Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 82
4 Beach Nourishment Project Design and Prediction A sound technical basis for beach nourishment design and prediction is important because beaches are dynamic systems that typically experience signifi- cant short- and long-term changes. Further, placement of sand during nourish- ment rarely follows the cross-section profile that would occur naturally. Indeed, the constructed profiles are expected to change significantly during the first several years following construction or renourishment. The constructed profile may not follow the exact design prediction because the coastal processes were different than the available data revealed at the design stage or environmental conditions subsequent to project construction or renourishment varied substan- tially from the predictions supported by the data. A sound technical basis for design and prediction is necessary for: · determining costs and benefits, · decision making on whether the project is economically viable and whether it merits implementation, · forming the ground rules for assessing project performance, · evaluating project performance, · validating assumptions, identifying design deficiencies, identifying and developing design refinements and corrective action re g~mes, · decision making on whether and when to proceed with renourishment, · evaluating design and prediction procedures, and · improving the design process. 82
OCR for page 83
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION THE DESIGN PROCESS 83 Various methods exist for beach nourishment design and prediction that are complementary in the overall process of establishing optimum project character- istics. The design and prediction process is inherently iterative. Candidate de- signs are identified and evaluated at a preliminary level in which the performance of the project is predicted by using simple, rapid, relatively inexpensive methods. These performance characteristics are then compared with the design objectives of the project. The design is then refined until the performance predictions using the simple methodology confirm establishment of an optimal design. For sites without complex boundaries (straight beaches without terminal groins, inlets, or headlands), simple prediction tools are expected to allow quantification of time to renourishment to within approximately 30 percent of actual project performance, in the committee's estimation. Once the preliminary design is established, more detailed and comprehensive predictive methods are employed to "fine tune" the preliminary design. The advantages of employing this two-stage approach in- clude a check of both the simple and more detailed methods, a more rapid conver- gence to the final design than if only the detailed methods were employed, and a better perspective of the interrelationships among the overall project characteris- tics. If the predicted volumetric losses based on the simple and detailed methods differ by a considerable amount (more than 50 percent), the bases for the results obtained by both methods need to be reviewed. This chapter enumerates, in a general manner, the important design parameters and the prediction capability. Detailed discussions of prediction and design are presented as Appendixes C and D, respectively. NOURISHMENT OBJECTIVES AND CONSTRAINTS The usual nourishment objectives are to provide a wide beach that will reduce storm damage from flooding and waves and increase recreational benefits. For those projects that include federal funding, there is a requirement to identify a design as determined by federal guidelines. This requirement involves detailed calculations of storm damage reduction benefits expected to accrue from several designs and from a considerable number of storm scenarios. Projects funded entirely by nonfederal sources may be limited by the amount of available funds, and the objective then becomes placement of material to provide the greatest longevity and maximum dry beach width for the dollars available. SIGNIFICANT PROCESSES IN DESIGN The purposes of a beach nourishment project are to increase the dune and berm dimensions and to advance the shoreline seaward to reduce storm damage
OCR for page 84
84 BEACH NOURISHMENT AND PROTECTION and widen the recreational area. In addition, ecological advantages may accrue if the prenourished beach was not wide. Sand placement at a beach nourishment site during project construction or renourishment may or may not correspond to the natural profile of the beach at the time of placement. In the United States, use of a construction rather than a natural profile is the normal placement practice (see Appendix D). The sand can be placed either on the beach, immediately seaward of the beach (e.g., as a bar or mound), or a combination of the two. Where the initial placement of sand does not follow the natural cross-section profile of the beach, it is important for all parties interested in a project's perfor- mance to recognize that substantial changes in the profile are both normal and anticipated. It is also important to monitor and measure these changes to deter- mine whether they conform to predictions and to provide a basis for design refinements and corrective action that may become necessary to accommodate site-specific conditions. Although other, less significant processes are present, the two most domi- nant relevant to design and performance are profile equilibration and alortgshore spreading (or spreading lossesJ of sand from the project area to the adjacent shorelines (referred to as "alongshore equilibrations. Profile equilibration, a process leading to an equilibrium profile or equilibrated profile, refers to the tendency of a beach to take a characteristic shape or form in response to the integrated action of the local wave climate, as well as to the character and quan- tity of sediment available. Further discussion is provide later in this chapter (see Figure 4-1~. The time scales of these two processes are disparate: profile equili- bration occurs in a few years, whereas the alongshore equilibration varies in duration and is related to project length, sediment grain size, and wave environ- ment. For example, a reasonably long project (i.e., alongshore length) may re- quire decades before 50 percent or more of the sand volume is transported to the adjacent beaches. Profile equilibration is usually treated as if it occurs instantly in evaluating performance at the preliminary design level, distinguishing its ex- pected short-term effects from the longer time scale associated with alongshore equilibration. Profile Equilibration The most frequent placement is as an extension of the natural berm at a fairly steep slope (steeper than equilibrium) at the seaward limit of placement. A sec- ond type of placement is completely subaqueous in an offshore mound. These two types of placement are shown in Figure 4-2. Use of a mound relies on the expectation that the material will provide wave height reduction and eventually move ashore and widen the beach. This placement method is usually less costly and may allow use of finer material than should be placed on the subaerial beach. Profile equilibration is the process by which the beach takes its natural form in
OCR for page 85
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION )- - -- Original shoreline ................... ................ - ~ ~ ....... 3 ............ } ............... ~ .............. ~ ' "Spreading Out" losses \ |- "Spreading Out" losses a) Plan view showing "Spreading Out" losses and sand moving offshore to equilibrate profile Sand moves offshore to equilibrate profile Nourished Shoreline Dry beach width (fine sand) Dune' 85 . 1* Al Original profile Equilibrated profile (fine sane) Dry beach width (coarse sand) I ~ ~ ~ Initial placed profile \ ~ : ~ Equilibrated profile (coarse sand) ~ \ Sea level b) Elevation view showing original profile, initial placed profile, and adjusted profiles that would result from nourishment project with coarse and fine sands FIGURE 4-1 Sand transport losses and beach profiles associated with a nourished beach.
