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FEATURES | ada gazette SPRING 2014


Reducing fl ood risk – dredging and other management strategies


The fl ooding in January and February 2014 has instigated a large amount of media att ention on the diff erent management strategies to reduce fl ood risk. This feature uses information publicised by ADA exploring the role of dredging, providing an overview which also includes the views and experiences of Internal Drainage Boards (IDBs). The discussion continues to look at other management strategies for reducing fl ood risk.


Dredging


 Why should dredging occur? Over time sediments are deposited in slow-moving watercourses. Dredging, also referred to as desilting and mudding, removes the build-up of sediment from a watercourse. Dredging is therefore essential for maintaining the capacity of the watercourse to the required standard, especially in lowland (pumped) areas. For IDBs that are maintaining the transmission of water in low-lying areas where catchments are relatively fl at, fl ow velocities are slow and pumps are often used, dredging is a crucial activity. It is important to take full advantage of hydraulic gradients to achieve fl ow. This is most eff ectively achieved if the capacity of channels is maintained through dredging and other maintenance activity such as weed removal. Regular dredging ensures adequate


conveyance of fl oodwaters so that, when water levels in a channel are high, the channel’s greater capacity allows water to fl ow freely through the channel. Where an event exceeds the capacity of the watercourse and there is overtopping and fl ood storage areas are under water, higher capacities in the channel maintained by dredging activity means water can be evacuated at a faster rate, providing the ability to recover before the next rainfall event. If the watercourse is working as effi ciently as possible, the duration of the fl ooding is minimised.


Man-made channels will ‘naturalise’ if left


alone. Evidence from IDBs supports the view that, where rivers and watercourses in low-lying areas have not been maintained to ensure that they have a high enough capacity, the risk of fl ooding is higher and there is an increase in the time that fl ood storage areas are under water. The optimal frequency of dredging activity is


dependent on many factors, including soil type, type of deposits, gradient, infl uence of tides and whether the drain is used to transfer freshwater through the area. Ultimately, the scheduling of dredging activity is an operational decision that should take into account experience and local knowledge of the area. IDBs hold the view that dredging, along with wider maintenance activity, is a cost-eff ective solution for reducing fl ood risk in low-lying areas with artifi cial drainage. Maintenance work off ers a high return on public investment with an estimated return of £10 for each £1 outlaid.


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TIM VICKERS


 Benefi ts of dredging – ecological and wider Dredging activity, if carried out sympathetically


and as a planned operation, has no detrimental eff ect on wildlife or species using the watercourse as their habitat. Dredging and other maintenance activity can actually enhance the water body ecologically. Failure to carry out regular dredging can result in the watercourse becoming so silted up that there is insuffi cient water to sustain a balanced ecosystem. A build-up of silt and rott ing vegetation can also impact oxygen levels. Dredging watercourses on a rotational basis ensures that there is always a mixed and varied habitat at various stages of siltation. Dredging of rivers and watercourses is also


important in order to maintain an adequate depth of water for the purposes of navigation, abstraction and recreation.


 When is dredging not appropriate? As discussed in ADA’s 10 point plan for


managing fl ood water (see page 5), dredging is just one of a range of management strategies that should be considered within the perspective of the catchment as a whole. Dredging is critical in lowland (pumped) areas where there is the need to maximise the conveyance of water through drainage ditches, channels and rivers. Other techniques may be more eff ective, both in practice and economically, higher in the catchment, where gradients are greater. Dredging and maintenance work should


be planned across the total catchment area. Dredging only part of an area in response to an emergency may not be economical. It is more cost eff ective to carry out maintenance and dredging on a regular and systematic basis. There is no benefi t in dredging


watercourses below invert levels of culverts as this will not increase the potential capacity of the watercourse.


IDB Experience: the role of dredging for Boards


Black Sluice IDB Dredging has a pivotal role within the


annual maintenance works of Black Sluice IDB. The Board maintains over 750km of watercourses. Watercourses have a maximum


10 year cleansing cycle, although some watercourses require a higher frequency of cleansing than others. Scheduling the activity is an operational decision that also takes into account pre-inspections and local knowledge. The importance of maintenance activity is highlighted by the Board’s commitment of £300k+ expended each fi nancial year on annual cleansing of their maintainable watercourses.


Lower Severn IDB The Lower Severn IDB carries out dredging


and bank re-profi ling of channels approximately every 7 years. Due to their annual mowing and weed cutt ing maintenance operations, the quantity of silt that builds up is small as this other maintenance activity allows the free fl ow of water, reducing silt deposition. Bank re-profi ling is necessary in the area due to subsidence of the banks over a period of time due to the nature of the ground (clay layer on peat). The Board has recently undertaken dredging


and re-profi ling work to a high-level carrier channel which conveys water over a fl ood plain towards the Severn Estuary. This channel is tide locked during high tide. In addition to the highland water, the Board pumps approximately 1,500ha of lowland water into the high-level carrier. During the past 10 years, severe rainfall events have resulted in the levels remaining high for a period of days. The high-level channel has been regularly dredged but last June it was decided to dredge and re-profi le the channel. The Board has noted major improvements during the latest rainfall events in December 2013 and January 2014. The water still rises to the same level but when the tide allows, the channel discharges at a higher rate than before meaning receiving capacity is restored more quickly prior to subsequent high rainfall events, improving resilience.


North Level IDB De-silting is an essential part of North Level


IDB’s routine maintenance programme. The Board carries out de-silting on approximately 25,000m of watercourse per annum. The Board assesses its watercourses to determine the frequency of its dredging operations. Broadly speaking, some require mudding out every 10 years with others every 20 years. North Level IDB has direct experience of the problems associated with lack of maintenance, including dredging. Historically, Moreton’s Leam, a main river that runs through their district has been adversely aff ected by lack of maintenance. The Board recently signed a ten year agreement with the EA to “take-over” the maintenance of Moreton’s Leam. Maintenance is conducted in co-operation with Natural England and the RSPB.


Martin Redding


Martin Redding


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