The Saga of the Bot Estuary Breach 2020

The Saga of the Bot Estuary breach involves an artificial breach which caused massive environmental damage, rapid closure of the estuary and then a natural breach which started to repair some of the environmental damage

The data and reasons for the artificial breach have been proved to be incorrect and the extensive environmental damage to the old, stable dunes and early closure of the estuary can be attributed to the extremely poor decision of the position of the breach. The additional environmental damage that occurred within the estuary is also due mostly to the poor position of the breach. (This is well documented in ongoing posts with pictures on Botfriends Facebook page and on the previous post on the website) and further developments.

Kleinmond Bot Estuary links

The observed evidence from this breach has established that the management decisions to link the breaching policy of the Bot Estuary to the Kleinmond Estuary are invalid.

The link with the Kleinmond Estuary seems to have been established when the sea planes were stationed here. Good rains in the Lamloch area have shown water flowing towards the Bot Estuary. Generally, this flows towards Kleinmond from the Lamloch river.

The flow of Bot water towards Lamloch only occurs when the Bot is around 2.4m-2.5m, heading towards a natural breach and is clearly full with a potential for flooding. In theory the link is established at 1.7m, but the rate of fill of the Bot Estuary only declined on the day before the breach to about 25mm. This can also be explained by the large increase in the area of the Bot Estuary once it gets above 2.0m -2.4m. The flow from the Bot Estuary towards Lamloch is less than the normal input from the Bot River. This is mainly because Bot water does not flow readily over the long distance to Lamloch which is also chocked with reeds. This also means that no fish or very little fish exchange occurs through the Bot and Kleinmond link. The only real link, for fish, is via the sea. It seems that the only valid reason for Bot water to flow to Kleinmond is to help scour out the pollution problem that occurs in Kleinmond. This scouring is only effective between the exit of Lamloch and the Kleinmond open mouth, and seems to be only after the Bot Estuary is above about 2.4m.

Should the management of the much bigger, more important Bot Estuary be compromised with attempting to solve a pollution problem in Kleinmond?  Does the slow movement of water through the Lamloch swamps achieve any significant scouring in the Kleinmond Estuary?

The establishment of a better link between the Bot Estuary and the Kleinmond Estuary was established when the sea planes were operating from the Bot Estuary. It was in their interest to avoid a breach of the Bot Estuary to maintain a high-water level for the planes landing and take offs.

The data given in the management plan of the Kleinmond Estuary closer at 2.5m appears to be incorrect. At 2.5m there would be extensive flooding of the picnic area and of some of the sewage installations. When last did this ever get to 2.5m before breaching naturally. The mouth area is protected by the rocky peninsular from building up to 2.5m.

The 1.7m link above Lamloch appears to be also incorrect. The engineered channel from the war days has silted up with blowing sand and “Die Keel” appears to be naturally getting raised to above 1.7m. At a 2m height in the Bot Estuary very little if any water was flowing towards Lamloch.  It was considered before to construct a berm across “Die Keel”. This was discounted as it required a large engineering structure and the environmental impacts were negative for Kleinmond.

A suggestion is a compromise to raise the level of “Die Keel” by about 500mm. This will assist the natural process which is occurring. It will reverse the actions done during the war to stop the Bot Estuary from breaching. Obviously, it will increase the likelihood of a natural breach occurring in the Bot Estuary, leading to a more stable estuary. It will reduce the link between the Bot Estuary and the Kleinmond Estuary making the management of the Bot Estuary easier and more natural.

A potential disaster

In the dune field to the west of the Bot Estuary mouth is an area that is normally flooded at 2.5m in the Bot Estuary. The erosion of the of the old established large dunes from the artificial breach opened this up to the sea. Had the natural breach not occurred and the Bot Estuary filled up to more than 2m this would have caused estuary water to start flowing out through the dune field, potentially developing into a major breach through the dune field. This would have been an even greater environmental disaster than the erosion of the old established dunes.

Unless the sea builds these dunes back to above 3m there is a very real threat that the environmental disaster that occurred from the position of the artificial breach could still develop into a much larger disaster.

Fish exchange and breeding

The movement of fish from Kleinmond Estuary to the Bot Estuary is probably purely by chance. The fresh water from a Bot breach tends to move east, and the fish exchange is mainly with the Klein River Estuary in Hermanus. There appears to be some confusion in Kleinmond Estuary and the Klein River Estuary in Hermanus/Stanford.

Do the relevant fish species still exist in the polluted Kleinmond Estuary? This is given as one of the reasons for managing the estuaries together.

