Tidal Flow

One of the many advantages of allowing tidal flow to enter the lower lakes and those areas denied tidal access by the barrages, is that silt and sand will be cleared from the vicinity of the mouth without the need for continual dredging. Channels will form naturally. 

Because the lakes have a large surface area (the very reason that evaporation is so high) each rising tide has to pass very high volumes of water through the restriction of the mouth area before the water level inside the lakes is the same as the level at sea. 

Only then does flow cease. On a falling tide, the same large surface area of the lakes provides a very high volume of water which then pushes out through the mouth as the tide falls at sea. 

If the level fell only 10cm in the lakes on a single outgoing tide, then that represents about 80 gigalitres of water that would have passed out through the mouth. In volume, that’s equal to 80 sq km x 1 metre deep of water. 

That amount of water would have carried sand and silt with it, out of the system, mostly scouring where there is greater restriction and is therefore travelling at a higher flow speed. Fast flow is what carries silt along with it, so that the scouring action is where it’s most needed. 

We can adjust how fast the rising tide flows into the lakes by allowing the inflow to access the lakes through all available channels, and limiting the outflow on falling tides to a few channels. The latter will add to the pressure. 

Less restriction coming in = less flow speed = less scouring of silt back into the system. More restriction is applied to outflow by limiting the flow to less channels, which = more flow speed = more scouring of silt out of the system. 

Pressure (tide height or, more precisely, difference in height) will dictate how fast water will flow from one point to another and the narrowest restriction along the way will be the limiter on how much water will pass through in a given time. Refer to Tidal Flows in The Coorong and Lower Lakes for more information.

Thoughtful manipulation of daily tides in this way can bias the movement of silt by the action of tidal flow to clean it from the system. 

Mike Young & Ken Jury