Examining terrain elevation assumptions used in current extreme precipitation estimation practices: A modeling study of the 2013 Colorado Front Range floods

Many mountainous regions are particularly susceptible to flash flooding due to the combination of steep, complex terrain and the potential for sustained heavy rainfall via orographic precipitation generation mechanisms. However, past paleohydrologic studies have suggested that there is little evidence of extreme rainfall and floods above 7,500 feet (~2280 meters) elevation in the Rocky Mountains of the U.S.

From a practical perspective, there are many U.S. dams built in high elevation locations (i.e., above 7,500 feet), and water resources managers and engineers require information about the potential for heavy precipitation to ensure structural and operational safety of dams and other water management infrastructure. While reference documents and procedures have existed for decades to estimate extreme-precipitation potentials across the U.S., recent events (most notably, the extraordinary Colorado Front Range flooding of 2013) have highlighted gaps in our understanding of extreme precipitation in mountainous areas, particularly with respect to the seasonality of extreme precipitation, elevation limits on heavy rainfall, and the types of weather systems that produce such events.

The elevation at which dams are located directly affects the design, construction, and ultimate operating standards via elevation “corrections,” or adjustment factors, which reduce the theoretical calculated upper limit of precipitation with increasing elevation. To more closely examine approximations and assumptions currently used in creating these adjustment factors, this study employs a high-resolution regional modeling study of the 2013 Colorado Front Range flooding event. Specifically, sensitivity to elevation is examined via a series of numerical model simulations in which both model terrain and storm environments are separately modified to assess both precipitation sensitivity to terrain in this particular extreme event, and also the impact of the storm environment on maximum elevations affected by heavy precipitation. Results are intended bring state-of-the-art climate and weather modeling capabilities to bear on an important and widely used risk-assessment approach, and gauge the added value of these types of sensitivity studies.