Flood frequency analysis over the western United States is complicated by annual peak flow records that frequently contain annual flows generated from distinctly different flood generating mechanisms. Bulletin17B (B17B) and its update Bulletin 17C (B17C) recognized the difficulties in determining flood frequency estimates with streamflow records that contain a mixed population of flood generated peaks, and recommend developing separate frequency curves when the hydrometeorologic mechanisms that generated the annual peak flows can be separated into distinct populations. Yet challenges arise when trying to consistently quantify the physical process that generated the observed flows. This thesis examines the role played by different flood producing mechanisms in generating annual maximum floods throughout the western United States using process-driven mixed populations.
First I evaluate the impacts of hydrometeorological processes on flood frequency in the western United States, with emphasis on the spatial and fractional contributions of atmospheric rivers (ARs) and eastern North Pacific tropical cyclones and their remnants (TC events) on annual maximum flows throughout this area. Six main areas in which flooding are impacted by ARs at varying degrees are found throughout the western United States. The Pacific Northwest and the northern California coast have the highest fraction of AR-generated peaks (~80–100%), while eastern Montana, Wyoming, Utah, Colorado, and New Mexico have nearly no impacts from ARs. The individual regions of the central Columbia River Basin in the Pacific Northwest, the Sierra Nevada, the central and southern California coast, and central Arizona all show a mixture of 30–70% AR-generated flood peaks. Analyses related to the largest flood peaks on record highlight the strong impact of ARs on flood hydrology in this region. Conversely, TC events play a limited role in controlling the upper tail of the flood peak distributions across the western United States. Southern California, Arizona, southernmost Nevada and Utah, southern and western New Mexico, central Colorado, and Texas have the highest fractional contributions of TC-event-generated annual maximums flows (~5-14%).
I then build on these insights to develop a statistical framework to perform a process-driven flood frequency analysis using the AR/non-AR-generated annual peak flows identified at 43 long-term U.S. Geological Survey (USGS) streamgages in the western United States. I use a simulation framework to perform flood frequency analyses in terms of mixed distributions and quantify the corresponding uncertainties by accounting for mixed populations. Sites with notably different quantile estimates in the upper tail of the distribution between the single (homogeneous) and the weighted (heterogeneous) population methodologies are found when (i) potentially influential low floods (PILFS) are identified and/or (ii) when the composite distribution contains markedly different at-site log-unit skews (shape parameter) among the AR/non-AR subpopulations compared to the single homogeneous population.