Beach Nourishment: A Guide for Local Government Officials
Beach Nourishment with Emphasis on Geological Characteristics Affecting Project Performance
by Robert G. Dean, Richard A. Davis and Karyn M. Erickson
This paper reviews the objectives of constructing beach nourishment projects, the processes that are affected through the placement of the beach nourishment project, the elements of the design features employed, the potential benefits and adverse effects of beach nourishment projects and concludes with several case studies that emphasize the physical performances of these projects.
Beach nourishment projects are usually constructed along shorelines where an erosional trend is present that could be either natural or a result of human activities. In those cases where this trend is due to human activities, the project will perform much better if the cause of the erosional trend can be minimized or eliminated in conjunction with construction of the beach nourishment project. The setting of a beach nourishment project is key to design and potential performance. Possible settings include a long straight beach, a project adjacent to an inlet that may be either a natural or modified inlet and a pocket beach. Projects constructed on rocky or seawalled shorelines, that otherwise have no sediment, present unique problems.
The design capabilities for beach nourishment projects have improved markedly over the last few decades, in part due to the better understanding of the associated processes, but also because of the data provided by the significant monitoring that has been conducted and documented for constructed beach nourishment projects.
Response Alternatives on an Eroding Beach
The response alternatives available in the presence of an eroding beach are surprisingly few and include: structural, retreat, nourishment or some combination of these.
Benefits of a Beach Nourishment Project
It has been demonstrated from both field studies and theory that a wide beach provides significant benefits in the form of storm damage reduction. During storms with elevated water levels and high waves, a wide beach performs as an effective energy absorber with the wave energy dissipated across the surf zone and wide beach rather than impacting on the upland structures. Figure 2a presents data collected after Hurricane Eloise that impacted the Florida Panhandle in 1975. This figure presents the results of assessment of damages to 540 structures (Shows, 1976). The horizontal axis on this figure represents the location of an individual structure relative to a "Control Line" that is a jurisdictional line established by the State of Florida approximately parallel to the shoreline. It is seen that if a structure were located 150 ft Gulf-ward of the Control Line, the damage to that structure was approximately $200,000 (1975 Dollars). However, if a structure were located on the Control Line (150 feet landward of the previous example), the damage to that structure would be approximately $14,000. Of particular significance, is the rather steep nature of the damage curve, indicating the very substantial reduction in damage due to the width of a beach fronting that structure. Although coastal construction procedures have improved substantially since 1975, Figure 2 still illustrates the energy-absorbing characteristics of a wide fronting beach. Beach nourishment is equivalent to shifting the damage curve seaward. Figure 2b demonstrates the storm damage reduction benefits of an additional beach width of 50 feet.
More recently, Rogers (2001) has reported on the storm damage reduction benefits due to beach nourishment projects constructed in the North Carolina area as documented through Hurricanes Bertha and Fran in 1996 and Dennis and Floyd in 1999. It was found that for Hurricane Fran, within the limits of the beach nourishment projects, no structures were damaged by waves or erosion whereas "outside the project limits, in similar surge conditions, an estimated 500 buildings were destroyed." For Hurricanes Dennis and Floyd, Rogers compared damages within the limits of three projects designed by the U. S. Army Corps of Engineers and smaller projects that are regularly maintained. Rogers stated "Hurricanes Floyd and Dennis threatened or destroyed 968 buildings outside the three Corps-designed projects’ manmade dunes. Remarkably, not even one building behind the project dunes was threatened by erosion – that’s ZERO." In summary, the storm damage reduction benefits of beach nourishment projects have been well established.
