Soil conditions and flooding
Current estimates are that the damage to property from the recent floods will cost in excess of a billion pounds. The average cost of repairing an individual flooddamaged house is at around £25,000. While the unusually intense and heavy rainfall is clearly to blame, two questions remain unanswered.
The influence of soil structural conditions on the amounts and speed of runoff from green field land is uncertain as is the contribution to flood management that an integrated approach to soil and water management could make.
Back in the 1980s, the Hydrology of Soil Types project established an underlying relationship between the nature of the soils within a river catchment and the overall hydrological regime of the river. By studying the soils and hydrology of several hundred gauged catchments, Boorman and colleagues were able to identify a strong statistical relationship between soil type and the speed of response of a river - whether it is flashy and rises in response to individual rainfall events or very constant as are chalk streams.
This relationship is detectable without including the effects ofland use and management on individual soils and is at the heart of the currently used Flood Estimates Handbook and the Environment Agency's predictions of flood frequency.
Following the Year 2000 floods in the Yorkshire Ouse, Severn and Uck/Bourne catchments, the Environment Agency funded NSRI staff to examine soil structural conditions across the affected areas. Well structured soils which break up easily into fine crumbs when handled, allow the rapid infiltration of rainwater and thereby slow down the transfer of rainwater to the river. If the structure is degraded either through formation of a surface 'cap' or crust, the puddling of grassland by stock or compaction by machinery, infiltration rates are much slower. With reduced infiltration, rainwater ponds on the surface or runs off rapidly into ditches and then to the river.
Using a whole-field soil structure degradation assessment method, NSRI reported finding degraded structure on between a fifth and a third of land within the
catchments. Severe or high degradation was found on 55 per cent of land used for late harvested crops.
Using different estimates of the link between degraded structure and run-off into the river, we predicted between only 1.5 per cent and up to 20 per cent increase in run-off as a result. This was regarded as a very significant finding and we recommended large scale field-based studies to explore this relationship so as to narrow the range of these predictions and better understand the impact on main river flooding.
Since then, there have been further catchment studies of soil structural condition that have corroborated the view that soil structure is not good, particularly on certain soils and under particular cropping systems. Work by other organisations, such as the studies by Imperial College and CEH at Pont Bren in Wales, has proved that infiltration rates are affected by soil use and management.
A major Defra/Environment Agencyfunded science review, known as FD2114,confirmed this but failed to find evidence from previous research of affects on main river flows and flood peaks.
The floods of the month have brought this issue back into the public's attention. With a bill to society of over one billion pounds, our opinion is that we should conduct the research required to fully understand the contribution to the severity of individual floods that is made by soil conditions.
Not only that, we should identify whether and, if so, how the land and the underlying soil can be managed in order to contribute to flood management and control.
Soil conditions and flooding
by Dick Thompson