In field studies on the Essex and Kent coasts following the North Sea 1953 storm surge, academics, Cooling and Marsland listed four possible causes for flood embankment failure. Three of these causes are particularly useful to consider regarding the failure of river walls at Hazelwood marshes and Havergate Island. These are, a) erosion of sea-facing embankment wall by wave activity, b) erosion of land-facing embankment due to over-topping, c) slippage or slump of land-facing wall due to water dripping through the bank.
The three causes of failure described above can be said to be examples of two distinct processes, Scouring and Desiccating Cracking. Of the causes listed above, a and b, can be associated with scouring and c can be attributed to desiccated cracking. The elements of each process and how they contribute to flood embankment failure will be considered in more detail.
The process of scouring can occur when water overtops a flood defence and reaches the ground on the landward side of an embankment in a state of turbulence, therefore, it could be said, erosion begins the moment the wave reaches the border between soil and water. The force that moves the wave interacts at speed with soil at the base of the landward side of a flood embankment. At this point, two processes are said to be at work, the immediate movement of the water directed by the physical space it hits and the state of the soil when the wave meets the ground.
The immediate area the wave hits is said to contribute to scour due to water meeting an obstruction, presumably this could be a rock or the edge of the base of the river embankment. Meeting this obstruction can interrupt flow and decrease its space and redirect surge water. As this alteration is very sudden and occurs at speed it can multiply the rapidity of the energy directing the water which can cause eddies to form.
The state of the soil the wave meets when it hits the ground, contributes to what is known as shear stress. Britannica.com define shear stress as an energy whose impact can distort a substance causing sliding along a horizontal surface alongside the source of the stress. The shear that occurs correlates to the descending progress of earth impacted by this process. The extent that shear stress causes a deep scour hole is related to the make-up of the soil at the base of the embankment, depending on soil makeup, sheer stress can lead to an eventual lifting of sediments particles causing scour.
A photo that was taken after the storm surge of December 2013 showing evidence of a shallow slippage, caused by scouring after wave overtopping.
The second process that can cause an embankment to breach is Desiccated Cracking. This is particularly said to occur in alluvial clay, a material used in some flood defences in the Alde Ore estuary.
Desiccated Cracking or fissuring relates to the formation of an intersected web of internal vertical and horizontal fissures, about 60 cm deep within the surface layers of a flood embankment. It is thought repeated wetting and drying of estuary embankments can contribute to desiccated fissuring.
In a flood surge, large amounts of water drip through desiccated fissures, in extreme conditions, this can cause hydraulic fracture, when the flow transmits through fissures to the landward side of a river wall. Rather than a wave overtopping, water flows through fissures below the crest, into the embankment. This can cause the lifting of blocks of material, leading to gradual slope failure and the eventual breach of a river embankment.
Photo from: https://eprints.hrwallingford.com/1291/
Regarding, the process of water seepage that causes failure on the land-facing embankment, a member of the Alde Or Association visited Hazelwood marshes during the storm surge at its peak on December 6th 2013. It was reported that the water level reached the top of the embankment with minor overtopping at low points. However, the observation of real interest was amount of free water flowing through desiccation cracking issuing from the landward bank. This is indeed the puzzling aspect that strikes an outside observer of photos of the after-effects of the storm surge at Hazelwood, that they all seem to show water flowing outwards from the land-facing side of the embankment.
Photo from https://eprints.hrwallingford.com/1291/
The two processes of Scouring and Desiccated Cracking are separate yet are linked in that they interact and are influenced by the physical space and soil make-up of the flood embankment they detrimentally affect. Scouring interprets then shapes the space and the particles it interacts with, the scouring out of the base of a flood embankment, being the physical result of this interpretation. Whilst desiccated cracking, develops over time within the structure of the embankment, with the fissures functioning as vehicles for the rapid movement of water and its mechanisms of erosion.
The tendency of recent storms to become ever more powerful and unpredictable alongside rising sea levels, make the complex processes of scouring and desiccated cracking increasingly useful to understand, so the effects of storm surges can be assessed to reduce the likelihood of flood embankment failures.
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