83. GENESIS allows representation of two general boundary conditions, 

 termed a "pinned -beach" condition and a "gated" condition. If the position of 

 the shoreline can be assumed to be stationary, this condition defines a pinned 

 beach. A pinned beach boundary is appropriate if the sediment budget is 

 balanced at the boundary segment of the beach, meaning that the input and 

 output volumes of beach material at the boundary are equal on an average 

 annual basis. A pinned beach boundary may also be imposed if the beach is 

 constrained (e.g., by a rocky cliff or seawall), but sediment can still move 

 alongshore and past the boundary area. 



84. A gated boundary condition describes the case of some preferential 

 gain or loss of sand at the boundary; in other words, the boundary influences 

 the transport rate. As a simple example, if a jetty is very long, no sand is 

 expected to flow onto or off the grid at that location. As another example, 

 at some inlets sand may move alongshore and off the grid into the navigation 

 channel running through the inlet, but sand cannot move onto the grid from the 

 inlet (except possibly in an extreme wave event). The inlet thus acts as a 

 gate or rectifier of transport, allowing sand to escape from the project reach 

 but not to enter. Specific examples and hands-on experience in prescribing 

 these conditions are given in Part VI. 



Variability in Coastal Processes 



Problem of variability 



85. Waves bring an enormous amount of energy to the coast, and this 

 energy is dissipated through wave breaking, generation of currents, water 

 level changes, movement of sand, turbulence, and heat. Incident waves vary in 

 space and time, and their properties also change as they move over the sea 

 bottom. The beach is composed of sediment particles of various sizes and 

 shapes which move along and across the shore controlled by laws that are not 

 well known. This sediment is transported by complex three-dimensional 

 circulation patterns of various spatial and time scales and degrees of 

 turbulence. The beach and back-beach also exhibit different textural proper- 

 ties that vary alongshore, across - shore , and with time. In light of the 

 profound variability of coastal processes, it is clear that a single answer 



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