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1、CaliforniaBeach Restoration Study January 20024. NOURISHMENT CONCEPTSSimply stated, beach nourishment is the introduction of sediment onto a beach. In most cases, thesediment is sand and the beach is in an eroded condition. The process supplements thediminishing supply of natural sediment. Nourished
2、 shorelines provide two primary benefits:increased area for recreation, and greater protection against coastal storms. Other tangiblebenefits include tourism revenues, restored wildlife habitats, enhanced public health and safety,increased coastal access, and reduced need for hard structures.Sedimen
3、t characteristics and sources, sediment placement methods, and maintenance Requirements, the key ponents of nourishment projects, are discussed in the followingsections.4.1 OverviewWhereas structural means of beach retention were mon 30 to 50 years ago, beach Nourishment has bee the preferred method
4、 in recent decades. Beach nourishment represents asoft method of shoreline stabilization, in contrast to hard alternatives such as groins. Hardstructures are designed to remain stable and stationary, fully resisting the actions of waves,currents, and sediment transport. Hence, they tend to be large
5、structures and may significantlyimpact the natural movement of sand. Soft stabilization alternatives, such as sand or cobblebeach fills, mimic nature and are intended to be dynamic, responding to changes in wave andcurrent conditions. In the case of beach nourishment with sand, the dry beach may bee
6、narrow during winter storms and then recover much or all of its original width under mildersummer wave conditions. Ideally, a beach nourishment project is designed so that this range of seasonal shoreline fluctuation remains within acceptable limits during the project design life.Ultimately, however
7、, nourishment material is sacrificial in nature and will require periodicmaintenance.Introducing new sand onto the beach can pensate for a reduced sediment supply delivered byrivers and streams. In this way, beach nourishment represents a means of restoring a morenatural system. Wider beaches, in tu
8、rn, reduce the need for hard structures while simultaneouslyincreasing recreational opportunities.4.2 Beach Nourishment MaterialThe characteristics of the available fill material are of utmost importance in the design of beachnourishment projects. At a minimum, the sediment must be uncontaminated an
9、d have a smallfraction of fine grain sizes . Mostnourishment projects use sand asthe fill material, although projects have been implemented using pebbles and cobbles.In addition to the foregoing properties, the fill material should possess grain sizes that areparable to or larger than those of the n
10、ative beach sand. parably sized grains will tend tobehave in a manner analogous to that of the native material, while larger grain size will tend tobe more stable. Smaller grains should be avoided whenever possible, as they are less stable andhence prone to accelerated erosion.4.3 Sediment SourcesSo
11、urces of nourishment material may include offshore deposits, inland areas, sedimentaccumulations from within the littoral system, and sand of opportunity . Each ofthese sources is described in one of the sections that follow.Sand of OpportunityThe majority of beach nourishment projects conducted in
12、California have utilized sand ofopportunity, which is derived from projects whose primary motive is not beach replenishment.mon sources of this material have been dredged sediment from harbor construction, harbormaintenance, and lagoon restoration projects . In these cases, the suitability of thesed
