2017 – Estimating the Duration of Flood Overtopping for Risk Assessment

D Stephens,  S Lang,  P Hill,  M Scorah

Robust estimates of the duration of flood overtopping are a key input into the dam safety risk assessment process. For embankment dams, the likelihood of erosion of the dam crest, downstream face and eventual unravelling of the embankment are heavily dependent on the duration of water flowing over the crest. Similarly, the chance of erosion of the abutments of concrete dams is strongly linked to the duration of floodwaters overtopping the dam. Previously, it has been difficult to define the annual exceedance probability (AEP) of the flood required to cause overtopping of a certain depth for a certain duration, and coarse assessments have typically been made based on critical storm durations of the dam crest flood (DCF). This approach carries significant uncertainty, particularly for structures on smaller catchments where the critical storm duration on outflow may be relatively short. In these cases, it has been difficult to confirm with any reliability that the flood required to achieve a significant duration of overtopping has an AEP close to that of the DCF. This paper describes a new algorithm that has been incorporated into the RORB hydrological model which allows for a frequency curve of flood overtopping duration to be determined within a Monte Carlo framework. The results of this analysis are presented for a case study of a quantitative risk assessment, to demonstrate how the outcomes influenced numerous aspects of the risk analysis process.