These studies may be used to develop the model tracer tests to determine the 

 optimum distance offshore for the breakwater relative to the longshore sedi- 

 ment movement. Model tracer tests can be used to qualitatively duplicate tom- 

 bolo development behind a breakwater, illustrating the points of tombolo at- 

 tachment for different wave angles and orientations of the breakwater. Since 

 the offshore contours do not adjust in a fixed-bed model, extrapolation of the 

 bathymetry change through time may not be valid, and a movable-bed model may 

 be desired. 



80. Movable-bed models may be used to determine beach planform and 

 bathymetry response to a detached breakwater and to simulate beachfill read- 

 justment shoreward of the breakwater. Ideally, a movable-bed model requires 

 verification of past beach changes in the study area. This in turn requires 

 significant amounts of prototype data. Movable-bed models require longer 

 running time per test to allow the development of an equilibrium beach con- 

 dition. In addition, the model results must be considered as qualitative due 

 to the difficulties associated with reconciling the different scaling require- 

 ments of waves and sediment transport. For any particular study one or all 

 three modeling approaches may be used. 



81. For any physical model study, prototype data are necessary. 

 Fixed-bed studies require up-to-date bathymetry and historical aerial photog- 

 raphy to ensure that the average beach planform is selected. Nearshore cur- 

 rent patterns (including locations of rip currents) are useful in ascertaining 

 the model's accuracy. Beach profile studies are useful in understanding the 

 beach dynamics and for planning the structure location. Good wave and water- 

 level data are necessary to select the appropriate test conditions. Detailed 

 sediment sampling also is desirable. If a movable-bed model study is under- 

 taken, more field data are required. Periodic seasonal bathymetric surveys of 

 the study area along with a detailed record of the waves, water levels, and 

 winds which occurred during the survey period are very valuable. A minimum of 

 two years of data should be collected. 



Numerical modeling 



82. Numerical models are destined to become a valuable and practical 

 design tool for predicting shoreline response to various structure configura- 

 tions. Simple models designed to predict zones of scour and erosion in the 

 fluvial environment have been available for many years from the Hydrologic 

 Engineering Center (HEC 1975) and recent site-specific models have been 



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