In many coastal areas, the recreational benefits of a wide beach can be substantial. An excellent example of this is the ten mile long Miami Beach, FL project that was constructed over the period 1976 and 1981, cost approximately $64,000,000 and has revitalized the economy of this area. Houston (1996) has investigated the recreational benefits of this beach nourishment project and found that in 1992, there were 20,000,000 tourist visits to Miami Beach, more than twice the total number of visits to the three most heavily visited National Parks combined: Yosemite, Yellowstone and Grand Canyon. Houston (2002) updated his study and found that the Nation’s beaches continue to be the key to our travel and tourism industry and that the Federal government is the primary beneficiary of taxes generated by this industry. Prior to nourishment of the Miami Beach project, it was quite difficult to walk along many portions of this beach, especially during periods of high tide. Ocean City, MD, Rehoboth Beach, DE, Jones Beach, NY and many other beaches in the United States are similar to Miami Beach in terms of the recreational attraction of wide beaches to local residents and tourists. The individuals who prefer urban beach settings contribute to reduced pressures on more pristine settings, such as wilderness areas, the National Seashore beaches, etc.. Additionally, many of the urban beaches are among the few natural environments available to low income residents who live in close proximity to these areas.
Potential Environmental Benefits
There are significant potential environmental benefits associated with beach nourishment projects. These include increased habitat for sea turtle nesting, nesting and foraging areas for sea birds, and habitat for beach flora such as sea beach amaranth and bitter panica. Although beach nourishment projects have been demonstrated to temporarily cause a reduction in the number of sea turtle nests within the project area, the nesting density usually rebounds to the same or to a greater level than occurred prior to the construction of the project (Trindell, et al, 1998; Ernst, et al, 1998; Ernst and Martin; 1999, Fletemeyer, 1999).
Possible Settings For Beach Nourishment Projects
There are a number of possible settings for beach nourishment projects. The settings have varying degrees of complexity that bear on their potential for performance and predictability. In the following sections, we commence with the most simple setting and progress to settings for which both the performance is likely to be reduced and the prediction of the performance has greater uncertainties.
Long Straight Beach
Nourishment on a long straight beach is the simplest setting possible and has the greatest history of construction and monitoring results. Figure 3a presents a case in which the planform of nourishment on a long straight beach represents a protuberance or "bulge" that the waves tend to straighten out, resulting in sediment flows that cause shoreline advancements on both sides of the project. It is seen that beach nourishment on a long straight beach acts as a "feeder beach" to the adjacent shorelines, and diminishes the need for shoreline stabilization at those locations. Beach nourishment profiles are usually constructed at slopes steeper than natural, resulting in profile equilibration that causes a narrowing of the dry beach width that may require several years to occur as shown in Figure 3b.
Beach Nourishment Adjacent to Inlets
There are three various sub-settings under the general situation of beach nourishment adjacent to inlets. One of these could be a natural inlet for which there may be an ebb tidal shoal that acts to bypass sand naturally (Figure 4a). A second case could be a deepened inlet but without the presence of jetties as shown in Figure 4b. A third possibility could be downdrift of a jettied inlet, a fairly frequent case (Figure 4c). Because of the presence of the inlet and the associated complexities in sediment transport processes including those of sediment transport induced by the navigational management practices such as jetty construction and channel deepening, the performance of projects adjacent to an inlet and the associated predictive capability are substantially less than that for the long straight beach discussed earlier. In the case of nourishment downdrift of a jettied inlet, the wave direction is of critical importance to the prediction of the life of the project. Without the presence of a supply of sediment, the nourishment in this area will erode at the rate of the net longshore sediment transport. The rate and direction of the net longshore sediment transport depends critically on wave direction. It can be shown theoretically and has been substantiated through monitoring, that, as a result of beach nourishment, the bulge resulting from beach nourishment downdrift of a littoral barrier will move downdrift and decrease in magnitude with distance and time.
Pocket beaches are defined as those that comprise a complete or partial littoral entity with headland controls on the two ends of the beach. In an idealized setting, there is very little or no exchange of sediment between the pocket beach and the adjacent shorelines. Pocket beaches can be natural or artificial. Many natural pocket beaches exist throughout the world and along the Pacific coastline of the United States. Artificial pocket beaches are usually constructed in areas where natural beaches are fairly narrow or absent. Examples of artificial pocket beaches include over 100 such systems on Chesapeake Bay, each consisting of several individual pocket beaches; the Fisher Key, FL project (Bodge and Olsen, 1992) constructed on a dredge spoil island originally consisting of cobble dredge material;, and the Fred Howard Park Beach that was constructed offshore of a muddy mangrove shoreline (see Figure 5). Additionally, there have been many pocket beaches constructed in the Caribbean where resorts have been developed along rocky shorelines with minimal natural beaches.
