Influence of the Garden Island Causeway on Mangles Bay, Cockburn Sound

Environmental & social considerations for modification or removal of the Causeway

Cockburn Sound Management Council Community Summary Paper

June 2003 In the early 1970s, the Garden Island Causeway was constructed across the southern entrance of Cockburn Sound. This Causeway consisted mainly of a 3.3 km long, solid rock-fill wall, reducing natural water exchange between the ocean and Cockburn Sound by about 40%. Thirty years on, the influence of the Causeway on the marine environment has been examined in detail. The potential environmental benefits of modifying the Causeway’s current design—or removing it completely—have also been assessed. This paper presents a summary of the key findings of this study. Figure 1. The Garden Island Causeway (looking south).

Background Cockburn Sound is located approximately There has been ongoing discussion about 20 km south of Perth, and is the most the degree to which the Causeway’s intensively used marine embayment in restriction of exchange between the waters Western Australia. With its sheltered, deep of the southern end of Cockburn Sound and waters and diversity of marine life, Cockburn the open ocean has influenced—and is still Sound is a very popular area for water influencing—the marine environment. sports, fishing and beach use. Tourism, fisheries, and aquaculture are important To address this issue, the Cockburn Sound commercial industries in this area. Management Council recently commissioned Cockburn Sound is also the site of a busy a modelling study to examine the impacts of port, an industrial area that depends on port the existing Causeway on: facilities, and a strategic naval base. Š Oceanic flushing and water quality; At the southern end of Cockburn Sound, a Š Marine communities, including seagrass Causeway connects nearby Garden Island to meadows and rocky reef communities; the mainland (Figure 1). This Causeway and was built in the early 1970s to provide road Š Coastal processes, such as shoreline access to and from the naval base, and is erosion and accretion, in Cockburn Sound. still the only road access to Garden Island today. The Causeway is approximately The study also assessed the potential 4.2 km long, comprising 3.3 km of solid, environmental benefits of modifying the rock-fill wall, with two trestle bridges—one existing Causeway design by widening 613 m long at the northern end, and the and/or adding additional openings, or other 304 m long at the southern end. removing it completely. This paper presents Limited oceanic exchange occurs through the major findings of this study. the two bridges. The work in this study was closely directed by scientific experts from the Mangles Bay Project Team, and has been externally reviewed by CSIRO Land & Water.

1 Flushing and water quality Water circulation and flushing patterns in Cockburn Sound are due in part to the Sound’s semi-enclosed nature (Figure 2.). The presence of Garden Island on western Cockburn Sound naturally constrains the exchange of water between Cockburn Sound and its adjacent oceanic waters. In addition, the southern entrance of Cockburn Sound is naturally shallow, and this also restricts Figure 3. Model showing improved levels of oceanic oceanic flushing. flushing (shown as blue) entering southern Cockburn Sound with the Causeway removed. Modelling estimates from this study have Changes to the flushing regimes from shown that the Causeway further reduces complete Causeway removal would have the natural flushing of Cockburn Sound by about following effects on summer water quality: 40%. Š The largest improvements in the Mangles Removal of the Causeway would result in: Bay deep basin; Š Small improvements in the shallows of Š Large (more than 100%) improvements to Mangles Bay, Rockingham and Kwinana; flushing of Mangles Bay deep basin, Š Little effect on Southern Flats, in southern Southern Flats and i