Flood risk management

Open spaces and green infrastructure can make a valuable contribution to managing surface water run off. This is a particular concern associated with the effects of climate change, notably the anticipated increased frequency of heavy rain events. Open spaces and green infrastructure can form part of critical flood risk management systems by providing space for managed flooding, protecting built up areas.

Urbanisation has significant effects on rainwater interception, storage and infiltration processes. Alteration of these hydrological processes can lead to increased surface runoff and greater vulnerability to flooding, with its associated physical and psychological effects.

Proper utilization of green infrastructure and Sustainable  Drainage Systems (SuDS)  can reduce the impacts caused by flooding. The primary benefits of SuDS to flood management are rainfall interception, increased soil infiltration, water uptake, water storage and the delay of peak flows, all of which reduce the quantity of water requiring management. Soil infiltration is a key method of managing flood waters and root penetration aids the process of infiltration. In addition, increased evapotranspiration releases watervapour to the atmosphere, increasing the storage potential of the ground. 

While it is obvious that blue infrastructure such as ponds, lakes and rivers store water, green areas such as parks and fields within the flood plain also have the potential to temporarily store storm water and aid with infiltration. Surface attenuation is a key benefit of green infrastructure and it has been shown that trees and pastureland can decrease peak flows by up to 60%.  

SuDS have been developed to improve urban drainage and therefore reduce the amount of surface runoff. Swales, infiltration trenches and retention basins capture water where needed and encourage infiltration into the ground.

One of the difficulties regarding storm water management appears to be that the function of green and blue infrastructure is not properly understood and engineers tend to lean towards engineered solutions when assessing the options for managing excess surface water. This is due to the ease of calculating the capacity and efficiency of engineered solutions, yet the benefits of SuDS are often less clear. The implementation of SuDS requires multidiciplinary working and  the intergration of urban design, landscaping and engineering considerations. The challenge can be overcome when urban design is at the heart of the design process.

Another problem derives from the fact that responsibility for flood risk management, and the infrastructure that supports it, is split between local authorities, individual water companies and the Environment Agency. This makes smart and sustainable planning of flood management measures virtually impossible to coordinate. The Flood and Water Management Act (2010) provides some clarity on these responsibilities and the proposed National Standards for Sustainable Drainage will help ease the difficulties around the delivery of SuDS.

CIRIA has developed a suite of guidance regarding SuDS and the implementation of sustainable water management measures. For more infomation, please visit the CIRIA SuDS website here or our Local Authority network LANDFORM here. To learn more about our range of SuDS training courses, please click here.