What is Natural Flood Management?
Natural Flood Management (NFM) takes inspiration from nature, catching and detaining storm water where it falls, increasing ‘roughness’ across all parts of the catchment, to reduce flood risk. NFM measures should be many and small, installed across wide areas, at a landscape or catchment scale.
The principal purpose of NFM is to reduce and slow peaks of surface and river water which follow storm and other intense rainfall events, ‘slowing the flow’, such that damaging flooding is reduced. The concept is illustrated in the diagram below. Note that the overall volume does not change, but the time over which it occurs is extended, and therefore the height of the peak is notably reduced.
There are many additional Green Infrastructure (GI) benefits to NFM compared to traditional hard engineering solutions, although often both are needed and can complement each other well.
These multiple benefits are set out across the guide below in the form of simple bar-graphs showing the relative benefits and costs.
Benefits and challenges
Is it easy or difficult to implement NFM techniques?
The amount of work required to create various NFM techniques varies, according to their scale, complexity, and to legal requirements such as planning and ordinary watercourse consents. As they are often small in scale, maintenance of NFM features is usually undemanding and straightforward, similar to maintaining any interventions (e.g. drains or fencing) in the landscape.
This guide is based on a range of previously published information on the subject of NFM, notably:
Natural Flood Management Measures – A Practical Guide For Farmers (Yorkshire Dales National Park Edition)
Working with Natural Processes (WWNP) Evidence Directory – Environment Agency
These documents were used to inform the benefits graphs and to provide technical information. They are highly recommended if you wish to obtain more detail about NFM, with the Practical Guide setting out the agricultural benefits, techniques and providing more links to follow.
The WWNP Evidence Directory provides considerable detail on multiple benefits of NFM, and how they were calculated.
The purpose of this guidance, however, is to present a brief introduction to the main techniques of NFM – a first ‘toe in the water’, so to speak.
How does catchment woodland help?
• Planting and woodland debris creates a rough surface for the water to run across (hydraulic roughness) which slows down its passage across the woodland area;
• Trees reduce the amount of water that reaches the ground by up to 45% due to interception in the canopy;
• Trees help to prevent soil erosion and bind the ground together using their root network. This improves the ground’s water absorption capacity, and reduces silt in downstream watercourses.
Riparian and Floodplain Woodland
How does riparian and floodplain woodland help?
• Creates shade in areas of the river creating a good habitat for aquatic wildlife;
• Tree roots help to stabilise the banks and prevent erosion as well as collapse;
• Planting in the floodplain increases the amount of water infiltrated in storm events;
• Trees help to reduce the amount of siltation that occurs when the water returns to the watercourse;
• The woodland helps to slow runoff entering the system by adding roughness to the surface.
Riparian woodland in different settings: a steep-sided Pennine valley, a gentle chalk stream, and a valley floodplain in the North York Moors
Cross-slope Woodland and Hedgerows
How do cross-slope woodland and hedgerows help?
• Up to 45% of rainfall does not reach the ground as it is caught by trees;
• Flow is slowed due to the rough ground surface created by vegetation;
• Woodland soil acts as a sponge and soaks up some of the runoff;
• Hedgerows intercept and absorb surface water flows.
Soil and Land Management
How does soil and land management help?
• Prevents channels being created which speed up the flow of water;
• Improved soil structure reduces the amount of runoff and allows more water to be infiltrated;
• Soil erosion can be reduced and less sediment will get carried into the watercourse.
• Soil nutrients and moisture are retained and crop yields increased, whilst reliance on fertiliser is reduced.
How do leaky barriers help?
• Time taken for storm-water to travel downstream is increased and maximum flood peak reduced;
• Disperses water onto the floodplain to slow the flow and help with infiltration;
• Drains down once the main flood has passed.
Re-profiling Land and Re-routing Ditches
How does re-profiling land and re-routing ditches help?
• Existing drainage infrastructure can be re-evaluated and re-purposed to meet today’s needs and changing climate;
• Fairly small changes in topography can have a substantial impact on where storm water flows, and can direct it away from vulnerable roads and properties;
• In combination with the above, the capacity of ditches and drains can be improved using other techniques, like attenuation areas and leaky dams;
• These are relatively low-cost solutions, using features that already exist.
Storage Ponds and Basins
How do storage ponds & basins help?
• Creates an area for a large volume of water to be held back during heavy rainfall events, reducing flood risk downstream;
• Can create new habitats by holding some water all year round, creating small ponds;
• Drains back slowly into the main waterway, so there is no rush of water when released.
River and Floodplain Restoration
How does river and floodplain restoration help?
• Creating bends in the river slows the water down by making it take a longer route, giving more capacity and a shallower gradient;
• Floodwater can be held on the floodplains and discharge slowly back into the watercourse once the storm peak has passed;
• Restoration can help to create a more biodiverse environment by creating wet woodlands and wetlands.
How does headwater management help?
• Improves the efficiency of bogs, allowing more water to be soaked up, like a huge sponge;
• Can reduce the amount of sediment entering the watercourse;
• Dams and drain blocking slow down the amount of water leaving moorlands to a more steady rate;
• Climate regulation through the increase in captured carbon.