人工沙滩养护外文翻译.doc

California Beach Restoration Study January 20024. NOURISHMENT CONCEPTSSimply 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. Nourished 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 structures.Sediment characteristics and sources, sediment placement methods, and maintenanceRequirements, the key components of nourishment projects, are discussed in the following sections.4.1 OverviewWhereas structural means of beach retention were common 30 to 50 years ago, beachNourishment has become 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. 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 nourishment 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 ofseasonal shoreline fluctuation remains within acceptable limits during the 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 restoring a more natural system. Wider beaches, in turn, reduce the need for hard structures while simultaneously increasing recreational opportunities.4.2 Beach Nourishment MaterialThe characteristics of the available fill material are of utmost importance in the design of beach nourishment projects. At a minimum, the sediment must be uncontaminated and have a small fraction of fine grain sizes (“fines”, such as silt and clay). Most nourishment projects use sand as the 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 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 whenever possible, as they are less stable and hence prone to accelerated erosion.4.3 Sediment SourcesSources 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 described in one of the sections that follow.4.3.1 Sand of OpportunityThe 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 dredged 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 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 SourcesDuring recent decades, offshore sand deposits have served as the most common source of borrow material. 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 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.4.3.3 Inland SourcesThere are a number of inland sources of beach-quality sand. In southern 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-establishment 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 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 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 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 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 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 NourishmentDune nourishment (Figure 4.1) 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 objective.Figure 4.1 Dune nourishment4.4.2 Dry Beach NourishmentNourishment of the dry beach is a very common approach. In this scheme, sand is placed on the dry portion of the beach and near the waterline, and results in an immediate increase in beach width available for recreation (Figure 4.2). However, because no sand is placed on the submerged portion of the beach, sand will be redistributed offshore across the entire profile until a stable configuration is established. The equilibrating process results in a substantial narrowing of the initial dry beach width. The loss of sand from the beach face, sometimes rather quickly, has been a major source of criticism of beach nourishment projects in the past. This misunderstanding about the redistribution of the fill sand could be eliminated by better public education on the part of coastal engineers, scientists, and planners. It should be made clear that the sand will adjust to a more stable configuration resulting in a substantial narrowing of the initial beach width. However, the project is designed so that the desired beach width is provided after the sand has been re-worked by waves and currents, and this narrower design width should be the public expectation.Figure 4.2 Dry beach nourishment4.4.3 Profile NourishmentProfile nourishment involves placing the sand across the entire beach cross-section, both above and below water (Figure 4.3). The placement method attempts to build the beach in an already stable configuration. Because the equilibrium condition develops immediately, there is little offshore redistribution of sand and changes in the dry beach width are minimal. However, this placement scheme is more difficult and also provides less storm protection because there is no extra reserve of sand on the beach as there is with the dune and dry beach nourishment schemes.Figure 4.3 Profile nourishment4.4.4 Nearshore Bar NourishmentThis method involves the placement of beach fill material in a sand bar just offshore of the surf zone (Figure 4.4). To be successful, the placement must be within the active portion of the beach profile. 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 be moved up onto the beach varies with wave conditions. Although the nearshore bar placement scheme is the most technically challenging, it may be the most cost-effective alternative.Figure 4.4 Nearshore bar nourishment4.4.5 Beach Nourishment with Sand Retention DevicesSand retention devices are often used to prolong the effectiveness of a beach nourishment program (USACE, 1995). These devices are designed to reduce the amount of fill lost alongshore or offshore. Examples of natural sand containment are common in California. Many naturally wide beaches exist where sand is retained by sediment-blocking features such as headlands, reefs, rocky stream deltas, and other irregular bottom contours (Everts, 2000). In concept, the use of sand retention devices with nourishment is appealing; however, it should be considered cautiously. Undesirable effects, including accelerated erosion at adjacent downcoast beaches and loss of nearshore recreational opportunities, may result if these devices are not utilized properly.4.5 MaintenanceAs indicated at the outset of this chapter, nourished beaches typically require periodicreplenishment. Waves and currents will redistribute the beach fill sand in the alongshore and cross-shore directions, background erosion may persist, and extreme storm events may cause large losses of sediment from the dry beach. As a result, maintenance should be scheduled in the original project plan and monitoring should be performed to insure that the maintenance schedule is appropriate. Typical re-nourishment intervals range between two and ten years.Depending on local site conditions and sediment availability, it may be more economical to place a smaller fill initially and perform frequent re-nourishment. Conversely, if the beach fill project is dependent on a single large dredge project in a nearby navigation channel, then a larger initial fill will be placed and the interval between maintenance operations will be greater.4.6 ReferencesEverts, C. H., and C. Eldon, 2000. “Beach Retention Structures and Wide Sandy Beaches inSouthern California”, Shore & Beach, Vol.68, No. 3, pp. 11-22.National Research Council, 1995. Beach Nourishment and Protection, Washington D.C.:National Academy Press, 334 pp.U.S. Army Corps of Engineers, 1995. Design of Beach Fills, USACE Engineering Manual1110-2-3301, 86 pp. + appen.Wiegel, R.L., 1994. “Ocean Beach Nourishment on the USA Pacific Coast”, Shore & Beach,Vol. 62, No. 1, pp. 11-35.中文翻译：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给的减少。采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抵抗海岸风暴能力。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，增强公共健康和安全，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部分，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迎。 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，以便足以抵抗波浪、流和泥沙输运的作用。因此，这些建筑物尽量做得足够大，可以抵抗天然的泥沙运动。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，在冬季期间干燥的海滩面变得狭窄，然后在狂野的夏季波浪条件下干燥的海滩面会恢复到原先的大部分或者全部宽度。 理论上来讲，人工海滩养护项目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，然而，同时增加娱乐的可能性。4.2.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项目中是至关重要的。至少，泥沙必须是不受污染的，允许一小部分细小颗粒（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，但是大部分海滩养护项目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。尺寸可以比较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，更小的颗粒都应避免，因为这些小颗粒不稳定，会倾向于加快其侵蚀速度。4.3.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，1995）。每个泥沙来源会在下面的章节中描述。4.3.1.运动中沙子的随机性 在吉利福尼亚州所进行的大部分海滩养护项目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、港口维护和瀉湖的恢复项目中的泥沙疏浚（Wiegel，1994）。在这些案例当中，作为合适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，海水里沉积物的成本被排除在外，还有海滩养护项目能够从疏浚作业中获得实在的效益。4.3.2.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。来源于残积物的沙子被疏浚和放置在干燥的海滩上。而这个方案的主要好处有低成本、在物质输入区的海滩上高的沉积率，当这个项目在进行中，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、黏土的比重比较高，需要溢出量大才能达到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跃部分，以便养护的沙子没有让波浪、流作用排回到输出区。4.3.3.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，这个现象就好比如一份很好的备份文件证明。困在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达到两个目的：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。4.3.4.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，也不是因为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成本方案。与沙的冲涮相似，这个操作过程要在正常的基础上引导进行。4.4.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达到一个平衡状态，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，在干燥的海滩上和还参与海水的分界线附近，横跨一个干燥的海滩面延伸到，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。4.4.1.沙丘养护 沙丘养护对于保护抵抗风浪的高地发展，是特别有效的。然而，在分水线正上方高处的物质放置，没有延伸到干燥的海滩面的宽度。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。4.4.2.干燥的海滩养护 干燥的海滩养护是一个非常普遍的方案。在这个计划中，沙子放置到海滩