In its idealized form, this type of beach is one where there is no sand present other than that resulting from beach nourishment. This could be the result of seawall construction or could have happened naturally in the vicinity of a rocky shoreline. The main feature of a seawalled beach is that prior to nourishment, the sand transport potential exceeds the sand available to be transported. It has been shown (Dean and Yoo, 1992) that a nourishment project placed along a seawalled beach will migrate in the downdrift direction and that the rate of migration of the centroid of the nourished beach will increase with time.
Establishing Project Objectives
Establishing project performance objectives for a beach nourishment occurs at the initiation of the design development process in consideration of the project’s setting, benefits, socio-economics, and environmental resources and concludes during the alternatives design evaluation phase by the project decision makers and stakeholders groups affected by the project. In the development of a project’s design objectives, the design professional identifies and considers site-specific factors that are to be evaluated in the design development phase and later refined during the permitting phase. Typically the key factors that require investigation and subsequent evaluation include:
Sand Quantities and Requirements
Typically, beach nourishment projects require huge volumes of material; at least several hundred thousand cubic meters. In addition to the volume requirements, certain specifications of sediment composition, color and texture must be met. All natural beach areas have a specific set of characteristics. Generally, beach sands are fairly well-sorted but might also have a second mode that ranges in relative proportion up to more than 50 percent of the sediment. This second mode is either shell gravel or rock gravel depending on latitude and adjacent geology. In the United States shell is at least a significant constituent in the Florida panhandle, Hawaii and the Caribbean. Gravel is an important part of beach sediment primarily in glaciated areas such as New England, Alaska and the Great Lakes.
The objective of nourishment is to provide borrow material that is well-suited from an engineering, economic, and environmental standpoint for the construction site in order to enhance the potential for good performance of the project. In the next section we will consider how beach quality sediment is found; here the emphasis is on the nature of the desired sediment. The objective is to nourish with sediment that is as close as possible to the natural sediment. The texture is typically the most important characteristic to be matched. It is critical that the mean or average grain size of the borrow area be at least as coarse as the natural sediment; preferably coarser. This can range from fine sand to coarse sand depending on the location and source. In Florida, the presence of a significant shell component has been shown to be a positive factor in nourishment performance but an adverse component to sea turtles and recreational beach use.
Color and composition are also factors to be considered. In most beach nourishment projects, the composition of the natural beach is not altered by borrow material. Where the natural beach is altered typically occurs where shell or rock gravel comprise large fractions of the borrow material but are absent or only a minor constituent on the natural beach. This is not a problem for performance of the project but generally brings complaints from state and local government regulators and beach users. Sediment color is also a similar, but generally temporary problem associated with beach usage and public opinion of a project. Borrow materials may be stained, typically black or dark gray from organic matter, or brown from iron oxide. Generally after 6 months to a year, this dark coloration is gone, for the most part, and the color of the nourishment material resembles the color of the natural material.
In addition to the characteristics of the borrow sediment, the volume available is a major limiting factor in many nourishment projects. The location of this available volume is also an important factor because the transportation costs associated with placing the borrow material on the beach often are the biggest cost in a nourishment project. Various geophysical and geologic approaches are necessary to complete the task of sand search that is essentially an exploration effort.
There are several steps toward achieving the goal of locating suitable material for a beach nourishment project. The first is examination of detailed bathymetric charts of the area to find potential sediment bodies. Magnetometer surveys are required to determine if any potential artifacts are present such as sunken ships, artifacts and buried historical sites. High-resolution seismic surveys and side-scan sonar surveys provide information on the surface sediment and the potential volume of material present. The final exploration effort is focused on coring the potential area for study and analysis of the sediments.
Not only must the borrow area contain enough sediment with the appropriate properties for nourishment, but there are some materials that cannot be included in the dredge and construction process. Mud layers have to be avoided because the borrow material placed on the beach must have no more than 5 percent of the grains of a site smaller than 63 microns (0.063mm) in diameter. If mud layers can be recognized and mapped, the excavation site can be designed for dredge operators to avoid them, although there have been problems of this nature in some projects, e.g. Jacksonville Beach, Florida. Rocks from nearby bedrock or beachrock must also be avoided in the borrow material. The presence of scattered cobbles of irregular rocks on a beach is not acceptable. Once on the beach, these rocks may have to be removed at a significant cost. These problems have also been factors in projects on the Gulf Coast of Florida, e.g. North Sand Key.
