When waves break the turbulent water surges up onto the beach… then runs back again. The returning waters can cause strong rips and underwater currents which take sand with them.

Mini rip at Middle Park (April 2010)

Each beach has many rips of varying size and strength, which can change location from one tidal cycle to the next. The stronger the wave action the stronger the rip. These currents carry inter-tidal sands into deeper water. The moving sands are usually picked up by longshore drift to eventually return to shore further along the coast. 

The Hydrodynamics and Coastal Processes Head Technical Report (January 2007) prepared for the CDP noted that tidal currents are the major sediment transport mechanism within ‘the Sands’ (south of the Bay). The report also predicted an increase tidal current strength but generally confined to the area of the deepened channel.

Baykeeper suspects that in the Portsea case it’s likely that the deepened channel is swallowing sand which would ordinarily have returned to the beach. Time and tide will eventually tell!