February 10, 2014

expert reaction to continued flooding

As storms continue to sweep into the UK and flooding in the Somerset Levels and many other areas worsen, experts comment on what impact dredging could have made, how rare this prolonged bad weather is, how we can expect conditions to develop, and the role of climate change. 

See further comments here. 

Andrew McKenzie, British Geological Survey, said:

“In early December 2013 groundwater levels over much of the UK were normal for the time of year.  The extreme rainfall in the southern part of England over the New Year and into early January had a dramatic impact on levels in the faster responding aquifers and spectacular rises occurred in groundwater levels in some aquifers. For example, groundwater levels in the Chalk at Tilshead on Salisbury Plain rose by about 20 metres in just two weeks. These rapid rises triggered some localised groundwater flooding, mainly in the upper parts of Chalk catchments. If rainfall had eased at that point the empty storage in the lower catchment might have been able to absorb the inflows. However, as rainfall has continued to fall the lower parts of many catchments have been saturated, and prolonged high river levels will have contributed to exceptional recharge.

“At the beginning of February record monthly groundwater levels were measured across many aquifers, and as levels in February are naturally high, these records are often all time records for the sites concerned.  Record levels were established in the Chalk of Wessex, the North Downs and Hampshire.   The wet weather also reversed a declining trend in some of the North West Permo-Triassic aquifers.

“Consequently, emergence of groundwater has been observed at many localities across southern England where groundwater flooding has been previously recorded.  The high water levels on the interfluves (the upper parts of groundwater catchments) will gradually feed into the lower parts of the catchments, over a period of weeks to months, so groundwater flooding is expected to persist.”

Kevin Hiscock, Professor of Environmental Sciences, University of East Anglia, said:

“Water management in the Somerset Levels is similar to that in The Broads in East Anglia and relies on pumped drainage off the land into the ‘high-level carriers’.  These carriers, or rivers, are above the elevation of the surrounding land surface and so are not connected to the flood plain as typical for more natural rivers.  Hence, some dredging of these high-level carriers will most likely be of help in discharging the pumped land drainage.  Even so, the unprecedented rainfall totals this winter will most probably have overcome the drainage of the Somerset Levels, even with dredging.  In more natural river systems outside of the lowest lying areas in Britain, then the future of flood control must be to allow the reconnection of a river to its flood plain.  Dredging of rivers and the placing of the sediment (mud) on the river bank only serves to deepen the channel, to slow down the flow and cause further sediment to drop out.  This silted, over-deepened channel is also of less interest in terms of its biodiversity.  It is better if the river channel is narrower and the flow is faster and so keeping the river bed clean and promoting a more diverse biodiversity.  By reconnecting the river to its floodplain in appropriate areas, away from human habitation, then flood water can be spread over the floodplain where it is stored before being released slowly back to the river.  This water storage (or ‘washland’) benefit then reduces the threat of flooding downstream, unlike in a dredged channel in which the water is sent further downstream and so potentially flooding regions in the lower part of the river’s catchment.  Hence, working with nature is better, and less expensive in terms of ongoing maintenance costs, than maintaining a highly managed river.  Some good examples of these river restoration principles are seen on the River Wensum in Norfolk.”

Dr David Boorman, Centre for Ecology & Hydrology (Science Area Lead for Water Resources), said:

“A key question in any unusual hydrological situation, such as a drought or a period of wet weather as we are experiencing now, is how long will it continue?  We are working with a number of partners to provide a long-range hydrological forecast for the UK. Forecasts of this type are already produced on an operational basis in a number of countries including the USA and Australia.

“The project involves bringing together information on the current weather, soil moisture, river flows and groundwater levels from across the UK, and using a number of methods of exploring possible future hydrological conditions, for example, using numerical models of river flows and groundwater models driven by long-range weather outlooks.”

Nick Reynard – Centre for Ecology & Hydrology (Science Area Lead for Natural Hazards), said:

“The cluster of drought and flood events through the early years of the 21st century and the recent runoff and recharge patterns, are near to the extreme range of historical variability as we currently understand it , and therefore also raise the question that they may reflect anthropogenic climate change. It is important to note, however that how river flows respond to extreme rainfall varies greatly because of geology, soils, land use etc, meaning that it is difficult to detect signals of climate change in observed events, and to generalise the impacts of climate change for future flooding. Tidal flood risk is increasing as sea levels rise but the outlook is more complex in relation to fluvial flooding.”

Professor Alan Jenkins, Centre for Ecology & Hydrology (Director of Water and Pollution Science), said:

“The long duration of the current flooding episode and its wide spatial extent make it an exceptional event. Relentless storm events over many weeks have combined to produce widespread flooding at a scale rarely seen in the UK. Over 5000 homes have flooded and transport infrastructure has been disrupted across the UK.

“The key to mitigating the impact of any flood is understanding that every catchment works in a different way. For example the current flood situation on the Somerset Levels is the result of a different range of processes to those currently affecting the lower reaches of the River Thames. The operational agencies are doing a sterling job on the ground and it’s important to recognise that hundreds of thousands of homes have been protected by the UK’s flood defence infrastructure.

“Many potential solutions have been talked about over the last few weeks as ways of minimising flood risk. This is a complex issue and over the next few months it is important that experts from the key agencies and research institutes sit down and carefully analyse whether there are lessons to be learnt from the current flood episode.”

On link with Climate Change:

Professor Myles Allen, University of Oxford, said:

“There are simple physical reasons, supported by computer modelling of similar events back in the 2000s, to suspect that human-induced warming of the climate system has increased the risk of the kind of heavy rainfall events that are playing a major role in these floods. But it is important to remember that other meteorological events that have caused flooding in the UK, like the rapid melting of late Spring snow in 1947, may have been made less likely by global warming. So just saying “climate change is increasing flood risk” is too simplistic: these kinds of floods (those driven by heavy winter rainfall) may be becoming more frequent, others perhaps less frequent. What is clear is that just looking back at the historical record to plan flood defences or set insurance premiums is increasingly misleading. The climate is changing, and the sooner we understand in detail what these changes mean for Britain, the better.”