Beach Nourishment and Protection (1995) / Chapter Skim
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D Design of Beach Nourishment Projects
D Design of Beach Nourishment Projects
Pages 189-250
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From page 189... ...
The design process for beach nourishment projects determines the quantity, configuration, and distribution of the sediment to be placed along a specific section of coast in order to restore natural storm protection, recreational area, or both. The design objective is to identify a unique project that best addresses and accommodates site conditions, erosion rates, wave climate, available sand, costs, funding sources, and environmental considerations.
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From page 190... ...
Analytical and numerical models of alongshore sand transport and cross-shore transport are examples of coastal process models that are important in beach nourishment project analysis. Judgment is also needed in evaluating candidate designs or elements of a design because coastal processes are complex (NRC, 1987, 1989, 1990, 1992)
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From page 191... ...
Early nourishment projects did not consider the offshore profile properly, often using unrealistic slopes, which subsequently caused an excessive loss of subaerial (dry) beach (Vallianos, 1974; Jarrett 1987; Hanson and Lillycrop, 1988; Davison et al., 19921.
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From page 192... ...
These concepts are generally accepted in the industry but are not widely used by designers, because overfill and renourishment factors are used to determine fill compatibility. Since profile shape change and winnowing occur on nourished beaches, both measures need to be considered in a design when the borrow sand is finer than the native beach sand.
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From page 193... ...
It was applied to a number of beach nourishment projects in the late 1980s and early l990s (Coastal Planning and Engineering, 1992a, b; USACE, 1989b) and has shown good correlation to monitored projects in Delray Beach, Florida (USAGE, l991c)
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From page 194... ...
Within the first year or so after placement of beach fill, the construction profile will be reshaped by waves to an equilibrium profile, causing the berm to retreat to the design and advanced-fill profile (see Figure 4-7~. For design purposes, the construction profile is treated as an anomalous temporary feature.
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From page 195... ...
In such cases, the use of an equilibrium profile defined by the grain size of the borrow material or an adjacent natural beach profile can be used to approximate the nourished profile. Further, if seawalls, groins, or other structures or features have caused the profile to deviate significantly from the anticipated equilibrated profile after nourishment, then Equation (D-1)
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From page 196... ...
If rock, clay, or peat outcrops (or, in the special case of the Arctic, permafrost) exist, the shape of the native beach profile will be affected.
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From page 197... ...
Figure D-4 shows the design cross-section of a nourishment project planned for Captiva Island, Florida. The shaded portion of the profile is that amount of sand needed to bring the beach level up to the proposed berm level before the beach berm is widened.
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From page 198... ...
The use of a narrower berm reduces or mitigates the littoral drift gradients associated with overly wide sections of nourished beaches in front of seawalls. The design beach is optimized by computing costs and benefits and determining the beach that would return the maximum net benefits (USAGE, l991b)
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From page 199... ...
As mentioned previously, the design beach is optimized by maximizing total net benefits, including storm protection and recreation. Recreational benefits are generated when a nourishment project rebuilds or maintains a public beach area.
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From page 200... ...
Although this may be beneficial to the adjacent beaches, the spreading losses from the project must be included in the advanced-fill design in order to achieve performance objectives for the project area. Delray Beach, Florida, is an example of a beach nourishment project where spreading losses represented the greatest component of the erosion rate on the nourished beach.
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From page 201... ...
Over half of the sand lost from Delray Beach can be accounted for as accretion on adjacent beaches in Highland Beach and Gulfstream (Beachler, 19931. This example demonstrates the importance of estimating spreading losses in the design of beach nourishment projects.
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From page 202... ...
Further evidence and quantification of the dependence of transport on sand grain size have been shown by the USACE (1985~. The transport rate variance with grain size can be applied through analytical and numerical models to predict the erosion rates of dissimilar borrow sands.
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From page 203... ...
Therefore, the selection of coarser sand for a nourishment project may increase the dynamic behavior of beach profiles, leading to greater erosion during storms of limited duration with small storm surges. By considering the above described performance characteristics of sand, assessment of alternative borrow areas can be made.
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From page 204... ...
using the nourished sand to build a wider and higher berm above the mean water level, (3) distributing the added sand over the entire beach profile, or (4)
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From page 205... ...
If the material comes from offshore dredging, it is usually more practical to place the sand on the beach and near the shore or to build an artificial bar. After construction, sand is redistributed in the cross-shore direction to form a more natural profile, governed by the sediment size of the fill and the prevailing wave conditions.
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From page 206... ...
The profile of 04/20/89 in Figure D-8C shows the effect of the first major storm that enhanced the formation of the alongshore trough and bar, with significant erosion of the nourished sand placed on the subaerial part of the beach. That erosion of the berm did not represent a permanent loss of sand from the beach, however, as it was deposited on the offshore bar and therefore was still landward of the closure depth of profile changes.
