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Ouse and Humber IDB The Ouse and Humber IDB has a 5 year
rolling programme in place which sustains regular desilting of the Board’s watercourses. The Board desilts over 110km of watercourse each year and the area it drains would suffer serious problems if it did not.
Other methods for reducing flood risk In the media, other ways of reducing flood
risk have also been discussed. Natural flood prevention measures have been examined, such as planting more trees to increase soil infiltration and take up more water from the soil; storing water in wetlands, ponds, ditches, channels and reservoirs; and slowing water by increasing resistance to its flow, for example, by planting floodplain or riverside woods. Environment Secretary Owen Paterson has said he would seriously consider innovative solutions like rewilding. However, the reality is that a range of land management techniques are needed to manage water in the upper parts of a river catchment,
Swale and Ure Drainage Board Swale and Ure Drainage Board aims
to maintain the bulk of their watercourses every two years. The maintenance operations involve dredging but also flailing the banks and overhanging brash, and removing the failings and weeds from the channel. The conveyance of the channel is managed for the benefit of the environment, land drainage and flood risk. The channel needs optimal conveyance in wet times to reduce flood damage.
Welland and Nene IDB Welland and Nene IDB covers an area of approximately 32,400ha and much of this is below sea-level. Watercourses here have very shallow gradients and thus 'self-cleansing' velocities are not always available. Dredging, therefore, plays a very important role in not only maintaining conveyance but also to create freeboard to provide capacity for storage of water in the drains during wet periods. The Board has a rolling dredging programme that means all drains are usually dredged every 5-12 years. In some parts of the area where the drains run through gravel strata the frequency for dredging is less, as accumulated material is slower to build up. However, in areas where peat and silt is prevalent there is a need for more frequent dredging. Without regular dredging many channels would not perform to their design parameters and efficient evacuation of water would not be possible.
Witham Fourth IDB Routine maintenance is the key to the efficient
drainage of Witham Fourth IDB’s catchment. De-silting programmes are based on need (some watercourses require more regular dredging than others) which ensures that work is only carried out where necessary. The Board has moved away from a simple time-based rotation to a more risk-based approach (still very much based on experience and local knowledge of catchments/watercourses but also using survey or modelling data where necessary). The Board has experienced some fluvial flooding problems in recent years linked to where EA managed watercourses have not been dredged. When these watercourses fail, water enters the IDB system placing additional burden on the Board’s assets.
and these need the co-operation of farmers and land managers. Good land management can reduce flooding and siltation whilst also preventing soil loss and providing farmers and land-owners with a more even and reliable water supply. Managing land to reduce flooding must
be planned in a coordinated way if it is to be successful. In urban areas, sustainable drainage systems (SuDS) can be integrated, including green roofs, areas designed to hold water safely within developments and permeable surfaces, reducing the run-off from rain falling directly on the town. This means that the integrated drainage (SuDS and piped drainage) in the town can cope with the storm event so there is no property flooding and diffuse pollution is also reduced. SuDS allow groundwater to recharge and the water that does leave the town is released back to the river slowly and is relatively clean, not causing problems for farmers and communities downstream. On the matter of the range of possible
solutions to managing flood risk ADA believes that it is important to consider the management of water across the whole catchment. ADA believes this requires a range of methods, including: working with upland farmers and landowners to increase soil infiltration and store more floodwater in the upper parts of the catchment; providing flood storage at critical points in the catchment; managing surface water sustainably in urban areas; and maintaining capacity downstream in lowland areas. The plan should be implemented collaboratively by a partnership involving all the key interests and organisations. A balanced approach is key and this should
perhaps also be reflected better in funding mechanisms with the need to concentrate greater funding on maintenance, both in rural areas where maintaining the capacity of rivers and watercourses is key, and in urban environments where physical assets already exist. Different types of catchments across the UK will require different management solutions. For example the Levels and Fens will require a different balance of management strategies in comparison with the Severn or Thames.
ada gazette SPRING 2014 | FEATURES
Case study – Somerset Levels The River Parrett is a main outlet to the sea
for water on the Somerset Levels. However, over recent years it has become silted up and is currently operating at around 50% of its capacity, leading to many people calling for it to be dredged. About 1,000 farmers, covering over 68,000 hectares of land, manage the Somerset Levels. Farmers here have claimed that a lack of river dredging has worsened the impact of the flooding in the area. The National Farmers’ Union has echoed these calls, writing to the Floods Minister expressing the need for the re-introduction of significant and consistent river maintenance work. However, the Environment Agency (EA) has previously stated that dredging would only make a small difference and is not the comprehensive answer to the severe flooding in Somerset. The work the EA is able to do needs to
be set in context. Government sets criteria determining where the flood budget is spent. The EA then acts as its operational arm based on funding available and the criteria set. The EA has pointed out that strictly following the Treasury rules has meant the funding has not been available to dredge the Somerset Levels, with funding prioritised in urban areas where there are more properties. Since the onset of the severe flooding in Somerset the debate has become more nuanced, with the EA apparently now agreeing that dredging of the tidal sections of the rivers is needed.
ADA believes dredging is essential in
low-lying areas. Jean Venables, speaking on the Today Programme on BBC Radio 4 (28 January) said of the situation in the Somerset Levels: “It’s a disaster area here and it could have been avoided if we had actually kept up with maintenance of the rivers. We’ve got a 20 year backlog of inactivity there and it’s actually very urgent that those rivers are dredged. There are other actions that can be taken as well upstream of the catchment, but it is very important that when you have an area of special drainage need that you actually have an outlet. The river is actually the outlet to that drainage area and it has to have sufficient capacity to be able to drain that area.” Jean Venables went on to add that they accept
that the land regularly floods on the Levels and the land surrounding the rivers acts as a flood plain. However, the flood plain needs to drain after a storm in order to recover before the next storm, but without sufficient capacity for the rivers to drain the area, water has been accumulating on the Levels. In Somerset, the Somerset Consortium of Drainage Boards (SCDB) have recognised the importance of considering management techniques within the perspective of the water catchment as a whole. The SCDB published their 10 point plan to managing flood risk, which was the basis for ADA’s 10 point plan.
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