13、iment as a beach fill material must be carefully examined both in terms of size fraction andpollutants. The primary advantage of sand of opportunity is the low cost. By placing thesediment on the beach, offshore disposal costs are eliminated and the nourishment projectprovides a tangible benefit fro
14、m the dredging operation.Offshore SourcesDuring recent decades, offshore sand deposits have served as the most mon source of borrowmaterial. Sand from these relict deposits is typically dredged and placed on the dry beach. Theprimary advantages of this approach include low cost, high placement rates
15、 on the receivingbeach, and minimal disturbance onshore while the project is underway.Although the use of offshore sand deposits also has disadvantages, careful planning andcoordination with resource and regulatory agencies can minimize the potential drawbacks. Onesuch drawback is the tendency for o
16、ffshore sediments to contain a higher percentage of silt andclay, necessitating a large overfill volume to account for anticipated losses. Additionally, theoffshore borrow areas must be sited well seaward of the active portion of the beach profile sothat the nourishment sand is not drained back into
17、 the borrow area by waves and currents.Inland SourcesThere are a number of inland sources of beach-quality sand. In southern California, the loss ofsediment reaching the coast due to the damming of rivers is a well-documented phenomenon. The sediment trapped behind the dams represents a significant
18、source ofnourishment material. The use of this sediment acplishes two objectives: re-establishment ofthe reservoir capacity and nourishment of the beaches. Other inland sources that have beenexploited in the past include sand dunes and deserts.Sources within the Littoral SystemSand bypassing and bac
19、kpassing operations redistribute sand within the littoral system. Neithermethod 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 ismost needed.Sand bypassing is the practice of
20、 transporting accumulated sand from the upcoast side of asediment barrier, such as a jetty, to the eroded side. The process attempts to restore the naturaldowncoast flow of sand. Many harbors in California conduct sand bypassing in conjunction withmaintenance dredging operations.Sand backpassing inv
21、olves the mechanical transport of material from a wide stable beach to anupcoast sediment-starved beach. This method often is utilized in locations where the sand froman eroding reach moves alongshore and is deposited in a more sheltered area. Backpassingessentially recycles the sand back to the ero
22、ding beach. If the sand volumes are moderate andthe haul distances are short, the practice can provide a cost-effective scheme for beachmaintenance. Similar to sand bypassing, the process must be conducted on a regular basis.4.4 Beach Fill PlacementOnce sand is placed on the beach, waves and current
23、s redistribute the material offshore andalongshore until a stable configuration is achieved. Depending on local conditions, a nourishedbeach 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 n
24、earthe waterline, across an extended portion of the profile that stretches from the dry beach to welloffshore, or pletely offshore as a sand bar . In some cases, hard structures mayprolong the life of the nourishment material. The various placement strategies are discussedbelow.Dune NourishmentDune
25、nourishment is particularly effective in protectinguplanddevelopment againststorm waves. The placement of material high above the waterline does not expand the width ofthe dry beach, however, and therefore is not appropriate when the enhancement of recreationalopportunities is an important project o
