These procedures are described by Curren and Chatham (1977, 1980), Bottin 

 (1982), Seabergh (1983), and Dally and Pope (1986). A number of 

 undistorted fixed-bed models, tracer studies, and/or movable-bed models have 

 been constructed at WES and used to predict the performance of detached 

 breakwaters in minimizing shoreline erosion. Fixed-bed models are used to 

 investigate the interaction of existing structures (i.e., groins) with the planned 

 breakwaters and their effect on wave-generated currents. Tracer studies are 

 then used in the fixed-bed model to qualitatively illustrate sediment movement 

 for existing structures and planned breakwater additions. 



In movable-bed models, wave conditions are first generated in the 

 movable-bed model to match or create an existing shoreline condition (base 

 case). Then, the same wave conditions are generated with the model detached 

 breakwater in place, and the shoreline effects are observed and documented. 

 Several plans for the breakwater placement and/or characteristics are usually 

 modeled to determine the optimum design. 



Prototype data. Well-documented information regarding prototype condi- 

 tions over a sufficient period of time is crucial for this type of modeling. 

 Wave characteristics, water level, bathymetry, shoreline history, sediment 

 characteristics, currents, and sediment budget are necessary. Curren and 

 Chatham (1980) indicate that the essential data are littoral transport 

 computation, sediment size distribution analysis, and the simultaneous 

 measurement of incident wave characteristics, bottom bathymetry, and littoral 

 and offshore-onshore sediment transport over a period of erosion and 

 accretion. Movable-bed modeling requires the most field data and a minimum 

 of two years of prototype data are recommended by Dally and Pope (1986). 

 Since data requirements are project specific, it is important that the client and 

 modeler communicate to determine whether the necessary data are available or 

 need to be collected prior to the modeling effort. 



Fixed-bed model. Froude similitude is normally used for fixed-bed 

 models and the geometric scale is selected as large as possible. Factors 

 considered in scale selection are depth of water required to prevent excessive 

 bottom friction effects, model wave heights, available model area, wave 

 generating and instrument capability, efficiency, and cost. The beach and 

 bathymetry are constructed of concrete mortar to reproduce the bathymetry 

 contours for a known prototype condition documented at some date and time. 

 Existing shore protection structures are also constructed and incorporated in 

 the model. For example, groins are usually constructed of galvanized metal 

 or stone and placed in the model according to prototype maps and survey 

 data. The detached breakwaters are constructed of stone and each stone is 

 scaled so wave reflection and transmission are correctly modeled. The 

 undistorted fixed-bed model of sufficient size correctly reproduces wave 

 refraction, shoaling, diffraction, breaking, and nearshore circulation cells (rip, 

 feeder, and eddy currents). The important parameters to be modeled are wave 

 height, period and direction, water levels, and wave-generated currents 

 alongshore and adjacent to structures. Dye injected into the water has been 

 used to measure and document current patterns and magnitudes. Waves are 



Chapter 3 Tools for Prediction of Morphologic Response 



65 



