CROSSTEX: A Description of the Experimental Condtions, Wave Climate, and Inner Surf Zone Hydrodynamics

Abstract

The Cross Shore Sediment Transport Experiments (CROSSTEX) are a group of experiments examining sediment transport processes in the near shore environment with the goal of improving process based models for sediment transport. This is accomplished via large scale lab experiments utilizing advanced instrumentation and controlled, repeatable wave conditions as well as a natural sand beach. One section of these experiments examined sediment transport in the swash zone, the alternating wet and dry portion of the beach. Velocity data was collected from outside the breaker line into the surf zone, along with free surface measurements throughout the tank. Additional instrumentation to measure optical backscatter, sediment grain velocity and water pressure was deployed in the near shore. Waves consisted of regular, 5th order Stokes waves with wave heights ranging from 12 cm to 30 cm, two runs with a simple bi-chromatic wave train and one run with random waves. An analysis of wave climate stability and wave repeatability indicates the wave tank approaches a steady, repeatable wave climate after approximately 5 minutes although bathymetry changes ultimately affect repeatability in the near shore. Comparison of the time averaged mean free surface with empirical estimates shows good agreement with other studies conducted in both the field and wave tanks. Phase averaged velocity profiles taken in the offshore by an Acoustic Doppler Current Profiler (ADCP) indicate a 2-D flow environment with minimal along shore flow and repeatable wave conditions. An alternate processing scheme was developed for the ADCP data to allow redundant estimates of each velocity component which was used to assess flow uniformity. Phase averaged velocity data in the surf zone taken from Acoustic Doppler Velocimeters (ADVs) revealed occasional strong along shore flow and more complex flow structure, but supports wave repeatability. Analysis of the bathymetry data in the surf zone revealed strong along shore gradients, which contribute to the more complex flow seen in the surf zone. Surf zone turbulence obtained by applying a linear predictive filter to velocity signals and differencing the filtered and original signals indicates plunging to weakly plunging breaking conditions as well as providing order of magnitude estimates of turbulent dissipation in the surf zone. Recommendations for the conduct of future large scale experiments (field or lab) in the surf zone and in general, and future work on the present data are provided.