26、bjective.DuneFigure 4.1 Dune nourishmentDryBeach NourishmentNourishment of the dry beach is a very mon approach. In this scheme, sand is placed on thedry portion of the beach and near the waterline, and results in an immediate increase in beachwidth available for recreation . However, because no san
27、d is placed on thesubmerged portion of the beach, sand will be redistributed offshore across the entire profile untila stable configuration is established. The equilibrating process results in a substantial narrowingof the initial dry beach width.The loss of sand from the beach face, sometimes rathe
28、r quickly, has been a major source ofcriticism of beach nourishment projects in the past. This misunderstanding about theredistribution of the fill sand could be eliminated by better public education on the part of coastalengineers, scientists, and planners. It should be made clear that the sand wil
29、l adjust to a morestable configuration resulting in a substantial narrowing of the initial beach width. However, theproject is designed so that the desired beach width is provided after the sand has been re-workedby waves and currents, and this narrower design width should be the public expectation.
30、Profile nourishment involves placing the sand across the entire beach cross-section, both aboveand below water . The placement method attempts to build the beach in an alreadystableconfiguration. Because the equilibrium condition develops immediately, there is littleoffshore redistribution of sand a
31、nd changes in the dry beach width are minimal. However, thisplacement scheme is more difficult and also provides less storm protection because there is noextra reserve of sand on the beach as there is with the dune and dry beach nourishment schemes.DesignThis method involves the placement of beach f
32、ill material in a sand bar just offshore of the surfzone . To be successful, the placement must be within the active portion of the beachprofile. The sand will gradually move onshore under the influence of waves and currents,increasing the beach width. The period of time required for the sediment to
33、 be moved up onto thebeach varies with wave conditions. Although the nearshore bar placement scheme is the mosttechnically challenging, it may be the most cost-effective alternative.Sand retention devices are often used to prolong the effectiveness of a beach nourishmentprogram . These devices are d
34、esigned to reduce the amount of fill lost alongshoreor offshore. Examples of natural sand containment are mon in California. Many naturallywidebeachesexistwhere sand is retained by sediment-blocking features such as headlands,reefs, rocky stream deltas, and other irregular bottom contours . In conce
35、pt, the useof sand retention devices with nourishment is appealing; however, it should be consideredcautiously. Undesirable effects, including accelerated erosion at adjacent downcoast beaches andloss of nearshore recreational opportunities, may result if these devices are notutilized properly.4.5 M
36、aintenanceAs indicated at the outset of this chapter, nourished beaches typically require periodic replenishment. Waves and currents will redistribute the beach fill sand in the alongshore andcross-shore directions, background erosion may persist, and extreme storm events may causelarge losses of se
37、diment from the dry beach. As a result, maintenance should be scheduled in theoriginal project plan and monitoring should be performed to insure that the maintenanceschedule is appropriate. Typical re-nourishment intervals range between two and ten years.Depending on local site conditions and sedime
