway is considered well placed with respect to minimising the impact on circulation.

Numerical circulation modelling investigations undertaken by the DEP (1996) used a three-dimensional model to simulate the vertical currents profiles. In addition to the inclusion of the vertical dimension, the effects of density gradients due to salinity and/or density were also investigated. The modelling of exchange through the Causeway concurred with the findings of Speedy (1994) on exchange through the Southern Opening. This study also highlighted the importance of density gradients between Cockburn Sound and the surroundings waters as an important flushing mechanism.

Both model investigations indicate that the exchange through the Southern Opening has decreased by a factor of 40%, indicating that there is general agreement between the three- and two-dimensional modelling studies in this region. The overall reduction in the flushing of Cockburn Sound was reported to be between 30% (three-

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dimensional model) and 50% (two-dimensional model). The two-dimensional model overestimates the decrease in exchange for Cockburn Sound, however the two- dimensional approach provides an accurate indication of the exchange in the southern region of Cockburn Sound.

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3. CAUSEWAY CIRCULATION MODELLING

3.1 APPROACH The approach used for the circulation modelling undertaken in this study was to:

• Determine the influence of removing the Causeway on Cockburn Sound as a whole, with a circulation model previously used (and validated) for examining Cockburn Sound; and • Determine the influence of different Causeway configurations on the Mangles Bay region. This was undertaken by refining the previously used circulation model so that the circulation in the southern opening was calculated at 100 m intervals. The choice of grid cell size was based on the recommendations of Speedy (1994) and the need for the grid to resolve different Causeway configurations.

To provide further veracity of the current velocity predictions within the southern opening of Cockburn Sound and within Mangles Bay, a validation study was conducted. Two current meters were deployed, one on the eastern edge of the Southern Flats and the other in the southern entrance of Cockburn Sound, to the west of the Causeway. These measurements were used to provide an indication of the forcing mechanisms that were the principal drivers of exchange through the Causeway, and thereby indicate whether the circulation model was able to predict circulation patterns over a wider region.

3.2 MODEL FORCING The circulation model was driven by tidal elevations (and in most cases measured water levels), and wind measured at Garden Island for the summer simulations. In addition to the forcing used for the summer simulations, surface heat fluxes were also applied to the model to heat and cool the water column during autumn (i.e. variations in water density were included). Other circulation forcing mechanisms such as groundwater inflow and alongshore pressure gradients were not included in any of the simulations.

Model forcing was chosen for typical periods in summer and autumn and have been used for previous work (Cockburn, 2000). The summer simulation was undertaken during the 17th to the 30th of January 1998. The autumn simulation was undertaken during the 1st to the 16th of May 1997. The seasons and model forcing were chosen to aid ecological interpretation in the changes in circulation from the Causeway. Different forcing periods such as a winter storm, would need to be used if erosion through the northern Causeway opening was to be considered (this was outside the scope of this exercise).

3.3 MODEL GRID Two bathymetry grids were used for the circulation modelling study. The first bathymetry grid was developed for previous circulation modelling undertaken for proposed developments along the eastern margin of Cockburn Sound, and was able to resolve the circulation along the eastern margin at intervals of 100 m, but resolution for the southern entrance and Mangles Bay was coarser (between 200 m and 250 m intervals). This bathymetry grid was used to determine the effect of removing the Causeway on Cockburn Sound as a whole.

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To adequately resolve the different Causeway configurations a second bathymetry grid was developed to determine the circulation every 100 m through the southern opening and Mangles Bay. An image of the model bathymetry, and model grid, is show in Figure 3.1. The bathymetry was supplied by the Department of Planning and Infrastructure, 1994. The bathymetry datum is Low Water Mark Fremantle, and was adjusted to Mean Sea Level to model the circulation.

Figure 3.1 Bathymetry (water depth in metres) of the southern region of Cockburn Sound with each of the four Causeway configurations, with model grid overlay

3.4 CAUSEWAY CONFIGURATIONS MODELLED The EFDC model was used to simulate the circulation and flushing characteristics of Causeway configurations in both summer and autumn, as shown in Table 3.1.

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Table 3.1 Summary of simulations undertaken using the EFDC model

SIMULATION CAUSEWAY SEASONAL FORCING MODE* CONFIGURATION A Causeway Summer Barotropic B No Causeway Summer Barotropic C Causeway Autumn Baroclinic D No Causeway Autumn Baroclinic Validation Existing Causeway Summer (with tide and Barotropic wind only) Validation Existing Causeway Summer (with tide, surge Barotropic and wind) 1 No Causeway Summer Barotropic 2 Existing Causeway Summer Barotropic 3 Double size of openings Summer Barotropic 4 Additional openings Summer Barotropic * Barotropic mode incorporates changes in water level, baroclinic mode incorporates changes in both water level and water density

Simulations A through to D were undertaken using a model with the original grid (cell size within the southern entrance ~250 m square). This grid was refined and used for the validation simulations and simulations 1–4. The initial intention was to also simulate Causeway options 1–4 under autumn conditions using the refined grid, but despite repeated attempts it did not prove possible to stabilise the model in baroclinic mode (i.e. incorporating changes in both water level and water density). Instead—and in response to DEP requests to consider effects on other areas of the Sound as well as Mangles Bay—the Sound was divided into nine areas, and flushing characteristics in each of these areas were examined under summer and autumn conditions using the original grid.

The study brief also originally required simulations to be undertaken of the effect of dredging the existing channels at the Causeway openings. These simulations were not undertaken for the simple reason that the additional cross-sectional area provided by the dredging (through which flushing could occur) would be minimal, and any benefit would further lessened if the dredged channel depth was not extended west to the Sepia Depression (which would entail extensive dredging through Shoalwater Islands Marine Park).

3.5 THE INFLUENCE OF THE CAUSEWAY ON COCKBURN SOUND Results indicated that the presence/absence of Causeway altered circulation patterns in the region south of James Point and the northern end of Careening Bay to a greater extent than within the northern regions of Cockburn Sound. The altering of circulation patterns affects the exchange of water between Mangles Bay and the surrounding water bodies, with potential effects on water quality. The strength of velocity near the seabed also changed, with potential effects on sediment movement and/or seagrass disturbance due to the Causeway openings focusing flow through the Causeway while retarding water velocities at other areas.

3.5.1 The effect of the Causeway on the flushing of Cockburn Sound The flushing of Cockburn Sound was determined by releasing a patch of dye within the whole of Cockburn Sound and recording the concentration of dye within every grid cell. The rate of dye reduction caused by transport and mixing within Cockburn Sound under different configurations and seasons gives a relative measure of the effect of the Causeway on flushing of Cockburn Sound.

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Overall flushing of Cockburn Sound Graphs showing the changes in dye concentration in Cockburn Sound in summer and autumn, with and without the Causeway, are shown in Figure 3.2, and the e-folding times are given in Table 3.2. It is important to note that the model runs were undertaken for a period of 16 days (in both summer and autumn): e-folding times greater than 16 days were calculated by extrapolation of the line that describes the decline in dye concentration, but this could not be undertaken with much confidence beyond about double the modelled period. For this reason, e-folding times over 30 days are reported as ‘>30days’ rather than specific values.

Figure 3.2 Changes in dye concentration in Cockburn Sound in summer and autumn, with and without the Causeway Note: red line denotes changes in dye concentration with Causeway present, blue denotes changes in dye concentration with Causeway absent

Table 3.2 The effect of the Causeway on the flushing of Cockburn Sound (summer and autumn)

SEASON APPROXIMATE FLUSHING TIME* (days) CAUSEWAY NO CAUSEWAY Summer >30 days 20–22 days Autumn >30 days 24–28 days * e-folding time

The results indicated that the Causeway reduces overall flushing of Cockburn Sound by about 30–50% in both autumn and summer. The results in Table 3.2 are consistent with those of previous work undertaken with the EFDC model (Table 2.1) and the findings of earlier work undertaken with other models (Section 2.3).

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Flushing of various areas within Cockburn Sound As noted in Section 3.4, the Sound was divided into nine areas (Figure 3.3), and flushing in each of these areas examined under summer and autumn conditions, with and without the Causeway.

Figure 3.3 Areas of Cockburn Sound examined for flushing characteristics

Graphs showing the changes in dye concentration in each of the areas are shown in Figure 3.4 (summer) and Figure 3.5 (autumn), and the e-folding times are given in Table 3.3 (summer) and Table 3.4 (autumn). Changes in dye were examined over both the full depth of the water column, and for the bottom 10% of the water column. However, in summer no difference was found between full depth of the water column and the bottom 10% of the water column (as expected, given that the water column is vertically well mixed in summer), while in autumn the bottom layer of the deep basin areas was only slightly less well flushed than the entire water column. Therefore, only results for the full depth of the water column are shown in Figure 3.4, Figure 3.5, Table 3.3 and Table 3.4. It must also be pointed out that the boundaries of the areas were guided by bathymetry but the areas are of different volumes, and flushing characteristics would change if they were defined differently.

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Figure 3.4 Changes in dye concentration in various areas of Cockburn Sound in summer, with and without the Causeway Note: red line denotes changes in dye concentration with Causeway present, blue denotes changes in dye concentration with Causeway absent

Table 3.3 The effect of the Causeway on the flushing of various areas of Cockburn Sound in summer

AREA APPROXIMATE FLUSHING TIME* (days) CAUSEWAY NO CAUSEWAY 1. Mangles Bay shallows 16-17 9–10 2. Mangles Bay deep basin 25 8–9 3. Southern Flats 16–17 7–8 4. Southern Cockburn Sound basin 25 8–9 5. Central Cockburn Sound basin >30 19–20 6. Rockingham/Kwinana shallows 25 17–18 7. Eastern Garden Island shallows 30 16-17 8. Eastern Flats 30 30 9. Northern Cockburn Sound basin >30 >30 * e-folding time

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Figure 3.5 Changes in dye concentration in various areas of Cockburn Sound in autumn, with and without the Causeway Note: red line denotes changes in dye concentration with Causeway present, blue denotes changes in dye concentration with Causeway absent.