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From page 207... ...
Post-Storms 04/20/89 9 months 01/20/89 12 months ~ ~-=~ ·-.W 0 100 200 300 DISTANCE (m) FIGURE D-8 Ocean City, Maryland, project profiles.
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From page 208... ...
There is a need for public education at the onset of a project so that the public understands that some initial offshore sediment movement and erosion of the berm are expected and recognizes that, so long as the sand remains in the littoral zone within the envelope of beach profile changes, the sand has not actually been "lost." Although the profile adjustment will in most cases result in shoreline recession, the material will still be present in the active beach profile; much of it will be in the offshore bar and on the berm. Further, the presence of sand in the offshore bar acts to break stories waves and to dissipate their energy before they reach the shoreline; accordingly, the nourished sand within the bar is still meeting the objective of protecting the coast from property erosion.
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From page 209... ...
Dredged material is deposited in shallow water, typically using split-hull barges, either as a mound or in the form of a long linear ridge that simulates a naturally occurring alongshore sand bar (the term "offshore berm" is generally used for the constructed bar but will not be used here because of potential confusion with the subaerial berm of the beach profile)
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From page 210... ...
This activity of the nourishment mound or bar placed at a depth that is shallower than the closure depth is not surprising because this placement in effect immediately introduces the sand into the nearshore zone of active profile changes where the nourished material can be readily incorporated into the overall beach profile. More uncertain, Hands and Allison (1991)
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From page 211... ...
(1994) have described a method employed in Germany that represents the volumetric losses over time from a beach nourishment project using the assumption that the volume decays exponentially with time.
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From page 212... ...
The design volume needs to be (consistently) based on shifting each natural beach profile seaward by the design width in lieu of a single straight-line design template.
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From page 213... ...
DESIGN FOR SEA-LEVEL RISE Since background erosion rates are used to design beach nourishment projects, these designs include the effects of relative sea-level rise over the period of the shoreline change data (see NRC, 1987~. If sea level rises at the same rate over the next 50 years, the nourishment design will include the effects of sealevel rise, as this effect is "built in" to the background erosion rates.
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From page 214... ...
The amount of sand to be bypassed is established by the natural coastal processes in the region. The quantity needed for downdrift beach nourishment may be greater than the amount trapped in the entrance and harbor, and bypassing only this amount may not be sufficient to adequately maintain the downdrift beaches.
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From page 215... ...
lllllllllllsloxllllllllb-lllllllllllllllllllll 1"III~III~ I~=~I~I IIIBI 273 These and other instaNadons and 1beir operadonaI perchance He described in 1be OSACE's engineering and design manual ! ~ -~f~ ~~ 3~/~ho~ (1991~\ Rich provides guidance far 1be design and evaluation of sand bypassing systems A coastal processes Judy far ~ project is very important (USAGE, 1991b)
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From page 216... ...
These include shoreline and berm positions, total volume, and the response of the beach to a storm. The first measure of success should be the longevity of the fill volumesthat is, the evolution of the fill from the construction volumes (design and ad 3.8 3.6 3.2 3.0 2.8 1 .e 1.6 1.4 3.4 ~ 2.6 _ 2 _ o, _ o 1.2 _ i, _ 1.0 _ D :~ 0 0 co I Z FIGURE D-9 Nourishment fill performance at Delray Beach, Florida.
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From page 217... ...
to the design volume over the renourishment interval. Figure D-9 shows the volumetric change of the Delray Beach project with time, which can be used to analyze the erosion rate for the beach and predict the time for periodic nourishment.
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From page 218... ...
of detached breakwaters and their effects on beaches (see, for example Dally and Pope, 1986; Rosati, 1990~. The detached breakwater and beach fill at Redington Shores on the Gulf of Mexico coast of Florida was constructed in late 1985 and early 1986 (Terry and Howard, 19861.
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From page 219... ...
When used, it must be understood by all concerned that a long-term maintenance program is required. There is extensive technical literature on groins, some of which includes information on their use with sand fill.
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From page 220... ...
Beach nourishment and dune construction were part of the plan, but the sand fill in the 10 compartments between the first 11 groins was not made owing to local economic problems. Dune and beach fill was placed in the four westerly compartments when the additional groins were built (Nersesian et al., 1992~.
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From page 221... ...
1 950 221 1 954 Sk ~ in_ ~ -~ ~ 1 958 fit _ i\ ~ C(t _\ ~ 1 962 Atlantic Ocean I ~I ~ I I I I I I I · I I I Groin 15 Groin 11 Groin 1 GROIN FIELD NOTE: Groins 12 through 15 not constructed BREAK INLET 1000 500 0 500 1000 tIlll1....1 1 1 Scale in Meters FIGURE D-12 Historic barrier island breaching and inlet creation near Westhampton, Long Island, New York. From 1963 to 1967, the city of Deerfield Beach, Florida, built a series of relatively short, low-profile groins with rock mounds at their seaward edge.
