Mapping Bluffs and Beaches to Quantify Sediment Supply 2021
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This project conducted by the Washington State Department of Ecology Coastal Monitoring & Analysis Program (CMAP) will help ESRP aim to understand how much sediment input is needed to sustain coastal processes and maintain nearshore ecosystem functions within a drift cell. Baseline boat-based lidar data collected between 2013 and 2016 will be compared to boat-based lidar collected as a part of this project in 2021 to quantify beach and bluff on a drift cell scale. Quantifying and characterizing morphologic change on beaches and bluffs can help to identify target conditions for assessing restoration effectiveness.
Sediment supplied from eroding bluffs is important for sustaining Puget Sound beaches and their ecosystem functions, however sediment supply rates are not well understood. As efforts to reconnect coastal bluffs to beaches by removing hard shoreline armoring become more common, it is essential to gain an understanding of typical sediment dynamics on unarmored shorelines as a target for restoration.
This project draws from existing high-resolution bluff and beach topographic data collected in 2015 and 2016 to direct efforts for collecting repeat surveys of beaches and bluffs in 2021. The high-resolution topographic data is collected using boat-based lidar technology. The horizontal look angle of the vessel-mounted laser scanner provides an advantageous view of the coastal landscape because it captures more detail on vertical faces like bluffs or shoreline armor than traditional airborne lidar systems.
The topographic data can be used to quantify physical aspects of shoreline morphology including beach width, beach slope, backshore width, bluff toe elevation, and bluff height. Comparing beach metrics between surveys can offer insight into the rate and volume that sediment is supplied to the beach by feeder bluffs. Review of beach and bluff change spatially within a drift cell can help us understand alongshore sediment dynamics.
In 2021, CMAP conducted lidar surveys at 17 sites around the Salish Sea, totaling 136.8 kilometers of coastline. Of these sites, 9 were repeat surveys and 8 were new sites.
CMAP primarily uses boat-based lidar supplemented with GPS topography data collected on foot to comprehensively map beach and bluff topography. A laser scanner mounted to the top of our research vessel, the R/V George Davidson, scans the nearshore environment. Returns from the laser scanner collectively create a lidar point cloud, which is stripped of vegetation and man-made structures to produce a high-resolution bare-earth digital elevation model (DEM).
The surveyed drift cells in 2021 were selected based on quantitative ranking of all drift cells within the Washington Salish Sea followed by a qualitative review for the final selection. The factors considered include:
- Ratio of mapped feeder bluff length to feeder bluff exceptional length
- Total length of feeder bluff exceptional
- Whether the site was previously surveyed during 2015 and 2016
- Proximity to existing or planned beach restoration efforts
- Visual inspection of bluff erosion likeliness based on oblique photos
- Feasibility and logistics of mobilizing for surveys
Repeat survey locations were ranked highest in the quantitative ranking, however some new sites were selected to increase the geographic and geomorphic distribution of baseline datasets, and to capture baseline surveys of planned restoration efforts.