Table 3.4 The effect of the Causeway on the flushing of various areas of Cockburn Sound in autumn

AREA APPROXIMATE FLUSHING TIME* (days) CAUSEWAY NO CAUSEWAY 1. Mangles Bay shallows >30 25–30 2. Mangles Bay deep basin >30 25–30 3. Southern Flats 30 30 4. Southern Cockburn Sound basin >30 30 5. Central Cockburn Sound basin 30 30 6. Rockingham/Kwinana shallows >30 >30 7. Eastern Garden Island shallows 20–25 16 8. Eastern Flats 30 30 9. Northern Cockburn Sound basin 25–30 20 * e-folding time

In summer the effect of the Causeway is—as would be expected—most pronounced in areas close to the Causeway, and diminishes with distance from the Causeway. With the absence of the Causeway, the strength of the tidal signal also increases in the shallows of Mangles Bay (Area 1), but diminishes on Southern Flats (Area 3). In general terms, the flushing characteristics of the northern basin of Cockburn Sound and Eastern Flats are little affected by the Causeway; the flushing of the Mangles Bay deep basin, Southern Flats and the southern basin of Cockburn Sound improves by a factor of 2–3 fold with the absence of the Causeway; and improvements of around 50% are experienced in the areas between. The results for areas close to the Causeway are consistent with the results of Speedy (1994), discussed in Section 2.3. The only area close to the Causeway where a 2–3 fold improvement in flushing times

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is not found is the shallows of Mangles Bay (Area 1): this is because the coastline shelters this area from the south/south-west winds that predominate in summer, and so even without the Causeway, calm conditions prevail. In point of fact, the model also probably over-predicts improvements in flushing in this area because it does not use site-specific wind data, nor incorporate a benthic drag coefficient (the dense seagrass meadows in these shallow waters would act to baffle water movement).

In autumn, the results indicated that the Causeway has less influence on the flushing characteristics of the various areas of Cockburn Sound: this is not unexpected given that autumn is the calmest time of year, while the shallow bank across much of southern entrance of Cockburn Sound would impede baroclinic flow of denser bottom waters out of the Sound even with the absence of the Causeway.

3.6 VALIDATION Current meter measurements taken by the Centre for Water Research (CWR) have been analysed and discussed by Pattiaratchi (2002). Current meter measurements were collected seaward and adjacent of the Causeway opening (Figure 3.6), and results indicated that the flow through the southern entrance of Cockburn Sound was influenced by low frequency currents, diurnal tides and sea breeze winds, semi- diurnal tides and sea breeze winds and seiches. The influence of alongshore pressure gradients could not be measured and may also be responsible for driving exchange between Cockburn Sound and adjacent waters.

6 x 10 Current Meter Locations 6.434

6.433

6.432

6.431

6.43

6.429

3.75 3.76 3.77 3.78 3.79 3.8 3.81 5 x 10

Figure 3.6 Location of current meters used in the validation study Previous modelling studies have included the effects of wind, tides and pressure gradients as driving mechanism for exchange between Cockburn Sound and the surrounding waters. Identification of low frequency currents that drive exchange has

DALSE: CSMC: INFLUENCE ON THE GARDEN ISLAND CAUSEWAY 25

not been explicitly identified or modelled previously. Low frequency currents were identified as having a period of 5 days and most likely driven by the passage of weather systems (principally change in air pressure). The energy of the low frequency currents was equivalent to those attributable to tides and wind and therefore was included in the model validation process.

3.6.1 Low frequency forcing Two model simulations were undertaken for the validation to determine the influence of the low frequency forcing identified from the field data. Water levels were imposed on the open boundaries comprised of the tidal response only for the first simulation, and both the tidal response and the low frequency water level response (assumed to drive the low frequency currents) for the second simulation. The open boundaries were forced with time series of water level elevations. On the western open boundaries each open boundary point was specified with a time series of surface elevation determined from an interpolation of the tidal phases and amplitudes between Warnbro Sound and Fremantle.

Water level measurements at Penguin Island (CWR), Mangles Bay (Department of Planning and Infrastructure, and CWR), Stirling Channel (Fremantle Port Authority) and Fremantle (Fremantle Port Authority) were compared and found to have a similar amount of energy at the 5 day frequency. Visual inspection of the low pass filtered water level for each of the water level measurement sites showed a similar response of the low frequency surface elevation at all sites.

As the low frequency time series of the water level at each site was similar the low frequency surface elevation (estimated from data collected at Fremantle) was summed to the tidal forcing for the first validation run. All data used on the boundary conditions was benchmarked to Australian Height Datum and referenced within the model to Mean Sea Level.

A full comparison between the two validation runs is presented in Figure 3.7, and a more detailed comparison (of Day 26) is presented in Figure 3.8. These two figures show velocities are well represented in general terms for each validation run. Comparison between modelled data and field data showed that velocity magnitudes were similar in range and direction.

A quantitative comparison between validation simulations and field data was undertaken by spectral analysis. Spectral analysis was undertaken to determine whether the tidal amplitudes and phases only or tidal amplitudes and phases with tidal residuals were the preferred open boundary forcing mechanism. Spectral analysis plots in Figure 3.9 showed that the addition of the tidal residuals provided a better fit to the field data than forcing the open boundaries with tidal constituents only. A better fit using the tidal residuals is found specifically with low frequency (> 1day) forcing.

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Southern Channel 30 Model: Tide 20 Model: Tide + Surge Field 10

-10

-20 20 22 24 26 28 30 East-West Velocity (+E Velocity cm/s) East-West Year Day (2002)

30 Model: Tide 20 Model: Tide + Surge Field 10

-10

-20 20 22 24 26 28 30

North-South Velocity (+N cm/s) Year Day (2002)

Figure 3.7 Comparison of modelled and field data in the southern entrance of Cockburn Sound

Southern Channel 30 Model: Tide Model: Tide + Surge 20 Field 10

-10 26 26.2 26.4 26.6 26.8 27 Velocity (+EEast-West cm/s) Year Day (2002)

30 Model: Tide Model: Tide + Surge 20 Field 10

-10 26 26.2 26.4 26.6 26.8 27

Velocity (+NNorth-South cm/s) Year Day (2002)

Figure 3.8 More detailed comparison of modelled and field data in the southern entrance of Cockburn Sound

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6 Southern Channel: East-West Currents 10 Model: Tide Model: Tide + Surge /Hz) 2 4 Field 10

2 10

Density ((cm/s) 0 10 0 2 4 Frequency (Hz) -4 x 10 6 Southern Channel: North-South Currents 10 Model: Tide Model: Tide + Surge /Hz) 2 4 Field 10

2 10

Density ((cm/s) 0 10 0 2 4 -4 Frequency (Hz) x 10

Figure 3.9 Spectral analysis of modelled and field data for the current meter in the southern entrance of Cockburn Sound The current meter on the eastern edge of the Southern Flats was moored right on the edge between Southern Flats and the deep basin of Cockburn Sound. The water depth changes rapidly within 100 m of the site and the model grid was unable to resolve the depth change adequately for comparison between the field data and the current meter. The data was measured at a depth of 8–9 m while the depth of the grid cell (represents average bathymetry of a 100 m2 region) was 13–14 m. As a result, a comparative analysis for this site was not undertaken. Validation of the data collected at the eastern edge of the Southern Flats could be undertaken if the grid was refined to resolve the large depth change to the deep basin. The cell size would need to be refined to at least less than 50 m, but this level of grid resolution was beyond the scope of this study.

3.7 THE INFLUENCE OF THE CAUSEWAY ON THE MANGLES BAY REGION Changes in circulation patterns and flushing due to the absence of the Causeway may alter water quality within the Mangles Bay region. The distribution of seagrass may also alter due to effects on current velocity, especially in regions close to openings in the Causeway. The output of the numerical circulation model was therefore focused on these two areas of interest. As noted in Section 3.4, this modelling exercise was undertaken for summer conditions using the refined grid, and examined four Causeway configurations: no Causeway; existing Causeway; double the width of existing openings; and additional openings in the Causeway.

3.7.1 Flushing Flushing was determined by releasing a patch of dye within Mangles Bay and recording the concentration at every grid cell in the domain to determine how rapidly dye concentration reduced due to transport and mixing of the dye between Mangles

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Bay and surrounding waters. The initial dye distribution is shown in Figure 3.10. A snapshot of the dye distribution for each of the Causeway configurations several days later is shown in Figure 3.11.

Figure 3.10 Initial dye distribution used to investigate the circulation within Mangles Bay. Dye concentration varies between 0 (blue) and 10 (red)

In Figure 3.11 the blue colour represents water from west of the Causeway or to the north of James Point. The red dye shows the water initially present at the start of the simulation within Mangles Bay. The snapshot of the dye distribution several days later shows the movement of dye is readily transported along the shallow regions of Cockburn Sound where the water circulation is driven by wind along the eastern and western margins of Cockburn Sound and by the tide, wind and low frequency forcing through the southern channel.

Animations of the dye movement show that the dye rapidly moves in and out through the openings of the Causeway. Modelled velocities within the southern entrance were up to a maximum of 50-60 cm/s. The majority of dye exchange is limited by the western boundary of the southern opening. The improved flushing in the absence of the Causeway is clearly apparent, with the ‘Doubled openings’ and ‘Additional openings’ configurations producing a mixture of improvements in most areas but apparent decreases in flushing in some areas (especially for the deep basin in Mangles Bay with the ‘Additional openings’ configuration).

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Figure 3.11 Instantaneous dye concentration distribution generated by circulation transporting and mixing the initial dye distribution on the 26th of January 17:30, 1998

3.7.2 Bottom shear stress The presence of the Causeway was found to produce regions of high ‘near-bottom’ velocity (and therefore high bottom shear stress) through the trestle bridges at the southern entrance of Cockburn Sound, due to the constricted flow into and out of the Sound. The velocities are high in this region due to the shallow water depth (less than 3 m deep) and the comparatively narrow width of the openings.

