THE PERSISTENCE OF HYDRAULIC DREDGE TRACKS FOLLOWING SURFCLAM HARVESTING: RELATIONSHIPS BETWEEN SHORELINE EVOLUTION AND NEARSHORE SEDIMENT TRANSPORT

Along a low to moderate wave energy, erosional shoreline hydraulic dredge tracks have persisted for more than 800 days in shallow water (4-8m). Hydraulic dredging for the Atlantic surf clam (Spisula solidissima) occurred off Provincetown, MA, from November 2014 through April 2015. Acoustic surveys following the dredge activity were conducted in the summer of 2015 with a phase-measuring sidescan sonar which collects co-located swath bathymetry and sidescan sonar imagery. A second survey was conducted in October of 2017, 875 days after the 2015 hydraulic clamming season ended. A subset of 1.05 km² or approximately 20% of the 2015 survey area was mapped in 2017.

For analysis, the 2015 survey area was divided into 100 m² grids and track density was calculated by the bottom disturbance related to the dredge tracks within each grid. The resulting track densities ranged between 0% and 53%. Dredge tracks were documented in 7.9% (0.4 km²) of the 5.1 km² surveyed in 2015. Grid cells with track densities greater than 10% in 2015 had not fully recovered by 2017, and the rate of dredge tracks remaining was proportional to disturbance (LR, r² = 0.95; F = 110.6; df = 1,7; P < 0.001; SE = 0.206). Results related to other parameters such as ADCP wave and tidal current measurements, dredge mark orientation, water depth, grain size, and onshore and offshore profiles will be discussed. The eroding shoreline and the persistence of dredge tracks reinforce the idea that nearshore energy regimes should not be assumed based solely on more visible shoreline processes.