38、nt availability, it may be more economical to placea smaller fill initially and perform frequent re-nourishment. Conversely, if the beach fill projectis dependent on a single large dredge project in a nearby navigation channel, then a larger initialfill will be placed and the interval between mainte
39、nance operations will be greater.4.6 ReferencesEverts, C. H., and C. Eldon, 2000. Beach Retention Structures and WideSandyBeaches inSouthern California, Shore & Beach, Vol.68, No. 3, pp. 11-22.National Research Council, 1995.Beach Nourishment and Protection WashingtonD.C.:NationalAcademy Press, 334
40、pp.U.S. Army Corps of Engineers, 199f5 Design of Beach Fills, USACE Engineering Manual1110-2-3301, 86 pp. + appen.Wiegel, R.L., 1994. OceanBeach Nourishment on the USAPacificCoastShore & BeachVol. 62, No. 1, pp. 11-35.中文译:4.人工养护理念简单来说,海滩养护就是将其他地方的沙补充到沙滩上.在大多数情况下,泥沙是沙和海滩处于侵蚀条件下的产物.而这个过程弥补了天然泥沙供应的减少.采
41、用 人工海滩养护的海岸线有两个主要的好处：扩大了娱乐场所的区域和增强反抗海 岸风暴水平.另外直接的好处包括增加旅游业收入 ,恢复野生生物栖息地,增强公 共健康和平安,增加海岸的活动范围和减少刚性水工建筑物.泥沙的特性、来源、放置方法和维护要求,这些关键的人工海滩养护局部,将会 在下面的章节中讨论.4.1. 概况在三十到五十年前利用结构性建筑物来保护海滩是很普遍的,然而,在近几十 年来,人工海滩养护的方法已经很受欢送.人工沙滩养护代表着一种稳定海岸线的软性治理方法,与其相对的是刚性方 法,如堤坝.刚性的建筑物设计是用来保持海滩的稳定、不变,以便足以反抗波浪、流和泥沙输运的作用.因此,这些建筑物尽
42、量做得足够大,可以反抗天然的泥沙运 动.软性的稳定方法,如沙子或者往海滩填充卵石,来模拟自然运动,希望变得有活 力,来应对波浪、流作用下而产生的变化.对于用沙子来进行人工海滩养护而言,在冬季期间枯燥的海滩面变得狭窄,然后在狂野的夏季波浪条件下枯燥的海滩面 会恢复到原先的大局部或者全部宽度.理论上来讲,人工海滩养护工程的设计是为海岸线波动在设计生命周期内保持合 理的范围内.然而,最后海滩养护物质在自然情况下的消耗需要周期性维护补充.往海滩引进新的沙子能够补充在河流、溪流作用下流失的泥沙减少量.通过这个方式,人工海滩养护代表着一种恢复得更加自然化的体系.相应地,更多宽阔海滩减少对刚性建筑物的需求,
43、然而,同时增加娱乐的可能性.人工海滩养护的物质可用于填充的养护物质特性在设计人工海滩养护工程中是至关重要的.至少,泥沙必须是不受污染的,允许一小局部细小颗粒如淤泥、黏土颗粒等虽然项 目已经用卵石或者鹅卵石作为填充物,但是大局部海滩养护工程采用的是沙子作 为填充物质.除了前滩地以外,填充物应该拥有的颗粒尺寸能够比得上或大于当地海滩的 沙子尺寸.尺寸可以比拟的颗粒倾向与当地物质类似的运动方式,然而,更大的颗粒会倾向于更加稳定.无论何时,更小的颗粒都应防止,由于这些小颗粒不稳定, 会倾向于加快其侵蚀速度.泥沙的来源养护物质的来源可能包括近海沉积物、 内陆区域、源于海滨系统内的泥沙随 机性累积.NRC
44、,1995.每个泥沙来源会在下面的章节中描述.运动中沙子的随机性在吉利福尼亚州所进行的大局部海滩养护工程都应用了运动中沙子的随机性 这一特性,这源于那些主要不是以海滩沙子补充为动力的自然海滩演变规律.这 种物质的普遍来源于港口建设、港口维护和潟湖的恢复工程中的泥沙疏浚 Wiegel,1994.在这些案例当中,作为适宜的海滩填充物质必须要经过一系列 的颗粒尺寸、污染物质的认真检验符合.运动中沙子的随机行的主要优势是低成 本.通过在海滩放置泥沙,海水里沉积物的本钱被排除在外,还有海滩养护工程能 够从疏浚作业中获得实在的效益.近海物质来源地近几十年间,海沙沉积物已经作为最常见的输出物质来源. 来源于
45、残积物的沙 子被疏浚和放置在枯燥的海滩上.而这个方案的主要好处有低本钱、在物质输入 区的海滩上高的沉积率,当这个工程在进行中,物质输运的距离是最小的.虽然海 洋沉积物也有其缺点,但认真的规划以及适当的媒介协调,就能够使潜在的缺点最 小化.其中,有这样一个缺点是倾向于海里的泥沙含淤泥、黏土的比重比拟高,需要溢出量大才能到达补充预期泥沙流失量.此外,海上泥沙输出区域必须位于正面朝向岸滩轮廓最活泼局部 ,以便养护的 沙子没有让波浪、流作用排回到输出区.内陆物质来源有大量的海滩质沙子来源于内陆物质.在加利福尼亚州南部,流失的泥沙到达海 岸取决于在河流建堤坝,这个现象就好比方一份很好的备份文件证实.困在
46、大坝 后面的泥沙代表着一种极其重要的养护物质来源. 这种泥沙的使用能够到达两个 目的：能够重建储存库容,海滩的养护物质.另外那些过去已经被利用的内陆物 质来源包括沙丘、荒地.在海滨系统内的物质来源沙子冲刷和淤积,这些作用在海滨系统内重新分配.既不是一种方法能代表着 一种真正的沙子来源,也不是由于一种新的物质添加到系统中.然而,这两种冲刷 和淤积的作用原理已经广泛地应用到及利福尼亚州最需要填沙的地方了.沙子的冲刷就是来自上行海岸边的沙挡被搬运到已经侵蚀的海岸.而这个过程试图去恢复那自然状况的下行海岸沙流. 在及利福尼亚州很多港口都与疏浚作 用联系起来引导沙子的输运.沙子的回淤是关于来自一个广阔稳
47、定海滩的物质机械化地输运到上行海岸缺少泥沙的海滩上.这种方法经常应用于来自侵蚀区域的沙子沿着海边运动,最后在一个更受庇护的区域沉积下来.从本质上来讲,回淤可重新地将沙子输运回到受侵蚀的海滩上.如果沙子的体积适中,加上被输运的距离短,那么这种情况能够提供一个更为节省海滩维护的本钱方案.与沙的冲涮相似 这个操作过程要在正常的根底上引导进行.海滩填充物的放置一旦沙子放置到海滩上,海浪和流会重新分配这些物质到海里和周边的海岸上,直到一个稳定的组合形成.养护一片海滩可能要花上几个月或者数年去到达一个平衡状态,这取决于当地的实际情况.那填充物可能会正好放置在海岸线上作为沙丘的养护物质,在枯燥的海滩上和还参
48、与海水的分界线附近,横跨一个枯燥的海滩面延伸到,恰好与海水接触的轮廓面,或者作为海里的 一个沙坝.在某些情况下,刚性建筑物可能延长养护物的生命期限.多种不同的放置策略将 会在下面讨论.沙丘养护沙丘养护对于保护反抗风浪的高地开展,是特别有效的.然而,在分水线正上方高处的物质放置,没有延伸到枯燥的海滩面的宽度.因此,当增加海滩面积的娱乐可能性是非常重要的项目目标时是不适合的.枯燥的海滩养护枯燥的海滩养护是一个非常普遍的方案. 在这个at划中,沙子放置到海滩面枯燥的局部和 分水线附近,会迅速增加娱乐的可用海滩面宽度.然而 ,由于在没有沙子放置下的海滩水下部 分,沙子将会在海里重新分配到海滩的整个轮廓面范围 ,直到稳定的海滩组合建立完成.这个 平衡过程将会导致起初海滩面宽度的大面积减少.海滩面上流失的沙子,有时变化相当快,在过去已经成为海滩养护物质的主要来源争议之 处.受过良好公共教育的海岸工程师、科学家、设计者排除了对填充沙子重新分配的错误理解.应该要明确熟悉到沙子会适应更加稳定的海滩组合,这导致大面积缩窄原先海滩面的宽度.然而,这个工程的设计是为了当沙子重新被波浪