COMPARISON OF MEASURED AND PREDICTED BEACH NOURISHMENT PERFORMANCE

Beach nourishment has been the shoreline stabilization method of choice in Florida for several decades with nourishment activity increasing rapidly over the last decade. All projects require monitoring of the nourishment evolution with the requirement that these data be available to the general public. Monitoring results are available for most projects over an average of six to eight years. These include projects on long coastlines, projects adjacent to inlets and one short project stabilized by 11 groins. Concurrent with the accumulation of the monitoring data has been an effort to develop methodology to predict the evolution of these projects.

In 1989 we developed and documented recommended methodology and forcing (wave characteristics) for the prediction of the evolution of beach nourishment projects in Florida, thus providing a basis for "blind folded" testing of this methodology (Dean and Grant, 1989). The methodology has been applied recently to eight beach nourishment projects. Consistent patterns have emerged, some of which were expected and others unexpected. It was found that the prediction capabilities are best for the "overall" characteristics of the project and least for the detailed characteristics. For example, total project volume remaining in the nourished area is predicted much better than the additional shoreline width at a point. The rate of profile equilibration from the steep placed profile to a milder equilibrium profile has been found to require on the order of two to three years for most projects. Finally, it has been found that the methodology predicts the volumetric decreases within the nourishment area reasonably well for the first few years after which the methodology predicts decreases which are greater than measured. One possible cause of these differences, which is presently under investigation, is the validity of the longshore sediment transport equation employed in the methodology.

The presentation will describe the methodology and provide results for seven projects followed by a synthesis of the findings.