BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

Table 8-14: Advantages and Disadvantages of Jetties as Shorezone ProtectionBMPADVANTAGES• Dissipate the energy of approaching wavesand reduce their ability to shoal theentrance to the channel.• Provide and maintain deep-water entrancesto marinas that help preserve boatnavigation.• Allow a more even deposition of littoralsediments along the foreshore andnearshore for permeable jetties whencompared to impermeable jetties.DISADVANTAGES• Scour a deeper channel and in doing so, maydisrupt littoral drift and depositional patterns.• Reduce the ability of waves to entrain andtransport sediment.• Often cause the accumulation of sedimentbehind and around the structure.• May deprive downdrift beaches of theirnormal sand supply and result in beacherosion.• Can create scenic impacts• Can cause navigational hazards.8.6.3 DESIGN CONSIDERATIONS AND PLANNINGDetermine the spacing between paired jetties with consideration of hydraulicprocesses, wave protection requisites, and safe navigation requirements. Bydesign, jetties modify the natural sediment regime in a designated place in thelittoral zone. In the past, they have been most commonly used in conjunctionwith marina development. These fixed jetties were designed to flush and trapsediment that clogs marina inlets. However, they also produce someundesirable results, mainly accelerated sedimentation and beach erosion inadjacent downdrift areas. Jetties are not appropriate where cross- or offshoremovement is the dominant means of sediment transport. The inclusion ofbeach nourishment as a maintenance measure can mitigate the negative impactsof jetties over time.The downdrift beach starvation caused by jetties can be mitigated by providingsand bypasses in the structures. Pumps placed on the updrift side can pass thesand in pipelines under the marina entrance. While this may be a costlyalternative, it may be more environmentally acceptable than continuedmaintenance dredging.A typical jetty in Lake Tahoe will be constructed of boulders. A jetty is usuallyoriented perpendicular to the shoreline. Jetties that are designed low relative tothe existing beach profile do not trap as much sediment as higher jetties.However, they are less effective in reducing sedimentation in the channel.Higher jetties can effectively block the sediment supply to downdrift beaches,resulting in erosion of these adjacent areas.Due to the complexity and site-specific requirements, jetties must be designedby licensed professional civil engineers who regularly design in-water structuresof this type. The design must demonstrate the following:• Design should not adversely affect erosion or deposition at the intersectionof the jetty and shore land.TRPA BMP HandbookCHAPTER 8: Shorezone Protective Structures and BMPs2012 8.6 JettiesPage 8-49