Reginald A. Beach, Rob A. Holman, Richard W. Sternberg
Office of Naval Research, Coastal Dynamics Program
The long-range goals of this program are to improve our understanding of the processes and physical mechanisms responsible for beach morphology changes and patterns of erosion and deposition in the nearshore zone.
Our objectives focus on bottom boundary layer processes. Specific science objectives include:
a. Characterization of cross-shore and longshore bottom boundary layer velocity profiles.
b. To understand the initiation and vertical distribution of suspended sediment by turbulence. To partition the observed sediment concentration into local resuspension and advective components.
c. To understand the temporal dependence and spatial gradients of cross-shore suspended sediment flux that ultimately lead to changes in nearshore morphology.
The overall approach is to determine the fundamental relationships between fluid forcing and sediment response by conducting detailed field studies using instrumentation deployed within the surf zone. These measurements are used for direct sediment transport calculations, in addition to guiding development and evaluation of bottom boundary layer, fluid-sediment interaction models. New instrumentation has been developed which allows better resolution of these bottom boundary layer processes.
A Vertical Electromagnetic current Meter Array (VEMA) for use in studying velocity profiles in the bottom boundary layer was brought from the prototype stage to actual field instrumentation. The array is comprised of 4 2-component (u,v) sensors located at z= 4, 10, 24 and 36 cm above the bed. A companion probe, the Fiber Optic Backscatter Sensor (FOBS) measures suspended sediment concentration at 19 elevations in the lower 50 cm of the water column. The lower eight nearbed sensors are spaced 1 cm in the vertical and indicate bed level fluctuations as they are alternately covered/uncovered by accretion/erosion, respectively. In conjunction with a pressure sensor to measure sea surface fluctuations, these instruments provide insight into the dynamics of sediment resuspension and transport in the wave- current bottom boundary layer.
Three sets of these instruments were deployed during both low (August) and medium (October) energy experiments in the longshore trough during DUCK94. These experiments had multiple objectives; 1) to characterize bed level fluctuations, both short and long-term (using FOBS), 2) to characterize longshore and cross-shore velocity and suspended sediment profiles in the nearbed region (using pairs of VEMA and FOBS), and 3) to investigate the influence of tidal stage on sediment resuspension, made possible by continuous monitoring over extended periods. Post calibration of the DUCK94 instrumentation has been completed and the task of data reduction is in progress.
In a parallel investigation conducted during August, 1994, a vertical array of 5 hot-film anemometers and a FOBS was deployed from the Field Research Facility Pier. This investigation focused on bottom boundary layer turbulence and its role in the initiation and vertical distribution of suspended sediment.
Results from the analysis of the paired hot-film anemometer and suspended sediment study (5 paired sensors in the lower 5 cm of the water column) have clearly shown that 1) nearbed fluid velocities in the wave bottom-boundary layer lead those higher in the flow, and 2) suspended sediment concentration and high-frequency velocity fluctuation time-series are highly correlated and show considerable temporal variability as well as rapid decay with elevation above the bed. Results of a 1-dimensional analytic bottom-boundary layer model show similar velocity profiles to those observed and are being used to investigate a bottom-boundary layer instability mechanism for the rapid, vertical resuspension of sediment from the sea bed (Foster et al., 1994).
P Beach, R. and R. Sternberg (in press), Suspended Sediment Transport in the surf zone: Response to Breaking Waves, Mar. Geol.
P Foster, D., R. Holman and R. Beach, 1994: Correlation Between Sediment Suspension Events and Shear Instabilities in the Bottom Boundary Layer of the Surf Zone, Proc. Coastal Dynamics '94, Barcelona, Spain, Feb. 21-25, 1994.
For a list of addresses of the investigators involved in Duck 94 click here.