The filter material must be porous enough to allow water to pass through and not build 

 up a dangerous hydrostatic head behind the rrevetment, but tight enough to prevent wave 

 action from pumping the underlying material through the voids. A graded gravel or crushed 

 stone blanket can be used as a filter but the gradation of the blanket material is dependent 

 on the size and gradation of the bank material. The filter layer should be spread in a 

 uniform layer 8 inches to 1-foot thick on the trimmed slope. Woven plastic filter cloth, 

 placed in accordance with the manufacturer's directions, is equally satisfactory and often 

 less expensive (Fig. 32). It has the added safety feature of ensured continuity, as it cannot 

 be worked through voids in the protective armor layer by eddy currents or wave turbulence. 

 The cloth should be of woven monofilament polyvinylidene or polypropylene. 



Below the water surface the filter also serves as a bedding layer for the armor stone. It 

 should be placed with extra care to ensure reasonable uniformity of gravel thickness or 

 full-design overlapping of cloth strips. The revetment stone should be placed on the 

 underwater part immediately after the filter has been layed to prevent being disturbed by 

 wave or currents. 



The revetment stone may be designed in accordance with formulas for breakwater armor 

 layers, using the anticipated maximum wave height in the basin or channel whose perimeter 

 is being protected. Since patrol boats may traverse a basin or channel at high speeds in 

 emergencies, and generate large wake waves, a lower hmit of about 4 feet should be placed 

 on the design-wave height, and the thickness of a rubble revetment should seldom be less 

 than 3 feet. The result will be an armor that can withstand almost any tidal or river current. 

 The top of the revetment must be above the limit of uprush of the design wave at extreme 

 high tide, and the bottom must be the design-wave height below extreme low water. Where 

 strong currents are anticipated, the revetment should continue to the bottom. Prefabricated, 

 overlapping concrete blocks may be used for the armor layer where stone is too expensive 

 (Fig. 33). The blocks should be sized according to the manufacturer's recommendations for 

 the design wave, and the toe should be well secured to prevent the entire revetment from 

 sliding down the slope. If the armor material does not extend to the top of the bank, the 

 remainder of the slope must be paved or otherwise stabilized to prevent rainwash and 

 possible displacement by pedestrians or animals. 



An alternative to the rubble or concrete -block revetment is the grout-filled cloth 

 mattress. Some proprietary fabrics manufactured especially for this purpose, may simply be 

 unrolled empty into place and then pumped full of grout (Fig. 34). The fabrics have the 

 advantage of requiring no bedding layer ensuring continuity of the protection and 

 facilitating placement. Also, they can be placed on a slope of 45° if the underlying material 

 will safely stand at that angle considering possible static-head fluctuations. Filter -point 

 mattresses are available for those parts of the revetment that must be "weep-holed" to 

 release hydrostatic pressures. 



69 



