Port Beach under winter erosion.
Port Beach under summer accretion.
Sand from Port Beach is being lost to Leighton Beach- 30%, the sand bar adjacent to Rous Head- 50% and around Rous Head - 20%. The calm years from 2000 to 2002 have probably resulted in a seasonal imbalance in longshore transport and the higher erosion rate at Port Beach. However, it also appears that sand returning from north to south during the winter may become trapped in the offshore bar, and not reach the beach.
The available data indicates there is a contemporary, longer-term erosion trend at Port Beach, particularly at the southern end. The cause of this erosion trend is a net sediment loss from this sector of the coast and insufficient compensating supplies from elsewhere.
Dredge material disposed offshore and on the beach has provided an artificial supply of sand to Port Beach for many years. There was a long period of sand build up at Port Beach that was largely sustained by the 10 million cubic metres of dredge material disposed of along the shoreline and further offshore between 1890 and 1970. This dredge material has been moving onshore and providing an artificial supply of sand to Port Beach for many years. This sand feed has now stopped.
Presently, the southern part of Port Beach is eroding at a rate of about two metres per year. This is realigning the southern corner of the beach and is a longer-term trend rather than a purely seasonal effect.
The shoreline adjacent to the main car park is showing a mild erosion trend of up to 0.8 metres per year, while there is no discernible trend at the northern end of Port Beach. Leighton Beach has accreted since 1945.
There are ongoing losses of sand from Port Beach to Leighton Beach and to both the sand bank north of the Rous Head extension and to the southern side of the Rous Head extension. This erosion trend is expected to continue. This will progressively reduce beach amenity and leave car parks and existing buildings increasingly vulnerable to storm erosion.
NPQ has engaged coastal engineers MP Rogers, who have extensive experience of coastal processes along the WA Coast, to undertake coastal modelling to meet the requirements of the Department for Planning as part of the approval processes for NPQ.
The horizontal geometry of the 3.5km NPQ breakwater will be designed to assist with sand transportation onshore, i.e. to accrete sand on the beaches, not erode them.
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