Beach berm
Towards a conceptual understanding of the mouth dynamics of the Mgobezeleni Estuary – the role of the beach sediments.
Some first rough ideas:
I am starting to get a feel for the estuary dynamics and how it breaches. Conceptually I have the following points – based mainly on personal observations and knowledge from other estuaries. (Note – the sub-paragraphs in italicstry to answer the question ‘why?’ and to give something quantitative- usually a guess – something to be measured and monitored)
- The mouth breaches when the water level rises to overtop the beach berm; But breaching only occurs if the seaward slope is steep enough (1a), or the velocity and volume of water is great enough (1b). i.e. the water has to be erosive – so this also depends on sand-grain characteristics (1c). As it erodes, so the channel gets wider and deeper(1d). Breaching is affected by the width and steepness of the beach(1e).
1a The slope affects the speed of the flowing water. As the outlet is often sinuous, or takes a long route to the north, this reduces the gradient. We are talking about a gradient that is determined by the height of the berm and the length of the estuary channel as it crosses the beach. If the channel is shortcut by taking the shortest route, this is the route with the greatest gradient. We are dealing with a maximum height of the beach berm that is 4.5 m (Huizinga’s figure) but at Sodwana it is seldom more than 1.5 to 2 m – else it starts to damage the bridge.
1b Erosive water has to move fast enough to move sediment ( I think there is an engineering equation for this). At Sodwana the speed would be 0.5 m/sec ( guess).
1c The erosiveness also depends on sand grain size and sorting – relatively large and well sorted grains ( i.e. ones that do not pack tightly) are most erosive. We do need to measure this – my guess is that average grain size is large ( say 1 mm in diameter – I do not know what this is as a phi value) and well sorted – and does not change much over the seasons – but is coarser the further south one gets (closer to the Point). There may also be elevation differences up the beach – as the mechanism changes from wave deposition to wind deposition – i.e. getting finer up the beach?
1d As the estuary breaches, the channel seems to stabilise at a width of about 6 m. Water depth is about 30 cm and speed is about 1 to 2 m/sec. Standing waves are set up – indicating erosion. The whole channel will cut down by about 1.5 m ( upper beach and less near the sea as gradient is less steep)
1e Beach width is very changeable – it may be a seasonal effect. Average minimum width ( from vegetation to sea at mid tide is about 60 m
2. On breaching, sand is washed into the sea. It may be trapped in the Bay and redeposited on the beach at some later time, or it may be taken out of the system (2a). This is obvious, but the impacts of whether it accumulates or not, are quite profound (I believe). The sand sometimes seems to get washed away and sometimes not (2b). There is a complex story on sediment balance that we need to think about. It also links to my feelings about the casuarinas on the beach which affect the accumulation/losses of wind-blown sediments (2 c).
2a This is important for the ski-boaters as accumulated sand fills up the bay – and so the ski-boaters want to breach to the north to avoid this. We can calculate the volume of sand – width of cut of estuary ( we have said that beach width is 60 m, channel width is 6 m and cut down ranges from 1.5 m (i.e. 1.4/2 m on average) to almost nothing – so volume of sand is 60*6*(1.5/2) = 270 cubic metres of sand. Is this large enough to affect boating?
2b It is possible to measure the bathymetry of the bay – or even plot the beach margin at low tide to see the shape and dimensions of the sediment delta and how quickly this is broken down by wave action. We had a student at St Lucia several years ago who put coloured glass beads into the beach sand to plot how they moved – in this case we could see how much returns to the beach and where
2c there is a need to do a sand budget – measure the volume in the beach and see how it changes
3. The mouth does not have to be completely closed for breaching to occur. The breach can happen if there is a constricted outflow and the rate of water flowing in increases to a point where the berm begins to erode significantly (3a). This can only happen if the constricted outflow is meandering or goes to the north - creating a low-gradient outflow path, and then the flow path shifts so that the water takes a shorter and steeper path. (3b)
3a Hopefully we can have measured what is needed to erode – this would occur if the water exceeds the critical value – see (1b)
3b See (1a)
4. Water inflow from the catchment is likely to be in the form of a very constant baseflow – i.e. a very consistent volume per unit time (4a). Rain events do increase the inflow - but we do not know by how much at this stage. So, there will be occasions when a storm flow will breach the estuary.
