Rapidly changing Dynamics of the Bot estuary breach

Bot estuary 21 Aug 2020, showing old dune erosion and moving spit of sand

The initial breach was reported in estuary breached, and further developments. Further rapidly changing dynamics of the Bot River estuary breach have occurred leading to extensive erosion of old vegetated sand dunes and the very quick closure of the breach. The estuary is filling rapidly since it closed and with the washed out sand moving eastwards may cause a new natural breach to occur. This is likely to be further west than the July breach causing further damage and semi permanently changing the dynamics of the mouth. The objectives of the artificial breaching of the estuary have not been met due mainly to the position of the breach.

Great picture from Jean Tresfon which clearly shows how much of the old dune has been eroded since the breach of the Bot estuary. This is a much bigger volume of sand (about 3x as much) as the initial breach which took out the blowing sand dune. Generally the front section of the dunes is much higher than the dunes behind them.

Generally high dunes on the sea side removed
Section of front dunes removed, the white sand between the vegetation is usually flooded when the estuary is high

High estuary before the breach, with flooding behind the high sea side dunes

It is all this extra sand which has increased the size of the spit out to sea. This reduces the wave action, deposits more sand in the estuary mouth and caused the estuary to close so quickly, just over 3 weeks after the breach.

The spit is already moving eastwards and wave erosion action on the old dune seems to be reducing. A comparison with the older diagram I posted also shows how much more of the old dune has been removed.

Shows the height of the eroding seaside dunes, now removed a week later

Should the erosion of the old dune continue much more, it will open up an area that was flooded by the full estuary (as can be seen in the third pic). The estuary will not then overflow through the sand dunes before flowing to Lamloch swamps and finally into the Kleinmond estuary. We will have to wait and see.

The lowest point at which the estuary is likely to have breached itself was well to the east of where it was artificially breached. The hand dug trench on the day of the breach, chosen by the people was the lowest point of the bund at that stage. Comments and pictures of the breach are in two posts before this one. The rapidly changing dynamics of the Bot estuary breach were not expected to have developed so far, and no explanation has been forthcoming of why the estuary was breached so far to the west of normal.

In time the old dune will probably re-establish with sand from the west. On the other hand, we may have changed the dynamics of the system semi-permanently, with what has happened after the estuary was breached too close to the dunes in 2020. Considerable water has accumulated in the estuary, over 1.5m since it closed.

Estuary filling rapidly after closing, just over 1.5m above sea level

Should we not breach it again further to the east where it was done for the last three times. A big storm may still breach the estuary even further west than the July 2020 breach, directly into the deep water where the spit of sand has moved further to the east. This is at present the lowest part of the bund closing the estuary. This would probably take out even more of the old dune right back into the estuary.

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