In the lead-up to the two-year anniversary of alteredmarshes, a series of discussions will consider whether it is the big storm surge events that cause the most coastal erosion, or smaller processes that develop over a long period, that weaken flood defences and erode shingle and cliffs.
The first of these discussions will consider The Compact Oxford English Dictionary, definition of a crack, which states that a crack is a narrow opening between two parts of something which has split or been broken. The relevance of this description will be considered in relation to the effects of drought conditions on clay flood embankments. The Met Office recently reported England experienced its driest July since 1935, with the driest on record for East Anglia. Met Office maps for actual rainfall show figures in the range of 0 to 25 mm for the Suffolk area in July 2022.
The collapse of a peat levee during a drought in 2003 in the Netherlands, prompted increased research into the effects of drought conditions on peat levees and earthen clay flood defences, such as we have in the UK. A paper Managing drought effects on levees in The Netherlands and England, focuses on a process known as Desiccated Cracking. Clay flood embankments have what is known as a Phreatic Line – a line that separates the section of embankment under water and the area above the water line. When water in a river or lake evaporates, causing the Phreatic Line to drop, areas of earthen embankments exposed above this line, can become subject to metrological conditions.
The effect of drought conditions on earthen embankments is further developed in a paper Experimental and theoretical analysis of cracking in drying soils. A process called Tensile stress can alter the structures of soils on top of earthen embankments. Tensile stress refers to the highest level of pressure soil can bear before it fails. Tensile refers to pressures that are trying to pull the soil to extreme lengths. Therefore, the point when tension within a material exceeds the ability of the substance to resist the pressure.
Once cracks form in the top section of a clay flood embankment, they can extend into the embankment and reach lengths of at least 1 metre deep. If these fissures form a web, this can convert the clay layer into a rubble like material which increases the permeability of the surface of the earthen embankment. Should a flood occur shortly after a drought, water is able to flow through the fissures which can cause the inward collapse of the inner slope by continual upheaval of chunks of rubble-like material.
Grass revetments over the top of clay embankments can also suffer under drought conditions, with substantial retreating of grass coverage, especially if overgrazing by sheep or mowing of grass occurs around the time of drought conditions. However, should rain return, but not under flood conditions, cracks in embankments and grass coverage can recover, but this is not always the case.
However, there are very few examples of earthen embankments collapsing due to drought conditions. Which brings the consideration back to the debate of whether erosion is caused by large scale events or the effect of smaller long-term processes that can cause a flood defence to fail. In some ways, it is a question, of whether the processes caused by drought conditions, cause more damage than the high-energy, high-volume events in storm surge conditions.
In the case of Desiccated Cracking under drought conditions, the process can change the solidity of a soil mass. A process strongly influenced by moisture content in the material, which is why it is particularly applicable to drought conditions.
This is similarly the case with mud flats and open shorelines which could have negative impacts on coastal nature reserves. As estuary levels evaporate, the progression of tensile stress through a soil mass can lead to fissures in established pressure areas. In this context, [Lee et al., 1988], usefully apply the word brittle to describe the soil state. However, in the case of a flat expanse of mud, the ground could also be hard and compacted, with deep running cracks, that make it hard for water to be absorbed, especially in large volumes following a drought.
In conclusion, it is said that water finds its own level and it is the flood events that erode networks of cracks in embankments. However, it could also be the case that the process of desiccated cracking, is itself an early manifestation of erosion that reaches an advanced state and is ultimately concluded by flood events when they happen.
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