OCR for page 86
86 Dune -- ~Natural berm ..... BEACH NOURISHMENT AND PROTECTION Nourished profile Mean sea level Prenourisheed profile a) Usual method of nourishment with added material placed as seaward extension of the natural berm where waves will distribute sand to an equilibrium profile seaward of the original profile Dune Nourished profile b) Placement of nourishment material in an offshore mound with expectation that it will move on shore by wave action to nourish the profile FIGURE 4-2 Two placement methods for beach nourishment material. response to the physical forces that are present. A significant advantage of the beach placement option is the initial additional dry beach widths over the time required for profile equilibration to occur. During initial construction and renourishment, sand is usually placed along the shoreline at slopes steeper than equilibrium. The steeper slopes allow easier documentation of the volumes of materials placed, and they also provide a tem- porarily wider beach during the equilibration phase. Under the mobilizing action of waves, the sediment will be transported seaward, gradually approaching an equilibrium profile. The equilibrium profile as generally used by designers of beach nourishment projects is defined as the natural form that the beach would take for a given volume of sand of a particular grain size under the prevailing
OCR for page 87
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION 87 wave environment. The equilibrium profile is affected by the presence of struc- tures or natural features such as headlands that affect physical processes at a project site and that would have to be accommodated for in estimates of the profile. The equilibrium profile is an approximation; therefore, local variations to that profile would need to be accounted for. The profile is dynamic during the course of a year because of seasonal variations in the wave climate. For planning and design, these variations are accounted for in an average or baseline for the site. The physical changes that occur at the site are more pronounced near the shoreline. Physical changes to the seafloor decrease with distance offshore be- cause the wave action in deeper water is diminished near the bottom and less sand is suspended in the water column. The extent of sand movement is determined through various measurement techniques (see Appendix H). The term closure is a volumetric measure that is applied to a position off- shore at which changes in profile elevations are so slight as to be difficult to measure accurately within the limits of existing monitoring technology. There may be some sand movement past closure, but it does not normally result in measurable elevation changes (Hallermeier, 1981~. Depth of closure is an ap- proximate and straightforward reference for the seaward extent of measurable sand movement and is typically used by designers to analyze the degree of profile widening that would be associated with any given volume of sand placed. Al- though depth is not the only factor associated with the movement of sand, design- ers believe that there is a reasonable correlation between closure and the depth of closure that fosters this practice. A strong correlation has been observed in some major projects being monitored (Kraus, 1994), but further monitoring and analy- sis are needed to validate the correlation scientifically. In evaluating project performance it is necessary to be able to predict the equilibrium dry beach width. If the native sand and the nourishment sand are nearly the same grain size, it is reasonable to assume that the equilibrium profile form will be the same as that of the native beach before nourishment but is simply displaced seaward, and the equilibrium beach width can be calculated using simple equations. However, sand finer or coarser than the native sand will have equilibrium beach profiles that are of flatter or steeper slopes than the native sand, respectively. In such cases, methods are available for approximating the equilibrium dry beach width (see Appendix C). Calculation of the equilibrium beach width requires estimation of the depth to which the profile will equilibrate; this depth is usually estimated on the basis of the statistical wave height and period characteristics. Alongshore Spreading The rate of alongshore spreading of the placed sand is a dominant engineer- ing measure of the success of a project and is fundamental to determining success relative to economic measures as well. If, for example, one-half the placed sand
OCR for page 88
88 BEACH NOURISHMENT AND PROTECTION were transported from the region within 2 years and a substantially more frequent renourishment cycle than anticipated was required, the project could probably not be considered successful. However, if this process were to take a decade or more, equaling or exceeding the planned renourishment cycle, the project would likely be judged favorably. Both simple and detailed methods for predicting the rate of alongshore spreading depend primarily on wave height, background erosion rate, and sedi- ment grain size. For projects constructed in the vicinity of engineered structures, a littoral barrier, or a sink, such as an inlet, wave direction also is important. Because nourishment projects are constructed along an eroding coast, in addition to the spreading caused by the project planform anomaly, it is assumed that the beach will continue to erode at the same rate as before the nourishment. However, if the nourishment sand is of different size than the native sand, adjustment to the background erosion rate may be appropriate and needs to be considered. Sand transported in an alongshore direction from a nourishment project on a long, straight beach will also provide benefits to the beaches adjacent to the project. DIVERSITY OF SETTINGS FOR BEACH NOURISHMENT Beach nourishment projects are undertaken over a wide range of shoreline conditions. As noted, an eroding shoreline can result from jetty or groin construc- tion, natural causes, or development too near the shoreline. Figure 4-3 presents four relevant situations of interest. Figure 4-3a depicts the simplest case of nour- ishment on a long straight shoreline. Here it is somewhat surprising that, when the nourishment sand is equal in size and shape to the native sand, the perfor- mance depends only weakly on wave direction. Therefore, at the preliminary design stage, it is usually not necessary to consider wave direction. Also relevant to design for this situation is the fact that there exists a single wave height that will cause the same average spreading losses as the actual wave climate. This fact facilitates calculations at the preliminary design stage. For the case shown in Figure 4-3b, in which the nourishment area is downdrift of a complete or partial littoral barrier, approximate methods are less effective and wave direction is important in addition to wave height. Further, the se- quences in which wave events occur influence the planform at any particular time. In this case, the capability to predict performance using simple methods is reduced and may be further limited by the available knowledge of wave condi- tions, particularly wave direction. Figure 4-4 presents an example of shoreline change associated with the Delray Beach, Florida, nourishment project. Figure 4- 5 presents an example of computer-modeled planform evolution for an initially rectangular planform and a uniform background erosion rate of 0.6 rnlyear.
