Probabilities of future flooding are the basis for planning and regulation of flood hazard areas in communities throughout the U.S. Estimates of future precipitation and flood discharge are essential to determine flood risk, but often these estimates are highly uncertain. In the Front Range region of Colorado, high spatial and temporal variability of summer storms and precipitation creates an especially high degree of uncertainty. Methods prescribed by the Federal Emergency Management Agency (FEMA) require that the estimates be used to define floodplain boundaries, and maps developed in accord with FEMA standards are used to identify which areas of a community are at risk of flooding and to plan adequate protective measures. Yet the maps often have turned out to be quite inaccurate when compared to actual flood events. Local decision makers and flood hazard managers express considerable skepticism about the accuracy of estimates of flood risk variables.

One factor contributing to uncertainties in flood risk variables is the uncertainty in local precipitation and runoff information. This study is intended to determine whether analysis of relationships between extreme precipitation and flooding and use of summer precipitation forecasts can contribute to flood hazard planning in the Colorado Front Range by reducing uncertainty. To be useful, the information must be accepted within the regulatory process and tailored to the needs of decision makers, floodplain administrators, and technical experts. Thus a major focus of the project is to understand the structure of flood hazard planning and regulation at federal, state, and local levels; interact with flood hazard planners and technical experts to learn about procedures, problems, local vulnerabilities, and information needs; and provide information that is operationally useful for local flood hazard planning and mitigation.

This talk will summarize what we have learned about decision making processes in floodplain management at local, state, and federal levels, how weather and climate information is presently used for flood hazard planning in Colorado, and how the users deal with uncertainty in that information.