Bradley Werner, Steven Elgar
Office of Naval Research Coastal Dynamics Program
Naval Research Laboratory and Office of Naval Research Young Investigator Program
The morphology of a beach is determined by complicated interactions between waves, currents, sediment transport and the beach morphology itself. Numerous competing hypotheses and models based on diverse conceptions of the important forces and interactions have been advanced to explain beach morphology. The overall goal of this research is to identify the dominant feedbacks and physics actually operating to determine nearshore morphology in the natural ocean. A subsidiary objective is to quantify the role of LOCAL feedbacks between sediment and fluid flow (self-organization) in beach morphology. The general approach will be to:
a. Use computer simulations to determine optimal initial beach and wave conditions for discriminating between hypotheses for beach morphology.
b. Create those conditions on a beach through manipulation of the morphology.
c. Observe the resulting evolution of the beach and the wave/fluid flow.
d. Couple these observations with simulations to refine the experiments on-site.
Wave, fluid flow and sediment transport processes in the swash zone can be probed through the study of beach cusps. Two primary competing hypotheses for beach cusp formation have emerged. The Guza/Inman/Bowen model proposed that the pattern of standing subharmonic edge waves is impressed on a beach to form beach cusps. Recent computer simulations of flow and sediment transport in the swash zone show that local flow-morphology feedback, i.e., self-organization, is a viable alternative means for producing uniformly-spaced beach cusps, in a manner similar to the earlier hypotheses of Johnson and Dean/Maurmeyer. The two models, standing waves and self-organization, are based upon very different physics; however, they make similar predictions for beach cusp spacing and for the conditions under which beach cusps form. Both models are in rough agreement with field observations. The specific objective of this investigation for DUCK94 is to determine the dominant physical processes operating in the swash zone by discriminating between these two models for beach cusp formation.
A variety of experiments on "plane" and cuspate beaches will be performed at DUCK94 to discriminate between the standing wave and self-organization models. In these experiments the shape of the beach will be altered and then its subsequent evolution will be observed. Computer simulation models will be used to predict the optimal wave conditions and the optimal initial and altered beach morphologies for hypothesis discrimination. Comparisons between the results of initial experiments and on-site simulation runs will permit optimization of plans for further experiments.
A combination of a bulldozer, a backhoe and shovels, guided by conventional surveys, will be used for altering beach morphology. The motion of the swash front will be recorded on video. Quantitative changes in beach morphology will be measured on different time and spatial scales with conventional surveys, stereo photography and video observation of stakes placed in the beach.
In addition to discriminating between hypotheses for beach cusp formation, the quantitative information obtained on flow and morphology change will be used to test specific components of beach cusp formation models, including their sensitivity to the details of small-scale sediment transport and flow models.
For a list of addresses of the investigators involved in Duck 94 click here.