OCR for page 89
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION INLET Pre-Nourished Shoreline Nourished Shoreline ~ >~ \~ Pre-Nourished .~ Shoreline '\ 1` ' '; ., Nourished .. ~ Shoreline ::: ,, : ~ : ., . a, . . :. . . . ';; :, :.: . . . :.': ; ~ ll 1 1 ~ Jetty a) Nourishment on b) Nourishment a Long straight Downdrift of a Beach 1 Terminal ~ j S~ucture ',, Ares ~ 1 Pre-Nourished Shoreline | Nourished ' Shoreline | e =~, ,- '.' JO I .: Terminal Structure 1 ~ I . c) Nourishment Stabilized by Complete Littered Terminal Structures Barrier 1 1 89 Hi ·-., ..\ . . : :\ . ,' . t .# ;S I ''.C fit 2 1 ..1 _-.'' '.< AMP '' ..'/ ~ . of/ ~s ~ .. :-: .. . :~ A? Groin -Pre-Nourished Shoreline Nourished Rhoreline d) Nourishment Stabilized by a Tapered Groin Field FIGURE 4-3 Planviews of various scenarios of nourishment placement and stabiliza tion. USE OF STRUCTURES AND OTHER SHORE PROTECTION DEVICES IN CONJUNCTION WITH BEACH NOURISHMENT The use of traditional shore protection structures and nontraditional] shore protection devices (including structures) is controversial, both within and outside the coastal engineering profession. From an engineering perspective, structures can sometimes benefit beach nourishment projects. Nontraditional devices are more problematic because there is little definitive information on their perfor- mance capabilities as well as a history of innovative devices that have failed to live up to their claimed potential. Use of Hard Structures with Beach Nourishment In some cases, particularly when the project is relatively short or signifi- cantly affected by inlets, it may be desirable to limit alongshore losses by con 1Nontraditional structures may be described as shore protection structures of an experimental nature whose performances cannot at this time be predicted to a reasonable degree (see further discussion in the following section).
OCR for page 90
go BEACH NOURISHMENT AND PROTECTION 60 40 a) Cal Cal a) 20 - a o oh -20 - < Fill area R165 R170 R175 R180 R185 DNA Monument .. . . . ..... ..... ...... , R190 R195 R200 FIGURE 4-4 Shoreline change at Delray Beach nourishment project, 1974-1990, show- ing shoreline change outside the project area; nourishments involved 2.78 million m3 of matenal. structing terminal structures, as shown in Figure 4-3c. This approach must be employed with knowledge of the potential adverse effects on the adjacent shore- lines, especially if the net alongshore sediment transport is substantial. Terminal structures are especially appropriate at project ends where potential damage to the adjacent shorelines is small to nil (e.g., at a so-called littoral sink, such as at an inlet or submarine canyon). If structures are used on the downdrift end of a project on a long shoreline, it may be appropriate to place sand downdrift of the structure in anticipation of adverse effects of the structure and to develop a monitoring plan that responds to structure-related erosion. One possibility is to use an adjustable structure to regulate sand transport from the nourished beach without significant impacts to adjacent beaches. An example is a groin con- structed from "H" piles with panels that can be added or removed (Dean, 1975~. Prediction of project performance in the presence of terminal structures requires knowledge of both wave height and wave direction, and capabilities are limited for both the preliminary and detailed methods (see Appendix C for further discus- sion). A different use of structures is their placement in the interior of a nourish- ment project, such as the groins shown in Figure 4-3d. The same general precau
OCR for page 91
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION 35 30 25 - ~ 20 c, ct Q In ._ a) - a) U' 15 10 5 o 5 ~ Initial As: A,,,.""""" ~' ; #~ _ l-~P tlumen~''l i - it,,. , ~ 1 ~ , l ~ m.: 1 1 1 1 1 1 1 _ 1Year -2 Years 5 Years 1 0 Years ~11. -~16511111181518111111 I """J -5 -4 -3 -2 -1 0 1 2 3 4 5 Alongshore Distance (km) FIGURE 4-5 Calculated example of beach nourishment project evolution. 91 lions apply as for terminal structures. Although the intent of the groins is to increase the longevity of the project, if the project is in an area of strong unidirec- tional alongshore sediment transport, updrift accretion and downdrift erosion often result. One approach is to taper the groin field toward the project ends in order to make the planform less abrupt and thereby induce the ambient sand transport to move around the groin ends, minimizing any adverse impacts. In addition, nourishment material can be placed on the downdrift side of the project in anticipation of any erosional effects. Predicting the detailed effects on adjacent beaches when structures are employed is relatively unproven, and it is generally necessary to develop and carry out monitoring to identify adverse effects and establish a contingency plan to mitigate such effects when they occur. Nourish- ment on a beach with a seawall results in downdrift migration of the placed material (see Appendix C). Use of Nontraditional Shore Protection Devices Nontraditional shore protection devices have been offered commercially as countermeasures for shore erosion problems. Such devices have often been in
OCR for page 92