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The bottom stress through the area of the southern entrance was calculated to determine the force per unit area exerted upon the seabed. Regions of high stress indicate that velocities near the sea-bed are high and may cause movement of sediment and/or seagrass loss. This is discussed more fully in Section 5.

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4. FIELD SURVEY

4.1 INTRODUCTION A field survey was conducted on the 8th of February 2002 to determine the physical characteristics of water in Mangles Bay. There has been little information collected in this region historically, especially with respect to dissolved oxygen—a factor that can affect nutrient cycling processes and hence nutrient-related water quality (see Section 5.1). Measurements of dissolved oxygen were therefore considered an important parameter to measure in the area adjacent to the trestle bridge opening.

The shallow region of Mangles Bay is typically sheltered during summer from persistent easterly and south-westerly winds during summer. Wind speeds measured at the Garden Island meteorological station ranged between 4 and 7 m/s and were east to south-easterly for the duration of the survey. The field survey was undertaken under calm conditions due to the sheltered position of Mangles Bay. Although not measured at Mangles Bay, the calm conditions experienced indicated that wind speeds were appreciably lower than those measured in the more exposed position at Garden Island.

4.2 HYDODYNAMIC INTERPRETATION Four transects were undertaken between 6 am and 12 pm over a distance of 1600 m (see Figure 4.1). Vertical profiles of salinity, temperature and dissolved oxygen were taken at fixed points. Contour plots of the results are shown in Figure 4.2 through to Figure 4.4.

Salinity remained relatively unchanged throughout the survey (Figure 4.2). Higher salinity water was found in the region between the coastline and the Causeway compared to the deeper regions of Mangles Bay. Presumably, this is due to the limited exchange between this region and the rest of Cockburn Sound. In addition to the limited exchange of this region, the shallowness of the water column means that greater evaporation and hence higher salinity will occur here.

Figure 4.3 and Figure 4.4 show part of the diurnal cycle of dissolved oxygen and temperature. In shallow regions the water column responded more readily than deeper regions to atmospheric forcing. Thus, the water column in the shallow region was cooler in the morning, reflecting the loss of heat during the night (Figure 4.3, top panel). During the day, the temperature over the shallow region increased at a greater rate than the deeper water, as solar radiation reaching the water surface has to heat a greater depth of water. By midday the difference in temperature between the shallow and the deep portions of Mangles Bay had decreased, and presumably, by early evening the shallow region may be even warmer than the deeper regions of Mangles Bay (Figure 4.3, bottom panel).

For dissolved oxygen levels, these were lowest in the morning in the shallows (Figure 4.4, top panel). This would have been due to respiration of seagrass and algae and sediment oxygen demand during the night. During the day seagrass and algae produce oxygen and so dissolved oxygen levels are elevated beyond saturation (Figure 4.4, bottom panel) through active vertical mixing and transfer of oxygen across the air-sea interface. Daily fluctuations in oxygen levels were more pronounced in the shallow regions containing seagrass than in the deeper unvegetated areas: oxygen levels in the former being both lower in the early morning and higher in the afternoon.

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The field survey results indicated that over the day of sampling, hydrodynamically, the shallow regions of Mangles Bay, particularly in close proximity to the Causeway, had limited exchange with the waters of the Cockburn Sound basin. Characteristics of these waters were primarily due to atmospheric cooling, heating, vertical mixing and low wind speeds during the day of sampling. Examination of the water characteristics in the deeper portion of Mangles Bay show these waters respond more slowly to atmospheric forcing and do not change their characteristics as rapidly.

Figure 4.1 Transect location

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Figure 4.2 Salinity profiles obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon Note: The profiles show a vertical cross-section of Mangles Bay from the shoreline (on the right) to the deep basin (on the left). The characteristics of the water at different depths are shown using different colours.

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Figure 4.3 Temperature (degrees Celsius) obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon Note: The profiles show a vertical cross-section of Mangles Bay from the shoreline (on the right) to the deep basin (on the left). The characteristics of the water at different depths are shown using different colours.

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Figure 4.4 Dissolved oxygen (% saturation) obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon Note: The profiles show a vertical cross-section of Mangles Bay from the shoreline (on the right) to the deep basin (on the left). The characteristics of the water at different depths are shown using different colours.

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5. THE EFFECTS OF VARIOUS CAUSEWAY CONFIGURATIONS ON ECOLOGY, COASTAL PROCESSES AND HUMAN USES IN COCKBURN SOUND

5.1 ECOLOGY The specific aspects of ecology this study was asked to address included algal blooms (phytoplankton in the water), oxygenation of the water, water clarity, nutrient loads and seagrass health. The first four aspects are discussed in Section 5.1.1 under the general heading of water quality. Predicted effects on seagrass health are discussed in Section 5.1.2.

It is important to precede the discussion on ecology by emphasising that assessment of the effects of various Causeway configurations is based on circulation modelling results. While the validation of the model provides confidence that key hydrodynamic processes are adequately represented, modelling results should be viewed in a comparative rather than definitive sense. It is not, for example, possible to say that ‘chlorophyll levels will decline by 50% with Causeway Configuration X’. The accuracy of any model depends on both the model itself, and the data used in the model (e.g. wind, tide, long-term changes in water levels). Observations made during the field survey described in Section 4 indicated that future modelling would particularly benefit by wind data that better represent the Mangles Bay region.

5.1.1 Water quality Different Causeway configurations can potentially affect water quality in Cockburn Sound via several processes, as follows:

• Improved flushing, resulting in: • More rapid dilution of existing nutrient inputs (pipeline discharges, stormwater runoff, groundwater discharge, atmospheric deposition and sediment nutrient release), and more rapid loss of nutrients from Cockburn Sound. Based on the assumption that nutrient availability is limiting phytoplankton growth, both of these factors will lead to decreased phytoplankton growth; • More rapid dilution—and more rapid loss from Cockburn Sound—of dissolved and particulate matter in the water column (including phytoplankton), leading to improved water clarity; • Improved oxygen levels in the water column, particularly near the seabed during calm conditions; • Changes to sediment nutrient cycling processes (due to increased supply of oxygen to the sediments via the water column), and therefore changes in sediment nutrient release (note that oxygen-related changes in sediment release of nitrogen are more to do with the form in which nitrogen is released than the total amount released, except in extreme cases); and • A decrease in the amount of organic matter that settles out on the sediments, and therefore a decrease in the amount of sediment nutrients that can subsequently be released into the water column to fuel phytoplankton growth (again, this is based on the assumption that nutrient availability is limiting phytoplankton growth). • Altered directions of water movement (horizontal and vertical) can affect the movement and dilution of existing nutrient inputs within an area.

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The first four of the above effects would be relatively ‘immediate’ in that any improvements should be quickly seen. The fifth of the above factors (a decrease in the amount of organic matter that settles out on the sediments) is not an effect that would be apparent in modelling results, as the ecosystem would take some time (about a year) for a new equilibrium to become established (between settlement of organic matter to the sediments and re-release of sediment nutrients to the water column).

The sixth of the above factors (altered direction of water movement) is unlikely to have a significant effect on water quality, as modelling of different Causeway configurations produced no major changes in current directions near the shore, where pipeline, stormwater and groundwater discharges of nutrients occur.

The fourth effect of the above effects—potential impact of changes in sediment nutrient processes due to improved oxygen levels—also needs to be placed into some perspective. This is briefly discussed below.

Organic matter that settles to the sediments (plankton, detached seagrass and seaweed, faeces of marine fauna) decomposes, and the nitrogen present is released as ammonia. In the presence of oxygen, the ammonia can be converted to nitrate. Both ammonia and nitrate can be used by phytoplankton (and other marine plants), although ammonia is the preferred source by many species of phytoplankton. Nitrate can also undergo a process called ‘denitrification’, where it is converted to nitrogen gas, and can be lost to the atmosphere. Denitrification can only proceed in the absence of oxygen. Thus, for denitrification to proceed, oxygen must be both present (to convert ammonia to nitrate) and absent (to convert nitrate to nitrogen gas)! This seeming contradiction in oxygen requirements is met in most sediments as their oxygen levels vary between day and night, and with sediment depth.

The proportion of nitrogen in sediment organic matter that is returned to the water column for use by phytoplankton generally depends of the amount of nitrogen lost via denitrification. Denitrification usually results in 10–50% of the nitrogen in sediment organic matter being lost as nitrogen gas (international literature indicates denitrification losses in Cockburn Sound are probably at the lower end of this range), with the remainder released into the water column as nitrate and ammonium. If conditions of low oxygen are maintained in sediments for extended periods (i.e. weeks), denitrification rates may decrease (because ammonia isn’t converted to nitrate), and so the proportion of nitrogen released from sediment organic matter may increase—and furthermore will be largely in the form of ammonia.

Modelling by the DEP (1996) indicated that extended periods of calm may lead to Mangles Bay deep basin sediments using up to 70% of the oxygen in the bottom 1 m of water after about 10 days, with such conditions most likely to occur in autumn. Data loggers deployed in the bottom waters of the deep basin in Mangles Bay during extended periods of calm in March 1993 recorded oxygen levels fluctuating between 1.3 mg/L and 5.2 mg/L over six days, but never the complete absence of oxygen (Bastyan et al., 1994), and similar findings have been made by the DEP (1996). For most of the year, however, bottom waters remain well oxygenated. It therefore appears that—even in the deep basin in Mangles Bay—sediment oxygen demand is rarely outstripped, and so the conversion of ammonia to nitrate (and some subsequent denitrification) can occur. The organisms that convert ammonia to nitrate are also slow-growing, and therefore slow to respond to changes in conditions: as bottom waters in Cockburn Sound are generally well oxygenated, it is probable that populations fluctuate little (although the depth at which they occur in sediments may

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change). Significant changes to denitrification rates due to improved flushing should therefore also be infrequent.

