QUANTIFYING THE EFFECT OF CEDAR REVETMENT TO MITIGATE BANK EROSION IN RICEFORD CREEK, MINNESOTA, USING HIGH-RESOLUTION UAV IMAGERY

Southeastern Minnesota has entrenched streams that are susceptible to bank erosion. Efforts have been made to identify sections of Riceford Creek, which is a tributary of the Root River in southeastern Minnesota, that have high erosion susceptibility using Rosgens’ Bank Erosion Hazard Index (BEHI). In 2014, locally harvested cedars were used as a revetment to stabilize and mitigate erosion of the stream banks prioritized by the BEHI analysis. This study utilizes high-resolution aerial imagery acquired with a small UAV to 1) determine if cedar revetment effectively mitigates bank erosion in Riceford Creek and 2) determine if the BEHI method is an effective way of quantifying erosion hazard in Riceford Creek.

A detailed stream survey and aerial imagery were collected in spring 2016. In September 2016 a large flood, qualitatively estimated as a ~50 year event, effected the reveted stream reaches. Additional aerial photography was collected in spring 2017 along with another detailed survey that summer.

GIS software was used to digitize the position of the stream banks in both 2016 and 2017 based on the aerial imagery from the UAV. Distances between the 2016 and 2017 stream banks were then used to quantify the magnitude of erosion along the reach. These were used to determine areas and distances of bank retreat or accretion that occurred during the flood in September 2016. Preliminary results from a comparison of the amount (and direction) of movement between reveted and non-reveted reaches reveal that areas with revetment had an average erosion of 0.18 m and areas with no revetment had average erosion of 0.23 m, suggesting that the cedar revetments reduced the net erosion. Preliminary results indicate that there is at least some correlation between bank erosion and BEHI classification. It is interesting to note that the BEHI bank classes that received the revetments experienced much less erosion than the untreated classes.

This study suggests that the Cedar revetments in Riceford Creek effectively mitigated erosion during a large flood event. Furthermore, UAV acquired aerial imagery was an effective tool for quantifying the changes due to the flood.