Geomorphic Monitoring of Coastal Marsh Restoration Sites: Insights from Field and Remote Sensing Approaches in Louisiana, USA
Authors: Shawler Justin , Ostojic Aleksandra , Harwood Joseph , Macon Christopher , Reif Molly , Schad Aaron 
Published: February 2025

Restoration of coastal marshes presents unique geomorphic monitoring challenges because these sites are often remote and/or inaccessible, and time and financial resources for field-based geomorphic monitoring may be limited. Yet, the geomorphic trajectory of coastal marshes controls the overall system health and longevity. The expansion of Unmanned Aircraft System (UAS) technology and new satellite platforms offer opportunities to complement ground-based geomorphic monitoring and overcome the challenges associated with traditional field methods. Here, we compare field-based and remote sensing approaches to monitor two restored coastal wetlands in Louisiana. At Spanish Pass, a restored marsh and ridge system, methods for measuring site elevation, quantifying shoreline position and classifying shoreline geomorphic types were compared. Field elevations measured with RTK GNSS were highly correlated (R2 = 0.97, RMSE = 0.08 m) to site elevations from the UAS lidar digital elevation model (DEM). Both UAS RGB imagery and satellite imagery collected within the same month as field data were within 3 m of field shoreline positions (mean absolute error 2.88 m UAS RGB imagery; 2.86 m October 2022 Google Earth imagery), though datum and slope-based shorelines derived from the UAS lidar DEM had the lowest error (2.09 m datum; 2.32 m slope) compared to the field. National Agriculture Imagery Program (March 2021) and Google Earth (January 2021) imagery collected >18 months prior to field data collection had the least accuracy compared to the field shorelines (mean absolute error 3.13 m and 5.08 m, respectively), likely reflecting shoreline change since that date. In general, field classifications and remotely sensed geomorphic shoreline classifications did not compare well (similarity values 0.4 to 0.6), which supports previous results from the literature that show large differences between field and remote sensing classifications. At La Branche, the second marsh restoration site, access was limited by thick vegetation and ground-based RTK transects were not collected. Thus, UAS lidar DEM elevation data were compared to elevations measured by airborne lidar (2017) and ground surveys (2013) to extend the monitoring record and track elevation change through time. The results demonstrated that minimal measurable elevation change occurred during that time. The airborne lidar data showed consistently lower elevations than field data (within the 0.2 m vegetated accuracy of the lidar), but UAS lidar elevations matched field data, with some spots at slightly higher elevations due to lack of bathymetric lidar data as well as possible accretion of the marsh platform. Overall, the utility and accuracy of both satellite and UAS remote sensing techniques were demonstrated for monitoring shoreline positions and platform elevations. However, marsh shoreline classifications can be improved with additional detail and/or quantification using elevation profile data (e.g., slopes, tidal datums). Practitioners and researchers monitoring coastal marsh restoration sites can use the information presented in this study to assess the tradeoffs and benefits of various methods, including a multi-methods approach as resource and monitoring needs change through time.