In overview, different Causeway configurations could potentially improve water quality in the short-term largely through more rapid dilution and greater exchange (with waters from outside Cockburn Sound), while in the longer-term there could be further improvements (albeit of lesser degree) in water quality due to reduced settlement of organic matter to the sediments. Improved oxygenation of bottom waters, and altered directions of water movement, are considered of lesser significance in affecting nutrient-related water quality. In Section 3.5.1 it was also noted that removal of the Causeway had only a small influence on the flushing characteristics of basin waters during calm conditions in autumn, presumably because the shallow sill across much of the southern entrance would prevent movement of dense, deep waters in the basin out of Cockburn Sound, even if the Causeway was absent. It can be inferred that extended periods of calm conditions in autumn are still likely to cause conditions of lower oxygen in deep basin waters, even with the Causeway removed.

Summer water quality The modelling results presented in Section 3.5 clearly indicated that the improvement in flushing due to removal of the Causeway was greater in summer than in autumn. Potential changes in summer water quality (water clarity, oxygen levels and phytoplankton levels) due to different Causeway configurations were gauged using modelled results on the amount of tracer dye remaining in various areas after a set time (see Sections 3.5 and 3.7).

Appreciable reductions in flushing times do not necessarily translate into appreciable improvements in water quality if chlorophyll levels are already low in an area. For this reason, the most recent chlorophyll data for the nine areas modelled in Section 3.5 are provided in Table 5.1 as context for predictions about the effects of different Causeway configurations. The data are from the Cockburn Sound routine water quality monitoring programme for 15 sites monitored weekly in summer 2001/2002.

Table 5.1 Median chlorophyll values in various areas of Cockburn Sound, summer 2001/2002

AREA SUMMER CHLOROPHYLL MEDIAN Outside northern and southern entrances 0.4 µg/L to Cockburn Sound (represents ‘background’ water entering the Sound) 1. Mangles Bay shallows Unknown, but likely to be <<1.8 µg/L* 2. Mangles Bay deep basin 1.8 µg/L 3. Southern Flats 0.8 µg/L 4. Southern Cockburn Sound basin 0.8–0.9 µg/L 5. Central Cockburn Sound basin 0.9 µg/L 6. Rockingham/Kwinana shallows 1.7–2.3 µg/L 7. Eastern Garden Island shallows 0.8–1.3 µg/L 8. Eastern Flats 1.2–2.1µg/L 9. Northern Cockburn Sound basin 0.7–0.8 µg/L * Seagrasses—and more importantly their associated epiphytes—would out-compete phytoplankton for nutrients in this area

In Section 3.5 it was noted that summer flushing in areas 2, 3 and 4 could potentially improve by 2–3 fold with removal of the Causeway. This is unlikely to produce a major difference in water quality in Areas 3 (Southern Flats) and 4 (southern Cockburn Sound basin), as chlorophyll levels in these areas are already relatively low (similar to waters at the north-western end of Cockburn Sound). In Area 2 (Mangles Bay deep basin) considerable improvements in chlorophyll levels and light attenuation could occur with removal of the Causeway, and lesser improvements

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could occur in Areas 1 (Mangles Bay shallows) and 6 (Rockingham/Kwinana shallows): little change is expected elsewhere.

On the basis of direct comparison to median chlorophyll levels in Area 7 (Eastern Garden Island shallows, where healthy seagrass occurs) (Table 5.1), water quality that is potentially poor enough to adversely affect seagrass meadows in deeper waters presently occurs in Area 2 (Mangles Bay deep basin, Area 6 (Rockingham/Kwinana shallows) and parts of Area 8 (Eastern Flats). Modelling results indicate that removal of the Causeway is unlikely to significantly affect water quality in the Eastern Flats area, but is likely to cause improvements in water quality sufficient to improve the health of seagrass meadows at their depth limit in Mangles Bay, and for that depth limit to be extended. Water quality may also improve sufficiently to potentially allow seagrasses to grow in the deeper parts of the Rockingham/Kwinana shallows (excluding areas affected by shipping-related turbidity in the area between the CBH jetty and James Point, and the presence of aquaculture lines). Although removal of the Causeway should result in water quality suitable for seagrasses to grow in these areas, it cannot be stated with certainty that they would re-establish, or how long it would take.

The above interpretation of the changes in summer water quality that might result with the removal of the Causeway provides context for more detailed examination of changes in Mangles Bay with different types of Causeway configurations. Graphs of changes in tracer dye can be produced for every grid cell in the model, but results for four sites of particular interest are shown in Figure 5.1. The four sites are:

• Mangles Bay shallows, a site in 3.2 m of water where routine monitoring of seagrass health is carried out every summer by Edith Cowan University; • Mangles Bay deep basin; • Southern Cockburn Sound basin ; and • ‘Nearshore Mangles Bay’, extremely shallow water near the Causeway.

The graphs in Figure 5.1 show changes in tracer dye concentration for all vertical layers of the model. For some sites there is little difference in between surface and bottom waters, but in others (e.g. Mangles Bay deep basin) the thickness of the coloured line—and in some cases splitting of the coloured line into multiple strands—indicates that surface layers are losing dye more rapidly than deeper layers. Dye concentrations also fluctuate as dye moves in and out of an area with the tide (particularly noticeable at the nearshore Mangles Bay site), and in some cases due to incoming dye from adjacent areas.

Predicted effects on summer water quality (water clarity and phytoplankton levels only: oxygen levels are already generally good in summer) at these four sites are summarised in Table 5.2. In relative terms, doubling the size of the existing channel openings produced the least improvement in water quality and removal of the Causeway the most, with the ‘additional openings’ configuration producing an effect intermediate between these two. There was, however, an exception to this at the Mangles Bay deep basin site: the ‘additional openings’ configuration acted to reduce flushing, probably due to the interaction of currents through the multiple openings producing a gyre in this region. This effect may change with different sizes and/or placements of the additional openings.

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Mangles Bay - Shallow Mangles Bay - Deep 10

4 Causeway No Causeway 2 Additional Openings

Dye Concentration Widened Openings 0

Southern Cockburn SOund Basin Nearshore Mangles Bay 10

2 Dye Concentration Dye 0 21 22 23 24 25 26 27 28 2930 31 21 22 23 24 25 26 27 28 2930 31 Year Day Year Day

Figure 5.1 Changes in dye concentration with different Causeway configurations at selected sites in summer (refined grid)

Table 5.2 Relative impact of various Causeway configurations on summer water quality at the southern end of Cockburn Sound

CAUSEWAY SITE CONFIGURATION Mangles Bay, Mangles Bay, Mangles Bay, Cockburn Sound inner seagrass site deep basin southern basin Doubling of existing o 9 9 o openings Additional Causeway o 9 8 o openings Removal of 9 99 99 o Causeway 9= small improvement, 99= significant improvement o = little change 8= small deterioration

As noted earlier, some additional (albeit lesser) improvements in water quality may occur in the longer-term due to reduced settlement of organic matter to the sediments: this would apply primarily to the Mangles Bay deep basin.

5.1.2 Seagrass Improvements in water quality can directly benefit seagrass health via reduced phytoplankton levels, improved water clarity and less growth of epiphytes on seagrass (due to reduced nutrient supply)—if these are the factors limiting seagrass growth in an area. These factors will combine to reduce shading of seagrass meadows, and so will be of most benefit to seagrass meadows in deeper waters where light levels are lower.

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The waters of Mangles Bay adjacent to the Causeway are very shallow and dominated by extensive stands of Posidonia seagrass that are heavily epiphytised. In some areas the seagrass leaves are emergent at low tide, and subject to desiccation stress. In its present state, there is little opportunity for phytoplankton blooms in this area (due to competition for nutrients by seagrass epiphytes and microscopic algae on and in the sediments). Increased current speeds in this area could, however, reduce the loads of epiphytes on seagrass and lessen accumulations of detritus within the meadows, and so improve seagrass health. However, increased current speeds can have both beneficial and detrimental effects, as discussed below.

For seagrass meadows, current speed is a matter of finding the right balance, with extremely low currents almost as bad for seagrass meadows (in some situations) as currents that are too fast. Changes in current speed can cause the following effects:

• Changes in the degree of erosion or ‘scour’ on existing seagrasses (either via the currents themselves, or sand mobilised by the currents); • Changes in seedling re-establishment (both the settling out of seedlings, and their ability to remain anchored in the area); • Changes in the amount of ‘drag’ on—and therefore erosion of—seagrass epiphytes (epiphytes can grow to a larger size in calmer waters); and • Changes in the degree of accumulation of organic matter in and around seagrass meadows. The seagrass site in Mangles Bay routinely monitored by Edith Cowan University is an example of a calm area where a layer of loose organic matter has accumulated around the seagrass leaves to the extent that that the meadows are shaded to a considerable degree.

Between 1954 and 1978, Cambridge (1979) estimated that approximately 260 ha of seagrass were lost in the Mangles Bay/Southern Flats area between Parkin Point (where the Causeway joins Garden Island) and Rockingham Beach (note that this includes losses to the west of the Causeway), and 440 ha between Rockingham Beach and James Point. Loss of seagrass in these areas took place mainly between 1967 and 1973. Nutrient-related water quality in the Sound was at its worst in the late 1970s, and the pattern of loss documented by Cambridge (1979) indicates that most losses in the area from Mangles Bay to James Point occurred before construction of the Causeway and were linked to poor water quality. Further minor losses in the Mangles Bay area have occurred since the late 1970s, but also some minor gains, particularly in recent years. In Mangles Bay, seagrass meadows near their depth limit appear stressed, with this stress appearing related to water quality and/or accumulation of organic material around the seagrass plants. In the shallows of Mangles Bay, seagrass distribution has remained relatively stable since 1967, other than losses in the area directly opposite the southern Causeway opening, and beneath boat moorings. Losses on Southern Flat were coincident with the opening of the Causeway, initially in areas directly opposite the Causeway openings, and then subsequently on the eastern edge of the Flats. There has been an ongoing pattern of minor gains and losses on Southern Flats since the late 1970s, with the gains predominating in recent years. The seagrass meadows on Southern Flats are in shallow water, appear to be healthy, and the pattern of gains and losses unrelated to water quality. The inference drawn from available data is that seagrass losses on Southern Flats are due to altered current speeds and/or sediment movement, rather than water quality.

