Schematic three-dimensional (3-D) models 



32. Three-dimensional beach change models describe bottom elevation 

 changes, which can vary in both horizontal (cross -shore and longshore) 

 directions. Therefore, the fundamental assumptions of constant profile shape 

 used in shoreline change models and constant longshore transport in profile 

 erosion models are removed. Although 3-D beach change models represent the 

 ultimate goal of deterministic calculation of sediment transport and beach 

 change, achievement of this goal is limited by the capability to predict wave 

 climates and sediment transport rates. Therefore, simplifying assumptions are 

 made in schematic 3-D models, for example, to restrict the shape of the 

 profile or to calculate global rather than point transport rates. Perlin and 

 Dean (1978) extended the "two -line model" of Bakker (1968) to an n-line model 

 in which depths were restricted to monotonically decrease with distance 

 offshore for any particular profile. Larson, Kraus , and Hanson (in prepara- 

 tion) treated longshore and cross -shore transport independently in an itera- 

 tive process and allowed for nonmonotonic depth change, i.e., formation of 

 bars and berms . Schematized 3-D beach change models have not yet reached the 

 stage of wide application; they are limited in capabilities because of their 

 complexity and require considerable computational resources and expertise to 

 operate. This class of model will probably be the next to be introduced into 

 engineering practice. 



Fully 3-D models 



33. Fully 3-D beach change models represent the state of the art of 

 research and are not widely available for application. Waves, currents (wave- 

 induced and/or tidal), sediment transport, and changes in bottom elevation are 

 calculated point by point in small areas defined by a horizontal grid placed 

 over the region of interest. Use of these models requires special expertise 

 and powerful computers. Only limited applications have been made on large and 

 well-funded projects (for example, Vemulakonda et al . 1988, Watanabe 1988). 

 Because fully 3-D beach change models are used in attempts to simulate local 

 characteristics of waves, currents, and sediment transport, they require 

 extensive verification and sensitivity analyses. 



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