BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

is generally limited in Lake Tahoe. Masonry or reinforced concrete bulkheadsare also examples of the massive gravity type of bulkhead. 22The minimum height of a bulkhead and lake wall are determined by themaximum calculated wave height of the designated design storm including runup(the 80 mile an hour – 1 hour duration wind event modeled at the legal highwater limit of 6,229.1 feet). Severe wave overtopping may lead to underminingat the back of the wall and to eventual failure and collapse of the structure.Waves can also wash away backfill along the sides of these structures. This losscan be prevented through flanking return walls that are keyed in at the ends ofthe bulkhead or by tying each end into existing shore protection devices orbedrock. In addition, the foundation embedment must be deep enough toprevent overturning Hyporeic groundwater percolating through the soil maybuild up pore pressures behind the structure and cause it to fail. Install andspace weep holes appropriately along the bottom of the structures andequipped with filters to relieve the pressure.Since toe scour can undermine the structure base and cause failure, toeprotection is necessary for stability. Typical toe protection consists of sizingrocks large enough to resist movement by large wave forces, with anunderlying layer of granular material or filter cloth to prevent the soil fromwashing through voids in the scour apron. 23There are two calculations that must be done when designing a bulkhead orlake wall. First, wave conditions and scour must be estimated such that thedesign wave is broken at the toe of the bulkhead or lake wall, recommended tobe modeled at the higher lake stand levels (e.g. perhaps 6,228 feet and higher).Furthermore, the scour depth at the base of the bulkhead is approximatelyequal to the height of the wave at the toe. Calculate the height of this brokenwave, Htb, based on there being no scour. The maximum scour depth smaxshould then be calculated, using Equation 2.1, as follows:s max = H tb (CEM Equation VI-5-259) (Equation 2.1)This scour depth is then subtracted from the level of the beach immediatelyadjacent to the structure, and the wave conditions at the base of the structurerecalculated. Rock toe protection should normally be installed to limit thedepth of the scour. The toe protection should extend a distance from the wallof at least twice the wave height, Htb, at the base of the structure (CEMEquation VI-5-277).22USACE, 1995, Engineer Manual EM 1110-2-1810, Engineering and Design: Coastal Geology, andUSACE, 2008, Coastal Engineering Manual - Parts I-623USACE, 1995, Engineer Manual EM 1110-2-1810, Engineering and Design: Coastal Geology andUSACE, 2008, Coastal Engineering Manual - Parts I-6CHAPTER 8: Shorezone Protective Structures and BMPsTRPA BMP Handbook8.2 Bulkheads and Lake Walls 2012Page 8-26