Sediment transport near the entrance to Port Phillip Bay (The Entrance) has been studied previously (Cardno, 2011b). Sediment is moving along the Bass Strait coast from west to east and it is estimated that approximately 400,000 m ³/yr of sand reaches Point Lonsdale from the west. This value was estimated using expert judgement, knowledge of the area and the yearly volumes of sediment dredged from The Cut at Queenscliff. The source of this material is likely to be a combination of beach-dune erosion in the study area and areas further west of the study area through the dominant sediment transport pathway. Material is also likely to come from offshore, as well as riverine inputs.

Approximately half of the 400,000 m ³/yr is thought to be carried offshore by the ebb tide or into the Entrance Deep. There are then two pathways for sand moving into the bay from Point Lonsdale. It is estimated that between 50,000 and 80,000 m ³/yr are moved by wave-generated longshore currents along the beaches of Lonsdale Bight. A further volume of the same order is moved across Lonsdale Bight outside the surf zone, but in depths of less than 15 m under the action of the tidal currents and waves. There are obviously much larger gross movements, but this is an estimate of the net annual movement.

The sand transported across Lonsdale Bight offshore is assumed to be carried by the tidal currents with additional sediment put into suspension by wave activity. The sea bed in a large proportion of this area is rocky with a veneer of sand (Vantree, 1998), suggesting that the energy available for sediment transport exceeds the sediment supply. This reduces the sensitivity of the transport to small changes in the energy.

When the sand reaches Shortland Bluff, some is carried further north within the littoral system to Queenscliff Creek where dredging records indicate an accumulation of approximately 82,000 m ³/yr (unpublished records, Parks Victoria, 2006). Some of the sand reaching Shortland Bluff is likely to be carried further into the bay by tidal streams and longshore currents, some will be carried out into the deeper water by the ebb tide currents and eventually out of the bay. The estimated patterns of sediment transport through the heads are indicated in Figure 3-10.

Much of the sand which heads north past Queenscliff probably makes its way into the Great Sands, and some of this would move on to the beaches of the north east Bellarine Peninsula. There is little natural accretion evident along the majority of the beaches within the bay, which would imply that sand is shoaling with minimal transport to local beaches. There is little evidence to support significant accretion of the Great Sands themselves. This sand may be available to supply nearby beaches with SLR, however at present there is no evidence that the dominant wave directions are able to facilitate this. The dredging at Queenscliff and pumping to the southern shore of Swan Island (a sink) also contributes to the lesser volume of sand able to be transported north during the summer.

Sediment transport along the St. Leonards and Portarlington shorelines was assessed as part of the Bellarine Coastal Process Study (Cardno 2011c). The net sediment transport directions are shown in Figure 3-9.

Figure 3-10     Sediment transport near the entrance of Port Phillip Bay (Cardno, 2011b)