The Environment Agency (EA) has published a variety of documents, ‘The evidence behind natural flood management’ which contain data, case studies and evidence about the role of natural flood management in reducing flood risk. This is the first time all the evidence has been brought together, with the intention of enabling more uptake.
The report contains more than 60 case studies from across England and explores how successful the approach is, how it could be used elsewhere and what research may still be needed.
Natural flood management is when natural processes are used to reduce the risk of flooding and coastal erosion. Examples include: restoring bends in rivers, changing the way land is managed so soil can absorb more water and creating saltmarshes on the coast to absorb wave energy.
The Environment Agency has developed a Working with Natural Processes (WWNP) Evidence Directory which looks in detail at the effectiveness of different measures at reducing flood risk. This is supported by maps which help practitioners think about the types of measure that may work in a catchment.
To view a presentation to give you an overview of the Working with Natural Processes – the evidence behind Natural Flood Management project click on image below.
The data is presented across 3 areas –
• Evidence Directory that summarises the effectiveness of Working with Natural Processes measures from a flood and coastal erosion risk (FCRM) perspective as well as the wider ecosystem service benefits they may deliver.
• mapping the potential for WWNP which is intended to be used alongside the Evidence Directory to help practitioners think about the types of measure that may work in a catchment and the best places in which to locate them.
• research gaps that need to be addressed to move this form of FCRM into the mainstream are identified in the Evidence Directory
The report highlights examples of different forms of natural flood management, describes the level of confidence in the flood management examples, areas and actions the EA feels still need to be actioned or explored, benefits accrued from using the management scheme plus further reading, case studies and maps. The report also comments on the degree of scientific confidence for each topic i.e. the level of confidence in the science that underpins the individual measures is given a confidence level (high, medium or low) based on the potential effectiveness of each measure at reducing flood risk.
The report looks at the following key areas:
River Restoration – River restoration reintroduces meanders to rivers and restores physical process. Making a river more sinuous can reduce flood peaks, water velocities and attenuate flow by slowing and storing flood water. The extent of this flood risk effect depends on the length of river restored relative to the overall size of the river catchment.
Floodplain Restoration – River floodplain restoration restores the hydrological connectivity between the river and floodplain, which encourages more regular floodplain inundation and flood water storage. This can decreases the magnitude of the flood peak and reduce downstream flood depths especially for high frequency, low return period floods. The extent of this flood risk effect depends on the length of river restored relative to the overall size of the river catchment.
Leaky Barriers – Leaky barriers are usually formed of wood and they are either formed naturally or are installed across watercourses and floodplains. They reduce flood risk by intercepting the flow of water in a river, this can can help restore river-floodplain connectivity which can reduce flood peaks, slow water velocities and attenuate flow by storing water on the floodplain.
Offline Storage Areas – Offline storage areas, are areas of floodplain which have been adapted (with a containment bund, inlet, outlet and spillway) to store and then release flood waters in a controlled manner. They provide temporary flood storage which can reduce peak flow. The extent of their flood risk effect depends on the number of storage areas provided throughout a catchment and their total storage volume.
Catchment Woodland – Catchment woodland can intercept, slow, store and filter water. This can help reduce flood peaks, flood flows (from 3 to 70%) and flood frequency. Largest reductions in flood risk have been seen for small events in small catchments, the extent of this reduction decreases as flood magnitude increases.
Cross-slope Woodland – A cross-slope woodland is a woodland which is planted across a hill slopes. It intercepts the flow of water as it runs down the hill reducing rapid runoff and encouraging infiltration and storage of water in the soil. There is an absence of measured data to show the flood risk impact of cross-slope woodland at the catchment scale.
Floodplain Woodland – Woodlands in floodplains can slow floodwaters and increase water depth on the floodplain. This can help reduce flood peaks (0-6%), delay peak timing (2 hours or more), desynchronise flood peak and reduce peak height. It can also enhance sediment deposition on the floodplain. Floodplain woodlands have greatest flood risk effect in the middle and lower river reaches of medium to large catchments.
Riparian Woodland – Riparian woodlands are planted on land immediately adjoining a watercourse, they can slow flood flows and can help reduce sediment delivery to the watercourse and reduce bankside erosion. They also have high evaporation losses and can create below ground water storage. Largest reductions in flood risk have been seen at the reach scale, in middle and upper catchments
Soil and Land Management – Soil and land management techniques can reduce peak flow by slowing and storing surface water runoff and encouraging infiltration with the soil. They can include a wide range of different measures as shown in the following flow chart.
Headwater Management – Headwater drainage management techniques can delay and flatten the hydrograph and reduce peak flow locally for small flood events by intercepting, slowing and filtering surface water runoff and encouraging attenuation and infiltration with the soil.
Runoff Management – Run-off pathway management techniques can delay and flatten the hydrograph and reduce peak flow locally for small flood events by intercepting, slowing and filtering surface water runoff. They can include a wide range of different measures as shown in the following flow chart. They usually work best as a cluster of features working as a network throughout the landscape.
Saltmarsh and Mudflats – Saltmarsh and mudflats reduce and dissipate wave and tidal energy in front of flood defences and can extend their design life. They can reduce the forces impacting on flood defences, and also reduce tidal surge propagation and lead to slightly lower water levels at defences.
Sand Dunes – Beach-dune systems form a natural barrier that reduce the risk of tidal inundation landward of the dune, they also act as reservoirs of sand to nourish beaches during storms. They act as a buffer protecting flood defence structures or cliffs behind from direct wave attack and erosion, this in turn enhances the design-life of other flood risk management infrastructure. They can also protect estuaries and lagoons through restricting the passage of storm surges and waves (Pye et al., 2007).
Beach Nourishment – Beaches provide an effective form of coastal defence, but only if they are of sufficient width and level. Where beach systems become depleted this affects their flood risk management value. Beach nourishment is the process of adding material to the shoreline. It is undertaken to improve or restore beach and their coastal defence function, it helps retain the standard of flood protection to the section of coast where implemented. To be effective it is a long-term maintenance activity usually repeated annually.
All the ‘Working with natural processes to reduce flood risk’ documents, maps and case studies can be read here