1、 1 California Beach Restoration Study January 2002 4. NOURISHMENT CONCEPTS Simply stated, beach nourishment is the introduction of sediment onto a beach. In most cases, the sediment is sand and the beach is in an eroded condition. The process supplements the diminishing supply of natural sediment. N
2、ourished shorelines provide two primary benefits: increased area for recreation, and greater protection against coastal storms. Other tangible benefits include tourism revenues, restored wildlife habitats, enhanced public health and safety, increased coastal access, and reduced need for hard structu
3、res. Sediment characteristics and sources, sediment placement methods, and maintenance Requirements, the key components of nourishment projects, are discussed in the following sections. 4.1 Overview Whereas structural means of beach retention were common 30 to 50 years ago, beach Nourishment has bec
4、ome the preferred method in recent decades. Beach nourishment represents a “soft” method of shoreline stabilization, in contrast to “hard” alternatives such as groins. Hard structures are designed to remain stable and stationary, fully resisting the actions of waves, currents, and sediment transport
5、. Hence, they tend to be large structures and may significantly impact the natural movement of sand. Soft stabilization alternatives, such as sand or cobble beach fills, mimic nature and are intended to be dynamic, responding to changes in wave and current conditions. In the case of beach nourishmen
6、t with sand, the dry beach may become narrow during winter storms and then recover much or all of its original width under milder summer wave conditions. Ideally, a beach nourishment project is designed so that this range of seasonal shoreline fluctuation remains within acceptable limits during the
7、project design life. Ultimately, however, nourishment material is sacrificial in nature and will require periodic maintenance. Introducing new sand onto the beach can compensate for a reduced sediment supply delivered by rivers and streams. In this way, beach nourishment represents a means of restor
8、ing a more natural system. Wider beaches, in turn, reduce the need for hard structures while simultaneously increasing recreational opportunities. 4.2 Beach Nourishment Material The characteristics of the available fill material are of utmost importance in the design of beach nourishment projects. A
9、t a minimum, the sediment must be uncontaminated and have a small fraction of fine grain sizes (“fines”, such as silt and clay). Most 2 nourishment projects use sand as the fill material, although projects have been implemented using pebbles and cobbles. In addition to the foregoing properties, the
10、fill material should possess grain sizes that are comparable to or larger than those of the native beach sand. Comparably sized grains will tend to behave in a manner analogous to that of the native material, while larger grain size will tend to be more stable. Smaller grains should be avoided whene
11、ver possible, as they are less stable and hence prone to accelerated erosion. 4.3 Sediment Sources Sources of nourishment material may include offshore deposits, inland areas, sediment accumulations from within the littoral system, and “sand of opportunity” (NRC, 1995). Each of these sources is desc
12、ribed in one of the sections that follow. 4.3.1 Sand of Opportunity The majority of beach nourishment projects conducted in California have utilized “sand of opportunity”, which is derived from projects whose primary motive is not beach replenishment. Common sources of this material have been dredge
13、d sediment from harbor construction, harbor maintenance, and lagoon restoration projects (Wiegel, 1994). In these cases, the suitability of the sediment as a beach fill material must be carefully examined both in terms of size fraction and pollutants. The primary advantage of sand of opportunity is
14、the low cost. By placing the sediment on the beach, offshore disposal costs are eliminated and the nourishment project provides a tangible benefit from the dredging operation. 4.3.2 Offshore Sources During recent decades, offshore sand deposits have served as the most common source of borrow materia
15、l. Sand from these relict deposits is typically dredged and placed on the dry beach. The primary advantages of this approach include low cost, high placement rates on the receiving beach, and minimal disturbance onshore while the project is underway. Although the use of offshore sand deposits also h
16、as 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 l
17、osses. 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. 4.3.3 Inland Sources 3 There are a number of inland sources of beach-quality sand. In sout
18、hern California, the loss of sediment reaching the coast due to the damming of rivers is a well-documented phenomenon (Chapter 7, this report). The sediment trapped behind the dams represents a significant source of nourishment material. The use of this sediment accomplishes two objectives: re-estab
19、lishment of the reservoir capacity and nourishment of the beaches. Other inland sources that have been exploited in the past include sand dunes and deserts. 4.3.4 Sources within the Littoral System Sand bypassing and backpassing operations redistribute sand within the littoral system. Neither method
20、 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,
21、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 a
22、n upcoast sediment-starved 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
23、 are short, the practice can provide a cost-effective scheme for beach maintenance. Similar to sand bypassing, the process must be conducted on a regular basis. 4.4 Beach Fill Placement Once sand is placed on the beach, waves and currents redistribute the material offshore and alongshore until a sta
24、ble 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
25、 that stretches from the dry beach to well offshore, or completely 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. 4.4.1 Dune Nourishment Dune nourishment (Figure 4.1) is particularly effective in protecting upland
