Extreme floods in the Pacific coastal region

The research program developed hydrograph procedures for estimation of extreme rain-on-snow floods on ungauged watersheds in the Pacific coastal region. A multi-disciplinary investigation was undertaken encompassing the areas of hydrometeorology, snow hydrology and hydrologic modelling. Study components include assessment of flood producing mechanisms in the coastal region; analysis of regional rainfall characteristics for input to a hydrograph model; examination of the role of a snowpack during extreme events; and application of a hydrograph model. Based on an assessment of atmospheric processes which affect climate, examination of historical flood data, and analysis of flood frequency, it is shown that the area bounded by the crests of the coastal mountains forms a hydrologic region with similar flood characteristics. Extreme floods in the coastal region are rainfall-induced, either as runoff from rainfall-only or as a combination of rain and snowmelt. Recorded storm rainfall along the coast was examined to determine whether regional characteristics could be identified from available data even though the magnitude of rainfall varies between stations. Multi-storm intensity data available from Atmospheric Environment Service and rainfall intensities occurring within single storms that were identified as part of this study were analyzed. Results show that ratios of shorter duration intensities to the 24-hour rainfall are in a relatively narrow range in the coastal region for both multi and single storm intenstity data, and this range set limits on the hourly intensities that need to be considered as input rainfall data to a hydrograph model. With regard to basin response to extreme rain-on-snow, available literature suggests that for a ripe snowpack, development of an internal drainage network within the snowpack is the dominant routing mechanism for liquid water. Consequences of this conclusion on hydrograph procedures are that a watershed undergoes a transition from snow-controlled to more terrain-controlled water movement and basin storage characteristics approach conditions which would occur on the same basin without a snowcover. Lag and route hydrograph techniques were investigated to assess whether this method can be applied to rain-on-snow floods. Results from analysis of two rain-on-snow floods suggest this procedure can be applied when the following methodology is adopted: 1) estimate travel time through the basin from channelized and overland flow considerations; 2) select a storage coefficient which simulates basin response; 3) take water inputs as the sum of snowmelt and rainfall; and 4) consider there are no losses to groundwater. The combination of results from each study component provides a methodology for estimating input rainfall data and for undertaking hydrograph analysis for extreme rain-on-snow floods in the mountainous Pacific coastal region.Applied Science, Faculty ofCivil Engineering, Department ofGraduat