92 BEA CH NO URISHMENT AND PR O TECTI ON stalled without the benefit of objective laboratory or field evaluations, although there are notable exceptions. In general, nontraditional devices, often of propri- etary design, have taken the form of fences, walls, mazes, or flexible elements and have been designed to interfere with wave-driven motions and to "trap" in shallow water sand that otherwise would not be available to the littoral system. In other cases, prefabricated structures have been designed to interact with incident waves in an attempt to trap or retard the alongshore movement of sand. However, no device, conventional or unconventional, can create sand in the surf zone. Any accumulations must necessarily be at the expense of an adjacent section of the shore. This effect sets structures and other devices apart from beach nourishment, which is the only demonstrated technology that addresses the basic problem in coastal erosion-a shortage of sand. Some of the nontraditional devices involve large concrete structures placed near the shore. They may or may not be beneficial. If they are not, any unfavor- able conditions that develop could be difficult and expensive to correct, including the necessity of removing devices that do not perform well or become hazards to beach users. Further, in the committee's view, some nontraditional devices have been oversold and, with respect to their performance, have shown no lasting capability for shore protection. Specific research would be needed to determine the performance capabilities of such devices and their suitability for use in conjunction with beach nourish- ment. Evaluation of any beach protection system is expensive because of the large size of any meaningful experiment, and it is time-consuming because of concerns for testing under a full set of climate conditions; however, a uniform and effective methodology could be developed. A performance demonstration specification is needed for evaluating the effectiveness of nontraditional shore protection and beach stabilization and restoration devices. The results of such a program would be expected to provide a more complete basis for the probable performance before any interested agency or private buyer commits to their use. With respect to a testing methodology, wave tank experiments could be conducted for preliminary evaluation of nontraditional structural alternatives. These experiments appear to have been done only to a limited extent, but they would be a wise investment before commitment is made to field trials. Ulti- mately, performance monitoring of sufficient duration would need to be con- ducted to ensure that actual performance over the long term is not masked by positive or negative performance in the near term. Such monitoring would need to be in terms of years because of seasonal and annual variations in environmen- tal conditions. For a fully valid performance assessment, field testing would need to be conducted of the technology in the absence of beach nourishment, with beach nourishment, and with beach nourishment only at comparable sites to establish actual capabilities.
OCR for page 96
96 TABLE 4-1 Estimated Prediction Capabilities BEACHNOURISHMENT AND PROTECTION Scenario Variable Being Predicted Percentage Error in Prediction Long straight beach, compatible sand Nourishment near an inlet (first kilometer) Volume losses from project area Shoreline changes owing to profile equilibration Shoreline changes owing to volumetric losses Shoreline changes owing to combined profile equilibration and volumetric losses Volume losses from project area Shoreline changes owing to profile equilibration Shoreline changes owing to volumetric losses Shoreline changes owing to combined profile equilibration and volumetric losses + 25 + 25 + 25 + as + so + 25 + 50 + 60 compatible with, finer or coarser than the native beach material; these methods , are presented in Appendix C. Methods developed by Pelnard-Considere (1956) can be employed to address the question of volumetric longevity due to along- shore spreading especially for such cases as nourishment on a long' straight beach. These methods are available in the form of equations or graphs. A detailed summary is presented in Appendix C. One finding from Pelnard-Considere (1956) for the case of a long straight beach is that the volumetric longevity of material placed in a project is proportional to the square of the project length and inversely proportional to the wave height to the 2.5 power. For preliminary design, esti- mates of wave height are required, and if the setting involves alongshore sedi- ment transport and structures such as groins or jetties, estimates of wave direction also are required. Usually, at the preliminary level, it is assumed that after equili- bration the nourished berm height will be the same as the native berm height, and attention is not directed to other profile characteristics (e.g., dune design). Detailed Design At the detailed design level, for some applications it may be important to consider detailed design of the dune cross-section to obtain certain flood protec- tion benefits. In addition, the most detailed wave data can be employed in com- plex numerical models. The numerical models in general consider the alongshore and cross-shore sediment transport components separately. The cross-shore mod
OCR for page 97