The location of appropriate sediment in the desired amount is usually the most important factor in determining the viability and cost of a beach nourishment project.
Role of Background Erosion
The rate of background erosion can be a limiting factor to the performance of a beach nourishment project. As shown in Figure 3a, one component of the shoreline changes in a beach nourishment project is due to the "spreading out" losses. A second component is due to the ambient or background erosion rates that have resulted in the need for the project. Figure 1 has shown the relationship between the appropriateness of beach nourishment and background erosion. Usually beach nourishment is quite feasible in areas where the background erosion rates range from 1 to 3 ft per year, are marginally feasible in areas where the background erosion rates are 3 to 5 ft per year and are generally not economically viable if the rates are greater than 5 to 7 ft per year. It will be shown that by considering nourishment on a long straight beach and only the spreading losses, the longevity of a project is proportional to the square of the length of a project. Thus, the background erosion rates are secondary for "short" projects and can be the dominant cause of volumetric reduction in a project area for long projects. However, this is simply a reflection of the low spreading out losses that occur for long projects.
Signification of Evolution Time-Scales
There are two time-scales associated with the evolution of a beach nourishment project. These time-scales pertain to the volumetric reduction within the project area (Figure 3) and the profile equilibration time-scale (Figure 3b). Either of these scales could govern the economic viability of a project. Projects that require renourishment within one to three years will usually be considered ineffective, whereas those with renourishment intervals of five or greater years will be considered quite successful.
Design Tools For Predicting Project Performance and Some Illustrative Results
Rational tools or methods exist for the design of beach nourishment projects. These range from simple methods to numerical models requiring a computer for implementation. The advantages of numerical models include the capability of predicting more details and allowing greater specificity of project input conditions. The approach presented herein will be to provide some of the qualitative results from the simple methods that are key to a successful project.
Project setting, project length, wave height, background erosion rate and sand characteristics are all critical to the performance of a beach nourishment project. Some of these other factors can be controlled in the project design phase and others cannot. The effect of each of these factors is discussed briefly in the following sections (except for Project setting which was presented in the preceding section). For purposes of discussion, the "evolution" of a project will be considered as the time required for a certain proportion of the placed sand volume to be transported from the project limits through spreading (Figure 3a). In examples to be presented later, we will refer to the "half life" of a project, i.e. the time required for one-half of the project volume to be transported from the project limits through spreading.
Example Results Illustrating Effects of Various Design Factors
It is of interest to illustrate the dependency of project performance on some of the parameters discussed earlier through examples for a long straight beach. These results are approximate, but serve to illustrate the sensitivity of project performance to various parameters.
Tables 1 and 2 illustrate the effects of various design parameters. Table 1 presents the half-life of the nourishment project as defined earlier for various design parameters. Cases 1 through 3 illustrate the effects of project length on the half-life. It is seen that with other conditions the same, increasing the project length from 1 mile to 4 miles increases the half-life from 1.7 years to 27.2 years. Comparison of Cases 1 and 4 illustrates the effect of wave height. Decreasing the wave height from 2 feet to one foot increases the half-life from 1.7 to 9.6 years. Cases 6 and 7 illustrate the role of uniform background erosion. Comparison of Cases 5 and 6 show that for these project characteristics, an increase in the erosion rate from 0 to 3 feet/year decreases the project half-life from 38.4 years to 8.7 years. Comparison of the results for Cases 1 and 7 shows that the same increase in erosion rate for this project decreases the half-life from 1.7 years to 1.4 years. The reason that the background erosion rate affects the half-life for the longer project much more (reduction by 77%) than the short project (reduction by 18%) is that the spreading out losses are so much greater for the short project, that the effect of the background erosion is relative small by comparison.
Table 1. Examples of the Effects of Various Design Parameters on the Half Life of a Project.
