BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

weights or connected firmly to sandy substrate via temporary anchors. 59 Theuse of a turbidity curtain on cobble or rocky lake substrates usually requiresthe use of moderately heavy sandbags to chink voids between rocks throughwhich turbid water would otherwise escape or dislodge. The freeboard heightfor a properly installed turbidity curtain must be adjusted to prevent wavesfrom overtopping the curtain, yet be low enough to prevent high winds fromentraining the curtain and re-positioning it off location.There are multiple options available for placement of turbidity barriers. Boxcurtains can be especially effective when applied in enclosed private mooringharbors and other enclosed residential boat storage.A silt curtain is an impermeable synthetic barrier constructed of a flexiblereinforced thermoplastic material. Silt curtains are most effective when used ona project where they are not opened and closed to allow equipment access tothe dredging or disposal area. Silt curtains are also limited to project locationswith less than 0.5 m/s wave or current velocities.Gunderbooms are designed to allow water to flow through the curtain whilefiltering suspended dredged sediment from the flow. Gunderbooms are similarto silt curtains but are constructed of permeable geotextile fabrics. They arealso designed to extend from the water surface to the substrate bottom.Sheet piling is a reusable, watertight barrier commonly made of steel, vinyl,plastic, or fiberglass. Interlocks between sheets form tight connections andallow minimum shift once installed. Sheet piles are costly to implement andless adaptable to changing operational requirements and high wave velocityconditions, particularly where irregular surfaces or edges compromise theirstable mounting. 60Cofferdams are temporary barriers commonly made of sheet piling. A bracedcofferdam is primarily used for bridging new pier construction in shallowwaters. It is constructed from a single wall of sheet piling that is driven into thesubstrate and surrounds the excavation or dredging site. The cofferdam isbraced on the inside for structural support and dewatered.The advantages of using cofferdams include their easy installation and removal,as well as their potential reuse for future projects. Cofferdam disadvantagesinclude the requirement of special equipment; high cost, time-consuminginstallation; possible failure in high current or wave locations. 6159USACE, 2008, Coastal Engineering Manual - Parts I-660USACE, 1995, Engineer Manual EM 1110-2-1810, Engineering and Design: Coastal Geology61USACE, 1995, Engineer Manual EM 1110-2-1810, Engineering and Design: Coastal GeologyTRPA BMP HandbookCHAPTER 8: Shorezone Protective Structures and BMPs2012 8.10 Turbidity CurtainsPage 8-67