to 1:75) these factors may be adjusted for analytically. Friction reduces 

 transmitted wave energy, but previous model tests have provided scaling rela- 

 tionships for selecting the unit size used in the model to achieve the correct 

 wave transmission. Details on scale effects are discussed by Hudson et al. 

 (1979). Models used to reproduce short-period wave effects are normally un- 

 distorted. Therefore, wave patterns due to refraction (governed by the verti- 

 cal scale) and diffraction (governed by the horizontal scale) can be produced 

 simultaneously. 



77. The types of physical models which may be used to determine the 

 effect of detached breakwaters on the nearshore environment are: 



a. Fixed-bed models. 



b. Fixed-bed models with sediment tracers. 



c. Movable-bed models. 



78. Fixed-bed models are usually molded out of concrete to accurately 

 reproduce the preproject bathymetry. The model is then used to examine the 

 initial response of wave-generated current patterns to the breakwater as 

 superimposed on the preproject bathymetry. This is especially useful when the 

 preproject condition contains other structures such as groins. The inter- 

 action of the existing structures and the planned breakwater on wave-generated 

 currents can then be examined. The breakwater location may be adjusted to 

 reduce the possibility of strong rip currents and other undesirable effects 

 which may, in nature, cause local scour around the structures, offshore trans- 

 port of littoral material, and hazards to swimmers. Fixed-bed models also may 

 be used to determine wave heights both at the seaward toe of the structure 

 where it can be used for computing the structure cross-section and also behind 

 the structure where the wave attenuation characteristics can be compared for 

 various wave conditions, water level, breakwater lengths, and distances 

 offshore. 



79. Fixed-bed models with sediment tracers are an extension of fixed- 

 bed testing. A thin layer of tracer material is introduced on the fixed-bed 

 surface and its movement observed. Previous studies and analytical work indi- 

 cate which materials tend to best simulate prototype sediment movement (Hudson 

 et al. 1979). Also, information from the prototype may aid in determining an 

 appropriate material to simulate sediment movement. Field studies such as 

 tracer tests may aid in determining how sediment transported either alongshore 

 or cross-shore enters and leaves a project region and where it is distributed. 



56 



