Strategically planting trees along floodplains proves to be a successful flood mitigation strategy. Computer simulations created by the Universities of Birmingham and Southampton revealed that such trees could reduce the height of flooding in towns located downstream by as much as 20 percent.
For their study, researchers examined the New Forest river catchment, located upstream from the town in Brockenhurst, England. They used a digital terrain model of the landscape and a hydrological model simulation to estimate how tree planting, river restoration and logjams might affect the peak height of a flood in a downstream urban location.
"As our research shows, targeted tree planting and restoration can contribute to reducing flood risk," said Simon Dixon, lead author of the study from the University of Birmingham's Institute of Forest Research (BIFoR). "We believe that tree planting can make a big contribution to reducing flood risk, and should be part of a wider flood risk management approach, including conventional flood defences. Tree planting would represent an extra element that helps to slow down the arrival of rain water to vulnerable locations."
Researchers found that planting trees on the New Forest floodplain and increasing the number of logjams across 10 to 15 percent of the total river length could reduce the peak height of a potential flood in a nearby town by 6 percent once the trees had grown for 25 years. Furthermore, if trees were to be planted along 20 to 25 percent of the total river length, flood peak height could be reduced by as much as 20 percent.
However, within 100 years, as the trees age and forests become more mature and complex, their simulations showed even larger reductions in flood peak height.
Engineered logjams, which are man-made dams, have been implemented to slow the flow of rivers. While this may work on a local scale, researchers found that logjams are not successful in reducing flood risk at the catchment scale. In other words, they don't necessairly reduce the risk of flooding downstream. Therefore, researchers urge that detailed site studies should be conducted to identify the best locations for logjams.
"Logjams contribute to slowing the flow by backing up water and pushing it onto the floodplain," Dixon addded. "In locations where the floodplains are meadows or crops, the water may still be able to flow over the surface quickly. To make the best contribution to flood mitigation, it is important they are used in locations with complex bankside vegetation to slow water flowing over the floodplain."
"This research reminds us that natural processes, when targeted carefully, can reduce downstream flood risk alongside other societal benefits, including biodiversity and recreation," research supervisor David Sear, of the University of Southampton, concluded.
Their study was recently published in the journal Earth Surface Processes and Landforms.
