Mountain precipitation analysis for the estimation of flood runoff in coastal British Columbia

A study of the precipitation distribution in coastal British Columbia is described and a technique is proposed for the reliable estimation of the frequency of rainfall generated floods from ungauged watersheds in the region. A multi-disciplinary investigation was undertaken encompassing the areas of hydrometeorology, meteorological modelling and hydrological modelling. Study components included analysis of long- and short-term precipitation in two medium sized watersheds located in southwestern coastal British Columbia; development of a 24-hour design storm for coastal British Columbia; generalization of the results over the coastal region of British Columbia; examination of the precipitation distribution during flood producing storms; identification of the applicability of a meteorological model for the estimation of short-term precipitation; and development of a physically-based stochasticdeterministic procedure for the estimation of flood runoff from ungauged watersheds of the region. Based on an assessment of the atmospheric processes which affect climate, it was found that the strong frontal storms which form over the North Pacific Ocean and travel eastward generate the majority of the precipitation during the winter and fall months, whereas convective rainshowers and weak frontal storms produce the dry summer period precipitation. Examination of the annual, seasonal, and monthly precipitation in the two study watersheds, the Seymour River and the Capilano River watersheds, showed that the variation of annual and winter and fall precipitation with elevation follows a curvilinear pattern, increasing up to middle position of the watersheds at an elevation of about 400 m and then decreasing or leveffing off at the upper elevations. The summer precipitation is more uniformly distributed over the watersheds than the winter precipitation and accounts for about 25% of the total annual precipitation. The Bergeron two-cloud mechanism has been identified as the dominant rainfall producing mechanism during the winter and fall months. Analysis of regional data and results of other regional studies indicate that the curvilinear pattern found in this study is more general and is similar for the whole of coastal British Columbia and the coastal Pacific Northwest. Study of the 175 storms in the Seymour River watershed showed that the individual storm precipitation is distributed in a pattern similar to that of the annual precipitation and this distribution pattern is not affected by the type of the event. Furthennore, the analysis showed that the storm time distribution is not affected by the elevation, type of the storm, its duration, and its depth. Also, analysis of the data from three sparsely located stations of coastal British Columbia indicated that the time distribution of the storms does not change significantly over the region. With regard to the development of techniques for the better estimation of flood runoff, a 24-hour design storm has been developed by using the data from the Seymour River watershed. Analysis of its spatial distribution revealed that this 24-hour design storm is distributed in a similar patter to that of the annual precipitation. Also, it was found that the 24-hour extreme raiiifall of various return periods is a certain percentage of the mean annual precipitation. Comparison with regional data and results of other regional studies showed that the developed design storm can be transposed over the whole coastal region of British Columbia. A comparative study and rainfall-runoff simulation for a real watershed showed that from the widely used synthetic hyetographs, only the Soil Conservation Service Type IA storm or the 10% time probability distribution curve of this study can accurately generate the flood runoff from watersheds of the region. The above results of the short-term precipitation distribution with elevation and in time were tested for extreme storms. Five periods of historical large flood producing storms were analyzed and it was shown that the fmdiiigs of the short-term precipitation analyses are valid for these extreme storms. The BOUNDP meteorological model was used for the estimation of storm precipitation in the mountainous area which covers the two study watersheds, but the results showed that this particular model is not capable of simulating the precipitation observed in the area. As a result, the initial intention of coupling the model with a hydrological model for the estimation of the runoff was abandoned. The above results of the analysis of precipitation in coastal British Columbia and the findings of previous research on the watershed response of coastal mountainous watersheds have been combined and used for the development of a physically-based stochasticdeterministic procedure. The procedure uses the method of derived distributions and Monte Carlo simulation to estimate the flood frequency for ungauged watersheds of the region. The procedure has been tested with data from eight coastal British Columbia watersheds and compared with the results of other widely used regional techniques. This comparison showed that the method is reliable and efficient, and requires very limited data, which can be found from a topographical map and the Rainfall Frequency Atlas for Canada.Applied Science, Faculty ofCivil Engineering, Department ofGraduat