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【正文】 gh the use of offshore sand deposits also has disadvantages, careful planning and coordination with resource and regulatory agencies can minimize the potential drawbacks. One such drawback is the tendency for offshore sediments to contain a higher percentage of silt and clay, necessitating a large overfill volume to account for anticipated losses. Additionally, the offshore borrow areas must be sited well seaward of the active portion of the beach profile so that the nourishment sand is not drained back into the borrow area by waves and currents. Inland SourcesThere are a number of inland sources of beachquality sand. In southern California, the loss of sediment reaching the coast due to the damming of rivers is a welldocumented phenomenon (Chapter 7, this report). The sediment trapped behind the dams represents a significant source of nourishment material. The use of this sediment acplishes two objectives: reestablishment of the reservoir capacity and nourishment of the beaches. Other inland sources that have been exploited in the past include sand dunes and deserts. Sources within the Littoral SystemSand bypassing and backpassing operations redistribute sand within the littoral system. Neither method represents a true source of sand because no new material is added to the system.However, both operations have been utilized extensively in California to place sand where it is most needed.Sand bypassing is the practice of transporting accumulated sand from the upcoast side of a sediment barrier, such as a jetty, to the eroded side. The process attempts to restore the natural downcoast flow of sand. Many harbors in California conduct sand bypassing in conjunction with maintenance dredging operations.Sand backpassing involves the mechanical transport of material from a wide stable beach to an upcoast sedimentstarved beach. This method often is utilized in locations where the sand from an eroding reach moves alongshore and is deposited in a more sheltered area. Backpassing essentially “recycles” the sand back to the eroding beach. If the sand volumes are moderate and the haul distances are short, the practice can provide a costeffective scheme for beach maintenance. Similar to sand bypassing, the process must be conducted on a regular basis. Beach Fill PlacementOnce sand is placed on the beach, waves and currents redistribute the material offshore and alongshore until a stable configuration is achieved. Depending on local conditions, a nourished beach may take several months or years to reach the equilibrium condition. The fill may be placed well above the shoreline as dune nourishment, on the dry beach and near the waterline, across an extended portion of the profile that stretches from the dry beach to well offshore, or pletely offshore as a sand bar (NRC, 1995). In some cases, hard structures may prolong the life of the nourishment material. The various placement strategies are discussed below. Dune NourishmentDune nourishment (Figure ) is particularly effective in protecting upland development against storm waves. The placement of material high above the waterline does not expand the width of the dry beach, however, and therefore is not appropriate when the enhancement of recreational opportunities is an important project object
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