Current velocities in the Mangles Bay region during a spring tide in summer for different Causeway configurations are compared to the present Causeway

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configuration in Figure 5.2 , Figure 5.3 and Figure 5.4. It can be seen that widening the existing openings or adding more openings lessens current velocities in the region of the existing northern channel, but increases current velocities near the southern and/or additional openings. The ‘additional openings’ configuration modelled resulted in a lessening of current velocities in the region of the existing northern channel to a similar degree as the complete removal of the Causeway. With the ‘no Causeway’ configuration, however, current velocities are much more uniform across the whole of the southern entrance.

Figure 5.2 Current velocity vectors during a spring tide in summer in the Mangles Bay region: comparison between present Causeway, and doubling of existing Causeway openings

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Figure 5.3 Current velocity vectors during a spring tide in summer in the Mangles Bay region: Comparison between present Causeway, and additional Causeway openings

Figure 5.4 Current velocity vectors during a spring tide in summer in the Mangles Bay region: Comparison between present Causeway configuration, and with the Causeway removed

Bottom shear stress is a measure of the force per unit area exerted on the seabed by currents close to the seabed, and is proportional to the current velocity, squared. Bottom shear stress in the Mangles Bay region during a typical incoming and

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outgoing spring tide in summer, without the Causeway, is shown in Figure 5.5 and Figure 5.6. This was used as a basis for comparison for changes in bottom shear stress with various Causeway configurations. Changes in bottom shear stress relative to the ‘No Causeway’ configuration are shown in Figure 5.7 and Figure 5.8. As can be seen in Figure 5.7 and Figure 5.8, areas where the existing Causeway configuration has caused two to four-fold increases in bottom shear stress correspond quite well to the areas of seagrass lost immediately following the construction of the Causeway. Increases of this magnitude are also apparent near the region of the existing southern channel under the ‘Doubled opening’ configuration, and near the additional openings of the ‘Additional openings’ configuration, although both these configurations result in lessening of current velocities in the region of the existing northern channel—possibly to levels that may let seagrass re-establish, but this cannot be stated with any certainty.

Figure 5.5 Bottom shear stress during an incoming spring tide in summer (kg/cm/s2), in the absence of the Causeway

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Figure 5.6 Bottom shear stress during an outgoing spring tide in summer (kg/cm/s2), in the absence of the Causeway

Figure 5.7 Changes in bottom shear stress (relative to the ‘No Causeway’ configuration) caused by various Causeway configurations: incoming spring tide, summer simulation

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Figure 5.8 Changes in bottom shear stress (relative to the ‘No Causeway configuration) caused by various Causeway configurations: outgoing spring tide, summer simulation

The potential effects of various Causeway configurations on seagrass were examined in some detail for four sites of particular interest:

• ‘Mangles Bay, inner’, an extremely shallow site with seagrass in south-west corner of Mangles Bay near the Causeway; • ‘Mangles Bay, seagrass site’, a site in 3.2 m of water where routine monitoring of seagrass health is carried out every summer by Edith Cowan University; • ‘Northern opening’, an area where seagrass loss has occurred that is attributable to increased current velocities with the present Causeway configuration; and • ‘Southern Flats’, an area of existing seagrass meadows where the ‘Doubled opening’ and ‘Additional opening’ configurations will result in increased current velocities.

The potential effects of changes in water quality and changes in current velocity on seagrasses at these four sites are summarised in Figure 5.3. The two Mangles Bay sites presently experience very low current velocities, and the increases in current velocities with various Causeway configurations should be beneficial. The decrease in current velocities with various Causeway configurations experienced at the Northern opening site may produce conditions that seagrasses can tolerate (on both sides of the Causeway). The Southern Flats site may experience seagrass loss with the ‘doubled openings’ and additional openings’ configurations.

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Table 5.3 Relative impact of various Causeway configurations on seagrasses. Predictions on shear stress based on peak current velocities during a spring tide in summer

CAUSEWAY SITE CONFIGURATION Mangles Bay, Mangles Bay, Northern Southern Flats inner seagrass site opening Doubling of existing openings: - Water quality o 9 o o - Bottom shear stress 9 9 9(?) 88 Additional Causeway openings: - Water quality o 9 o o - Bottom shear stress 99 9 99 88 Removal of Causeway: - Water quality 9 99 o o - Bottom shear stress 99 99 99 99 9= small improvement, 99= improvement sufficient to benefit seagrass o = little change 8= small deterioration, 88= significant deterioration

Potential gains in seagrass meadows with different Causeway configurations As noted in Section 5.1.1, removal of the Causeway should result in improvements in water quality that improve the health of seagrass meadows at their depth limit in the Mangles Bay area, and theoretically allow that depth limit to be extended. The estimated area of potential seagrass gain is around 50 ha. Water quality may also improve sufficiently to theoretically allow seagrasses to grow in waters 3–10 m deep in the Rockingham/Kwinana area: the estimated area of potential seagrass gain (in areas not affected by shipping-related turbidity and the presence of aquaculture lines) is around 50 ha. Changes in current speeds and/or sediment movement due to removal of the Causeway may potentially allow about 200 ha of seagrass to re- establish on Southern Flats (if areas west of the Causeway are included).

The improvements in water quality from the ‘doubled openings’ and ‘additional openings’ configurations are lesser than with the removal of the Causeway, and so potential ‘water quality-related’ gains in seagrass may be less than the total of 100 ha estimated above. The exact ‘threshold’ water quality required to theoretically allow seagrasses to re-establish in waters 3–10 m deep in the Mangles Bay/Rockingham/Kwinana area is unknown, and may be achievable with modifications of the Causeway (rather than complete removal). However, it cannot be stated with any certainty that the ‘doubled openings’ and additional openings’ configurations would decrease current speeds in the north-eastern area of Southern Flats sufficiently to theoretically allow seagrasses to regrow, and there is the possibility that increased current speeds may cause some losses opposite the widened/new openings further south. Therefore, no estimates can be made for ‘current speed-related’ gains in seagrass with the ‘doubled openings’ and additional openings’ configurations.

A final point that must be emphasised in this section is that the above discussion is about potential gains in seagrass due to the creation of suitable conditions. As previously noted in Section 5.1.1, a change to conditions that are theoretically suitable for seagrass re-establishment does not mean that they will return. Water quality in the Rockingham/Kwinana shallows is similar to the Mangles Bay area and therefore theoretically suitable for seagrasses to re-establish in waters less than 3 m deep, yet this has not happened. Similarly, a patch of seagrass in the middle of healthy meadows at the northeast end of Garden Island that was lost more than ten years ago (due to urchin grazing) has also failed to regrow. It is possible that

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revegetation efforts will be needed to achieve the seagrass regrowth theoretically possible due to improved conditions (noting that cost-effective techniques to re- establish seagrass meadows in Cockburn Sound have yet to be developed).

5.2 COASTAL PROCESSES The Causeway has two types of effect on coastal processes:

• Changes in wave energy arriving at shoreline. This effect is most evident at James Point, which prior to the construction of the Causeway was a shoreline feature in balance between longshore sediment transport to the south due to northwest swell waves, and longshore sediment transport to the north due to southwest swell waves. With the construction of the Causeway, southwest swell energy entering Cockburn Sound was lessened considerably, and James Point has since been realigning due to the predominance of longshore sediment transport to the south; and • Direct interruption of longshore sediment transport due to the presence of a physical structures at the shoreline (as seen in the erosion/accretion problems around Parkin Point on Garden Island and along the Rockingham beaches).

5.2.1 Effects due to changes in wave energy As noted in Section 2, the offshore wave climate of Perth is composed of both swell- wave energy (generated distantly) and wind-wave energy (generated locally). The prevailing direction of these wave energy components is distinctly seasonal. In summer swell-waves typically arrive from the south-southwest and wind-waves from the south; whereas in winter the swell waves typically arrive from the west- southwest and the wind-waves from the west (Lemm, 1996). The narrowness of the southern entrance to Cockburn Sound, the presence of submerged reefs and the broad shallow Southern Flats sand shoal all combine to cause a significant natural attenuation of offshore wave energy to inshore of Cockburn Sound.

Five years of wave data were obtained by the Commonwealth Department of Construction prior to the construction of the Garden Island Causeway (Department of Construction, 1977). Three wave-rider buoys were deployed: one at an offshore location approximately 10 km west of Garden Island in 37 m of water; one near Careening Bay; and one on Parmelia Bank. Unfortunately, instrument malfunction confounded the data collection, with an overall data return of only 33% from the deployment (Riedel and Trajer, 1978; Lemm, 1996). Analysis of the available data indicates a wave attenuation of approximately 85% from the offshore buoy to Careening Bay within Cockburn Sound (Hearn, 1991). Analysis of the offshore wave data indicated a 100 year return period significant wave height of 7.5 m and a 10 year return period significant wave height of 6.0 m, whereas within Cockburn Sound the 100 and 10 year return period significant wave heights were 1.3 m and 1.0 m, respectively (Hearn, 1991). The wave periods recorded offshore ranged from 5 to 12 s whereas within Cockburn Sound the wave period ranged from 2 to 6.5 s which indicates that the waves within Cockburn Sound are typically locally (wind) generated and fetch-limited in the lee of Garden Island (Hearn, 1991). Wave roses generated from wind data indicate a predominance of south to south-westerly wind- wave energy in Cockburn Sound in spring and summer, whereas during autumn and winter a greater proportion of westerly and north-westerly waves occurs.

