Estimating design flood runoff volume

Abstract

Abstract Standard flood risk estimation methods in the United Kingdom have largely focused on peak flows at ungauged locations. However, the importance of whole‐hydrograph and event volume estimation in a design context is increasing with the application of unsteady‐state hydraulic models and construction of sustainable drainage systems. Here, we explore the relationship between peak flow estimation accuracy and flood volume estimation accuracy across 780 events in 81 catchments. Runoff hydrographs are modelled using ReFH2, a rainfall‐runoff model widely used by practitioners for design flood estimation in the UK. We find that strong performance in peak flow estimation is highly correlated with strong performance in event volume estimation, and that between‐event variation in performance is greater than the typical reduction in performance when moving from calibrated to design (regression‐based) model parameters. Unfortunately, evaluating model performance in terms of runoff volume is complicated by the fact that measured rainfall hyetographs and runoff hydrographs are themselves estimates that can disagree with each other for legitimate reasons. We demonstrate that it is not always possible, expected or realistic to close the water balance over an event in a topographically defined river catchment. Hence, ‘errors’ in modelled hydrographs cannot be solely attributed to modelling deficiencies.

Keywords

• hydrological modelling
• hydrology
• rainfall‐runoff