Mechanisms and Rates of Sand Bypassing along a Rapidly Evolving Inlet-Spit System

Chincoteague Inlet and the proximal barrier islands of Chincoteague, Assateague, and Wallops comprise a dynamic coastal complex that is experiencing rapid change associated with sediment redistribution (erosion and deposition) and a shifting tidal inlet and flood- and ebb-tidal delta sands. Understanding the morphodynamic and sediment-transport processes and underlying hydrodynamic and geomorphic controls within this rapidly evolving inlet spit system is critical to developing a regional sediment management plan that is protective of the Town of Chincoteague, southern Assateague Island, and the Mid-Atlantic Regional Spaceport and the NASA flight Facility on Wallops Island. Although this region experiences prevailing winds from the northwest quadrant during winter months with wind speeds frequently greater than 10 m/s, northeast storms and the infrequent hurricane events can, in part, control sediment transport processes and morphology of this system. From 1994 through 2013, the Chincoteague Inlet throat widened from 548 m to ~1,950 m, allowing greater wave transmission through the inlet and causing widespread erosion landward near Chincoteague Island. We hypothesize that inlet widening is in direct response to decreasing sand supply to the updrift inlet margin (southern Assateague Island) due to sand bypassing mechanisms to the downdrift Wallops Island. Here, we investigate sand bypassing rates and mechanisms associated with Chincoteague Inlet system under varying wave conditions by coupling the SWAN wave model with the flow and sediment transport module within the Delft3D modeling suite to assess the relative contribution of sediment transport to the morphologic changes of this system resulting from each event. Our results show that, during hurricanes such as Hurricane Dorian (Sept 2019), bottom orbital velocities increase three-fold (e.g. from ~0.5 m/s to 1.5 m/s) and wave-energy transport increases by an order of magnitude (1.5 * 104 from ~1.2 * 103 W/m). These storm conditions facilitate higher sand entrainment and transport rates throughout the inlet and ebb-flood-tidal delta as compared with quiescent periods, suggesting that sand bypassing during storms may be a key process through which sand is transferred through this system.