The construction of the Causeway has resulted in further sheltering of the southern region of Cockburn Sound from wave energy; however, as noted above, this area was already well sheltered prior to Causeway construction. Wave ray analysis of

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offshore swells (Figure 5.9) and wind-waves (Figure 5.10) shows the refraction of waves through the southern opening of the Causeway: in both of these diagrams, the solid lines show wave rays with the Causeway in place, and the dotted lines are indicative lines showing further penetration of wave energy (based on visual interpretation) that would occur if the Causeway was removed. It must also be noted that that the analysis of wave refraction in this area is a particularly complex task (Hicks et al. 1973a). Visual interpretation of the data to the east of the Causeway (assuming the Causeway is removed) shows that under both south westerly and north westerly swell there would be an increase in swell-wave energy in the Mangles Bay area and an increase in the eastward longshore current at the shoreline in this area. It is likely that the anticipated increase in swell-wave energy (with the removal of the Causeway) in areas eastward of Mangles Bay would become relatively less with distance from the Causeway. The removal of the Causeway will also result in a slight increase in the wind-wave energy that is experienced on the northern beaches of Cape Peron (Figure 5.10).

5.2.2 Longshore sediment transport and shoreline change A net easterly sediment transport direction prevails on the northern beaches of Cape Peron, with an annual average sediment movement of 5,000 m3 (Boreham, pers. comm). This sediment is trapped on the western side of the breakwater of the Cape Peron boat ramp. This sediment is bypassed, as required, to the eastern side of the Causeway by the City of Rockingham (Boreham, pers. comm). Hence, the Causeway is not the main trap for this longshore sediment transport, and removal of the Causeway would not release this trapped sediment for continued movement eastward.

Examination of sediment trapping within Mangles Bay and the prevailing wave energy conditions suggests a net eastward sediment transport direction prevails between the Causeway and Palm Beach. Between Palm Beach and Kwinana Beach the net sediment transport direction is not clear and the net sediment transport is likely to be small and seasonal with northward transport in summer and southward movement in winter. Between Kwinana Beach and James Point it appears that a net southward sediment transport prevails. It has been suggested that James Point was formed as a salient in the lee of Garden Island from the interaction of waves arriving through the South Channel and the northern end of Garden Island (Silvester and Hsu, 1991). The construction of the Causeway reduced the wave energy from the south and hence a net southward sediment transport prevailed at James Point following construction (Silvester and Hsu, 1991). It is likely therefore, that the removal of the Causeway would result in the reestablishment of a balanced sediment transport regime at James Point.

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Figure 5.9 Refraction of (a) south west and (b) north west swell-waves through South Channel (Modified from FPA, 1972) Note: Wave rays shown as solid lines are conditions with the Causeway present. Wave rays shown to the east of the Causeway as dotted lines are indicative of conditions with removal of the Causeway, and have been visually interpreted.

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Figure 5.10 Refraction of north east wind-waves through Mangles Bay (Modified from FPA, 1972). Note: Wave rays shown as solid lines are conditions with the Causeway present. Wave rays shown to the west of the Causeway as dotted lines are indicative of conditions with removal of the Causeway, and have been visually interpreted.

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A beach survey programme undertaken prior to the construction of the Causeway suggested that the beaches in Cockburn Sound were in a state of seasonal equilibrium (Hicks et al., 1973b). Visual inspection of shoreline position plots of the Mangles Bay region from 1942 to 2000 provided by the Department for Planning and Infrastructure indicated that the beaches of Mangles Bay from the Causeway to the Fisher Street Jetty have eroded slightly (ca 20–30 m) since the construction of the Causeway—likely due to the interruption of littoral sediment transport. The area of Palm Beach (from Fisher Street Jetty to Patterson Road) has shown some accretion (ca. 30 m) since the construction of the Causeway and from Patterson Beach to Kwinana Beach the shoreline position has been relatively stable with slight accretion to the south of the CBH Jetty and slight erosion to the north of this jetty. The shoreline position of the beaches between the Kwinana wreck site and James Point has been considerably influenced by the construction of numerous shoreline features and it is difficult to distinguish the effect of the Causeway construction on these areas.

It is likely that the majority of the shoreline changes noted above will be reversed with the removal of the Causeway. However, as noted above, the littoral sediment transport in this area would still be trapped at the Cape Peron boat ramp and therefore in Mangles Bay it is likely that the removal of the Causeway will result in further shoreline erosion area due to the increase in wave energy that would be experienced in this area. It is also possible that the increased wave energy may cause a flattening of the beach profiles in the southern Cockburn Sound area which may result in sediment deposition on some of the nearshore seagrass meadows.

There has been far less study on effects of the Causeway on coastal processes on Garden Island, although interruption of littoral sediment transport has resulted in erosion of the beach at southern Careening Bay, within Cockburn Sound, and accretion of sand at Broun Bay to the west of the Causeway and around Parkin Point at the commencement of the Causeway. This effect would be reversed with removal of the Causeway.

A final point that needs to be made in this section is that all the changes in longshore sediment transport and shoreline profiles caused by the Causeway (and those that would occur with removal/modification of the Causeway) are relatively small, and can be—or already are—addressed by sand bypassing and other management measures.

5.3 HUMAN USES

5.3.1 Current uses The current use of the Causeway is for transport of personnel and goods between the mainland and the Garden Island naval base. The southern end of the Causeway also protects boat launching activities at the Cape Peron boat ramp from north-easterly, easterly and south-easterly winds. This boat ramp is intensively used by the public (estimated boat use over 13,000 boats/year; DAL, 2001) second only to the Woodman Point boat ramp (estimated boat use over 16,000 boats/year; DAL, 2001). Many recreational fishers and boaters using small craft utilise the sheltered conditions in the lee of the Causeway: fishing for squid and crabs is particularly popular.

As noted in Section 1, the Mangles Bay area adjacent to the Causeway is not as popular for swimming as beaches further east (it is very shallow and full of seagrass),

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but it is heavily used for boat moorings. This area also has private boat ramps (including Mangles Bay Fishing Club, and The Cruising Yacht Club), a dog beach, a jet-ski and water-ski area, and is popular for recreational fishing.

The Southern Flats area is popular for yachting and recreational fishing, and there is mussel aquaculture at the northern edge of the Flats at the ‘drop-off’ into deeper water. Aquaculture lines are at the northern and north-eastern ends of Southern Flats, with the former experiencing greater current velocities than the latter: the sites have only been used for a couple of years and so trends are hard to infer, but mussel growth rates do not appear to differ much between the two areas (although lines at the north-eastern sites tend to have more algal fouling) (Glenn Dibben, pers. comm.). There is also mussel aquaculture near the CBH Jetty (i.e. Kwinana Grain Terminal): this is a sheltered area with low current speeds and relatively high levels of chlorophyll, and aquaculture is confined to the top 5 m of the water column—unlike the Southern Flats sites where aquaculture extends much deeper (Glenn Dibben, pers. comm).

A preliminary assessment on the potential impacts of removal of the Causeway on human uses of the Mangles Bay region is as follows:

• Any removal/alteration of the Causeway is likely to cause considerable disruption to the operation of the naval base on Garden Island. The Causeway could not be removed without a new bridge being in place first. Preliminary calculations undertaken by the Department for Planning and Infrastructure (DPI) indicate that removal of Causeway rockfill material would cost approximately $5.5 million (assuming that the material could be disposed of within 10 km of the site), while construction of a bridge between the two existing trestle bridges, and to land, would cost about $120 million. Removing the rockfill material and bridge construction would also affect the environment (e.g. turbidity, scouring), and effects would need to be carefully considered; • The Cape Peron boat ramp is intensively used by recreational boaters. The portion of the Causeway that protects the Cape Peron boat ramp from north- easterly, easterly and south-easterly winds cannot be removed—unless some other structure is put in place to provide similar protection. The need for some structure to protect the boat ramp will inevitably mean some reduction in the flushing of the shallow waters of Mangles Bay; • The activities of small boat users that presently shelter in the lee of the Causeway may be restricted; • Conditions will be less sheltered in the water-ski and jet-ski areas immediately adjacent to the Causeway, and these activities may be restricted to some extent; • Recreational vessel moorings may be affected by increased exposure to swell- waves and wind-waves; • Some coastal structures have been built ‘post Causeway’, and may not be designed for ‘pre-Causeway’ swell-wave and wind-wave conditions. The CBH Jetty, for example, was built in two stages, the first being completed in 1969. and so built to withstand ‘pre-Causeway’ conditions, but the second was completed in 1977. If ‘under deck’ clearance in any jetty/wharf is too low, then waves contacting the underside of the deck may create uplift forces sufficient to damage the jetty structure. Groynes and walls can also be directly impacted by waves. The ability of such structures to withstand changed conditions would need to be investigated; • Some erosion of beaches in the Mangles Bay/Rockingham area is likely, and may necessitate sand bypassing;

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• The reestablishment of a balanced sediment transport regime at James Point may affect activities at the BP Refinery (e.g. silting up of the water intake pipe); • Changes in water quality are unlikely to affect beach use, yachting, jet-skiing, water-skiing and pleasure boating; • The considerable improvement in water quality in the Mangles Bay deep basin, and lesser improvements in Mangles Bay shallows and Rockingham/Kwinana shallows will be due to decreased phytoplankton production in these areas. This may have some effect on the fisheries, particularly if there is no compensating gain in seagrass meadows. It would be extremely difficult to predict the nature, degree and extent of any such effect on the fisheries; and • The changes in current velocity and water quality on Southern Flats are unlikely to adversely affect mussel aquaculture in this area. However, mussel aquaculture at the CBH jetty may be adversely affected by the significant decrease in phytoplankton production predicted to occur in this area, although any effects may be partly offset if the mussels can be grown at depths greater than 5 m.

