Temporal and Spatial Variability of the Bathymetry of a Natural Beach


Investigator

Steve Elgar

Funding Source

Office of Naval Research, Coastal Dynamics Program

National Science Foundation

Objective

The overall, long-term objective of this investigation is to understand the feedback between complex topography, waves, and the three-dimensional nearshore circulation.

The immediate specific goal of this project ("Sonic Duck", which includes both the pilot experiment, DUCK94, and SandyDuck) is to experimentally determine the temporal and spatial variability of natural beach bathymetry as a function of wave-induced fluid flows. The measurements will both determine the important scales of bathymetric variability as a function of cross shore position, and better characterize the coupling between waves, currents, and morphological features with length scales between about 1 and 200 m (eg, megaripples and sand bars).

Specific goals related to megaripples including observing:

Specific goals related to sand bars include determining:

An additional objective is to continue development and testing with field observations, models that predict the evolution of surface gravity wave fields (broad banded in both frequency and direction) from outside the shoaling region through the breaking region and the surf zone. These models include the effect of linear and nonlinear shoaling and refraction as well as dissipation in the surf zone. Such wave propagation models are necessary for 3-D circulation and sediment transport/beach- response-to-fluid-forcing models. We plan to continue these efforts with the DUCK94 data, and use the results in the design of the SandyDuck experiment.

Approach

The evolution of waves, currents, and bathymetry on a natural beach was observed during the DUCK'94 field experiment on the North Carolina coast. In collaboration with R.T. Guza (Scripps) and T.H.C. Herbers (Naval Postgraduate School), a cross-shore transect of pressure gages, current meters, and sonic altimeters was deployed from the shoreline to about 8 m water depth (figure 1). The altimeters (developed and deployed by graduate student Edith Gallagher) measure the distance from a fixed frame to the seafloor, and when processed with a "bottom finding" algorithm yield seafloor locations approximately once per minute (the frequency of estimates depends on wave conditions). A dense, two-dimensional array of altimeters was deployed in the surf zone to observe propagating megaripples.


Figure 1

Previous observations in rivers suggest megaripple characteristics (eg, amplitude, orientation, wavelength, and propagation speed) depend on mean flows, while other bedform-generation models relate megaripple geometry to oscillatory currents. The observations will be used to test these models.

The cross-shore transect of instruments provide high spatial- and temporal-resolution measurements of the bar-scale bathymetry and flow field, allowing characterization of sand bar movement under a range of conditions, including energetic waves when the bar moves rapidly. Predictions from an energetics-type bar evolution model are being compared to observations.

Results

The cross-shore transect of instruments (figure 1) was deployed during July 1994 and maintained through October 1994. The sonic altimeters have a variable threshold-of-detection level that (rapidly) adjusts according to the strength of returns from the seafloor and through the water column. For 3 months (DUCK'94) of data collection the altimeters performed nearly flawlessly, even in the extremely turbid water present during a Nor'easter when the offshore wave height was greater than 4 m and mean currents in the surf zone exceeded 100 cm/s.

Megaripples were observed throughout the surf zone during the experiment. The two-dimensional array of altimeters detected 20-50 cm high propagating bedforms with length scales of a few m.


Figure 2

During the Nor'easter the sand bar moved about 100 m offshore, causing 1.5m of erosion near the initial location of the bar crest and 1 m of accretion 85 m farther offshore (figure 2). The sand bar moved rapidly, but smoothly in time (figure 2), suggesting a gradual evolution, as opposed to periods of no motion followed by sudden jumps in bar location. The observed offshore bar migration is well predicted by an energetics-type sediment transport model initialized with the pre-storm bathymetry and driven by the observed currents (figure 3). The model has some predictive skill with several other profile change events, although bar movement is not well predicted during relatively low energy wave conditions (offshore significant wave height < 1.5m). During high energy events, the suspended load transport driven by the mean offshore flow (eg, undertow) dominates the total transport. However, during lower energy events, the suspended load contribution from mean and oscillatory flows are of equal importance. The reasons for the inaccurate predictions, including errors in the oscillatory terms (ie, effects owing to threshold of sediment motion and phase lags between fluid and sand motion), are being investigated.


Figure 3

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