consistent small gains in material that appear to be decreasing as the project 

 slowly approaches a dynamic equilibrium. 



414. Figure 48 is a plot of the calculated and measured shoreline in 

 positions in December 1982. The calculation was begun in October 1977, and 

 the 1-year wave data set was repeated. GENESIS predicted major shoreline 

 change from 1977 through 1980 and only slight change thereafter, indicating 

 that the project had adjusted to equilibrium with the 1-year data set. 



415. Calculated and measured 1982 shorelines are in almost perfect 

 agreement along the eastern two-thirds of the project, with the model repro- 

 ducing the locations and shapes of the salients. It is also interesting to 

 note that the model predicts the small shoreline recession observed within the 

 distance of about 300 ft from the east groin. Erosion in the vicinity of the 

 west groin is qualitatively reproduced, but the magnitude is less than the 

 measured amount. Three reasons can be given for the underestimation: 



a. Inadequate wave time series. 



b. Wave diffraction by the groin, which was omitted in the model. 



c. Local effects, such as a rip current. 



It is believed that the three reasons are important in the order they are 

 given. In particular, the opening between the tip of the west groin and the 

 western-most breakwater is relatively great, making the exposed area in that 

 energy window more sensitive to variations in the wave climate than the 

 protected areas in the shadow zone of the breakwaters. Additional sensitivity 

 testing would easily shed light on whether the model prediction could be 

 improved in the vicinity of the west groin without degrading the prediction 

 elsewhere; this task is left as an exercise for the reader. 



Summary and Discussion 



416. The presented case study provides an example of data preparation, 

 interpretation of previously obtained results, calibration and verification 

 procedures, and, finally, use of the model to analyze alternative project 

 designs. A description of many of the intermediate simulations had to be 

 omitted, and it is emphasized that the treatment is somewhat schematic as 

 compared with actual design applications. 



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