Estuarine morphodynamic response and recovery to extreme events
Date
2020
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
This dissertation focuses on the estuarine morphodynamics of the Delaware Bay Estuary at Broadkill Beach, Delaware, a site approximately 11 kilometers northwest of the estuary mouth. Estuaries are typically shallow in depth and contain high amounts of organic material and fine sediments, are tidally influenced with sources of fresh and saltwater, and tend to serve as biodiverse areas where multitudes of flora and fauna live, visit to bred, or use as habitat during juvenile life stages for survival. These qualities make estuaries vital environments to understand but also incredibly difficult to observe with high spatiotemporal resolutions. This dissertation utilized traditional instruments (e.g. GPS, sonar, and satellite imagery) alongside autonomous systems (e.g. unmanned aerials systems, autonomous surface vehicles) along with historical monitoring data to study the coastal zone of the Delaware Bay Estuary in response to extreme storm events, non-storm conditions, and anthropogenic modifications. ☐ Individual tropical and extra-tropical cyclones (TC and ETC) of varying intensity and proximity to the study site were assessed using autonomous platform and traditional methods. These field campaigns showed both storm types can heavily impact the subaerial and subaqueous estuarine coast with the potential for accretional and erosional outcomes. Following the individual storm analysis, clustered storm impacts from March 2018, when four ETCs occurred in a single month, were compared to singular TC and ETC volumetric change. Extreme event thresholds for total water levels and significant wave heights within the Delaware Bay Estuary (DBE) of these and historical storms were determined. Using in situ wave measurements and historical water levels at nearby NOAA gauges, a significant event threshold of 1.05 m significant wave heights or TWL above 2.5 m above mean higher high water (MHHW) would result in significant morphological change and sediment transport. The study site at Broadkill beach was noted to be heavily influenced by aeolian transport for subaerial accretion while overall background bedload transport was out of the system and towards the DBE mouth. Finally, field sediment samples indicated that the existing subaqueous sediments are mixed sands and muds which impacts the expected movement and mobilization in the nearshore. ☐ Collectively, these findings support the need for shoreline monitoring before the TC season and following any singular storm events greater than the thresholds or clustered events (within 9 days) due to their compounded volumetric change. No profiles fully recovered following extreme TC or ETC events without manual placement of sediment by coastal managers due to the long-term erosional trend of the lower DBE. Thus estuarine shoreline projects should utilize the sediment fluxes, volumetric rates, and return period equations observed in this dissertation for project lifetime and maintenance planning. Additionally, the methods employed here to measure storm impacts is highly recommended for shoreline monitoring rather than traditional summer and winter only profiling. The estuarine site studied in this dissertation did not exhibit seasonal variability but rather episodic event response. These events may be strong winds during frontal passages, TCs and ETCs, or manmade sediment placements. Pre- and post-event surveys of the subaerial and subaqueous environments is the most representative way to monitoring the health and response of the DBE and other estuarine shorelines.
Description
Keywords
Coastal hazards, Cohesive sediments, High-resolution remote sensing, Sediment flux