Mgobezeleni lake-estuary dynamics
Ricky wrote: I want to build up a conceptual understanding of the section of the catchment that links Lake Mgobezeleni to the sea.
My understanding is:
The outlet from Lake Mgobezeleni used to pass directly from the Lake to the sea until about 6000 years ago when the sea level - which had been very low during the glacial Period - reached current day sea level. At this stage the dunes formed and deflected the outlet southwards - until it was 'fixed' in place by the Jesser Point Rocks.
The watercourse then accumulated organic material which formed peat. This peat now restricts water flow - and as such acts a a 'weir' that controls the water level in lake Mgobezeleni.
Water level in Lake Mgobezeleni is about 3.5 m above MSL
The peat could have been deposited in the 6000 to 3000 years BP period when the sea level was a little above current day sea level, this level - so if the peat erodes it could affect the well-being of the lake.
So, given this concept, what do we need to know?
1) Depth of the peat throughout the channel
2) Age of the peat - need some C14 dates
3) Elevation of the surface of the peat relative to MSL
4) How far backing up estuary water (after closure) is evident in the system.
Ricky, while I accept your concept I feel it is essential to establish where the restriction lies in the flow path from the lake to the mouth that can cause the backup of water. We have not established if the mouth closure is transmitted to the lake. The latest lake level data is currently not available to correlate to our logger data but once DWA update their database we should be able to see if there is a possible link.
If the tidal range is 2m at Sodwana and we are only getting the high tides flowing into the estuary when the mouth is open then it puts the mouth bern at about 1mMSL. Add 1m berm when the mouth is closed and we already have a 2m head at the mouth leaving only 1m gradient to the lake.
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