What Flooding and Dredging means for you

Flooding and river dredging became topical during the winter floods of 2013/14. However, the need to dredge a river merely treats a symptom of poor, uncoordinated management. Future Flood Risk Management must use natural processes at a catchment scale.

Apart from heavy rain, flooding is caused by:

  • Modern land management practices – e.g. land drainage and soil compaction lead to unnaturally rapid rainfall run-off.
  • Heavy rainfall washes unstable soils into rivers, silting up channels and making them less effective at moving large volumes of water swiftly.
  • Straightening river channels for drainage merely moves flooding downstream and erodes riverbanks, so exacerbating siltation. This canalising of river channels has also disconnected rivers from their natural floodplains.
  • Widespread building development has been encouraged on floodplains, leading to many properties being inundated at times of prolonged rainfall.

Solutions to flooding include measures on a catchment scale:

  • Slowing water flow down by:
    • Recreating wetland habitat to store water.
    • Protecting river corridors from potentially damaging land use.
    • Restoring meanders and reconnecting rivers with their floodplains.
    • Planting native trees in uplands to aid water retention.
    • Blocking drainage channels to increase water retention and naturally regulate flows.
  • Actively support the building of houses that incorporate flood resilience features, and discourage, where possible, further building development in floodplains.

Why dredging is not the whole answer:

  • Dredging may be necessary in some areas in the short term, but this only treats a symptom and should not be required under efficient catchment management
  • Dredging will almost certainly damage aquatic habitats
  • Dredging is expensive and requires on-going maintenance. Better to spend resources on stopping inputs at source
  • Allowing unregulated dredging puts the onus on landowners to avoid environmental damage, when they may not possess sufficient knowledge. This could result in the breaching of European environmental Directives.

Background

The winter floods of 2013/14 have inevitably attracted calls for rivers to be dredged by the Environment Agency (EA) to make them more capable of discharging water to the sea as quickly as possible in times of heavy rainfall. There have also been pilot de-regulation studies, supported by Defra, to allow landowners the freedom to de-silt their own rivers without seeking specific permission from the Environment Agency (EA), so taking away the Regulator’s coordination role in river management.

We appreciate that when a landowner’s fields are flooded, or a homeowner’s property left standing in feet of diluted sewage, then the wellbeing of the environment, let alone fisheries, is far from being a priority, and the causes of flooding are an irrelevance, or at least tantamount to being wise after the event. However, dredging rivers is merely a means of treating a symptom of poor land management. It is far better to manage rivers on a catchment basis and ensure that excessive silt levels do not enter rivers in the first place, and that particular attention is given to water retention and slowing down river flows. So, unless we understand the reasons why rivers flood, and how best to deal with the issues that we can influence, then we cannot hope to improve future Flood Risk Management policies.

What causes flooding in our rivers?

Flooding is a natural process. Rivers drain catchments (the entire area upon which rain falls that drains into a particular river system) and therefore prolonged heavy rain will eventually result in an excess of water in the system. However, several man-induced influences can drastically increase the effects of flooding:

  • Draining the uplands to improve livestock grazing, and the impaction of soils from overgrazing, results in rapid run-off, rather than water being retained in a natural ‘sponge’ effect at the top of rivers. This impact continues downstream, with land drained for agricultural purposes further adding to run-off.
  • Rapid run-off not only causes unnaturally high flows, it has the potential to wash unstable soils into streams and rivers, thereby increasing the silt input to channels, making them less effective at moving large volumes of water swiftly.
  • Many river channels have been straightened and canalised in recent decades in an attempt to move drained water quickly to the sea. We now know that this can be counter productive, often merely moving flooding further downstream and causing excessive erosion of riverbanks, so exacerbating the siltation problem.
  • Canalisation has disconnected rivers from their floodplains, which naturally allow excess flows to spill out over riparian land and so increase a valley’s capacity to carry water. Instead, widespread housing and business development has been encouraged on floodplains, leading to many properties being inundated at times of prolonged rainfall.

What are the Solutions?

We must work with natural processes as part of effective Flood Risk Management:

  • Slow water down. This can be done in a number of ways:
    • Recreate wetland habitat on marginal riparian land, using agricultural subsidies to compensate landowners for lost production.
    • Similarly, use agri-grants to compensate landowners for protecting river corridors with buffer strips and, where feasible, to alter farming practices – e.g. changing planting regimes so that soils are stabilised, and encouraging contour ploughing to aid water and soil retention.
    • Restore meanders in rivers and reconnect them with their floodplains, allowing them to naturally flood when necessary.
    • Plant native trees in upland areas, which again aid water retention. (Non-native conifers will potentially increase acidification, and so harm upland streams).
    • Block upland drainage channels and so increase water retention at the head of rivers, naturally regulating flows and protecting against floods and drought conditions.
  • Actively support the building of houses that incorporate flood resilience features, as well as discouraging, where possible, further building development in floodplains.

Even if we can accomplish all of the above, there will be periods of prolonged rainfall, like that experienced in the 2013/14 Winter, which will overwhelm any Flood Risk programme. However, a policy built around these solutions will certainly help guard against the more numerous medium-flood events, which presently have the capacity to cause misery to communities, economic loss and environmental damage.

Why dredging is not the whole answer

  • While dredging may be necessary in some areas in the short term, this only treats a symptom of poorly managed river systems and should not be required under efficient catchment management that works with natural processes and landowners to minimise inputs, such as unstable soils, at source.
  • Dredging will almost certainly damage aquatic habitats. Some ecosystems, such as those containing spawning and juvenile fish habitat, are extremely vulnerable to a digger bucket impacting deeper than anticipated. Total removal of weed, large boulders etc will denude fish and invertebrates of cover.
  • Allowing unregulated dredging puts the onus on landowners to avoid environmental damage, when they do not necessarily possess the required knowledge to assess the impact of their work. Any such activity must be coordinated under a catchment plan overseen by managers and regulators who fully understand the implications of measure such as dredging.
  • Dredging is expensive as well as environmentally invasive. Better to spend resources on stopping inputs at source, which would be a major part of effective catchment management, as laid out above.

Conclusion - Catchment Management

By using natural processes within Flood Risk Management, flow regimes will be naturally regulated and excess silts kept from entering river channels, so minimising the need to dredge and decreasing the amount of ‘hard’ engineering required further down river systems to protect property and livelihoods. This coordinated approach – referred to as Catchment Management – will deliver the maximum benefit to communities and their economies, and also protect river environments and their dependent species. Truly a win-win scenario.

Further information:

Dredging up trouble