4a Assume catchment to be 8000 ha, average annual rain to be 1000 mm and 20% of the rain enters the groundwater – and assume all the groundwater passes through the estuary mouth. i.e. about 0.5 cubic metres per sec as base flow
5. Breaching may also be influenced by the groundwater moving through the berm from the estuary to the sea (5a)– this may destabilise the beach sediments. Erosion may then be facilitated by this when there are big waves from the sea. We have little knowledge of how much groundwater percolates through the beach berm when it is closed - or whether this, by wetting the sand, promotes erosion from the seaward side. We need to look for the little runnels below the berm which are indicative of percolation. We should measure gradient of the watertable when the mouth is closed and the water has backed up.
5a Assume estuary water level after closure to be 1.5 m above msl and beach width to be 60 m. The gradient is then 1:40 – There are standard equations for percolation rates for water moving through well sorted sand of known grain size. Use this and then apply Darcy’s equation to estimate water loss. We also need to know the frontal width of the estuary – my guess is it is 40 m
6. Sediment supply on the beach is very important for controlling the mouth. The build-up of sediments affects the width of the beach as well as the height and rate that the beach berm can build up. Is there a general northerly movement of sand in this situation where the estuary is protected by Jesser Point? If we look at the formation of the estuary, it seems as if the dune has worked its way southwards over the centuries – indicating a general southward movement of sediments (not sure if this is directly wind-driven or also in the wave zone).(6a)
6a This is the sand-balance already referred to – gained by measuring change in beach volume over a specified time to get rates and seasons.
7. The probability of breaching may increase once the tide starts rising after a neap tide period - and may occur soon after there is sea overwash into the estuary. As high tides start rising after a neaps period, so seawater enters the estuary by overtopping – increasing the volume of water in the estuary and thus increasing the elevation of the backed-up water. The waves may cause some of the erosion to occur - on the seaward slope. But this may be the time when the beach berm has sediment depositions.. These are opposite impacts – but we do not know which will be larger – and under what conditions(7a)
7a We need to do a post-mortem on every breaching event – to determine tide state, sea state, estuary inflows, estuary level at breaching ( berm height) etc
8. Breaching is affected by sea state - rough seas and large sea-swells tend to breach the estuary. It would also be affected by the angle of the wave attack. This a dynamic that does need consideration. We need to consider the way the Jesser Point reef affects the angle of wave attack.
9. Closure is also affected by sea state, tide ( it is more likely to close over neap tides), the amount of sediment available in the beach zone, and the amount of water flowing out of the estuary.
10. Sand grain size may be important as it allows greater permeability - and also is more easily eroded by overtopping water - especially when the sand grains are well sorted
11. There are seasonal patterns in the beach morphology dynamics - winter tends to have more erosive seas - creating a steeper -sloped beach front and causing waves to remove sand. The opposite occurs during summer months.
12. There is another point to consider – and that is that estuary closure can occur very rapidly – and we do need to understand this process more. The scale of berm building relative to the size of the estuary needs consideration. It seems as if a closure can occur easily over one tide if conditions are right.(12a)
12a We need to analyse closure patterns – in the same way as we analyse breaching patterns
13. As breaching can happen at any time, we should also ask what post-mortem information we can obtain after a breaching event. We need to analyse each breach and work out tides, winds, sea state and beach berm height
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Each of the above statements needs to be challenged - and if possible backed by findings of other studies that have been reported in the literature. Then we need to develop a hypothesis for each. After this we think about what we should be measuring – and the frequency of measuring.
Ricky Taylor, assisted by Guy Bate
2 December 2013.
17 Jan 2014