BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

maintains its position under wave shear through the weight of or interlockingbetween the individual units. Static revetment types are referred to as mildlyflexible, semi-rigid, or rigid. Flexible armor retains its protective qualities evenwith severe distortion, such as when the underlying soil settles or scour causesthe toe of the revetment to sink.Another critical component of the design for static revetments is choosing anappropriate filter cloth geotextile or composite geogrid to place between therocks and the underlying soil. The use of an underlayer of stones between thearmor layer and the geotextile is common except when the stone size is lessthan 200 pounds. The underlayer should have a median weight no less thanone-tenth that of the revetment surface armor stones. 25The underlying filter layer helps support the armor against settlement, allowsgroundwater drainage through the structure, and prevents the soil beneathfrom being washed through the rocks by waves or groundwater seepage. Thetoe protection prevents settlement or removal of the revetment’s lakeward edgeor toe. Undercutting of the toe can lead to failure of the entire revetment; thetoe of the revetment must be protected by some type of rock apron.Overtopping of the revetment, which usually results in backshore erosion, canbe limited by designing the revetment height to be greater than the expecteddesign storm wave run-up height, or by protecting the area above therevetment with a protective apron composed of smaller material. Flanking canbe prevented by tying each end of the static revetment into adjacent shorezoneprotection structures or to competent bedrock in the existing shoreline. As theadjacent shoreline retreats, however, the ends may need to be extended in orderto maintain contact. Planting of vegetation on the lake side of the revetmentdiminishes wave energy, creates a more natural appearance, and serves tostrengthen the revetment.The steps in determining the design for a static revetment are extensive. It isstrongly encouraged that licensed professional civil engineers with directexperience around designing revetments be consulted during the design phase.The estimate of the required armor stone size from Hudson’s equation (refer toSection 8.16.1.8, Riprap Sizing for more details) is sensitive to wave height. Theproper wave height for Hudson’s equation as applied to the design of a lakeshore revetment is the regional design’s storm maximum calculated waveheight. 26Applicable wave height statistics for all Lake Tahoe settings are not alwaysavailable. Further, the design storm wave height, Hs, in Hudson’s equation is25USACE, 1984, USACE Waterways Experiment Station, Vicksburg MI, Shore Protection Manual, 4 thed. 2 Vol. and USACE, 2008, Coastal Engineering Manual - Parts I-626USACE, 1989, EM 1110-2 -5025, Environmental Engineering for Coastal Shore ProtectionTRPA BMP HandbookCHAPTER 8: Shorezone Protective Structures and BMPs2012 8.3 Static RevetmentsPage 8-33