Flood frequency estimation by a derived distribution procedure
An event rainfall-runoff simulation procedure based on the method of derived distributions is proposed for the estimation of flood frequency for ungauged watersheds. The procedure uses a stochastic rainfall generation model and a rainfall-runoff watershed model. The results of previous research on rainfall characteristics and watershed response are incorporated into the two models. These rainfall characteristics are storm depth, storm duration, space and time distribution. The simplified watershed model, used in the procedure, has previously been tested and given good simulation of the watershed response. Some of the rainfall and watershed model parameters are stochastic in nature and are assumed to follow various probability distributions. Monte Carlo simulation is used for the generation of the various parameter values and simulation of the flood hydrographs. After 5000 realizations, the frequency of the hourly and daily peak flow and the flood volume is estimated. The proposed procedure is applied to eight coastal British Columbia watersheds and the results compare well with the observed data and with the Extreme Value type I (EVI or Gumbel) fitted probability distribution. The method is easy to apply, requires limited regional data and is shown to be reliable for small and medium forested watersheds with areas ranging from 10 to 600 km2. Sensitivity analysis shows that the procedure is stable and is not sensitive to the number of realizations. It is suggested that, given an appropriate adjustment of the rainfall generation model and testing and validation, the procedure could be used in areas with climates other than that of coastal British Columbia.