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION 97 els are employed to evaluate the effects of severe storms on the design cross- section, whereas the alongshore transport models are used to address the volu- metric distribution of material remaining at various times in the future. In the cross-shore application, considerable attention may be directed to the effective- ness of placing sand in particular candidate geometries to provide flood protec- tion benefits. At this design level, it is possible to investigate in greater detail the stabilizing benefits of structures and the effects of particular hypothesized storm events in the vicinity of structures. In general, the detailed models provide greater flexibility to evaluate and compare the relative merits of particular alternative designs. SAND SOURCES A CONSIDERATION IN PROJECT DESIGN Over the past three decades, the materials for practically all large beach nourishment projects have been obtained from offshore deposits. A few medium- sized projects have been constructed by hauling the material from land borrow sites to the nourishment areas using large trucks or by moving sand from an onshore source via conveyer belts. It is essential that material obtained from the sea be located a sufficient distance offshore that the sand placed in conjunction with the nourishment will not be carried back into the borrow areas. In most cases, borrow areas need to be a minimum of 2 km from the shoreline, well seaward of the depth of closure. The most important borrow material characteristic is the sediment grain size. Borrow material grain size matching the native material is considered synony- mous with quality. A candidate borrow area may be considered unacceptable if the silt and clay fraction exceeds a certain percentage. This percentage needs to be related to the natural turbidity in the nourishment area. Fine material also adversely affects project performance. Early projects constructed without regard for grain size performed relatively poorly, and recent developments indicate that nourishment sand that is only slightly smaller than native sand can result in significantly narrower equilibrated dry beach widths compared to sand the same size as (or larger than) native sand. To identify potential borrow sources and to evaluate the material quality, a sand survey must be carried out that usually includes collecting geophysical profiles, surface samples, and cores. This report assumes that all sand sources are sufficiently free of contaminants to meet fed- eral, state, and local requirements. Therefore, contaminated sediments are not otherwise considered in these discussions. Sediment Sources and Construction of Projects The selection of a source of suitable material for a particular project depends on the design needs but also on environmental factors and on the cost of transport of the material from the borrow area to the placement site. These factors and their
OCR for page 98
98 BEACH NOURISHMENT AND PROTECTION long-term implications need to be considered with respect to beach nourishment programs and conveyed to all participants and parties of interest. The actual construction of a beach nourishment project normally involves (1) the search for a source of sediment that meets, as nearly as possible, the criteria specified in design documents; (2) the removal and transfer of the material to the nourishment site; and (3) its placement on the beach as prescribed by the design. These three components of a beach nourishment project are fundamental to its performance and often determine the cost and feasibility of a project. The search for viable sediment sources occurs early in the planning of a project because it can affect the design by determining the mode of delivery of the sediment and its placement on the beach; it also effectively defines the grain sizes of the fill. All these construction aspects are also important to the economic analyses and the environmental factors that must be determined early in the project. For these reasons, it is essential that project decision makers and design- ers have a basic understanding of sediment sources, transfer, and placement. The search for suitable material generally involves locating a deposit of sand and gravel of sufficient volume and grain size that could serve as a suitable source. Potentially, beach-quality sand and gravel can be obtained from inland, inlet, or offshore sources. Nonindigenous sediments imported from other areas or coun- tries and artificial materials are also potential sources. The general attributes of each of these potential sources are summarized in Table 4-2 and are described in greater detail in Appendix F. Locating and Assessing Offshore Sand Deposits The completion of a detailed geotechnical investigation is important in the search for offshore sediment sources on the continental shelf (Pries, 1980~. The investigation generally begins with high-resolution seismic reflection profiling that employs equipment towed behind a survey vessel (Williams, 1982~. The record is derived from reflected sound from the bottom and subbottom layers of sand or other sediments and with confirming observations from sediment cores taken from the area. The seismic data are used to map the stratigraphy and identify ancient fluvial and tidal inlet channels. Surveys may also include the use of side-scan sonar, which focuses a broad acoustic beam across a swath of seabed to define the small-scale shoals, bedforms, and variations in seabed texture. Records from the side-scan sonar can be used to produce photo-like images of the seabed (Williams, 1982~. Seismic reflection and side-scan observations are some- times augmented by diver observations, particularly to define the limits of poten- tially useful sediments. Such tools are important to determine the areal extent of sediment that potentially could be used in a project and also to locate any reefs or areas of hard bottom that are environmentally sensitive (e.g., Beachler and Higgins, 1992~.
