FLOODPLAIN REGULATIONS AND FLOOD INSURANCE FOR ALLUVIAL FANS IN COLORADO

Many Colorado municipalities and counties are affected by flood hazards associated with alluvial fans (including debris flows). Counties affected include Clear Creek, Boulder, El Paso, Eagle, Garfield, Pitkin, Gunnison, Hinsdale, Ouray, San Miguel, and Montezuma. At least two other counties, Rio Grande and La Plata, have had such problems greatly exacerbated by recent wildfires.

All of the above counties (and many municipalities within them) participate in the National Flood Insurance Program (NFIP). Alluvial fans are acknowledged by the NFIP, but mapping or regulating these hazards is not explicitly mandated. Flood insurance is available, but it does not fully recognize the unique character of alluvial fans. Alluvial fans have been mapped in detail by FEMA in three Colorado communities (Glenwood Springs, Ouray, and Telluride). No model regulations have been developed by the State of Colorado or FEMA to provide local governments or property owners with specific guidance for managing these hazard areas. There is a significant void in technical and administrative direction for mapping of hazards, design or review of proposed mitigation measures, or determination of appropriate insurance rates.

Mapping must include a combination of geologic and engineering methodologies. Both disciplines must join with planners and building officials to develop standards for the design and review of mitigation measures. Technical criteria for approving or disapproving mitigation measures for conventional riverine flooding have existed in Colorado for many years, but they do not exist for debris flows and alluvial fans.

Probabilistic analysis guides flood insurance rates. A “100-year” flood has a 1% chance of occurring in any year, and a “10-year” flood has a 10% chance. Alluvial fans involve a complication. Within the 100-year fan a single 100-year flood event may go to the left, to the right, or down the center. The center is typically higher in elevation because historic events have deposited more debris there. However, gravity may make the lower flow paths to the sides preferable for future floods. Quantifying risk in alluvial fans is not for the faint-hearted.