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Field observations of swash-zone dynamics with Vectrino velocimeter

Researchers from the University of Delaware and Texas A&M University have used the Nortek Vectrino Profiler velocimeter to understand swash zone hydrodynamics and foreshore response during beach recovery.

The swash zone is a dynamic region of the nearshore zone where wave energy is dissipated or reflected and often represents a region of active beach erosion or accretion. Swash-zone flows are commonly turbulent and sediment- and bubble-laden, increasing the difficulty of deploying in-situ instrumentation and collecting field data.

During February 12-25, 2014, a swash-zone study was conducted at South Bethany Beach, Delaware by Patricia Chardón-Maldonado (University of Delaware), Jack A. Puleo (University of Delaware), and Jens Figlus (Texas A&M at Galveston).

Understanding swash zone hydrodynamics

The study was done following a large storm and the study’s purpose was to understand the swash zone hydrodynamics and foreshore response during beach recovery. Conducting field measurements in this highly energetic zone was a challenging task because measurements had to be obtained close to the bed.

Vectrino Profiler velocimeters on scaffolding frame.

Measuring swash-zone flow velocities

During the field study, five Nortek Vectrino Profiler profiling velocimeters measured swash-zone flow velocities in close proximity to the bed, in addition to other hydrodynamic and sediment transport instrumentation. As seen in the pictures in this article, the sensors were deployed on a scaffolding frame, where five cross-shore stations were established to simultaneously measure near-bed velocity profiles, sediment concentration, and water level fluctuations on a steep beach.

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The Nortek Vectrino Profiler measures all three velocity components over a range of 0.03 m at 0.001 m bin spacing in a minimum elevation of about 0.05 m. The high-resolution data were used to estimate bed shear stress and turbulent dissipation rate in the swash zone during the rapidly changing morphology related to the beach recovery.

Scaffolding frame showing the sensors deployed in the swash zone. Severe cold weather and icy conditions (below 30° F) during the field study increased the difficulty of measuring flow velocities in the swash zone. Icicles formed on the frame, hindering sensor adjustment during foreshore changes.

Cross-shore velocity and more

The image below shows, as a function of time, an example of data of cross-shore velocity, bed shear stress and turbulence dissipation rate.

Gaps in the time series indicate the sensor was not submerged or correlations were too weak to allow for robust velocity estimation. The data covers 20 seconds of swash flows where cross-shore velocity magnitudes up to 2 m s-1 were observed. The bed shear stress magnitude reached 80 N m-2 during the offshore-directed phase of the swash-zone flow. The maximal values (O(10 N m-2)) were observed when the velocity magnitudes were at the extremes.

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The instantaneous data show that turbulence dissipation rates increase during bore arrival and ranged from O(10-4 m2 s-3) to O(10-1 m2 s-3). Velocity measurements obtained from the Vectrino Profilers were also coupled with detailed measurements of sediment concentration just above the bed in an effort to quantify sediment flux in this highly energetic region.

This project was funded by NSF-OCE.

Time series of: (a) water depth; (b) cross-shore velocity (positive values indicate onshore-directed flow; negative values indicate offshore-directed flow); (c) cross-shore bed shear stress magnitude; (d) turbulence dissipation rate from the five cross-shore stations. Discontinuities in the time series are due to bubbles, bore collapse, swash-swash interaction, and/or intermittent sensor submergence.
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