23. The shoreline change model predicts shoreline position changes that 

 occur over a period from several months to several years. The model is best 

 suited to situations where there is a systematic trend of long-term change in 

 shoreline position, such as shoreline regression downdrift of a groin or jetty 

 and advance of the shoreline behind a detached breakwater. The dominant cause 

 of shoreline change in the model is spatial change in the longshore sand 

 transport rate along the coast. Cross -shore transport effects such as storm- 

 induced erosion and cyclical movement of shoreline position as associated with 

 seasonal variations in wave climate are assumed to cancel over a long simula- 

 tion period. Cross -shore effects are implicitly included in the model if 

 measured shoreline positions are used in verification of predictions. 



24. Figures 2a-c show an example of shoreline change that is well 

 suited for modeling. The site is Oarai Beach, located about 180 km north of 

 Tokyo on the Pacific Ocean coast of Japan. A 500-m-long groin was constructed 

 to protect a fishing harbor from infiltration by sand carried by the longshore 

 current. Because of the availability of extensive wave, shoreline position, 

 and other needed data, this beach proved ideal for development and refinement 

 of a predecessor shoreline change model of GENESIS (Kraus 1981; Kraus and 

 Harikai 1983; Kraus, Hanson, and Harikai 1984; Hanson and Kraus 1986b; Kraus 

 1988a, b, c ,d) . Figures 2a and 2b show that the shoreline had a clear tendency 

 to advance on the updrift side of the long groin independent of season if the 

 interval between compared surveys is taken to be 1 year. Figure 2c gives a 

 plot of shoreline positions surveyed during each season of 1 year. The 

 tendency of the shoreline to advance is partially obscured because the 

 relatively short interval of 3 months includes the effects of individual 

 storms and other seasonal changes in wave climate, such as change in predomi- 

 nant wave direction, on shoreline position. 



Duration and Extent of Simulation 



25. The length of the time that can be modeled depends on the wave and 

 sand transport conditions, accuracy of the boundary conditions, character- 

 istics of the project, and whether the beach is near or far from equilibrium. 



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