Natural Flood Management or NFM
Natural flood management aims to store water in the landscape and slow the rate at which water runs off the landscape into rivers. There are 4 main points to Natural Flood Management: slowing water, storing water, increasing soil infiltration and interception of rainfall.
Every farm will have features that could play a role in natural flood management. Techniques to achieve this rely on one or a combination of the following fundamental mechanisms: By creating and maintaining capacity in bunds, ponds, ditches, swales or floodplains so they fill during rainfall events and slowly empty over 12 to 24 hours. Vegetation, especially tree leaves, intercept rainfall so it doesn’t reach the ground. This leads to drier soils underneath, which increases the amount of floodwater that the soil can store before generating run-off. Improving soil structure can increase the depth that water is absorbed to, which significantly increases the volume of water that can be stored in the soil to reduce saturation and surface runoff. Increasing the resistance to the flow of water helps to slow the rate at which it travels, reducing the risk of downstream
Planting trees is an important method of natural flood protection. Research has shown that trees can be up to 5 times more effective at soil infiltration than grazed grasslands. Trees also slow the flow of water across a downhill landscape, which in turn helps prevent soil and nutrients being lost to running water. Planting woodland strategically on and around the natural pathways can be more effective at interrupting, slowing and reducing flood water run-off. Key locations include run-off source areas, cross slope buffers/shelterbelts, and riparian zones.
Hydraulic Roughness of Different Vegetation on Floodplains
|Pasture No Brush
|1. Short grass
|2. High grass
|1. No crop
|2. Mature row crops
|3. Mature field crops
|1. Cleared land with tree stumps, no sprouts
|2. Same as above but heavy sprouts
|3. Heavy stand of timber, few downed trees, little undergrowth, flow below branches
|4. Same as above but with flow into branches
|5. Dense willows, summer, straight
This table clearly shows Willow and other SRC/SRF crops’ dominance as flood water manipulators. The higher the value, the greater the surface resistance for slowing the flow of water. Pastures made up of grassland don’t offer much resistance, but a balanced mixture of trees, hedges and coppice can significantly slow the flow of water.
Willow improves the land’s resistance to flooding by increasing infiltration and evaporation to reduce the volume of floodwater. Willow plantations can also prevent damage to downstream infrastructure by trapping flood debris and sediment, reducing the risk of damage to farm bridges, fences and other structures downstream. Willow increases evapotranspiration and slows the spread of water across a floodplain by inceasing ‘hydraulic roughness’. The crop is incredibly resilient and fast-growing – providing flood potection even in its first year. It also doesn’t need harvesting every year, so if there’s a flood one year, it can be left until the next harvest at no loss to the farmer.
Typically, energy crops require less tillage and less time with machinery in use, causing less soil compaction. Reducing soil compaction allows better water infiltration – again, reducing the rate at which rain water can reach streams and rivers. Adding energy crops to your grazed or arable land mix in the right places can help reduce runoff and erosion from your land.
Energy Crop Consultancy is not only able to advise you on the NFM benefits of the various energy crop types – they’re not the only way that you can improve your land’s NFM attributes. We can help you plan for and design other NFM features such as:
- Leaky Dams
- Cross Slope Woodland Planting
- Runoff Pathways
- Riparian Buffers
- Ponds & Swales
Energy Crops Consultancy is involved in lobbying both local and national government to have the Natural Flood Management attributes of energy crops recognised and remunerated.
When it comes to flood damage, prevention is better than cure. Government needs to do more to recognise the NFM work farmers are already doing to help towns and villages reduce peak flood levels.
Neil Watkins, ECC Director.