The Geomorphology and Sediments of Cockburn Sound
The Geomorphology and Sediments of Cockburn Sound
The Geomorphology and Sediments of Cockburn Sound
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<strong>The</strong> <strong>Geomorphology</strong> <strong>and</strong> <strong>Sediments</strong> <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong><br />
superphosphate plant). Subsequent sediment studies within the sound in 1994 (Department <strong>of</strong><br />
Environmental Protection, 1996) found that contamination levels had decreased significantly when<br />
compared with results reported in the 1976 to 1979 study due to large reductions in wastewater<br />
discharges. <strong>The</strong> 1994 study found metal levels in <strong>Cockburn</strong> <strong>Sound</strong> were below Department <strong>of</strong><br />
Environmental Protection draft guidelines except for arsenic <strong>and</strong> mercury in some areas near<br />
industries or harbours (DAL, 2001). <strong>The</strong> 1999 study found that levels <strong>of</strong> all metals were well below<br />
the national ‘Interim Sediment Quality Guidelines’ (ISQG; ANZECC, 2000) for the protection <strong>of</strong><br />
marine ecosystems. Attempts were also made in the 1999 survey to determine the natural levels <strong>of</strong><br />
metals (Cr, Cu, Pb, Ni <strong>and</strong> Zn). Based on these values, it was indicated that there was lead<br />
contamination near areas <strong>of</strong> shipping <strong>and</strong> widespread zinc contamination throughout the sound.<br />
At the southern end <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong>, a causeway links the mainl<strong>and</strong> at Point Peron to the<br />
southern end <strong>of</strong> Garden Isl<strong>and</strong>. It was completed in 1973 to provide vehicular access to the naval<br />
base at Careening Bay. Two openings were built into the causeway to maintain some water<br />
exchange, however, the southern channel has been reduced from 2 km wide prior to construction <strong>of</strong><br />
the causeway to two separate channels that are 305 m <strong>and</strong> 610 m wide (DAL, 2002). <strong>The</strong> causeway<br />
has significantly reduced exchange between <strong>Cockburn</strong> <strong>Sound</strong> <strong>and</strong> the ocean, especially for the<br />
southern area. <strong>The</strong> exchange through the southern opening has been restricted by approximately<br />
40%, <strong>and</strong> the overall flushing <strong>of</strong> the sound has been reduced by 30 to 50% (Department <strong>of</strong><br />
Environmental Protection, 1996). However, water quality was a problem in <strong>Cockburn</strong> <strong>Sound</strong> prior to<br />
construction <strong>of</strong> the causeway, as noted above. Extensive areas <strong>of</strong> seagrass were lost due to poor<br />
water quality between 1954 to 1978 <strong>and</strong> approximately 260 ha were lost in Mangles Bay <strong>and</strong><br />
Southern Flats, <strong>and</strong> 440 ha between Rockingham Beach <strong>and</strong> James Point (Fig. 1; Cambridge, 1979).<br />
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