How effective is stormwater attenuation on flood risk?

Stormwater attenuation is commonly used to mitigate the adverse effects of urban development on flood risk. Peak flows are commonly reduced for specific design events through on-site attenuation. But does it work? Could it be that flow attenuation actually increases downstream flood risk? The answer is yes, and it occurs probably more often than what you would think.

New urban developments are often called upon to achieve “stormwater neutrality”, where post development runoff peak flows are not to exceed pre development flows for specific events. This requirement is driven through a number of legislative frameworks, and generally assessed through comparing stormwater runoff hydrographs of the development area under pre- and post-development conditions.

Stormwater attenuation can have adverse impact on flood risk, depending on the wider catchment characteristics, the location of the development in a catchment, and the design flow adopted. A catchment wide assessment may therefore be necessary to ensure that downstream flood risks are not increased. The objective of this paper is to summarise legislative drivers that are often applied to flood risk, demonstrate the potential effect of flow attenuation on the wider catchment, and to emphasise key factors for assessment when looking at effective stormwater management of the flood risk.