Table 2 presents sample results of the effect on the dry beach width of nourishing with sands of different volumes and sizes relative to the native. Cases 1 through 3 are for a nourishment volume density of 100 cubic yards per foot and a native sand size of 0.2 mm. These cases illustrate the effect of nourishing with sands of the same size as the native, and sands that are coarser and finer than the native. It is seen that nourishment with sands that are the same size as the native results in an equilibrated dry beach width of 100 feet whereas nourishment with sands 0.18 mm and 0.22 mm results in equilibrated dry beach widths of 17 feet and 150 feet, respectively. Cases 4 through 6 present the same information as the previous three cases for a nourishment volume density of 50 cubic yards per foot and it is seen that the percentage variations in dry beach width for the dissimilar sediments are greater and for the 0.18 mm sediment, there is no additional dry beach width.
Table 2. Examples of the Effects of Sediment Size and Nourishment Volume Density on the Width of the Equilibrated Dry Beach.
Figure 3a illustrates the spreading out losses associated with beach nourishment projects and it has been noted that projects with very short lengths will have an associated short life. One potentially attractive approach to prolonging the life of beach nourishment projects is through the use of stabilization structures. One possible approach is presented in Figure 6 and in areas such as illustrated in Figure 5 for Fred Howard Park, where the use of stabilization structures is essential. The major concern with the use of stabilization structures is their potential adverse effects on the adjacent shorelines. For example, prior to the development of our present understanding of coastal processes, stabilization structures in the form of groins were the preferred approach to controlling beach erosion. However, groins function to trap sediment out of the system and have an associated adverse effect on the downdrift shorelines. The recognition of this effect was the impetus for the gradual evolution of beach erosion control toward beach nourishment that is the currently preferred methodology. Thus, in considering the use of stabilization structures for prolonging the life of beach nourishment projects, one must consider very carefully the potential adverse effects on the adjacent shorelines. One approach that is in current use is to require the sponsor of a nourishment project that involves structures, through a permit condition, to monitor the project and to include thresholds within the project that, if exceeded, require either the removal and modification of the structures or the renourishment of the project area in which the structures exist.
One setting in which structures can be used with relatively small concern of potential impact is adjacent to sediment "sinks"
such as adjacent to deepened inlets, etc. A second location in which structures have been used quite effectively is immediately downdrift of inlets where a bypassing bar is present. An example is South Lake Worth Inlet, FL where the bypassing bar is evident in Figure 7. The bypassing bar at an inlet reinstates, to some degree, the longshore sediment transport downdrift of the bar location. In this scenario, it appears reasonable to stabilize the portion of the shoreline updrift of the attachment point of the bypassing bar.
In summary, the use of structures to stabilize beach nourishment projects should be considered very carefully in the design process, recognizing both the potential impact of the structures on the adjacent shorelines and the perceived impact in those cases in which the background erosion or storm effects on the adjacent shorelines can be misinterpreted as due to the structures. The settings under which stabilization structures can be employed without consideration of adverse impacts to the adjacent shorelines are relatively few.
Planform Evolution of Multiple Nourishments
Following the construction of a beach nourishment project, the volume within the project area will decrease with time as described earlier, and eventually, in order to maintain a minimum project volume, it will be necessary to renourish the project area. An example is the Delray Beach, FL project that was initially constructed in 1973 and has now been nourished a total of five times (the initial nourishment and four renourishments). Fortunately, this project has been monitored on an annual basis. The results of that project in terms of total volume remaining within the project area relative to pre-project conditions up to the year 2000 (not including the most recent nourishment in 2002), are presented in Figure 8. It can be seen that of the total 4.65 million cubic yards of material added up to that time, as of the 2000 survey, a total of 2.86 million cubic yards remained within the project area (or 62 % of the total added). In addition, the adjacent areas have benefited substantially through the project performing as a "feeder beach" as illustrated schematically in Figure 3a. Beachler (1993) examined the benefits of this project to the adjacent areas and found that as of 1990, of the 41 % of the project volume that had been transported out of the project area, 7 % and 16% could be located north and south, respectively by surveys within 2 miles of the project limits. In summary, the Delray Beach project has been very successful within the project area and as a "feeder beach" to the adjacent areas.
