The real relationship between flooding and ground subsidence

The real relationship between flooding and ground subsidence

Although flooding and ground subsidence might appear at first to be unrelated they are actually different outcomes from similar climatic events. Future Climate Info’s recent Flood Seminar was the ideal forum in which to go in to a little bit more detail about this evident link:

Prolonged rainfall leading to pluvial, fluvial and ground water flooding events can initiate the destabilisation of underground voids. There is potential for air-filled voids to be present at depth in areas underlain by solution features formed by natural dissolution processes (affecting soluble rocks like chalk, limestone, gypsum and salt) and also to be associated with abandoned historical mines. Such voids will tend to form gradually and exist in a metastable state that is in equilibrium with the overlying ground sequence (a function of the stresses and strength of the ground).

However, downward percolating ground water can change the equilibrium conditions by adding weight to the void roof by the addition of water (saturation) and reducing the friction between soil particles and the rock contact. Water flows can also cause erosion and movement of fines through the sequence. These changes can trigger the void to start to move upwards to the surface. Often the upward migration of voids is a stop/start process taking a number of storm cycles before the void “daylights” at the surface to form a “sinkhole” (or crown hole over an old mine working).

It is feasible that particularly severe and prolonged rainfall may cause a number of sinkholesto appear– this was evident during the exceptionally wet winter of 2013/14 in southern England when around 20 sinkholes appeared in February 2014 alone!

Review of annual rainfall using Meteorological Office rainfall gauging station data suggests that, since 2000, there have been notable increases in rainfall in particular years in southern England, characterised by more intensive rainfall events. These changes in rainfall pattern suggest that the recorded number of sinkholes being triggered might be expected to increase.

Sometimes the triggering effect of rainfall is mimicked by a burst water main which suddenly introduces a large volume of water into the ground. Should the water penetrate and percolate down into ground containing existing upward migrating voids then surface collapse can be caused. There are a number of cases where water main leaks appear to have set off subsidence problems affecting the highway and sometimes adjacent properties, but in reality these are areas where there are pre-existing solution features or man-made cavities in the ground (e.g. SE London; Reading, Berkshire; Beaconsfield, Buckinghamshire; St Albans, Hertfordshire; Leeds and Margate, Kent).

Caution should also be taken when new developments employ SuDS in case the site might be underlain by old mine workings or solution features in particular geological settings. The concentration of surface water drainage disposal via shallow or deep soakaways, or even collection into attenuation basins, needs to be carefully considered to avoid triggering ground subsidence.

 

 

Dr Clive Edmonds was a speaker at Future Climate Info’s Flood Seminar on 2 March 2017.

Clive Edmonds

Clive Edmonds

Partner – Geotechnics/Geohazards

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