Near Bed Intermittent Suspension


Investigator

Daniel M. Hanes, Christopher Vincent

Funding Source

U.S. Office of Naval Research, Coastal Dynamics Program

Objective

The overall goal is to develop a model which will predict sand transport and bathymetric change due to waves and currents under time-varying conditions.

Accomplishment of the long term goal will require significant improvement of our understanding of the physical interactions between fluid and grains near the seabed, as well as the development of models derived from our understanding of the relevant physical processes. The accurate prediction of local bedforms and local sediment suspension are essential components of transport and bathymetric change models. Through field investigations we will characterize the fluid forcing and the sediment response. Phenomena such as fluid acceleration, shear stress, and wave groupiness are believed to have a significant influence upon the local bedform geometry, as well as the initiation, duration, and magnitude of near bed sediment suspension.

Approach

A combination of experimental observations and model development is being pursued simultaneously. Field observations of nearbed suspended sediment and hydrodynamics provide a basis for the discovery of phenomena and development of models. Model development is focusing on the coupling of bedload to suspended load in order to describe the reference concentration and the structure of near bed sand suspension under waves. The bedload model is based upon the mechanics of granular-fluid mixtures, with overlying saltation modeled through trajectory predictions based upon an equation of motion for individual grains.

Our field observations will focus on measurement of the small scale sediment processes, with emphasis on the measurement of suspended sediment concentration, and bedform geometry. An instrument array consisting of an acoustic concentration profiler, pressure sensors, optical backscatterance sensors, an electromagnetic current meter, an underwater video camera, and a data acquisition package was deployed. These data allow for examination of suspension processes on a variety of time scales.

The time averaged profile of suspended sediment concentration can be described by an exponential curve, with the exact shape dependent on ripple characteristics and flow conditions. Knowledge of the reference concentration gives a unified description of time-averaged suspended sediment concentration. The reference concentration, or concentration very near the bed, provides the boundary condition which determines a unique solution to the shape of the profile.

The influence of bedforms upon the local suspension of sand is believed to be important, and will be the focus of some of the experiments. In particular, we hope to document the transition from a rippled bed to plane bed conditions, and the associated suspended sediment field. The effect of this transition on the reference concentration will also be evaluated.

Based upon our previous investigations, we anticipate that time averaged models will not be sufficiently accurate to describe the suspended sediment flux. Previous results indicate the fluctuating component of the transport is highly significant. The importance of intermittent suspension upon the sediment flux will be evaluated by estimating the various temporal contributions to the cross-shore transport of sediment.

Results

During DUCK94, data were collected under a variety of conditions, including storm and storm recovery periods. The data have undergone initial quality control, calibration, and preliminary analysis. A new transducer array designed to measure small scale bedforms was deployed at Duck during July, 1995. Figure 2 describes schematically the ultrasonic, multi-element transducer array (MTA). This instrument allows the measurement of bedform geometry with millimeter vertical resolution and centimeter horizontal resolution over a profile length of 45 centimeters.

At DUCK94 we obtained a variety of measurements covering a wide range of wave and current conditions. Figure 1 shows some examples of some of the vertical profiles of suspended sediment measured at Duck. These data indicate strong vertical gradients in the concentration profiles, but only weak gradients in the mean sediment size profiles. We are currently working to interpret these data in the context of a combined diffusion/convection type model. We have also obtained numerous measurements of small scale bedforms at Duck in July 1995, using the MTA ripple profiler. An example of these data is given in Figure 2, where the small bedforms are easily seen. Data obtained on a cross-shore profile indicate large variations in the local bedforms and local sediment size.

The connections between small scale and large scale sedimentation processes are important in order to develop a comprehensive understanding of nearshore sedimentation processes, and an ability to model bathymetric change. Our research provides new information on small scale processes which will allow these connections to be discovered and verified.

Refereed Journals

Dick, J.E., M.R. Erdman, and D.M. Hanes, "Suspended sand concentration events due to shoaled waves over a flat bed", Marine Geology, 119, 1994, 67-73.

Hanes, D.M. and P.E. Dompe, "Field observations of fluctuations in coastal turbidity", Journal of Marine Environmental Engineering, Vol. 1, 1995, 279-294.

Lee, T.H. and D.M. Hanes, "Explicit solution to the acoustic backscatter equation to measure the concentration of uniform, suspended particles", Journal of Geophysical Research, 100, C2, 1995, 2649-2657.

Lee, T.H. and D.M. Hanes, "Comparison of field observations of the vertical distribution of suspended sand and its prediction by models", submitted to Journal of Geophysical Research, 1995.

Un-refereed Papers

Hanes, D.M., "Turbidity and suspended sediment associated with beach nourishment dredging", 24th International Conf. on Coastal Engineering, ASCE, Kobe, Japan, October 23-28, 1994, 3016-3029.

Hanes, D.M. and J. Lee. "Velocity measurement of granular flow utilizing high speed digital images", A.S.M.E. Fluids Engineering Annual Conference, Hilton Head, SC, August 13-18, 1995.

For a list of addresses of the investigators involved in Duck 94 click here.