Defensive move

3rd July 2014


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  • Water


Janice Tyler

The River Restoration Centre's Jenny Mant and the ADA's Jean Venables debate whether regularly dredging rivers is the answer to rising flood risk

The record winter rain focused attention on how the UK manages water to reduce the impact of flooding. To do this cost-effectively and sustainably depends on joining up the management of water across a river’s entire catchment. Success can be measured in reduced flood risk, improved water quality and wider biodiversity.

In the upper parts of the catchment, good agricultural and land management practice, such as tree planting, provides the groundwater recharge, evaporation and transpiration from plants to retain water as well as reduce peak flows and runoff. Providing flood storage further down the catchment – including restoring rivers in urban areas – retains water during high rainfall to prevent downstream flooding.

Open space next to waterways in towns stores excess water and doubles up as recreation and public open space during times of low flow. Other sustainable drainage systems (SuDS) – areas designed to hold water safely in developments and permeable surfaces – in the towns, including green roofs, reduce the runoff from rain falling directly in the area.

In the lowland parts of the catchment, where gradients are very low and drainage is managed to control water levels, removing silt to maintain the transmission capacity of watercourses is essential to maximise the conveyance of water to pumps and sluices where it is discharged into the sea or another watercourse. Removing silt in lowland areas where water levels have been managed for centuries to provide social and economic benefits, and otherwise would revert to marshland, is different from removing silt for navigational purposes in upstream sections.

In upper areas of a catchment, where gradients are higher, there should be no need to remove sediment because silt will be moved downstream naturally in high flow conditions.

Removal of silt is different from activities to straighten and alter the profile of river channels. However, the debate about where dredging is and is not appropriate has been characterised by misunderstandings driven by a lack of consistency in the definitions of the operation by different organisations.

The Association of Drainage Authorities (ADA) and the Internal Drainage Boards define dredging as “desilting”, a process of removing the build-up of silts or sediments from a watercourse to maintain its design capacity. The Environment Agency is now using this as the definition. Previously it made a distinction between dredging and desilting, with the former seen as enlarging the original channel through deepening and widening or reprofiling.

After the 2014 winter flooding, the ADA provided information on the purpose of desilting, which it regards as essential in low-lying areas where the land is relatively flat, flows are slow and pumping may be required to manage water levels. In such areas enough capacity in the channel must be maintained to take full advantage of hydraulic gradients to transfer sufficient quantities of water to discharge points. Higher capacity in the channel maintained by desilting moves water faster so the area can recover before the next heavy rain. Vegetation management, such as weed removal, is also essential to maintain design capacity.

Desilting is not “bad” for the environment when carried out sympathetically as a planned operation. It can increase the biodiversity and nature conservation potential of artificial water bodies without reducing their effectiveness as drains. Without desilting, watercourses can become so silted up that there is insufficient water to sustain a balanced ecosystem.

Dr Jean Venables, chief executive, and Tim Vickers, former technical researcher, Association of Drainage Authorities.

Historically, many rivers in the UK have been dredged, resulting in detrimental impacts on habitats. Although dredging may reduce some localised flooding, it would not have prevented the devastation of last winter.

Post dredging, a river will immediately start to deposit fine sediment, since dredged sections will have more storage capacity. Material can either enter the river from the surrounding areas – often contaminated, and adding additional pressure to already degraded habitats – or “mined” from upstream parts of the catchment, resulting in unpredicted bank erosion problems and further increases in sediments. As these natural river processes kick in, they negate any long-term benefit of the operation.

There is an argument in support of local desilting of berms – a level space or raised barrier separating two areas that are used to control erosion and sedimentation by reducing the rate of surface runoff. This is necessary where, for example, it is essential to maintain flow under road bridges if rainfall becomes more frequent and intense. However, routine dredging of rivers is an expensive, unsustainable river management activity, with limited benefit.

An 870m reach of the River Knaik, in Perthshire and Kinross, Scotland, was dredged in June 1995, and illustrates what can happen when a river is dredged. According to Scottish Natural Heritage, the morphology – and hence the habitat quality – was destroyed and the dredged section replaced by a shallow, flat, uniform, silt-covered bed. The increase in fine sediment was recorded at 23,000%.

Some argue that the blocking of upland ditches and erosion gullies to delay the flood peak downstream is preferable, but this technique in itself is not enough to manage floods. We need to rethink our land and water management approaches at the catchment level. If strategically implemented, measures could be taken to reduce the amount of fine sediment and water runoff entering rivers during floods by focusing on the cause rather than the effect, thus negating the need to dredge rivers.

Sustainable drainage, soil compaction reduction, buffer strips to reduce water flow connectivity by increasing resistance and storing water to “slow the flow” can all play their part.

Of course no two catchments will ever respond the same. The Parrett catchment in Somerset, centre of so much discussion last winter, is a perfect example. Much of this clay-dominated catchment has inland areas that lie up to 4m below the coastal plain, a situation that contributed to most of the flooding. Unlike the nearby Brue catchment, where flooding was restricted to controlled washland zones with limited impacts on the local community, the Parrett has no exclusion sluice. As a result, more than 32km of river are subject to tidal-related siltation. The tide flows in over two hours and ebbs slowly out for 10, carrying more silt upriver than it is capable of returning.

Freshwater flows normally occur only a few times a year in contrast to the twice daily tides. Since the channel size and resultant discharge capacity of the tidal reaches are determined by the tidal regime, it cannot handle major freshwater floods. Any enlargement of the tidal reaches by dredging, however, results in rapid resilting. So, without the implementation of a range of catchment-based measures, including a tidal exclusion sluice, dredging would need to be frequent or even continuous.

Surely by implementing a range of solutions tailored to specific catchment needs we should be better able to manage flooding and ensure fragile wildlife habitats are preserved. This makes economic sense and would ensure that the dredger bucket is used as a last resort only.

Dr Jenny Mant is science and technical manager at the River Restoration Centre. She would like to thank independent chartered engineer Dan Alsop for his assistance with this article.

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