Mapping Fluvial Geomorphic Hazards In Valley Margins: The Fluvial Hazard Buffer

Fluvial geomorphic hazards resulting from valley bottom fluvial process such as channel migration or the erosion and deposition of sediment and debris can impact valley margins adjacent to and well above the active geomorphic floodplain. Valley margin and hillslope impacts due to toe erosion, hillslope failure, and mass wasting events are typically not considered in riverine flood hazard assessments or floodplain inundation mapping. Fluvial geomorphic processes can impact property and structures well above a mapped floodplain as is evidenced by the high percentage of flood insurance claims in the United States that come from properties located outside of mapped floodplains. We present a method and framework for evaluating and mapping valley margin hazards, what we call the Fluvial Hazard Buffer, to fill in this gap of flood and riverine hazard analysis. Using pre- and post-flood digital elevation models as well as aerial imagery, we analyzed valley margin erosion as a result of flood events and from gradual channel movement over time in streams and rivers representing a variety of flood climatologies and geomorphologies in Colorado, U.S. With this analysis we develop numeric guidance for mapping the fluvial hazard buffer beyond the margin of the geomorphic floodplain. This buffer is a factor of stream or valley bottom width depending on the stream type. Though the regional dataset informing these guidelines may be specific to Colorado, the framework for mapping the fluvial hazard buffer may be applied elsewhere.