Floodplain Assessment for Two Large Braided Rivers in Denali National Park and Preserve

Abstract

The National Park Service (NPS) is required to assess risks of flooding to existing and proposed structures within the boundaries of the NPS system. Within Denali National Park and Preserve (DNPP and 2) There is a moderate to high potential for bank erosion. The development of a return period versus flood depth/bankfull depth curve is utilized to delineate inundated areas for a specific flood event. The bankfull depth is taken as the distance from the top elevation of mid-channel gravel bars to the thalweg. Flood depths for varying return periods are calculated using HEC-RAS and cross-sections from the Teklanika and Toklat Rivers. By determining the bankfull depth on rivers with similar hydrologic and hydraulic characteristics as the Teklanika and Toklat Rivers, the curve serves as a tool for calculating flood elevations and corresponding floodplains for specific flood events. A two-dimensional risk analysis based on potential bank erosion and consequence severity is used to assess localized bank erosion resulting from flooding. Bank characteristics and fluvial geomorphic variables, such as the bank height/bankfull depth ratio and channel width/depth ratio, are used to assess potential bank erosion. Potential loss of existing and/or proposed structures in a flood hazard zone is categorized with low, moderate, or high consequence severity. After NPS review, the method may be utilized on similar reaches.