Hydrometeorological analyses of extreme floods in urban and mountainous watersheds

This dissertation presents hydrometeorological and hydrological analyses of extreme floods in urban watersheds and mountainous regions. The first part of the dissertation deals with issues pertinent to implementing an enhanced flash flood forecasting system in an urban setting. Hydrometeorological analyses of rainfall and flood response are carried out in urbanizing watersheds in the Philadelphia and Baltimore metropolitan regions. These analyses are based on high resolution (1-km, 5-minutes) rainfall fields derived from WSR-88D (Weather Surveillance Radar - 1988 Doppler) radar reflectivity observations and USGS (United States Geological Survey) discharge observations. Bias-correction of the radar derived rainfall products is a key element in quantitative flash flood forecasting. Diagnostic and hydrologic model analyses show that stormwater management structures (e.g. storm drain network) control the extreme flood response of urban sub-watersheds (drainage area of less than 10 km 2). Initial soil moisture is also playing a secondary role to that of constructed storage (e.g. small reservoirs and detention basins). The second portion of the dissertation concerns the climatology of extreme rainfall from orographic thunderstorms in the Front Range of the Rocky Mountains. Analyses are motivated by problems of dam safety in which distributional properties of extreme rainfall are of most interest. A radar data set of sixty-six storm events from the summer months of 1995-2003 were compiled and developed. Climatological analyses of extreme rainfall are carried out both from an Eulerian perspective, in which distributional aspects of rainfall at fixed locations is examined, and a Lagrangian perspective in which distributional aspects of rainfall are based on storm tracking algorithms. Analyses focus on the spatial and temporal distribution of warm season rainfall in the complex terrain of the Upper Arkansas River basin. The pronounced diurnal cycle is a key element in the warm season rainfall climatology. Lagrangian analyses of storm initiation, motion, and development points to the observation that the upper 3000 mi2 of the Arkansas River basin does not contribute to extreme flooding downstream at Pueblo.