OCR for page 99
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION TABLE 4-2 Potential Sources of Beach Nourishment Sediment 99 Offshore source Inlet source Accretional beach source Upland source Riverine source Lagoon source Artificial or nonindigenous material source Emergency source The most difficult operational conditions because of exposure to open sea. Increasingly difficult to obtain permits because of concern for impacts on hard bottom and migratory species. Must consider the effects of altering depth on wave energy at the shoreline. May be combined with a navigation project. Sand between jetties in a stabilized inlet. Often associated with dredging of navigational channels and the ebb- or flood-tide deltas of both natural and jettied inlets. Generally not suitable to mine sand (1) from most of the stable shorelines or from any eroding shoreline, (2) where there are insufficient surveys to define volumes, or (3) where sediment size and type vary markedly in the cross- shore direction. Generally the easiest to obtain permits and assess impacts. Offers opportunities for mitigation. Both quantity and quality of economical deposits often limited. Adverse secondary impacts from mining and overland transport. Has the potential for large quantities. Generally high quality. Transport distance a possible limiting factor. May interrupt a natural supply of sand to the coast. Typically difficult to obtain permits unless in conjunction with lagoon restoration or navigation projects because of regulations against loss of wetlands. Often low quality because of deposition of fine material. Convenient to barrier beaches and in protected waters for ease of construction. Flood-tide deltas the principal sources. Seldom tested in the United States because of high transport and redistribution costs. Some laboratory experiments done on recycling broken glass. Aragonite from Bahamas a possible source. Includes deposits around inlets and local sinks and sand from stable beaches with a sufficiently wide buffer. Generally used only in emergencies following storms, where a change in the shoreline planform is desired, or where, in the short term, is the only affordable option. May be combined with a navigation project. Not "true" source in that sand is not added to the system. SAND BYPASSING AS A SOURCE In some regions the need for beach nourishment has resulted from sand being trapped by the construction of breakwaters in the nearshore area to protect a harbor or by jetties built to fix the location of an entrance to an inland harbor. Where there is a net alongshore transport of sand in a dominant direction, sand
OCR for page 100
100 BEACH NOURISHMENT AND PROTECTION can be trapped updrift of the structures, within the entrance and/or harbor, or in an ebb-tide shoal. This deprivation of sand to the downdrift beach will ultimately cause erosion of this beach. Sand trapped in an entrance channel or harbor may interfere with navigation and require removal. In some cases, harbors or en- trances are designed to trap sand in a preferred location to minimize interference with navigation and facilitate its removal by dredging. Good engineering practice requires that this sand be deposited on the downdrift, or eroding, beach to main- tain the littoral sand transport. This operation is referred to as bypassing and may be continuous or intermittent. Availability of Suitable Sources Reconnaissance studies have been completed by the U.S. Army Corps of Engineers (USAGE) and the U.S. Geological Survey to assess the quantities of sand available on continental shelves that could be mined for various uses, in- cluding beach nourishment. The Inner Continental Shelf Sediment and Structure program of the Coastal Engineering Research Center included surveys from many areas along the U.S. seacoast and from the Great Lakes. Williams (1986) esti- mated sand and gravel resources within the U.S. exclusive economic zone (EEZ) at more than 1,200 billion m3 in water not deeper than 60 m. Compared with the annual sand and gravel consumption in the United States, these estimated vol- umes might suggest that anticipated national needs can be satisfied for the fore- seeable future. However, their use for beach nourishment may be prohibitive because many of the sand deposits are considerable distances from the shore and are at water depths at which sand mining may not be affordable. In addition, the thickness of some offshore deposits may not be sufficient for cost-effective use. Although large volumes of sand are present in the EEZ, economically located deposits of suitable quality and quantity to meet beach fill requirements are often limited. The continuing use of beach nourishment in new areas as well as the mainte- nance of projects already in place will, in the future, place a burden on project planners to locate new and continuing sources of reasonably accessible borrow material for these projects. Although the estimated reserves of sand suitable for nourishment programs are large, there have been local shortages, a situation that is likely to become more common in the future. For example, in Florida numerous projects have nearly depleted economically recoverable sand reserves in state waters. Increas- ingly, distant sources are being considered for use, including colitic Aragonite sands found in the Bahama Islands (see Appendix F). The increasing shortages are particularly important to long-term nourishment programs that are expected to continue for 50 years or more. The shortages are likely to increase the costs of renourishment significantly because of the imposition of acquisition costs and increased transportation costs relative to local sources of beach-quality material.
OCR for page 101
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION 101 The economic viability of projects using these sources will develop as more distant sources foster the construction and use of dredges capable of removing sediment from deep water, combined with the use of larger transport vessels and appropriate materials-handling schemes for the placement of this material. Areas such as the Pacific coast or regions where the use of offshore or navigational dredging sources may not yield sufficient amounts of sand in the long run will be the first to develop methods of recycling sand within segmented areas so as to use the littoral transport process as a relatively closed cycle process. Development, planning, and implementation of these processes remain a challenge; they may include artificial sand trap basins with continually operating pumping systems to relocate sand updrift; use of inlets as sand sources, with transport of material both updrift and downdrift via a dedicated hopper dredge to maintain a material balance; or implementation of construction methods that retard the