AN ASSESSMENT OF BEACH NOURISHMENT SEDIMENT CHARACTERISTICS

D.K. Stauble, M. Hansen, W. Blake

Abstract


An important component of beach nourishment design is to assess the compatibility of the borrow material with the native beach sediment. One avenue to judge the "success" of a project is the amount of fill retained over a specific period of time after fill placement. Presumably, if the fill material placed on the eroded beach is compatible with the energy of the coastal processes, it will be resorted along the profile but be retained within acceptable limits in the vicinity of the project area. At present the selection of suitable borrow material is based only on theoretical criterion. Specifically, the Fill Factor and Renourishment Factor are based on models developed by Krumbein (1956), Krumbein and James (1965), James (1974), Dean (1974), James (1975) and Hobson (1977). These methods of judging borrow area suitability have not been fully tested in the field and the Shore Protection Manual (U.S. Army, 1977) warns that they should only be used as a general indication of possible fill behavior. A review of monitoring reports on selected recent beach restoration and sand by-pass projects revealed a lack of standardization on data collection and analysis. Little evaluation of the actual behavior of fill material on the nourished beach had been carried out. To assess the suitability of the fill material, projects with adequate data were investigated and the short (one year) and long term (two to three years) behavior of actual fill data was described. Detailed collection of native sediment before nourishment, representative borrow material at time of placement and samples at specific times after fill placement were used to determine the redistribution of fill grain size characteristics and determine the accuracy of the present beach fill models. It was found that present models do not take into account non-normal grain size distributions found at the projects studied or CaCO„ shell material. A safety factor assuming loss of fine material from the borrow should be used with the Adjusted Shore Protection Manual Fill Factor Method to give more accurate results. The delta variable in the Renourishment Factor was found to vary between projects and should be calculated for each project. At the present time use of the entire grain size distribution is necessary to understand the sediment redistribution after fill placement.

Keywords


beach nourishment; sediment characteristics

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