DEM Differencing as an Approach for Identification of Stream Restoration Projects and Quantification of Streambank Sediment and Nutrient Loads
Lisa Fraley-McNeal
Center for Watershed Protection, Inc.
Ellicott City, MD
Authors: Lisa Fraley-McNeal, Mike Hickman, Bill Stack
Stream restoration has become an increasingly common tool utilized to meet water quality objectives, including wasteload allocations associated with Total Maximum Daily Loads (TMDLs). As such, practitioners are seeking the most effective methods to evaluate and report credits for sediment and nutrient reductions. The Chesapeake Bay Program (CBP) Stream Restoration Crediting Protocols were developed to provide a general technical framework for quantifying the water quality benefits of a specific suite of stream restoration practices, focusing on sediment and nutrients. Erosion rate monitoring and modeling through methods such as cross section surveys, bank pins, and BANCS assessments were recommended in the original crediting protocols approved in 2014. A recent workgroup developed Consensus Recommendations for Improving the Application of the Prevented Sediment Protocol for Stream Restoration Projects Built for Pollutant Removal Credit that expands upon the alternative bank erosion monitoring methodologies that can be used to calculate prevented sediment loads, including an approach for Digital Elevation Model (DEM) differencing.
DEM differencing has gained popularity with advances in digital imaging and drone technology. LiDAR-derived DEM differencing is the subtraction of two subsequent DEMs to derive DEMs of difference to map erosion, deposition, and volumetric change. The value of this methodology is the ability to identify changes along the entire stream network within a watershed over broad time scales. This method also allows for the quantification of erosion and deposition that has occurred along the length of the stream network as opposed to traditional monitoring methods at selected locations which then must be extrapolated to obtain estimates for a stream reach. Use of DEM differencing to remotely identify opportunities for stream restoration can save time and money by reducing on-the-ground field assessments required to identify stream erosion hotspots.
The Center for Watershed Protection has been utilizing LiDAR-derived DEM differencing to evaluate the sediment and associated nutrient flux within watersheds in Maryland and Pennsylvania. This presentation will provide an overview of our experience with the methodology, utility of the results in comparison to other monitoring and modeling approaches, and suggestions for future improvements.
About Lisa Fraley-McNeal
Lisa Fraley-McNeal is a Senior Watershed and Stormwater Research Specialist that joined the Center for Watershed Protection in 2006. Her areas of expertise include GIS and field methods for watershed assessment, watershed planning, stream restoration, Chesapeake Bay TMDL crediting, and applied research on topics related to watersheds and stormwater. She served as one of the Sediment and Stream Corridor Restoration Coordinators for the Chesapeake Bay Program and helped with development of the Stream Restoration Expert Panel Recommendations. Lisa’s interests include fluvial geomorphology and urban hydrology. She has a B.S. degree in Geography and Environmental Systems, with a writing minor and cartography certificate from the University of Maryland, Baltimore County. Lisa also has a M.S. degree from the University of Maryland, Baltimore County in Marine and Estuarine Environmental Science.