Did the fish breeding even have time to adapt to the breach with the short time of opening? The position of the breach having been the main determinant of the early closure (23 days). The excessive amount of additional eroded sand been deposited back into the opening.

Five years of a closed estuary is longer than the breeding cycle of most of the fish species. Does this data even exist?

The EMP correctly states that it is crucial for either one of the Bot/Kleimond or Klein River Estuary to be open in spring/early summer every year for fish nursery purposes. The Kleinmond Estuary by itself is rather small to accommodate this requirement. In addition, it remains with pollution problems (that are unlikely to be resolved in the future) which is not healthy especially for fish breeding purposes or baby fish.

The management of the estuary should be primarily to maintain fish stocks, fish breeding and other marine or brackish flora and fauna.

Breaching height

At 2.55m the Afdaks section was flooded above the line of the natural Fynbos species. This caused many of the pioneer Fynbos species that had moved below that level to die. This was more evident near the mouth where salty seawater had caused higher salinity water flooding. The question then to be asked is at what level the estuary should be breached artificially?

Scouring of sediments

Unless there is a big storm when the mouth is already open, very little scouring occurs except near the exit to the sea. Obviously, the narrow channel at breaching removes large quantities of sand blocking the mouth and immediately behind the bund in a narrow channel. Further behind this the estuary is so wide that the flow of water from the west and east banks towards the centre is not strong enough to achieve much movement of the sediments, which then more or less remain in place. During an open condition, wave action into the estuary moves considerable volumes of sand into the estuary at high tide and removes most of this sand during low tide. The shallow or intertidal flooded areas near the mouth have had a net gain in sand as they have become higher sand or less water at low tide over the months that the estuary has been open. These are the areas prone to wave action at high tide as they dissipate into the estuary.

In the upper reaches of the estuary during closed conditions the silt load from the Bot River is deposited as soon as the flow of the river dissipates over a wider area. With lower water levels during the open condition, only the silt within the channel of the river gets scoured out and moved further down the estuary. A very small amount of this goes into suspension, and may get carried out of the mouth to the sea. The continued deep water dredged channel where the Afdaks meets the Bot River is evidence that the silting from the Bot River has not really moved down beyond the Afdaks/Bot junction. Only an exceptional storm during the open condition will widen the river channel and permanently remove sediments to the sea. This will also deepen the main channels.

There is clear evidence that massive silting is still occurring and not getting scoured out by breaching policy, especially in the upper reaches of the Bot and Afdaks rivers.

More frequent breaching of the estuary would restore the estuary closer to its natural condition and more scouring of sediments would occur. The position of the breach at the centre of its mouth would still achieve a deep-water channel to the west and a secondary channel more to the east. This would remove more sediment close to the mouth and stop the continued build up of sand within the estuary. A flood condition during the open phase would be more effective

The rapid expansion of reeds into more shallow sections causes more silt accumulation and further expansion of the reed beds. A single species now dominates much larger sections of the estuary with the consequent loss of biodiversity in these areas. Flora and fauna interactions occur mainly along the edges of reedbeds and not within dense areas of reeds. This interaction area has now been reduced considerably, especially without the presence of large herbivores opening up channels through the reeds which would considerably increase these interactions areas.

The estuary is not only tending towards a more, fresh water system, allowing the expansion of the reeds, but large areas of the estuary are becoming shallower, decreasing the size of the estuary with further expansion of dense single species reeds.

Salinity units

This needs more investigation, with a critical look at all the data gathered, most of which has been unavailable even when requested by interested persons.

Position of the breach

EMP states that the artificial breaching should be within 300m of the 2008 breach and well away from the dunes. This was not the case for the artificial breach of 2020, which caused massive erosion of the dunes and may cause an even bigger disaster which nature is repairing, but this may take to much time to avoid the potential major disaster which has been created by the location of the artificial breach.

The Estuary Management Plan (EMP)

The Estuary Management Plan (EMP) states that the breaching of the Kleinmond Estuary has been discontinued in the last decade, resulting in more breaching opportunities for the Bot Estuary. This is not logical. The closure of the Bot generally builds up to about 3m, the much lower closure of the Kleinmond cannot affect the potential breach of the Bot. The keel height between the Bot and Kleinmond affects the potential breach of Kleinmond only.

The EMP states that breaching at low water levels may compromise already stressed fish immunity to pathogens and cause an increase in fish fatalities. The fish are stressed anyway and starting to die. Breaching will introduce salty sea water, providing a healthier environment, removing the stress, and saving more of the fish population.

The EMP clearly states that, in the 4^th year, breaching should occur even if water levels are low. Also says breach ‘if feasible’. This is not clear and open to wide interpretation. Does this mean if the committee feel like it?