transport process. SAND TRANSFER EQUIPMENT AND METHODS Generally, sand is excavated and transported from the borrow site to the beach by one or more of three types of equipment: cutter-suction dredge, trailing- suction hopper dredge, or a dedicated sand bypass system. However, the vast majority of beach projects have either used self-propelled hopper dredges with pump-out capability or pumped the borrow material directly to the beach fill site via pipelines with cutter-suction dredges. As noted previously, transport via trucks and placement directly onto the beach nourishment site have been used for some projects in which sand and gravel were obtained from upland sources. At present, the major constraints on the transport and placement of material for beach nourishment from offshore borrow sites are weather-related delays owing to sea state and winds; restrictions on construction activity, methods, and timing relating to environmental concerns; equipment limitations for deepwater dredging; and distances over which sediment must be transported. The construction of beach nourishment projects may involve the use of one or many possible combinations of equipment and techniques, depending on the site, the size of the job, environmental and other constraints, and the level of competition at bidding time. A more detailed discussion of the types of equip- ment, particularly dredging equipment, and their use in the mining, transfer, and placement of sand on beaches is contained in Appendix F. Herbich (1992) pro- vides a detailed technical discussion of dredging engineering, including place- ment methods. EROSIONAL HOT SPOTS In the design phase it is assumed that the distribution of volumetric erosion along the project will conform to the detailed design calculations. However, in
OCR for page 102
102 BEACH NOURISHMENT AND PROTECTION most projects, for reasons that are not obvious, there will be one or more areas that will erode more rapidly than their neighbors and more rapidly than predicted using accepted methodologies. These areas are called erosional hot spots. In some cases, they may occur at locations where a high rate of background erosion existed prior to the project. In other cases, the location may not correlate with preproject problem areas. Although the causes of hot spots in the latter case are not known, it has been hypothesized that they may be due to wave refraction and possibly wave focusing. Wave refraction could occur as a result of preproject bathymetry or bathymetry resulting from the geometry of the placed material. The composition of bottom material may also be a factor in that varying bottom conditions could affect the rate of movement and deposition. Regardless of the cause, erosional hot spots require renourishment earlier than the overall project, and because mobilization of the required dredging equipment is expensive, it is desirable to exercise measures to increase longevity in these areas. One approach during renourishment is to place a greater volume of sand in hot-spot areas, thereby extending the time before required subsequent renourishments. FEDERAL DESIGN PROCEDURES The USACE has developed guidelines and procedures to be used in the design of nourishment projects in which the federal government is a cost-sharing participant (see Appendix H). The implementation of these guidelines and proce- dures is evolving. Based on a general review of documentation for various beach nourishment projects by the committee, application of the best physics to project design has not been uniform among the USACE districts. Modern design profiles in the United States began with development of the "Caldwell Section" for emer- gency sand dune protection used after the great Ash Wednesday 1962 storm that struct the Mid-Atlantic coast (see Appendix H; Podufaly, 1962~. The USACE standard design procedures have evolved since then, although the basic form of the Caldwell section is still reflected in design (Appendix H). Today, design procedures usually define a "design" cross-section and an "advanced-fill" cross- section, as shown in Figure 4-7. The concept is that the design cross-section is the minimum cross-section that yields the expected benefits prior to renourishment. Advanced fill is the material placed seaward of the design cross-section to allow for erosion between nourishment events. Procedures are applied to attempt to optimize these two cross-sections. Ideally, these procedures would incorporate the concepts of pro- file equilibration and "spreading losses." However, in some recent designs the volumetric loss rates were based only on the historical erosion rates, a practice that fails to recognize that the "bulge" created by the nourishment can cause spreading losses that may be at least as great as the historical values. In addition, present federal guidelines for beach nourishment recommend the use of a "com- patibility" factor to account for differences between the native and nourishment
OCR for page 103
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION ; Design Width Orgina P ~_ 103 FIGURE 4-7 Schematic of design and advanced-fill nourishment profiles. sediments and a "renourishment" factor to account for spreading losses. These two concepts were developed prior to recent developments in equilibrium beach profiles and in both preliminary and detailed methods for calculating spreading losses. POSTCONSTRUCTION DESIGN REFINEMENT AND CORRECTIVE ACTION As discussed earlier, performance of a beach nourishment project, once con- structed, often does not conform to predictions because of limitations in predic- tive models and supporting data or because the wave climate was different than assumed. Monitoring programs are needed to detect deviations from predicted performance that could compromise the design integrity of a new beach nourish- ment project unless they are corrected. Such programs need to be timely enough to support early detection of deviations in beach behavior from those predicted. Variations then need to be assessed for significance, and corrective action re- gimes need to be developed and implemented. Few monitoring programs identi- fied during the committee's assessment were either timely enough or sufficiently developed to meet this objective. Although experience has shown that erosion rates vary across a project, traditional construction practices with uniform levels of overfilling will result in the placement of too much material on slow erosion areas and the underfilling of erosional hot spots. Alternatively, placing fill where it is needed instead of over
OCR for page 104
