Patterns and rates of historical shoreline change in the Delaware Estuary

Date
2016
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University of Delaware
Abstract
Shoreline change in coastal and estuarine systems is a result of both natural and anthropogenic factors that influence sediment accumulation and erosion within the intertidal zone. The Delaware River and Bay estuary, a 215-km long coastal plain estuary on the U.S. Atlantic coast, is a submerging estuarine basin consisting of a tidal freshwater river (upper estuary), a stratified estuary (lower estuary), and a weakly stratified bay (Delaware Bay) at its mouth. Beginning in the late Holocene and continuing to present, marine transgression has caused the bay and estuary to broaden, its shores to retreat landward and upward, and its coastal environments to transition from tidal wetlands and tidal flats to sandy, barrier beaches. Superimposed on this natural change are anthropogenic influences on estuarine sedimentation, beginning in the 19th Century, such as construction of a shipping channel, maintenance dredging, shoreline hardening, and modification of tidal wetlands. Although previous research has shown that much of the estuary shoreline is retreating because of transgressive erosion, the nature of shoreline change in the estuary-bay as a whole has never been established. In this study, patterns and rates of shoreline change in the estuary from 1879 to 2012 were characterized using five shoreline datasets (1879, 1948, 1991, 2007, 2012) and the USGS Digital Shoreline Analysis System (DSAS) extension for ArcGIS. Linear rates of shoreline change were computed using both linear regression and endpoint methods to investigate temporal variations in shoreline extension and retreat. Volumetric rates of shoreline change where determined using DSAS and sediment bulk density data, to estimate the mass of sediment associated with shore erosion and accretion. Given that wind waves are a known agent of coastal change in the estuary, archived wave data (2007?2015) were examined to identify potential relationships between wave parameters (significant wave height, wave period, wave power) and rates of shoreline retreat. Results indicate that coasts of the lower estuary-bay have been in a state of net retreat during historical times. From 1879 to 2012 the long-term rate of shoreline change for the entire lower estuary?bay was -1.1 ± 0.13 m/yr. This rate of retreat equates to -1.5 ± 0.18 x108 kg/yr, assuming retreat is due to erosion of the shoreface. By comparison, the short-term (2007?2012) rate of shoreline retreat for the lower estuary?bay system was higher at -2.13 ± 0.47 m/yr. Long-term rates of shoreline change for the lower estuary region alone were -0.64 ± 0.13 m/yr and -1.3 ± 0.13 m/yr on the Delaware and New Jersey sides, respectively. In the bay region long-term rates on the Delaware and New Jersey sides were respectively -0.73 ± 0.13 m/yr and -1.7± 0.13 m/yr. Among the four different types of coasts classified for this study (barrier beach, tidal wetland, transitional wetland-barrier, and hardened), transitional and wetland coasts had higher rates of shoreline retreat than the barrier beaches. In sum, both long-term and short-term rates of shoreline retreat are higher on the New Jersey side of the lower estuary-bay. Comparison of modeled wave properties and shoreline change data indicates a general correlation between wave power and shoreline retreat, presumably due to wave erosion of the shoreface. However, further research is needed to identify the actual mechanisms and time-dependence of shoreface erosion. By documenting historical shoreline change in the estuary, the findings of this study can help identify vulnerabilities associated with sea-level rise, climate variability, and human pressures.
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