The performance of multiple beach nourishment projects compared to the initial beach nourishment depends to a surprising degree on the background erosion rates. In general, if the background erosion rate is small, the combined effects of the multiple nourishments perform much better than the initial nourishment. If the background erosion rate is small, the renourishment intervals increase substantially with successive renourishments. However, for large background erosion rates, the renourishment intervals required to maintain a beach are quite complex and have not been investigated thoroughly.
Time-Scale for Profile Evolution
We have previously discussed time-scales for planform evolution. There is also an associated time-scale for profile evolution. As a rough approximation, the initial additional dry beach width usually will be 1.5 to 2.5 times greater than the equilibrated dry beach width depending on volumes added and the associated sediment size. This decrease is due to profile equilibration. Thus, it is important for the stakeholder to understand that over several years the nourished beach width will decrease with the rate of this evolution decreasing and approaching a more or less equilibrium dry beach width. Subsequent decreases in dry beach width occur due to volume reductions within the profile due to longshore transport to project adjacent areas. This reduction in beach width due to profile equilibration can be seen in Figure 9 which compares the relative volume change for the Manatee County, Florida project with the relative plan area change. It is seen that the two relationships diverge over time. If the profile were placed at equilibrium and remained at equilibrium, the two curves would be identical. The time-scale of this evolution is of special interest to the stakeholder in anticipating and understanding the eventual stabilization.
The time scale for profile equilibration has been examined for several projects and it has been found that the project will stabilize to one-half the equilibrium adjustment in approximately 2 to 3 years. Time scales of profile equilibration are an area where very little investigation has been carried out and it appears that the available wave energy is a determining factor in the rate of profile equilibration.
Erosional "Hot Spots"
Erosional "hot spots" (EHSs) are defined as areas that erode more rapidly than predicted in beach nourishment design and/or more rapidly than the adjacent shorelines. EHSs can occur on natural or nourished beaches. A remarkable photograph of three EHSs adjacent to two shoreline advancement areas was presented by Combe and Soileau (1987) and is reproduced here as Figure 10. Dean, et al. (1999) identified twelve possible causes of EHSs and later Kraus and Gagliano (2001) added six additional possible causes of EHSs. EHSs occur in practically all beach nourishment projects and some of these causes are related to the design and construction of the project. The objective of examining and understanding EHSs is to reduce or eliminate their adverse effects on the performance of the beach nourishment project. Some of the EHSs have associated with them areas that erode less rapidly or advance as seen in Figure 10.
In summary, our understanding and capability to predict the occurrence of EHSs in beach nourishment projects is far from adequate. In many cases, identification of the locations and possible causes of EHSs following the initial nourishment provides a better basis for examination and design of the renourishment project such that these EHSs can be minimized or eliminated. The most prevalent approach in the past to address the problem of EHSs is simply to overfill the beach in that particular area. With greater future understanding, it is believed that more effective approaches will be developed to reduce the adverse effects of EHSs.
Monitoring has now been carried out for almost three decades on some beach nourishment projects. The performance of these projects provides valuable insight on the prognosis of this methodology as a future method of erosion control. Five projects are reviewed briefly below.
Delray Beach, FL Project
This project was first nourished in 1973 and has now been renourished three times for a total of four nourishments. Monitoring results are presented in Figure 8. To provide a basis for comparison with the monitoring data (shown as solid circles), the lines are so-called "blind-folded" predictions for no background erosion (BE = 0) and a background erosion of two feet per year. Clearly, this project has been subjected to a number of storms in the 29 years since its initial construction, including Hurricane David in 1979, the 1984 Thanksgiving Day Storm, and Hurricane Andrew in 1992. It is seen that although the volume remaining within the project area has varied with respect to the predictions, there are no major lasting effects of the storms. It is noted that these calculations are based on a constant wave height. This example illustrates the effects of a relatively large project absorbing the wave energy associated with storms without major and lasting effects.
Perdido Key, FL Project
This project was nourished in 1989 and comprised the placement of approximately 5.4 million cubic yards of good quality sand adjacent to the Pensacola Bay Entrance. This entrance has been dredged to a depth of 45 feet which is considerably greater than the natural limiting channel depth of 20 feet. This entrance is also stabilized by two very short structures on the west side of the entrance; however, the effects of these structures are believed to be minimal due to their relatively small size and location well back in the inlet throat.
