Natural Flood Management (NFM)
Information about how natural flood management works and what we are trying to achieve.
Natural flood management (NFM) aims to reduce the downstream maximum water height of a flood (the flood peak) or to delay the arrival of the flood peak downstream, increasing the time available to prepare for floods. It works by restricting the progress of water through a catchment by making the catchment rougher and more difficult for water to flow quickly over land and in stream. The phrases "Slow the flow" or "Working with natural processes" are sometimes used to describe this approach to reducing flood risk. NFM strategies can be loosely classified by their likely location and distribution in a catchment They rely on one, or a combination, of the following underlying mechanisms:
a. Attenuation by using, and maintaining the capacity of, ponds, streams, floodplains and soils to attenuate water
An empty pond in Workmans Wood, Sheepscombe, to collect and attenuate high flows on the Sheepscombe Brook
b. Increasing soil infiltration. Allowing water to soakaway can reduce surface runoff. Free-draining soil will make saturation less likely, and evaporation from soil can also make space for water.
Soakaways downstream of culverts in Workmans Wood, Sheepscombe
Large Tree trunk acting as a flow diverter on spring line in Workmans Wood, Sheepscombe
Tree Trunks deflecting flow onto the woodland floor to increase infiltration. Workmans Wood, Sheepscombe
c. Slowing water by increasing resistance to flow. For example, by planting floodplain or riverside woods or constructing large woody debris dams in channels & gulleys or on flood plains.
Very large woody debris in channel and on floodplain of Dillay Brook, Slad valley.
Benefits of Large Woody Debris in streams
The Stroud valleys NFM project is using Large Woody Debris to help slow down the rate at which peak flows travel downstream and to spread the peak over a longer time period. The Land Drainage Act 1991 requires owners of water courses to maintain and remove obstructions to flow and historically, wood and fallen trees have been removed from streams and water courses. Whilst managing banks and removing obstructions can be essential to reduce flood risk in some locations, there are places where leaving trees or adding woody debris dams will have a variety of beneficial effects.
Wood debris dams form naturally when large and long sections of tree fall into and across the channel. These large sections start to gather smaller sticks and leaves which provide a permeable surface that allows water through, but crucially, reduces the rate of flow of water in the stream during high flows. Research has demonstrated that both small and large woody debris are essential for a healthy, functioning water course.
Woody debris changes the physical shape of the channel and can push flows out of the channel onto the bank. It helps to create pools and riffles, providing a variety of habitats for fish and aquatic insects and therefore attracting mammals and birds. Importantly for our purposes, Large Woody Debris can slow the movement of silt and sediment downstream. Silt can increase flood risk by reducing the amount of space for water in a channel. Large Woody Debris slows the rate at which a flood peak travels downstream by providing a physical barrier to flows. A large number of debris dams in a short stretch of stream can be more effective than a single large structure. Large Woody Debris can help to store and attenuate water. This is particularly the case when water is pushed out of channel and onto the flood plain, such as on the Dillay Brook in the photos below;
Large woody Debris dams built using Alder coppice at Snows Farm, Dillay Brook in the Slad Valley: Before heavy rain
Using wood sourced from the immediate steam side is a sustainable and economic way of reducing flood risk and improving water quality. The Stroud valleys NFM project is seeking sustainable and long term solutions to reduce flood risk for residents and we will seek to use wood from normal woodland management and thinning to provide materials for the construction of large woody debris dams.