Development of a sustainable, catchment-based approach to flood protection
News from Forest Research: February 2008
Standing trees, fallen logs, branches and leaf litter form a very dynamic and complex network of multiple channels and dams, which help to slow down flood flows
Flood risk is a major international challenge and extreme floods are expected to become more frequent due to predicted increases in the amount and intensity of rainfall with climate change. This is focusing attention on the need to improve flood protection for homes and businesses on floodplains, as well as direct new build away from high-risk areas. The high cost of providing and maintaining hard engineered flood defences and their low ecological and aesthetic value is driving the development of a more sustainable, catchment-based approach to flood protection, working with land use to increase natural water retention and retard flood flows.
Advantages of floodplain woodland
Woodland provides a number of options for flood alleviation, principal amongst which is the ability of floodplain woodland to delay flood flows and enhance flood storage. This relies on the hydraulic roughness created by dams of woody debris within stream channels and by the physical presence of trees, shrubs and deadwood on the floodplain. The net effect of these features is to reduce flood velocities, enhance out of bank flows, and increase water storage, resulting in an overall smaller downstream flood.
Forest Research’s Huw Thomas has been evaluating the hydraulic impact of restoring floodplain woodland using computer-based model simulations to explore the interactions between woodland and flood flows at sites in South Wales, Yorkshire and Somerset.
Results confirm that floodplain woodland could help to desynchronise and thereby reduce downstream flood peaks in major river basins, as well as giving more time to issue flood warnings. The main concern surrounds an enhanced risk of upstream flooding above the floodplain woodland due to the backing-up of flood waters. Another concern is an increased risk of wash-out of large woody debris leading to the blockage of downstream culverts and bridges. However, it is believed that both of these issues can be resolved by careful site selection.
Huw is now carrying out fieldwork to measure the impact of floodplain and riparian woodland on flood flows with a view to testing and hopefully validating model predictions. Funding from Defra’s Flood and Coastal Erosion Risk Management Innovation Fund enabled Forest Research to carry out a major demonstration study for the River Laver catchment in North Yorkshire, but the selected site proved impractical.
Huw and Samantha Broadmeadow are now identifying other suitable locations. Samantha is assisting the Forestry Commission’s Yorkshire & The Humber Conservancy in mapping opportunities for floodplain woodland creation in the region. There is also strong interest in establishing a demonstration study in Scotland.
It is hoped that the field studies will strengthen the evidence for using floodplain woodland as a sustainable method for downstream flood alleviation. The results, models and suitability maps will be used to promote better integration of woodland and agriculture for flood risk management, as well as in the identification and prioritisation of sites for future action.