roughness of the sides. In actual practice, the dissipation of long-period or unusually large 

 waves in this manner is small, and the cost of roughening the sides sufficiently to be 

 effective is not justified by the results. If long-period waves (16 seconds or more) or unusual 

 height (15 feet or more) are common to the water body served, a detached breakwater set 

 athwart the entrance will probably be necessary to keep surge in the harbor within 

 acceptable limits, regardless of any interior dissipating devices that might be tried. Only a 

 hydrauUc model study should be used to make this determination. 



d. Bank Protection. Landward of its jetty-protected reach, the channel requires no 

 protection from the main water body, but does require protection from waves and currents 

 in the channel itself. Erosion of the channel banks is the main concern, and several types of 

 revetted slope have been used successfully to prevent erosion. The least expensive, where 

 quarrystone is readily available, is a rubble layer (Fig. 31) placed on a filter layer. Each bank 

 must first be built or trimmed to a slope that will be stable under the conditions of 

 placement of the revetment. The slope wiU depend primarily upon the nature of the bank 

 materials. If it is entirely a cut-slope in undisturbed natural soil, it may be stable with an 

 incUnation of 1 on 1.5. If any part of the bank is filled, it must be compacted and trimmed 

 not steeper than about 1 on 2. Some soils will require a flatter slope, and this determination 

 should be made on the basis of laboratory soil tests. 



Figure 31. Well protected entrance to boat basin, Harwick, Massachusetts. Rubble revetment 

 protects exposed bank. 



68 