104 BEACHNOURISHMENT AND PROTECTION filling slow erosion areas will usually conserve and reduce the overall amount of fill that is needed. Early detection and correction of erosional hot spots through the placement of additional advanced fill will also contribute substantially to maintaining the design integrity of the project. A well-founded project would provide for robust monitoring for the duration of the profile equilibration period (2 to 5 years) to enable early identification and implementation of design refine ments and corrective measures. SAND BYPASS SYSTEMS AND HYBRID SYSTEMS Some regions need beach nourishment because of sand trapped by a harbor constructed by the installation of breakwaters in the nearshore area, or by jetties built to fix the location of a natural or constructed entrance into a coastal harbor or waterway system. A net alongshore transport of sand can cause: · accreting sand updrift of structures, · trapping of sand within the entrance or harbor, · formation of an ebb-tide shoal seaward of the entrance, and · erosion of the downdrift beach. To maintain required navigation depths, sand must be dredged from the entrance channel and harbor or from a sand trap constructed contiguous to and updrift of them. In many cases, it is desirable that sand not accumulate updrift of the entrance structures. It may be appropriate to bypass the sand around the barrier to nourish downdrift beaches. Similarly, sand that accumulates in navigation chan- nels as a result of harbor protection works could also be placed on downdrift beaches to help restore the sand budget of the littoral system. The importance of this fact relative to more traditional beach nourishment, in terms of quantities, is reflected in Tables 14 and 16 in Shoreline Protection and Beach Nourishment Projects of the U.S. Army Corps of Engineers (USAGE, 1994~. Hybrid shore protection projects are combinations of beach nourishment and structures, such as detached breakwaters, groins, jetties, revetments, seawalls, and submerged sills. There is a considerable body of knowledge on the structural design of the components and some on their functional design. There are existing procedures for the functional design of detached breakwaters and fill, and for groins anc1 fill hut not for the other types of hybrid projects. Some details are given in Appendix D. PROFESSIONAL ACCOUNTABILITY FOR DESIGN A project must be both structurally and functionally sound in order to be successful. Therefore, it is imperative that the project designer tee qualified to assess coastal processes affecting the site and design shore protection projects
OCR for page 105
BEACH NOURISHMENT PROJECT DESIGN AND PREDICTION 105 that are well correlated to these processes. Selecting a qualified engineer is some- what difficult because, although coastal engineering is a demanding discipline requiring specific knowledge of coastal processes and the design of coastal works, it is not recognized as a separate engineering discipline by regulating bodies or certification entities. Possession of a professional engineer's license by itself does not mean that the holder has the necessary expertise to design coastal works, although possession of such a license generally helps to promote competent oversight and professional and official accountability. Because there is no formal licensing program for coastal engineers at the state level, federal agencies with coastal engineering interests could establish a federal certification program to encourage and enhance the professional development of federal employees in- volved in the planning, design, construction, and maintenance of coastal works. In view of the fact that coastal engineering expertise at the federal level resides primarily with the USACE, that agency is a logical choice to develop and imple- ment a program designed to improve the professional credentials of federal prac- titioners. REFERENCES Beachler, K. E., and S. H. Higgins. 1992. Hollywood/Hallandale, building Florida's beaches in the 1990's. Shore and Beach 60(3): 15-22. Dean, R. G. 1975. Compatibility of borrow material for beach fills. Pp. 1319-1333 in Proceedings of the 14th Coastal Engineering Conference. New York: American Society of Civil Engineers. Dean, R. G., E. P. Berek, C. G. Gable, and R. J. Seymour. 1982. Longshore transport determined by an efficiency trap. Pp. 954-968 in Proceedings of the 18th International Conference on Coastal Engineering. New York: American Society of Civil Engineers. del Valle, R., R. Medina, and M. A. Losada. 1993. Dependence of coefficient K on grain size. Journal of the Waterway, Port, Coastal, and Ocean Engineering 1119(5):568-574. Hallermeier, R. J. 1981. Seaward Limit of Significant Sand Transport by Waves: An Annual Zona- tion for Seasonal Profiles. Coastal Engineering Technical Aid No. CETA 81-2. Fort Belvoir, Va.: Coastal Engineering Research Center, U.S. Army Corps of Engineers. Herbich, J. B. 1992. Handbook of Dredging Engineering. New York: McGraw-Hill. Kraus, N. C. 1994. Importance of Beach Profile Surveying and Depth of Closure for Beach Nourish- ment Project Design. Presentation to the national meeting of the American Shore and Beach Preservation Association, October 6, 1994, Virginia Beach, Va. Pelnard-Considere, R. 1956. Essai de Theorie de l'Evt~lution des Formes de Rivate en Plages de Sable et de Galets. 4th Journees de l'Hydraulique, Les Energies de la Mar, Question III, Rap- port No. 1 (in French). Vicksburg, Miss.: U.S. Army Waterways Experiment Station, U.S. Army Corps of Engineers. Pilkey, O. H., R. S. Young, S. R. Riggs, A. W. S. Smith, H. Wu, and W. D. Pilkey. 1993. The concept of shoreface profile of equilibrium: a critical review. Journal of Coastal Research 9(1):255-278. Podufaly, E. T. 1962. Operation five-high. Shore and Beach 30(2):9-17. Prins, D. A. 1980. Data collection methods for sand inventory-type surveys. Coastal Engineering Technical Aid 80-4. Fort Belvoir, Va.: Coastal Engineering Research Center, U.S. Army Corps of Engineers.
OCR for page 106
106 BEACH NOURISHMENT AND PROTECTION USACE. 1994. Shoreline Protection and Beach Nourishment Projects of the U.S. Army Corps of Engineers. IWR Report 94-PS-1. Fort Belvoir, Va.: Institute of Water Resources, Water Re- sources Support Center, U.S. Army Corps of Engineers. Verhagen, H. J. 1992. Method for artificial beach nourishment. Pp. 2474-2485 in Proceedings of the 23rd International Conference on Coastal Engineering. New York: American Society of Civil Engineers. Williams, S. J. 1982. Use of High-Resolution Seismic Reflection and Side-Scan Sonar Equipment for Offshore Surveys. Coastal Engineering Technical Aid 82-5. Fort Belvoir, Va.: Coastal Engineering Research Center, U.S. Army Corps of Engineers. Williams, S. J. 1986. Sand and gravel deposits within the United States exclusive economic zone: resource assessment and uses. Pp. 377-386 in Proceedings of the 18th Offshore Technology Conference. Richardson, Tex.: Offshore Technology Conference.
Representative terms from entire chapter: