Abstract

Effect of wind speed and direction on nearshore processes and cross-shore
sediment mobility on an estuarine beach, Delaware Bay, New Jersey
by Nancy L. Jackson
Dissertation Director: Dr. Karl F. Nordstrom

This study examines the role of wind in influencing cross-shore flows and sediment mobility on an estuarine beach exposed to a 46 km fetch. The investigation was conducted over a lunar tidal cycle.  Four pressure transducers, three bi-directional current meters, four anemometers and a wind vane were deployed in a cross-shore array. Data were obtained at 0.5 s intervals in 17.1-minute records. Changes in surface elevation and depth of scour were determined from depth of activity rods.

Mean wind speeds ranged from near zero to 14.18 m s"1. Locally generated significant wave heights ranged from 0.07 to 0.56 m, with peak periodicities between 2.4 and 4.9 s. There is a reduction in wave energy variance with increasing wind speed during low speed offshore winds, and low frequency energy dominates. Low frequency energy dominates on the upper foreshore during low speed onshore winds. The magnitude of energy is low under low velocity winds (<5.0 m s"1), except when low frequency ocean swell dominates during northeast and southwest winds.  No detectable two-dimensional circulation system was found on the upper foreshore during low wind speeds.

During strong onshore winds, energy at locally generated incident wave frequencies dominates the cross-shore fluid motions. Flows are skewed onshore on the low tide terrace seaward of the breakpoint. A steady two- dimensional flow circulation occurs on the upper foreshore, with an onshore flow near the surface and an offshore flow at the bed. As the offshore mean flow increases, the magnitude of the skew decreases and reverses direction, resulting in offshore sediment mobilization.

Conspicuous geomorphic change was confined to the upper foreshore above the low tide terrace. Post-storm recovery occurred immediately following storms and was faster than noted on other estuarine sites, due to availability of sediment. Little net elevation change occurred on the low tide terrace, and the slope remained unchanged. Post -storm recovery is similar in magnitude during onshore winds and low speed offshore winds when low
Frequency progressive gravity waves are present. A two-dimensional circulation system with an offshore mean flow at the bed provides a more convincing explanation for the offshore movement of sediment across the profile than reliance on incident wave parameters.