ABSTRACT

Currents flowing along the shore transport sediments and pollution, place stress on the bottom and affect total water levels, flooding, beach erosion, and subsequent recovery. Yet few observations of these important alongshore flows exist in the inner-surf and swash zones, especially during storms, and the physics driving them during existing observations is undetermined. Along with wave-forcing and alongshore water-level gradients, evidence suggests wind-forcing also may be a significant driver of very shallow-water alongshore currents. Understanding of the physics driving shallow-water flows is needed for nearshore hydrodynamical, morphological and shoreline evolution, and wave overtopping models. This project will leverage funded efforts during the DUNEX study in Duck, NC in fall 2021 with additional moored instrumentation and remote (drone) sensing of the very-near shore to enable evaluation of the wave and wind processes driving alongshore flows near the beach. The project will provide an important new dataset to the community, train a post-doctoral scholar, and contribute to societal interests through informing management of nearshore zones subject to storm-forced erosion and inundation.

This project will obtain in situ and remotely sensed field observations for a range of wave conditions to estimate the terms in the momentum balance governing the nearshore zone. Hypotheses to be evaluated are, 1) the forcing of alongshore flows near the beach is cross-shore (depth) dependent; 2) opposing water-level-gradient and oblique-wave forcing drive alongshore flows that change direction between the surf and swash zones, and; 3) oblique-wave and wind forcing dominate inner-surf and swash flows during storms. The DUNEX-funded fieldwork (USCRP, USGS, NSF, USACE, and DoD), which this project builds upon, includes an alongshore array of surfzone ADVs, a single cross-shore transect from the dune to the inner surf with pressure gages, ADVs, and lidar, and frequent bathymetry surveys at the U.S. Army Corps Field Research Facility (USACE-FRF) in fall 2021. FRF also maintains a directional wave array at 8-m, a tide gauge, several anemometers, and a lidar that collects beach and dune topography hourly. The expanded observations proposed here include two cross-shore transects of pressure gages. Additionally, surface flows can be estimated by tracking breaking-wave-generated foam in sequences of images. Drone-based particle image velocimetry and the Optical Current Meter have been used in the past in surfzone conditions. This project will extend and hone these techniques for the swash zone. Observations during storms and moderate wave conditions will be compared and contrasted to examine how processes differ as wave and wind conditions change. The results could significantly improve the ability to predict flows near the beach, the corresponding storm-induced changes to coastal morphology, and the subsequent recovery.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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