Multi-Scale Assessment of Gully Erosion at Road Drainage Outlets
Gully erosion and sediment deposition from roads are underrated sources of sediments entering receiving waterways. While gully erosion has been studied throughout the world, the monitoring of the temporal and spatial erosional processes related to culverts and road drainage is rare. The objectives of this study are to quantify rates of gully erosion from Vermont’s transportation drainage networks at multiple temporal scales and report on insights gained from a multi-scale approach to monitoring gully erosion. To quantify event to seasonal timescales of gully erosion, high resolution terrestrial LiDAR surveys were conducted at 13 field sites. Field sites were monitored at least seasonally from September 2019 to May 2021. To evaluate longer-term rates of gully erosion, a coarser approach was implemented that used high resolution airborne LiDAR that covered a longer time interval than field surveys allowed. Culvert outlet locations were inspected for evidence of gully erosion and a sample of gully volume change was estimated via digital elevation model of difference. Results of this study indicate that gullies at road drainage outfalls are a common occurrence in our study region with a median gully frequency of one in every 2 kilometers of road. Gully presence is influenced by road length, elevation, and slope steepness, revealed by different scales of analysis. Estimates of gully change ranged -24 m3/year to 269 m3/year with 76% of features examined eroding less than 10m3/year. These features appear to be dynamically evolving, with some evidence that gully stabilization occurs in some sites over time. Gully stabilization may be achieved over time as these features reach an equilibrium slope or when grade controls such as fallen trees arrest incision. This study can be used as a building block for making roads more resilient to extreme weather events and reducing the environmental impact of roads.