The measured proportion of sand remaining within the eastern and western halves of the beach nourishment project area and the total are shown as symbols in Figure 11. Also shown by the lines in this figure are the "blind-folded" predictions for the same quantities. As for the case of the Delray Beach project, these predictions are based on a single wave height. This project has been impacted by a number of hurricanes as shown in Figure 11. The impacts of these storms are evident as reductions in project volume; however, as for the case of Delray Beach, FL, there are no dramatic long term effects of the project, thus underscoring the project acting as an effective integrator of wave energy.
Manatee County, FL Project
This project comprised the placement of 2.3 million cubic yards along a shoreline length of 4.2 miles and was completed in early 1993 immediately prior to the so-called "Storm of the Century." The effect of this storm on expedited profile evolution is evident in Figure 9 where it is seen that the average beach width decreased by some 70 feet accomplishing in a single storm the equilibration effects that would normally require two to three years. This is to be compared with the approximate total equilibrium beach retreat of some 140 feet.
Sand Key, FL Project
The largest nourishment project on the Gulf peninsula coast of Florida is at Sand Key in Pinellas County. This barrier island is 28 km long and is convex toward the Gulf of Mexico (Figure 12). Commercial and residential development extends throughout the island and its beaches are critical to protection of these upland properties and to tourism, the dominant economic element of the area. Beach erosion has been a problem throughout most of the island since development became widespread in the 1950s. Extensive seawalls and multiple groin fields did little to maintain the beach environment.
A nourishment project extending across most of the barrier island was formulated and approved by the U.S. Army, Corps of Engineers. Construction of the project was planned for four phases with a general design template having a +6 ft NGVD (1.8m) elevation and a 50-150 ft (15 to 45m) width with a beach face of 1:20. The overfill ratio was typically about 30 percent.
Because the Gulf Coast of Florida is sediment starved, borrow sources were a major problem. The first phase (Figure 12) was constructed at Redington Beach and completed in July 1988. The borrow material was taken by suction dredge from the ebb tidal delta at nearby Johns Pass and pumped directly to the construction site. Approximately 700,000 m3 of sediment was placed on 3 km of beach at a total cost of $2.9 million.
Phase II was at Indian Rocks Beach (Figure 12) and completed in December 1990. Because of the absence of enough suitable beach sand in the area, borrow material was taken from the huge ebb tidal delta at the mouth of Tampa Bay, 40 km to the south. Because of the distance, construction material was barged to the construction site and pumped onto the beach. This project extended for nearly 8 km and included 1,000,000 m3 of material at a cost of $14.5 million.
Phase III at Indian Shores Beach, between the first two phases (refer to Figure 12), extended for 4.5 km. It used the same distant borrow source but utilized a different removal and delivery technique as Phase II. Construction material was taken using a clam-shell dredge and loaded onto barges. After transportation to the site, the material was off-loaded to the beach using a clam-shell dredge and a conveyor belt (Figure 13). This innovative approach to beach nourishment was much less costly than conventional techniques. The project was completed in December 1992 with 800,000 m3 of material, at a cost of about $11 million, nearly $3 million below budget.
The last phase of this project, North Sand Key (Figure 12), was not constructed until 1998. It extended for about 13 km along the most critically eroding portion of Sand Key. The borrow material came from offshore of the construction site and was pumped directly onto the beach. A total of 1.6 million m3 of material was placed on the beach at a cost of $28 million; equivalent to a unit cost of $17.50/m3. A maintenance nourishment of the first three phases took place during the construction of Phase IV.
The entire Sand Key nourishment project has performed extremely well. Detailed monitoring has shown that the design template was maintained for several years: beyond the predicted duration except for a few locations. Although this overall project was very costly, it has been one of the most successful projects on the Gulf Coast. The benefit-cost ratios are high. One large hotel experienced a change in occupancy from about 40% before nourishment to over 80% after nourishment. Other commercial enterprises have had similar results.