5.3.2 Future uses The main future human use of the region considered was a marina in Mangles Bay, the concept of which has long been supported by local government and some State government departments as a potentially extremely valuable recreational and tourism asset for the Rockingham area. Part of the rationale for the marina is that boating and recreational pressure on Cockburn Sound is expected to almost double in the next 20 years, but opportunities for mooring pen space are extremely limited. For example, there are about 260 swing moorings on the southern shoreline of Mangles Bay, and these are unsatisfactory for many reasons: they are insufficient for demand; have caused significant loss of seagrass; and are unprotected from north-west storms.

A variety of designs for the Mangles Bay marina have been considered, with the most recent of these formally submitted for environmental assessment in October 1992. The Environmental Protection Authority concluded that the marina proposed in 1992 was environmentally unacceptable, mainly because of significant impacts on the remaining seagrass in the Mangles Bay area (direct loss of approx. 32 ha of seagrass, indirect affects due to water quality) and the ecological significance of preserving the small amount of seagrass that remains in Cockburn Sound. A series of revised configurations for a marina were subsequently considered by a Mangles Bay Steering Committee established by LandCorp, which presented a report to government in July 1998, recommending a canal-type marina constructed inland from the coast. The option proposed by the Steering Committee would result in an estimated loss of approximately 7 ha of seagrass, due to dredging of an access channel. The revised marina configuration has not been formally progressed further.

It is difficult to predict the impact of any alterations/modifications of the Causeway on any future marina without knowing the final design of the marina, and the management practices to be adopted. Due to increased residence times, water quality in any marina is generally less than in adjacent waters: the extent to which marina water will affect Mangles Bay water quality will depend on the size, shape, depth and design features of the marina, and may vary from negligible to measurable.

The removal of the Causeway is not expected to greatly improve flushing in the Mangles Bay shallows and, as noted in Section 5.3.1, the portion of the Causeway protecting the Cape Peron boat ramp (or some replacement structure) will need to

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stay, which will further lessen any flushing of this area. The ‘doubled opening’ and ‘additional opening’ configurations are expected to result in little improvement of water quality in the Mangles Bay shallows. The marina design will need to achieve sufficient dilution of marina water to avoid adverse environmental impacts in Mangles Bay, irrespective of the presence/absence of the Causeway. The direct seagrass losses involved in construction of a marina (e.g. due to channel dredging) may, however, be viewed as more acceptable if removal/modification of the Causeway improves water quality in some areas sufficiently to increase the likely success of any mitigation efforts carried out to offset the seagrass losses.

The marina will need groynes at the marina entrance, as well as a dredged channel leading to the entrance. Alterations in swell-wave and wind-wave conditions and longshore sediment transport with removal of the Causeway may necessitate alteration of groyne design and channel alignment, and affect maintenance dredging of the channel and management of any sand bypassing.

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6. CONCLUSIONS AND RECOMMENDATIONS

6.1 CONCLUSIONS

6.1.1 Flushing and circulation characteristics Hydrodynamic modelling results indicate that removal of the Causeway will produce a greater impact on the flushing characteristics of Cockburn Sound in summer than in autumn, and significantly affect current velocities within about 1–2 km of the Causeway. Based on e-folding times calculated from summer simulations, removal of the Causeway would produce:

• A 2–3 fold improvements in flushing times in the Mangles Bay deep basin, on Southern Flats, and in the southern basin of the Sound; • Approximately a 50% improvements in flushing times in the shallows of Mangles Bay, the Rockingham/Kwinana area and eastern Garden Island, and the central basin of the Sound; and • Little change on Eastern Flats or in the northern basin of the Sound.

Adding additional openings to the Causeway will produce a lesser effect on flushing than removal of the Causeway, and doubling the size of the existing Causeway openings will produce a lesser effect than additional openings.

6.1.2 Ecology Potential impacts on summer water quality due to modifications of the Causeway were inferred by considering changes in flushing time against the context of the most recent data on summer water quality: large changes in flushing times did not always equate to large changes in water quality if present water quality was good. It was inferred that removal of the Causeway should result in a considerable improvement in water quality in the Mangles Bay deep basin, possible small improvements in the shallows of Mangles Bay/Rockingham/Kwinana, and little change elsewhere. Adding additional openings to the Causeway or doubling the size of the existing Causeway openings will result in lesser improvements in the Mangles Bay deep basin, and little change in water quality elsewhere.

Potential impacts on seagrasses due to modifications of the Causeway were examined in terms of changes in water quality, and changes in ‘bottom shear stress’—a measure of the force per unit area exerted on the seabed by currents close to the seabed, and proportional to the current velocity, squared. This approach was taken because in the Mangles Bay/Rockingham/Kwinana area the historical loss of seagrass (and present stress on seagrasses at their depth limit) appears to be related to poor water quality, whereas on Southern Flats the historical loss of seagrass appear due to altered current speeds and/or sediment movement, rather than water quality.

Based on the theoretical recovery of seagrass areas lost in the past, removal of the Causeway should result in improvements in water quality that improve the health of seagrass meadows at their depth limit in the Mangles Bay area, and theoretically allow that depth limit to be extended. The estimated area of potential seagrass gain is around 50 ha. Water quality may also improve sufficiently to theoretically allow seagrasses to grow in waters 3–10 m deep in the Rockingham/Kwinana area: the estimated area of potential seagrass gain (in areas not affected by shipping-related turbidity and the presence of aquaculture lines) is around 50 ha. Changes in current

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speeds and/or sediment movement on Southern Flats due to removal of the Causeway may potentially allow about 200 ha of seagrass to re-establish (if areas west of the Causeway are included).

The above estimates are potential gains in seagrass due to the creation of suitable conditions, but there is no certainty that seagrasses will actually return. A feature of the species of seagrass found in Cockburn Sound appear to be that, once lost, they do not always readily regrow even if conditions are suitable. It is possible that revegetation efforts would be needed to achieve the seagrass regrowth theoretically possible due to improved conditions.

6.1.3 Coastal processes The Causeway has two types of effect on coastal processes: changes in swell-wave and wind-wave energy arriving at shoreline, and direct interruption of longshore sediment transport due to the presence of a physical structures at the shoreline. Analysis of coastal processes in the region adjacent to the Causeway is an extremely complex subject, and preliminary predictions on the effect of removing the Causeway may be summarised as follows:

• Under both south westerly and north westerly swell removal of the Causeway would result in an increase in swell-wave energy in the Mangles Bay area and an increase in the eastward longshore current at the shoreline in this area. It is likely that the anticipated increase in swell-wave energy in areas eastward of Mangles Bay would become relatively less with distance from the Causeway. The removal of the Causeway will also result in a slight increase in the wind- wave energy that is experienced on the northern beaches of Cape Peron; • There is a net easterly sediment transport direction on the northern beaches of Cape Peron, and sediment is trapped on the western side of the breakwater of the Cape Peron boat ramp. This sediment is bypassed, as required, to the eastern side of the Causeway. The Causeway is not the main trap for this longshore sediment transport, and removal of the Causeway would not release this trapped sediment for continued movement eastward; • As littoral sediment transport east of the Causeway would still be trapped at the Cape Peron boat ramp, it is likely that the removal of the Causeway will result in some shoreline erosion in the Mangles Bay area due to the increase in wave energy that would be experienced in this area. It is possible that the increased wave energy may cause a flattening of the beach profiles in southern Cockburn Sound which may result in sediment deposition on some of the nearshore seagrass meadows. It is also likely that the slight realignment southwards of

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James Point that occurred after the Causeway was constructed, may be reversed; • On Garden Island, interruption of littoral sediment transport has resulted in erosion of the beach at Careening Bay and accretion of sand in Broun Bay and around Parkin Point. This effect would be reversed with removal of the Causeway; and • The changes in longshore sediment transport and shoreline profiles that would be caused by removal/modification of the Causeway) are akin to those caused by construction of the Causeway, in that they will be relatively small and readily addressed by sand bypassing and other management measures.

6.1.4 Human uses

Present uses A preliminary assessment on the potential impacts of removal of the Causeway on present human uses of the Mangles Bay region is as follows:

• Any removal/alteration of the Causeway is likely to cause considerable disruption to the operation of the naval base on Garden Island. Nor could the Causeway be removed before a new bridge was in place; • The Cape Peron boat ramp is intensively used by recreational boaters. The portion of the Causeway that protects the Cape Peron boat ramp from north- easterly, easterly and south-easterly winds cannot be removed—unless some other structure is put in place to provide similar protection. The need for some structure to protect the boat ramp will inevitably mean some reduction in the flushing of the shallow waters of Mangles Bay; • The activities of small boat users that presently shelter in the lee of the Causeway may be restricted; • Conditions will be less sheltered in the water-ski and jet-ski areas immediately adjacent to the Causeway, and these activities may be restricted to some extent; • Recreational vessel moorings may be affected by increased exposure to swell- waves and wind-waves; • Some coastal structures have been built ‘post Causeway’ (e.g. the second stage of the CBH jetty) and may not be designed for ‘pre-Causeway’ swell-wave and wind-wave conditions. If ‘under deck’ clearance in any jetty/wharf is too low, then waves contacting the underside of the deck may create uplift forces sufficient to damage the jetty structure. Groynes and walls can also be damaged by waves. The ability of coastal structures to withstand changed conditions would need to be investigated; • The re-establishment of a balanced sediment transport regime at James Point may affect activities at the BP Refinery (e.g. silting up of the water intake pipe); • Changes in water quality are unlikely to affect beach use, yachting, jet-skiing, water-skiing and pleasure boating; • Localised improvements in water quality will be due decreased phytoplankton production in these areas. This may have some effect on the fisheries, particularly if there is no compensating gain in seagrass meadows; and • Predicted changes in current velocity and water quality on Southern Flats are unlikely to adversely affect mussel aquaculture in this area. Mussel aquaculture at the CBH jetty may be adversely affected by the significant decrease in phytoplankton production predicted to occur in this area, although

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any effects may be partly offset if the mussels can be grown at depths greater than 5 m.

