THE SHORELINE IS NOT THE STORYLINE: VARIABILITY IN ONSHORE AND OFFSHORE COASTAL SEDIMENT TRANSPORT

A multi-year project within Cape Cod National Seashore looking at, in part, the relationship between onshore and offshore sediment transport is nearing the end of a 2-year data collection phase-preliminary results are presented here. A recent period of accelerated rates of shoreline erosion has undermined a Seashore parking lot almost annually starting in 2012. The Seashore is committed, when feasible, to a program of managed retreat and relocation. A project is currently underway to move the parking lot landward. The erosion rate in the area the last few years has ranged from 4-10 m/yr, in an area with a long-term rate of ~1m/yr. In order to understand the relationship between onshore and offshore sediment transport quarterly surveys including Acoustic Doppler Current Profiler (ADCP) data, onshore and offshore shore-normal profiles, bottom grab samples, freefall penetrometer data as well as vessel-based, acoustic and unmanned aircraft system surveys for marine and terrestrial 3D surface production, respectively, were conducted. Preliminary results show a system that cannot be characterized by looking at either the onshore or offshore sediment transport processes, both need to be taken into consideration to understand the agents of coastal change and to provide managers with vital information to conduct science-based management.