Aerial Photogrammetry and Repeat Lidar as a Method of Quantifying Erosion Rates in Legacy Sediment as an Alternative to Traditional Methodologies

Pennsylvania leads the US in rate of removal of low head dams and within the state Lancaster County has the largest number of remaining historical dams. Motives for dam removal include safety, fish passage, and water quality. Few studies monitor how the removal of these dams actually affects habitat, water quality, or geomorphic processes (e.g. bank erosion and sediment transport). At present, conventional methods of measuring bank erosion include the BANCs qualitative approach, bank pins, and cross sections. Understanding how historic reservoir (“legacy”) sediment responds to the removal of a dam is crucial to meeting federal pollution reduction targets. Furthermore, stream restoration is commonly used by jurisdictions to meet mandatory pollutant reduction goals. Estimates of sediment loads from stream bank erosion are used to calculate potential reductions of sediment and nutrients that can be achieved through restoration. However, quantifying rates of bank erosion is difficult due to seasonal variability, spatial variability, and inherent challenges and limitations in measurement techniques.

Rapid advances with airborne lidar and drone technology, including Structure from Motion, make it possible to identify the best sites for restoration and to quantify the potential load reductions more accurately. In south-central PA, we use lidar dem differencing from 2006 to 2014 to calculate erosion rates along all streams and identify reaches with the highest rates of erosion. These streams are heavily impacted by historic mill damming and store an abundance of legacy sediment. Along one of these streams, Indian Run, we calculate 0.2-0.3 m^3 of erosion per m of stream length per year, a relatively high rate for this region. We are refining this estimate with analysis of volumetric change between 2018 (drone photogrammetry) and 2014 (airborne lidar) with the goal of increasing the resolution of our results, reducing volumetric error, and understanding the abilities and limitations of photogrammetry. Supplementing traditional methods with higher fidelity datasets is crucial to how we quantify erosion of legacy sediment. Our preliminary results suggest that using drone based photogrammetry could reduce the uncertainty of our results by an order of magnitude.