Sand Management What causes the Bar Why are we silting up? April Hamer History Dredging on a Flood Tide| No current Berths Avaliable |
What causes the Bar?
The bar at Lakes Entrance is a natural phenomenon and similar bar formations can be found at estuary entrances all over the world. The east coast of Australia has relatively high sand transport by waves and as a result, some very large bars occur at estuary entrances.
Waves reaching the coastline break stir sand into suspension. The angle the waves make with the coastline causes a longshore current and this transports the sand along the coast. The action is known as longshore transport.
Tidal currents have an effect, but on the open coastline the majority of longshore transport is caused by wave action. Along the 90 mile beach, the coast is subject to high wave energy from Bass Strait and consequently the longshore transport rate is high by world standards.
There is another form of sand transport known as Onshore/Offshore Transport which, as it's name implies, is where the transport is perpendicular to the coastline. The combination of these two forms of transport is often known as Littoral Transport.
There are many opinions regarding the overall direction and quantity of sand movement at Lakes Entrance.
One long-held view is that the weather from the east during summer transports around 1,000,000m3 to the west; the weather from the southwest during winter, featuring less consistent but stronger winds and waves, transports 1,500,000m3 to the east. Therefore, in this scenario, the gross longshore transport is estimated at 2,500,000m3 per year and the net longshore transport is 500,000m3 per year towards the east.
However, the findings from latest research are considerably different to previous estimates.
A study in 2004 (Gippsland Ports’ Sand Management Study 2004) concluded that the net longshore transport direction is uncertain and probably varies according to the year. Research using wave data showed a net direction towards the west during the period 1989-1996. The study also found the gross volume of sand was about 1,000,000m3 and the net volume was about 100,000m3. Further research released in 2004 (Riedel) showed that from 1999 to 2003 the gross volume was about 300,000m3 per year and the net amount was about 60,000m3 per year towards the west.
When the longshore transport is interrupted by a tidal jet moving in and out of an estuary, the energy in the longshore transport is destroyed and the sand sinks out of suspension and causes Accretion. It is this Accretion of sand that creates the bar.
Since the formation of the Gippsland Lakes there has always been an entrance to the ocean. The natural entrance would form a bar that would cause restriction to the flow and sand would build up. Eventually a flood would open a channel (often in a new location) and the process would start all over again. There are records of the entrance being as far east as Red Cliffs near Lake Tyers and as far west as The Barrier, some seven miles away.
Historical records contain many references to the 'entrance being closed'. This meant closed to navigation. On only one occasion are there records of the entrance being closed to tidal flow.
The sidecasting dredge April Hamer shifts about 600,000m3 per year to clear the channel but does not remove any sand from the bar. If dredging stopped entirely, the entrance would likely remain open as it did from 1900 to 1976 (before the April Hamer), with a channel of around 1.5 metres deep which could locate itself anywhere through the bar.
Many ideas have been studied to find a better solution for the bar. Building longer walls would interrupt longshore transport resulting in sand accretion and the bar would rapidly form again at the end of the walls. A fixed sand by-pass system would have to shift large quantities of sand in each direction to be effective. A hopper dredge or a vessel which pumps sand away from the bar may be an option when the time comes to replace the April Hamer.
Remember - crossing the bar is Very Dangerous
