DECLINING COASTAL SUSPENDED SEDIMENT IN THE NORTHEAST US FROM SATELLITE OBSERVATIONS

During periods of sea level rise, as we have experienced since the Last Glacial Maximum, coastal sedimentary environments such as salt marshes, mud flats, and beaches rely on a continuous supply of sediment to maintain quasi-equilibrium and adjust to marine transgression. Sediments may be derived from rivers, resuspension by tides and waves, and from coastal erosion of bluffs and upland soils. However, the built environment can reduce sediment delivery to the coast. Coastal armoring projects constructed to reduce erosion also reduce sediment sources to the coastal zone by halting the transfer of mass from eroding bluffs to the adjacent littoral cell. Dams along rivers can retain sediment within impounded reaches. Both of these impacts reduce sediment available to maintain beaches, shellfish habitat, and salt marshes.

Here we use satellite and GIS analyses to show how the sources of sediment to the Northeast US coast are declining, and map the resultant decreases in coastal sediment availability. We used historical shoreline change observations to estimate the average sediment addition to the littoral zone since the late 1800s and since the late 1900s. Shoreline change has slowed, reducing littoral zone sediment inputs. We used Google Earth Engine to examine the entire Landsat 5-9 catalogue, including 40,000 images, with an average of 800 images for any given location across the Northeast US littoral zone to assess changes in sediment availability to nearshore environments. Across our six sentinel sites, we observed a 20-50% decline in coastal suspended sediment concentrations. Satellite observations also show persistently higher sediment concentrations during winter versus summer, consistent with erosion from coastal storms (Nor-easters) providing a major source of coastal sediment during high energy wave events. With increasing rates of sea level rise and declining sediment delivery to the coast, there is an urgent need to find creative solutions to make up for deficits in coastal sediment. Results of our sediment mapping can be used to help design adaptive coastal management practices, including providing constraints for tidal sediment flux when building living shorelines or restoring tidal flow to restricted marshes.