A framework for assessing flood frequency based on climate projection information

Abstract. Flood safety is of the utmost concern for water resources management agencies charged with operating and maintaining reservoir systems. Risk evaluations guide design of infrastructure alterations or lead to potential changes in operations. Changes in climate may change the risk due to floods and therefore decisions to alter infrastructure with a life span of decades or longer may benefit from the use of climate projections as opposed to use of only historical observations. This manuscript presents a set of methods meant to support flood frequency evaluation based on current downscaled climate projections and the potential implications of changing flood risk on how evaluations are made. Methods are demonstrated in four case study basins: the Boise River above Lucky Peak Dam, the San Joaquin River above Friant Dam, the James River above Jamestown Dam, and the Gunnison River above Blue Mesa Dam. The analytical design includes three core elements: (1) a rationale for selecting climate projections to represent available climate projections; (2) generation of runoff projections consistent with climate projections using a process-based hydrologic model and temporal disaggregation of monthly downscaled climate projections into 6-h weather forcings required by the hydrologic model; and (3) analysis of flood frequency distributions based on runoff projection results. In addition to demonstrating the methodology, this paper also presents method choices under each analytical element, and the resulting implications to how flood frequencies are evaluated. The methods used reproduce the antecedent calibration period well. The approach results in a unidirectional shift in modeled flood magnitudes. The comparison between an expanding retrospective (current paradigm for flood frequency estimation) and a lookahead flood frequency approach indicate potential for significant biases in flood frequency estimation.