an opportunity for reducing the complexity of natural systems, for scaling 

 down dimensions, and for accelerating change over time so that detailed 

 interactions can be identified. Physical models can be applied in studies of 

 hydrodynamics, sediments, and structures. In studies of coastal processes and 

 responses, the wave tank is both the simplest and the most utilized physical 

 model. 



Physical models are typically either two- or three-dimensional. A wave 

 tank is considered to be a two-dimensional model because changes over length 

 and over depth can be examined. Where variations over width are also 

 investigated, the model is considered to be three-dimensional. A three- 

 dimensional model or basin may have a variety of types of bottoms, including 

 beds that are fixed, fixed with tracers, or moveable. Physical models require 

 precise scaling and calibration, and much design and construction expertise 

 must be devoted to their initial construction. Once set up, however, they 

 allow for direct measurement of process elements, repeated experiments over 

 a variety of conditions, and the study and isolation of variables that are 

 difficult to assess in the field. 



Some examples of physical model experiments (conducted principally in 

 wave tanks) that helped elucidate geomorphologic variability of coasts include 

 studies of littoral drift blockage by jetties (Seabergh and McCoy 1982), 

 breaker type classification (Galvin 1968), experiments of cliff erosion 

 (Sunamura 1983), relationships of storm surge or short-term water level 

 changes to beach and dune erosion, and studies of suspended sediment 

 concentration under waves (Hughes 1988). 



Large-scale physical models of harbors, rivers, and estuaries have been 

 built and tested at WES in order to examine the effects of jetties, weirs, chan- 

 nel relocations, and harbor construction on hydrodynamics and shoreline 

 changes in these complex systems. Measurements made by gages at prototype 

 (i.e. field) sites have sometimes been used to help calibrate the physical 

 models. In turn, the results of tests run in the physical models have identified 

 locations where gages needed to be placed in the field to measure unusual 

 conditions. An example is provided by the Los Angeles/Long Beach Harbor 

 model (Figure 28). In operation since the early 1970's, it has been used to 

 predict the effects of harbor construction on hydrodynamics and water quality. 

 As part of this project, wave gages were deployed in the two harbors at 

 selected sites. Figure 29 is an example of wave data from Long Beach 

 Harbor. Although the two gage stations were only a few hundred meters 

 apart, the instrument at sta 2 occasionally measured unusually high energy 

 compared to sta 1 . The cause of these energy events is unknown but is 

 hypothesized to be related to long-period harbor oscillations. 



Chapter 4 Laboratory Techniques and Approaches 



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