Coupling Traditional and Emergent Technologies for Improved Coastal Zone Mapping

Abstract

The shallow, turbid water of the Delaware Bay Estuary is the second most navigated waterway in the USA behind the Mississippi River and experiences tropical and extratropical cyclones from June through April bringing high winds, storm surge, precipitation, and coastal change. Mapping coastal areas within the Delaware Bay is particularly difficult due to inherent environmental factors such as low underwater visibility (less than 1 m), rapid tidal current (greater than 1 m/s), changeable weather, and strong mid-latitude winds. This study utilized four sonar systems, two manned vessels, two autonomous vessels, three real-time kinematics global positioning systems (RTK GPS), two unmanned autonomous aerial systems, and satellite imagery to quantify subaerial and subaqueous volume and feature changes following storm events. The study site was located at Broadkill Beach, Delaware, a microtidal beach inside the Delaware estuary and outside the National Coastal Mapping Program survey area. Six storms (four tropical and two extratropical cyclones) from 2012 to 2018 were surveyed pre- and post-storm. Key insights in sensors and platforms proved unmanned aerial systems (UAS) orthomosaic resolution to be superior for mapping shoreline, profile, and dune toe changes, swash zone features such as cusps, wrack lines, and infrastructure damage to beach crossovers. Subaqueous platforms performed well for most storm events with RMSE of 0.23 m or less. This uncertainty, including subaerial uncertainty (max = 0.016 m), can be reduced with precise GPS systems and ground truthing of elevations through additional sensors like RTK GPS and subbottom profilers. No clear pattern emerged between accretion and erosion for tropical or extratropical cyclones (TC or ETC). Low-energy beaches are rarely studied and therefore open-coast relationships are transplanted to these environments. Storm responses of the site did not match expected traditional cross-shore transport models as beach orientation and storm wind direction most likely induced longshore transport out of the system. UAS orthomosaics were critical in identifying wrack lines and debris which altered subaerial transport patterns at this low-energy site. This work highlights the importance of irregular, small-scale features such as relic jetties, wrack lines, and local wind waves in estuarine sites when traditional open-coast theory would diminish these processes. More importance should be placed on the use of high-resolution imagery and elevation data over widely spaced profiles when monitoring and managing low-energy sites experiencing episodic storms such as TC and ETC to accurately assess processes, relationships, and morphologic response.