Future uses The main future human use of the region considered was a marina in Mangles Bay, the concept of which has long been supported by local government and some State government departments. It is, however, difficult to predict the impact of any alterations/modifications of the Causeway on any future marina without knowing the final design of the marina, and the management practices to be adopted. The marina will need groynes at the marina entrance, as well as a dredged channel leading to the entrance. Alterations in swell-wave and wind-wave conditions and longshore sediment transport with removal of the Causeway may necessitate alteration of groyne design and channel alignment, and affect maintenance dredging of the channel and management of any sand bypassing.

Due to increased residence times, water quality in any marina is generally less than in adjacent waters: the extent to which marina water will affect Mangles Bay water quality will depend on the size, shape, depth and design features of the marina, and may vary from negligible to measurable. The removal of the Causeway is not expected to greatly improve flushing in the Mangles Bay shallows and, as noted earlier, the portion of the Causeway protecting the Cape Peron boat ramp (or some replacement structure) will need to stay, which will further lessen any flushing of this area. The ‘doubled opening’ and ‘additional opening’ configurations are expected to result in little improvement of water quality in the Mangles Bay shallows. The marina design will need to achieve sufficient dilution of marina water to avoid adverse environmental impacts in Mangles Bay, irrespective of the presence/absence of the Causeway. Any direct seagrass losses involved in construction of a marina (e.g. due to channel dredging) may, however, be viewed as more acceptable if removal/modification of the Causeway improves water quality in some areas sufficiently to increase the likely success of any mitigation efforts carried out to offset seagrass losses.

6.1.5 Overview Modelling results have indicated that removal of the Causeway would result in conditions that theoretically permit the re-establishment of 100 ha of seagrass in the Mangles Bay/Rockingham/Kwinana area (due to improved water quality) and 200 ha on Southern Flats (due to reduced current velocities), but there is considerable uncertainty about whether the seagrasses would return or how long this would take. Doubling the Causeway openings or adding additional openings would produce lesser improvements in water quality, and because the ‘threshold’ velocity tolerated by seagrasses is not known, it is uncertain whether the predicted changes in current velocities on Southern Flats (both increases and decreases) would all be beneficial to seagrasses.

Any removal/alteration of the Causeway has the potential to cause considerable disruption to the activities of the naval base on Garden Island, and some adverse effects on recreational use and commercial activities. Removal of all or part of the Causeway could not be undertaken before an alternative structure was in place. Nor can the portion of the Causeway that protects the Cape Peron boat ramp from north- easterly, easterly and south-easterly winds be removed—unless some other structure is put in place to provide similar protection. Even if other parts of the Causeway are removed/modified, the need for some structure to protect the boat ramp will reduce the potential for improved flushing of the shallow waters of Mangles Bay.

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The above conclusions are predictions of the environmental effects likely as a result of modifying the Causeway, not an evaluation of whether the environmental benefits justify the costs. The latter evaluation would require a formal cost-benefit analysis, and would be most appropriately carried out by the CSMC. As a ‘lead in’ to such an exercise, preliminary calculations undertaken by the Department for Planning and Infrastructure (DPI) indicate that removal of Causeway rockfill material would cost approximately $5.5 million (assuming that the material could be disposed of within 10 km of the site). Construction of a bridge between the two existing trestle bridges, and to land, would cost about $120 million. Removing Causeway rockfill material and bridge construction would also affect the environment (e.g. turbidity, scouring, loss of the ‘reef-like’ habitat to marine biota that the rockfill material has developed into), and these effects would need to be carefully appraised.

On the basis of available data it is concluded that the environmental benefits of removing the Causeway do not justify the associated costs. For the other modifications to the Causeway considered in this study (wider openings/additional openings), there is a risk of adverse environmental effects that might outweigh environmental benefits.

6.2 RECOMMENDATIONS Should the CSMC decide that further investigation into the potential effects of removing/altering the Causeway is warranted, it is recommended that the Department of Defence be invited to prepare a submission on the implications to the naval base of modifying or removing the Causeway. A number of recommendations are also provided below to improve confidence in modelling results, and predictions of effects on ecology and coastal processes:

• Hydrodynamic modelling: • Improvements in input data used in the EFDC model would provide greater confidence in model results. In particular, wind conditions vary considerably in Mangles Bay, especially due to the wind shadow during south-westerly wind conditions along the southern boundary, and so the model would greatly benefit from data collected by a weather station installed in the Mangles Bay region (data from Garden Island had to be used in the present exercise); • The model would also benefit from data collected by a current meter in Mangles Bay, and from incorporation of benthic drag coefficients; • The model grid could be further refined, and further work done to stabilise the model in baroclinic mode and • Simulations could be run for an entire year to improve estimates of e- folding times. • Ecology: • A key issue to arise from ecological interpretations carried out in this study is the current velocity ‘threshold’ that is detrimental to seagrass. Work is needed in this area. • Coastal processes: • Comments on wave effects and littoral drift/shoreline transport are preliminary and based on desktop analysis. To describe the full effects of alterations to the Causeway on these processes would require a more detailed study which incorporates wave modeling (including refraction and diffraction processes) and beach profile modeling, and also considers the effects on the numerous navigation channels and coastal structures that have been constructed in Cockburn Sound before and after the construction of the Causeway.

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7. ACKNOWLEDGMENTS This report was written by Karen Hillman and Guy Gersbach, and formatted by Katy Rawlings (DAL Science & Engineering Pty Ltd).

It is with pleasure that the enthusiasm, support and understanding of Heidi Bucktin and John Connolly (Cockburn Sound Management Council) are acknowledged. Members of the Project Advisory Group for this study were also unfailingly helpful and constructive, and contributed significantly to this report. The members of the Project Advisory Group were (in alphabetical order):

• Peter Boreham (Department for Planning and Infrastructure); • Rod Lukatelich (BP Refinery, Kwinana); • Des Mills (Department of Environmental Protection); • Paul Neilson (City of Rockingham); • Malcolm Robb (Water and Rivers Commission); and • Boyd Wykes (Department of Defence), with review comments also supplied via Commanding Office P.A. Higgins, HMAS Stirling, Royal Australian Navy.

Warm thanks also go to Stewart Barr (Department for Planning and Infrastructure) for information on coastal processes and preliminary costing of trestle bridge construction and Causeway removal, and Glenn Dibben (WA Fishing Industry Council Inc.) for freely offering his knowledge and advice about the mussel aquaculture industry.

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8. REFERENCES Bastyan, G., Paling, E.I. and Wilson, C., 1994. Cockburn Sound Water Quality Studies: Nutrient release from the sediments and water quality. Environmental Science Report 94/2, Nutrient Analysis Laboratory, Murdoch University, Western Australia.

Cambridge, M.L., 1979. Cockburn Sound Environmental Study. Technical Report on Seagrass. Department of Conservation and Environment, Perth, Western Australia

Cockburn, 2000. Long-term Shellsand Dredging, Owen Anchorage. Environmental Review and Management Programme. Prepared by DA Lord & Associates Pty Ltd for Cockburn Cement Ltd. Report 96/032/4.

D'Adamo, N., 1992. Hydrodynamics and Recommendations for Further Studies in Cockburn Sound and Adjacent Waters. Environmental Protection Authority, Perth, Western Australia.

DAL, 2001. The State of Cockburn Sound: A Pressure:State:Response Report. Prepared by DA Lord & Associates Pty Ltd for the Cockburn Sound Management Council. Report 01/187/1.

Department of Construction (Commonwealth), 1977. Wave Climate: Cockburn Sound. Report No. MW 79, Maritime Works Branch, Melbourne, 151 pp.

DEP, 1996. Southern Metropolitan Coastal Waters Study, Final Report. Prepared by the Department of Environmental Protection, Perth, Western Australia.

Fremantle Port Authority, 1972. Port of Fremantle Outer Harbour: Review and Report on Stage I Port Development Scheme—Point Peron Area. September 1972.

Hearn, C.J., 1991. A Review of Past Studies of the Hydrodynamics of Cockburn Sound and Surrounding Waters with an Appraisal of Physical Processes and Recommendations for Future Data Collection and Modeling. Prepared for the Environmental Protection Authority, May 1991.

Hicks, A.B., Buchannan, F.J., Fernie, G.N. and Tabert, P.W.J., 1973a. Design and construction of the Garden Island Causeway, Cockburn Sound. The Institution of Engineers Australia, Annual Engineering Conference, Perth.

Hicks, A.B., Meagher, T.D. and Waterman, P., 1973b. Planning and marine environmental investigations for the Garden Island Causeway, Cockburn Sound. The Institution of Engineers Australia, Annual Engineering Conference, Perth.

JPPL, 2001. James Point Port: Stage 1, James Point, Cockburn Sound. Public Environmental Review. Prepared by DA Lord & Associates Pty Ltd for James Point Pty Ltd. Report 00/059/6.

Lemm, A., 1996. Offshore Wave Climate: Perth Western Australia. The University of Western Australia, Department of Environmental Engineering, Honours Thesis.

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Pattiaratchi, C.B., Imberger, J., Zaker, N. & Svenson, T., 1995. Perth Coastal Waters Study: Physical Measurements. Project P2. Prepared by the Centre for Water Research for the Water Authority of Western Australia.

Pattiaratchi, C., 2002. Field Measurements of Current Flows through the Causeway and Mangles Bay. Prepared by the Centre for Water Research, the University of Western Australia, for the Cockburn Sound Management Council. Report No. WP 1772 CP.

Riedel, H.P. and Trajer, F.L., 1978. Analysis of five years of wave data, Cockburn Sound. Fourth Australian Conference on Coastal & Ocean Engineering, Adelaide, 143–147 pp.

Silvester, R. and Hsu, J.R.C., 1991. New and Old Ideas in Coastal Sedimentation. Reviews in Aquatic Sciences 4: 375–410.

Speedy, V., 1994. A Numerical model of the effects of the Garden Island Causeway on the flushing of Cockburn Sound, Western Australia. Honours Thesis, Department of Geography and Oceanography, University college, University of New South Wales, the Australian Defence Force Academy.

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