A follow up investigation of the estuary in November 2020 shows

• The old blowing sand dune has rapidly built up again on the west side of the breach to well above the existing water level.
• The main flow to the sea is east of the artificial breach position, and steadily moving further east, (see latest pictures of the estuary mouth).
• The sea is no longer eroding the old dune, to the west, and has recreated a wide beach.
• The old eroded dune section is slowly building up again. Should the estuary close and build up to 2.5- 3.0 m above sea level, the risk of a breach through the dune field has been reduced.
• An inspection of “Die Keel”, with the low water in the estuary, shows that the watershed point (i.e., highest point between Lamloch and the Bot estuary) is well west of the “Die Keel”. The water from the west of the Bird Hide flows to the east, back into the Bot Estuary. Much of the flooding of the dune field area occurs before water start to move towards Lamloch.
• This is almost certainly above the 1.7m used to define the watershed between the Bot Estuary and Kleinmond Estuary.
• The seepage of water towards Lamloch is much smaller than expected, finer sand mixed with clay and organic matter.
• The seepage of water from the Bot Estuary to the sea, through cleaner more coarse sand is more than doubled in length than the traditional estuary mouth area.
• The extensive flooding of very shallow water through the old dune area between “Die Keel” and the sea considerably increases the water area of the Bot Estuary at high water levels. Therefore, greater evaporation loses.

This information should be taken into account in determining the Bot Estuary management plan for the future.

The apparent loss of water from the Bot Estuary in years that it does not breach is more than double the expected evaporation losses, especially as even in the dry summer months there is still considerable input of water from the Bot River.

Of interest is that the dredging which was done in the Afdaks section to where the planes were parked off Fisherhaven still exists. Some of the deepest parts of the estuary are still outside of the main Bot River channel, towards the slipway in Fisherhaven. It is unlikely that this is maintained from the Afdaks River which is much smaller than the Bot River.

Recommendations

The Bot Estuary should be manged quite independently of the Kleinmond estuary. The 3m height in the EMP should be changed to 2.5m.  

• more frequent natural or artificial breaching
• more stable salinity situation
• higher biodiversity maintained of flora and fauna

“Die Keel” should be raised, probably west of the bird hide at Rooisand.

• This would assist the natural process which is occurring and lead to more stability of the Bot Estuary.
• It would assist in correcting the dredging done during the war at “Die Keel”.
• This could be cheaply and easily done without extensive engineering works.
• It would likely add more water to the Kleinmond Estuary by avoiding the observed eastward flow from Lamloch.

The artificial breach position should be at the same point, about 300m from the eastern side.

• This has been recommended for decades.
• The lowest point for a natural breach in 2020 was in fact in this position.
• The accumulation of the natural blowing sand dune on the west protects the deep-water channel from silting up and preserves it as the shortest distance to the sea. But this is no reason to artificially breach in this position.
• Nature has very quickly built this back up with all the additional sand washed into the sea.

There would be still be large changes in the shore line of the Bot Estuary, but over shorter periods of time and the estuary position would be more stable.

Maybe consideration should be given of assisting to close the estuary after say 4-5 months of a breached condition. After sufficient time has elapsed for the fish exchanges. This may not be necessary with the more regular and correct position of the breach. This would lead to more stability of the estuary.

Serious consideration should be given to establishing a separate fresh water system in the Afdaks section of the estuary.

• This tends to be fresh water anyway.
• A dam wall can be constructed by using some of the accumulated silt with minimal seepage at 3- 3.5m above sea level.
• The very large catchment of the Afdaks would provide enough water to fill and maintain a much larger water body than is been envisaged.
• Once full it will stay very stable, without the huge changes in the shore line, caused by the main breaching. The farm dams, which do not have water withdrawn from them, are good examples.
• Bird species will increase dramatically, because of huge increases in other fauna from a more stable shore line.
• Existing mud flats will be more stable, providing more food from increased flora and fauna.
• Islands can be easily made, which if constantly surrounded by water provide secure roosting places (as is evident from a periodic island which naturally exists in the Afdaks, at high water levels). Great flocks of birds fight each other for place when this island is operating. More of these birds may become resident for more of the year.
• Reeds are necessary for the system, but would need some periodic control to stop them spreading over the whole system.
• In the long run, this system will probably provide more water into the estuary once it stabilizes.
• Clearing of alien vegetation in the Afdaks catchment would considerably enhance the flow of clean water into the estuary.

A similar situation should also be considered in the upper section of the Bot River. However, this would be more prone to flood damage. The saga of the Bot Estuary breach will continue.

Michael Austin. A friend of the estuary.   15 January 2021