UC San Diego

The research presented in this dissertation is a prototype synthesis of hydrologic, meteorological, and geomorphologic principles and modeling toward the characterization of flash flood occurrence frequency in the southern California mountainous region. Globally, flash flooding has the highest fatality rate per person affected among natural disasters and occurs over small spatial (<500 km²) and short time (<12 hours) scales. This research characterizes regional flash flood occurrence potential with high spatial resolution (O[30 km²]) through an interdisciplinary modeling approach that involves diagnostic analysis to understand the small- and large- scale forcings and effects of flash flooding in southern California, geomorphologic principles to define the level of flood occurrence in streams draining flash flood prone basins, numerical modeling of orographically-forced precipitation with high resolution, hydrologic modeling of soil moisture evolution, and, as a synthesis of models and data, the simulation of flash flood occurrence potential. The research begins with an investigation of observed precipitation and streamflow at scales relevant to flash flood occurrence, i.e., hourly to daily timescales (Chapter 1). This analysis, based on a compiled regional database, demonstrates the influence of larger scale climatic forcing on the regional precipitation and streamflow occurrence associated with flash flooding conditions in southern California. Geomorphologic principles defining bankfull streamflow and the hydrologic index for flash flood occurrence are presented in Chapter 2, with region-specific relationships for predicting bankfull conditions with high spatial resolution. High spatial and temporal resolution estimates of precipitation are required in this research due to small scales of flash flood occurrence in nature. Chapter 3 intercompares three alternative methods for estimating orographically driven precipitation in the region based on raingauge data and numerical models, and includes the adaptation and application of a simplified high resolution orographic precipitation model for southern California. The research culminates in Chapter 4 with the characterization of soil moisture climatology (1949-2005), and a multi-model ensemble characterization of the historical regional flash flood occurrence frequency. Frequency is estimated by counting the instances of precipitation exceeding the threshold satisfying bankfull and soil moisture deficit conditions as defined from the earlier geomorphologic and soil moisture modeling study. Chapter 4 concludes with an application of the integrative methodology developed to identifying changes in flash flood occurrence frequency under projected climatic change