BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

of not being able to protect the backshore during a sustained severe storm. As the berm is eroded bystorm waves, the material is usually deposited offshore where it will continue to dissipate wave energy. 36If the project is built primarily for recreational purposes, the design goal may be to maximize retention offill volume within the limits of the project reach. This design objective may dictate whether or not filltransitions are used at all, and if so, how they should be designed. Usually, areas being considered for abeach nourishment project have experienced problematic erosion for some time. Sometimes there is arecord of previous studies and perhaps a record of past engineering activities at the site. 37Beach nourishment projects usually involve placement of a berm along a finite length of shoreline.Sometimes, beach nourishment projects include the creation of a feeder beach, in which fill material isintroduced at the updrift end of the area intended to receive the fill. Offshore and longshore littoraltransport then distributes the material to the rest of the project area. Feeder beaches work best in thoseareas that serve as a source of littoral material for downdrift beaches that are presently experiencing adeficit in the supply of littoral material, where the net transport direction is predictable, and where thenet transport rate is robust. 38The rate at which the new fill erodes depends on the coarseness of the fill relative to the native beachmaterial. Ideally, the grain size of the fill material should be slightly larger and of slightly greater densitythan the native beach materials. Generally, fill material that is coarser and denser than the native materialwill erode more slowly than the native material would; whereas finer fill will probably erode morequickly. The material used in artificial beach nourishment may be obtained from dredging of offshoresediments if clean and free of fine sediments and requires no overland hauling by trucks. Regardless of thesource, regional ordinance requires that the material be clean, nonorganic, chemically, and biologicallyinert material similar to the native lithology. Fill materials must meet the following conditions:Generally, the coarser the material, the steeper the beach slope will be. The addition of slightly coarsermaterial will produce a somewhat steeper beach profile in the foreshore and the new beach will convergeon the old beach beneath the surface, preferably just beyond the depth of maximum wave action. Inaddition, if fill is placed over a shorter length of shoreline, it may create a projection that is subjected toincreased wave action. Thus, it is preferable to make the transition to the existing shoreline over a longerdistance if possible.As stated, the elevation of the constructed berm must be at or higher than any natural berm analog. Thefinished grade of the fill should match the existing profile and slope based on the premise that the beachto be nourished was in an approximate equilibrium with the local wave climate. In principle, this refers toor means from the top of the unconsolidated beach (or the run-up elevation if backed by dunes) down tothe depth of closure (i.e. the effective limiting depth of wave effects on the lakebed profile).One estimate of the closure depth hc given by Birkemeier 39 (Equation 5.1), is:36USACE, 2008, Coastal Engineering Manual - Parts I-637USACE, 2008, Coastal Engineering Manual - Parts I-638USACE, 2008, Coastal Engineering Manual - Parts I-639Birkemeier, W.A., 1985, Journal of Waterway, Port, Coastal, and Ocean Engineering 111, Field Data on Seaward Limit of Profile Change,pp. 1507-1521TRPA BMP HandbookCHAPTER 8: Shorezone Protective Structures and BMPs2012 8.5 Beach Nourishment and ReplenishmentPage 8-43