Town of Palm Beach Nourishment Project
One of the most successful nourishment projects in the state of Florida was at the Town of Palm Beach, Florida. The site setting required the design of a nourishment project along a one-mile section of highly eroded and heavily armored shoreline located at a significant seaward offset. The State of Florida and the U.S. Army Corps of Engineers, sensitive to the high cost of maintaining nourished beaches along this highly erosional beach (a site determined by the engineers to be an EHS), granted permits to the Town of Palm Beach for construction of beach nourishment and 11 adjustable groins. Authorizations for these permits included a requirement that the local government place supplemental sand for the purpose of maintaining a threshold volume of sand within the groin field.
Construction of this project was completed in November 1995. Subsequently, the groin field was constructed using land-based equipment and a de-watering system to allow work in the trenched areas for the assembly of the groins. The eleven (11) variable lengths, adjustable height groins were completed in early 1996. A photograph of the project one year after completion is given in Figure 14. The initial construction project placed 675,000 cubic meters of quality sand (d50 was 0.33mm) along a berm at 2.7 meters above mean sea level. The average unit sand placement quantity was 390 cubic meters per meter of shoreline (125 cubic yards per foot), a high unit quantity relative to most beach nourishment projects located in Florida. Eleven groins varying in length from 27 meters to 52 meters were constructed at a 97 meter spacing. The groins were constructed using pre-cast concrete modules weighing approximately 25 tons placed on a wide foundation mattress.
Prior to sand placement, this one-mile length of shoreline was heavily armored with greater than 90 percent of the shoreline protected by vertical concrete seawalls and little to no visible beach at high tide. Nourishment required a substantial unit placement quantity to ameliorate for the expected longshore sand redistribution effects on the project area (i.e. end-losses). Expected high post-project background erosion, due to wave focusing impacts, and a permit stipulation both required a minimum threshold volume of sand within the groin field. Project engineers predicted an average annual loss rate equivalent to 15 percent of the initial sand placement volume.
Implementation of a monitoring program for the projects was undertaken to evaluate beach nourishment and groin performance based on measurements of pre- and post-construction surveys within the groin field and the adjacent beaches. A managed systems approach was developed by the sponsor’s engineer to satisfy the state and federal regulators that the project’s maintenance plan would eliminate potential adverse impacts to adjacent downdrift beaches. Analyses of profile surveys, as shown in Figure 15 were performed semi-annually following completion of the project to determine the quantities of sand within the groin field and the need for maintenance of remedial actions such as lowering the crest elevation of the groins or removing modules to shorten the length of a groin. The 1995 Town of Palm Beach Sand Restoration Project was one of Florida’s first projects built in conjunction with a groin field since the early 1980’s and included the requirements of an extensive monitoring program and authority for the local government to perform remedial corrective actions and maintenance over a 10 year period. An example of the pre- and post-project conditions is given in Figure 16. The success of this project demonstrates that both groins and beach nourishment can be made to work successfully together.
Beach nourishment is usually in response to an erosional trend that could be natural or due to human effects, usually interference with a natural sediment supply. Capabilities to predict the performance of beach nourishment projects has improved markedly over the past three decades as a result of the construction and monitoring of many projects and advances in modeling, both at the general and detailed levels, the latter being carried out with numerical models. Beach nourishment provides a number of potential benefits including: storm damage reduction, and recreational and environmental enhancement.
Beach nourishment projects usually place sediment at slopes steeper than equilibrium which also creates perturbations in the shoreline alignment. Nature responds by inducing sediment flows that tend to reduce these disequilibria. The planform disequilibrium occurs in the longshore direction as longshore sediment transport and results in sediment being transported from the project area to the project adjacent areas where it is deposited and widens the beaches there. The profile disequilibrium induces seaward sediment transport that results in a profile of milder slope and a reduced dry beach width. Adequate beach nourishment design requires an understanding of these evolutionary processes and the ability to predict these time scales with reasonable confidence. The time scale for the volumetric losses due to longshore transport is proportional to the project length and inversely proportional to the 2.5 power of the wave height that mobilizes the placed sediment. Background erosion rate can also be a determining factor. Less is known regarding the time scales of profile equilibration; however, the equilibration "half-life" is on the order of two to three years, but is related in an unknown manner to the mobilizing wave height.
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