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Number can represent the fluid mechanics. However, these two criteria cannot be simultaneously satisfied on a small scale. The Froude Number is essential because it incorporates gravity. Gravity is the dominant restoring force for wave motion, thus it is better suited for this application. Similarity criterion representing the sediment transport processes of overwash is more difficult to determine. There are many criteria that can be used but criteria based on beach profile erosion are considered to best represent this event. Utilizing beach profile erosion criteria requires that five parameters be included. These are: the horizontal scale; the vertical scale; the median diameter of the sediment; the fall velocity; and the specific weight of the sediment. These four parameters work in conjunction to give the movable bed model its ability to simulate prototype conditions. The U.S. Army Corps of Engineers (May 1979) conducted an intensive review of similitude criteria for beach profile erosion. A summary of their findings is given in Table 3. 1. Each of the criteria is significant for an appropriate case.
Noda (1972) combined the results of earlier studies and suggested a relationship for movable bed models based on ratios of length, depth, sediment diameter, and specific weight. For similitude to occur, the following relationship must be satisfied: n, V.3 (n) (n 0.8 (3.1)
and
n. (n7)1"8 = (n/)055 (3.2)
" n, = model length / prototype length
" nd = model depth / prototype depth
" nD = (sediment diameter) model / (sediment diameter) prototype n,~ = ratio of specific weight of sediment in model to prototype