What is Natural Flood Management?

Natural flood management uses techniques to restore or imitate the natural functions of rivers, floodplains, and the wider catchment, enabling water to be stored in the landscape and to slow the flow of water running into rivers. There are several different types of Natural flood management (NFM) and this is often referred to using different terms such as ‘nature based solutions’. NFM techniques rely on one, or a combination, of the following processes to function:

  • Slowing water by increasing the roughness of the landscape. This increases resistance to its flow for example, by planting hedgerows and trees, installing leaky dams using woody debris, or creating buffer strips (strips of vegetation such as trees and hedges).
  • Storing water in the landscape using bunds, ponds, ditches, swales or floodplains so they fill during rainfall events and empty slowly over 12 to 24 hours.
  • Increasing soil infiltration through improving soil structure can increase the depth to which water is absorbed, significantly increasing the volume of water that can be stored in the soil. This will make saturation less likely, potentially reducing surface runoff.
  • Intercepting rainfall using vegetation, especially tree cover is effective as a significant volume of water can be held within the tree canopy. This helps by slowing its journey to the ground with a proportion evaporating from the leaves, reducing the volume of water that could contribute to a flood. Trees can reduce the amount of water reaching the ground by 25 – 45 % for conifers and 10 – 25 % for broad leaves[1].

 

[1] CALDER, I. R., REID, I., NISBET, T. and GREEN, J. C. (2003). Impact of lowland forests in England on water resources – application of the HYLUC model. Water Resources Research 39, 1319–1328.

Soil Aeration

Soil aeration is a way of achieving multiple benefits across a large area through improving the condition of the soil.

Often soil is compacted by heavy machinery or livestock, particularly when the ground is waterlogged. Compact soil acts as a barrier to water being absorbed, increasing the rate and speed at which it runs off toward the watercourse. This water will often take fertilisers and nutrients with it. This compaction also affects the ability of plants including grasses to grow.

By understanding the condition of the soil and selecting the correct aeration machinery, the compacted earth can be cut into and broken up to enable greater water infiltration and consequently, absorbency.

The impact of doing this means more water is retained in the ground, reducing the volume entering waterways in high rainfall. In turn this reduces the flood peak. Holding the water in the soil also reduces local flooding as there is less surface water to run off. Furthermore, by improving the connection with groundwater, and increasing the ability of the ground to store water, grasses and crops can develop stronger root systems improving yield.

Grassland Conversion

By allowing and encouraging grasslands to grow, there is an improvement in the capacity of the ground to capture and store water than where soil is compacted or intensively farmed. The improved structure and natural aeration of the ground through the action of the roots from the grasses and plants in the grassland help to breakup the soil and increase its ability to hold onto water.

This creates a habitat better able to support a diverse ecosystem and also encourages the filtration of pollutants previously held in the soil (fertilisers etc) which would otherwise run-off into watercourses.

The grasses also prevent sediment running into waterways, improving water quality and because of the length of grasses and their ‘tussocky’ nature, the effective distance water has to travel is increased along with resistance to this, slowing the flow into rivers and streams.

Tree Planting (Woodland Creation)

There are several ways that trees can help in reducing flood risk.

Firstly, trees generally evaporate more water than other types of vegetation. This can reduce the volume of floodwater draining from the land and into the watercourse. More evaporation reduces the amount of storm rainfall reaching the woodland floor, and results in drier soils that can then store more of this water below-ground.

Secondly, soils under woodland tend to be better structured due to the action of root systems, enabling more storm rainfall to enter and pass through the soil rather than quickly run-off the surface. This promotes the retention of floodwater within soils and delays its passage to watercourses.

Thirdly, trees, shrubs and deadwood, particularly running alongside streams and within floodplains exert a greater drag on floodwaters, compared with grass, slowing the flow.

Finally, tree cover protects the soil from becoming eroded, preventing sediment being carried into watercourses, This reduced the effect of silt on river and aides water quality.

Hedgerow Planting

Hedgerows are an intrinsic part of the landscape, providing boundaries as well as important benefits to agriculture such as shelter and shade for livestock.

Hedges can be used to control rainwater run-off, they act as a permeable barrier to water, slowing the flow and reducing compaction of soil, thus increasing the opportunity for water to be absorbed into the landscape. They also prevent sediment and debris from being transported into the watercourse.

Hedgerows are also very beneficial to wildlife and biodiversity, providing habitat and shelter to a wide range of species and creating a ‘wildlife corridor’.

Bunded Hedgerows

The flood reduction benefit to hedgerows can be improved by planting them on an earth bank (bund). This increases their capacity to hold back water and slow the rate of water entering the watercourse.

Buffer Strips

Buffer strips are strips of vegetation that provide a physical barrier to slow the flow of rainwater flowing off the land. Using barriers consisting of vegetation also increases infiltration of the water and prevents soil, sediment and nutrient loss from the land.

Riparian buffer strips are positioned next to watercourses (between 4-12m away from the bank) whereas in-field buffer strips are found adjacent to field boundaries and across fields.

Buffer strips can contain long grasses, trees and shrubs. Alongside a river they usually require fencing to prevent livestock from accessing both the buffer strip and the watercourse itself.