however, by carrying the revetted slope or exposed wall face to the channel bottom, thus 

 obviating the need for a berm. In this event, care must be exercised during maintenance 

 dredging to avoid damaging the toe of the revetment or wall. 



The depth of the inner entrance channel must provide adequate clearance below the huUs 

 of the largest user craft, taking into account possible water level fluctuations, wave trough 

 depressions and vessel squat. Near the outer end of the channel, it is important to eliminate 

 breaking waves. This is a factor only when the channel outlet has no offshore breakwater 

 protection and waves can enter directly from long offshore fetches. In this case, the depth 

 may have to be increased to prevent the design wave from breaking as it enters the channel. 



Where river or tidal ebb-flow currents are present, the period of each incoming wave is 

 shortened, but its energy remains the same, and will break at greater bottom depth. This 

 effect must be analyzed and accounted for in the entrance-depth calculations. Moreover, 

 such currents, coupled with the incoming wave turbulence, may cause asymetric erosion of 

 the channel side slopes with resultant meandering of the channel itself (Fig. 29). This is a 

 special problem that may require the help of a stream -flow analyst. 



c. Wave and Surge Dissipation. Site selection and environmental considerations have 

 indicated that it is not always possible to prevent undesirable swell and surge from entering 

 a harbor, particularly along the Pacific coast. However, there are several methods of 

 reducing swell and surge to acceptable heights before they reach the interior basins. The 

 provision of harbor-resonator basins just inside tlie entrance has been discussed but they are 

 still experimental and, if used, might occupy land sorely needed for other purposes. A very 

 effective energy dissipator is a nonuniform array of large stones placed on a flat slope facing 

 the outer entrance at the first turn in an entrance channel (Fig. 30). Waves are broken by 

 the stones and their energy dissipated in turbulence and heat rather than being reflected off 

 a wall or revetted slope into the inner basins. 



A variation of this type of energy dissipator is the wave-absorption beach, recessed in the 

 elbow of the first bend in the channel. Another variation is the wave reflector, designed to 

 reflect the waves back toward the first leg of the entrance channel rather than to break them 

 up. The wave reflector is a series of short reflecting walls or revetted slopes set in echelon 

 around the first bend. Each is aimed back toward the entrance and separated from its 

 neighbor by a short training wall alined in the wave direction. The reflected waves are out of 

 phase with each other and undergo diffraction as they pass the ends of the training walls, 

 they then diverge in a scattered pattern of harmless wavelets as they move erratically back 

 through the channel. These reflectors have not as yet (1973) been installed in a prototype 

 channel, but their effectiveness in model analysis is encouraging. 



Some dissipation of wave energy can be achieved by the combination of a trapezoidal 

 channel-bed section and upper slopes roughened by large stones in the revetment. In theory, 

 the waves diverge away from the channel axis by refraction and are partly destroyed by the 



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