Bighorn River Inundation Risk Mapping
May 21, 2020
This study consisted of the following objectives:
Develop a series of maps depicting ground surface elevations relative to the river.
Approximate relative flood risk based upon ground elevations and lateral connectivity to the river.
The resulting data and maps can be used in conjunction with other Research Initiative products such as the Channel Migration Zone mapping to inform management decisions throughout the river corridor.
Elevations relative to the river surface were assessed using 2018 high-resolution LiDAR elevation data. A Relative Elevation Model (REM) was created by normalizing the elevation data set (LiDAR data) to approximate the trend of a water surface in the river using a cross section methodology (Figure 1). The resulting data display areas above the river grade as positive values and areas below the river grade as negative values as it extends laterally away from the river.
To relate relative elevation to inundation risk, a review of stage discharge relationships for the three USGS Bighorn River gages was used to define a range of elevations relative to the LiDAR river elevations that may be
at risk of inundation. Based on this assessment, elevations within two to three feet of the LiDAR river elevation are described as relatively high risk, three to six feet are considered moderate risk, and above 6 feet are considered low risk. Elevations at or below the LiDAR river elevation are assumed to be at very high risk of inundation. Areas greater than 10 feet above the LiDAR river elevation were determined to be above any likely inundation scenario and thus excluded from the maps. These risks are presented as a color gradient on the maps as shown in the map legend (Figure 2).
Figure 3 shows an area with a relatively narrow floodplain, but with some older channels with reactivation potential.
Inundation risk maps are a cost-effective way to estimate how land adjacent to a river may be impacted by flooding. Generally speaking, the higher you are above the river, the less risk of that land being inundated by rising water. The risk maps are designed to highlight those areas at highest risk. These risks will change depending local geography, constrictions, and distance downstream from Yellowtail Dam. Aside from inundation risk, low areas at high risk of inundation can help identify locations for side channel reactivation or riparian restoration. The work completed under this part of the Research Initiative provides an additional tool for land owners and managers to make informed decisions based on risk of inundation.
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