“Knee-jerk reaction by Government towards flooding of the Somerset levels?” asks Duncan Goodwin, Landscape Architect and University of Greenwich Senior Lecturer
This was the tone of the strapline in the New Civil Engineer on 6th February. The NCE reported that the £3-4M required to dredge the Parrett and Tone rivers in this part of the West Country is not supported by science and so may be money wasted. At the time of publication 40 properties were flooded due to prolonged and persistent rainfall, along with around 65km2 of farm land. Much of the Somerset Levels is below sea level and dredging the rivers is unlikely to have prevented the extent of flooding this year. The sheer volume of rainfall experienced would have overwhelmed any extent of river volume increases and increased flow is even likely to worsen the flooding further downstream in Taunton and Bridgewater.
By contrast, Boston, in Lincolnshire, was hit by storms and tidal surges back in December, causing the displacement of residents from 579 homes. Repairs are yet to be completed and many people are still living in temporary accommodation. Bostonians are keen that the installation of a tidal barrier within the River Haven, similar the Thames Barrier, is pursued without delay. This flood defence is contained within the Government’s National Infrastructure Pipeline and was projected to commence last year. If implemented, it would protect over 15,000 residential properties and nearly 700 businesses but is currently at the planning stage, with Central Government consulting on ways to deliver cost savings.
Since the introduction of the 2010 Flood Management and Water Act, Local Authorities are responsible for flooding caused by surface water and groundwater ingress, whereas, the EA are responsible for river flooding. Surface water flooding occurs when the existing drainage system is unable to accommodate the level or intensity of rainfall and surcharges. Groundwater flooding occurs when persistent high rainfall causes aquifers to overfill or high river levels to drive water through near surface levels in the ground. Often water will flow into basements or upwards through floors, directly from the ground as seepage or springs. Its effects tend not to be evident during storm events but follow behind, often, after some delay.
The Environment Agency estimates that approximately 5M homes in England are likely to be at risk of flooding. Revised maps showing where the areas of risk occur, have been published to enable flood defence measures to be more accurately targeted. The EA have reduced the number of properties at risk of surface water flooding by 0.8M and their consultants, ESI, have declared that the risk of ground water flooding is also reduced from what was previously predicted. These examples of downgrading are said to be due to higher sophistication of software models.
Traditional piped drainage infrastructure is typically designed to accommodate the flows associated with 1 in 30 year rainfall events (this equates to a significant downpour). For storms in excess of this, the surface water sewer capacity will be exceeded and appear on the surface as sewer flooding. This exceedance will typically cause ponding in roads and car parks with the excess volume being discharged across the ground with the intention that it soaks away. During continued high intensity rainfall overland flows will be conveyed, often in an uncontrolled manner, across the surface until they are able to settle in any depressions or are contained by walls and other obstructions within the urban built form. This often results in indiscriminate flooding of property. It is simply not economically viable to design sewers for the most extreme events as they would be operating well below capacity for the majority of their design life. As we already design surface water drainage systems to surcharge, we also need to manage any exceedance in a sustainable way if we are to reduce the prospect of more properties flooding. In 2006 CIRIA produced a document, C635, Designing for exceedance in urban drainage – good practice,(currently under review for update) which specifically considers ways to achieve this. Accurate flow path analysis around buildings can be used to create flood pathways and, if these can link to storage areas located as close to the source as possible (such as downpipes and impervious paved areas), a more robust and sustainable approach to drainage can be delivered. If this can be widely implemented, some of the pressure can be taken off the downstream drainage systems and main river catchments which are prone to flooding. It might not provide any respite to the farmers struggling on the Somerset Levels, where the deployment of better resilience measures may be more appropriate, but it could help those currently experiencing the misery of property damage along the Thames and Severn rivers.
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New Civil Engineer – 06.02.14