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From page 222... ...
, is at the south boundary of the sand fill. The other two are the jetties at Bakers Haulover Inlet, located 15 km north of the cut, about two-thirds of the distance to the Dade County-Broward County line.
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From page 223... ...
was obtained from two offshore borrow areas, pumped to shore, placed subaerially, and redistributed by use of bulldozers. The state of Maryland placed a recreational beach fill of 2 million m3 of sand between 3rd Street and the Maryland-Delaware border.
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From page 224... ...
The gross alongshore transport in the southern third of the bay was estimated to be 700,000 m3/year with a net of 41,000 m3/year (only about 6 percent of the gross) toward the north.
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From page 225... ...
This change in location was made based on observations of the performance of the first project. The new sill has performed in a more satisfactory manner, with sand moving in from the first beach fill, forming a wider beach along the section of coast in the lee of the new sill (Tomasicchio, 1994~.
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From page 226... ...
The following discussion deals only with this latter risk. Beach nourishment projects provide protection even when subjected to events that exceed their design level (see Appendix H)
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From page 227... ...
Engineering always requires making decisions with incomplete data but strives to minimize the probability of failure or loss using the data that are available. Risk Considerations in Beach Fill Design The purposes of performing a risk analysis for a beach nourishment project include: · identifying the physical and biological problems associated with beach nourishment, · comparing technologies to determine their relative effectiveness in reducing the risk associated with beach nourishment projects, and · setting management/operation priorities or selecting from among several actions.
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From page 228... ...
For example, an artificial wave environment having the same statistics of the real wave environment at a beach fill site may be generated synthetically. Based on knowledge of the physics of coastal processes important to the performance of beach nourishment projects, the synthetically generated data series is used to evaluate the response of the project for many possible, statistically similar scenarios.
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From page 229... ...
For beach restoration, there are basically two design elements for which risk needs to be considered separately. The first is the risk that the design cross-section that provides storm protection for upland properties could be impacted by a major storm.
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From page 230... ...
, and n is the number of years. Consequently, if a nourishment project with a proposed lifetime of 10 years is designed for a 100-year storm, the risk of a 100-year storm occurring at least once in that 10-year period IS: R = 1 _ :1- 1 )
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From page 231... ...
As the scale of a project increases, the level of protection it provides and the economic benefits also increase, so that protection is provided against more severe conditions; however, the cost of providing this additional protection also increases. For a beach nourishment project, the project's scale might be indicated simply by the berm width or by the volume of sand per unit length of beach.
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From page 232... ...
Figure D-16 presents the same information as Figure D-15 but in a different way. Here backbeach damage is plotted against wave height for various berm widths that could exist at various times during the renourishment cycle.
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From page 233... ...
Consequently, simulation of the performance of beach nourishment projects holds the promise of quantifying risks associated with such projects. Simulating the performance of proposed beach fills requires that a long time series be generated of the physical events-storms, waves, and water levels that could occur during the lifetime of a fill.
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From page 234... ...
There have been few coastal engineering simulation applications, mainly because of the complexity of the processes involved and the paucity of good data on the physical environment. In recent years, however, progress has been made in the development of computer models of the relevant coastal processes, and several simulations have been made.
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From page 235... ...
As listed in Table D-1, beach nourishment TABLE D-1 Budget of Littoral Sediments Credit Debit Balance Alongshore transport into area River transport Sea-cliff erosion Onshore transport Biogenous deposition Hydrogenous deposition Wind transport onto beach Beach nourishment Alongshore transport out of area Wind transport out Offshore transport Deposition in submarine canyons Solution and abrasion Solution and abrasion Mining Beach accretion or erosion SOURCE: After Bowen and Inman ( 1966)
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From page 236... ...
The spit, composed mainly of gravels and cobbles derived from the Elwha River and cliff erosion into glacial outwash sediments, was formed by the eastward alongshore transport of those sediments (Galster and Schwartz, 19901. Erosion of Ediz Hook began early in the century as the river was dammed, cutting off its estimated supply of 38,000 m3/year of sediment to the littoral zone, and then by the construction of a bulkhead along the eroding sea cliff, depriving its 200,000 m3/year sediment contribution.
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From page 237... ...
VENEER BEACH FILLS Veneer beach fills have been used in situations where beach-quality sand is not available in sufficient quantities to economically undertake a nourishment project. Veneer fills involve placing beach-quality sand over a relatively large volume of material that is generally unsatisfactory for beach nourishment.
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From page 238... ...
erosion if the underlying fines are not to be exposed during storms or before scheduled renourishment. Because of these constraints, veneer beach fills with underlying fines are less likely to be used on beaches exposed to large waves or beaches that experience a large tidal range.
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From page 239... ...
FIGURE D-l9 Veneer beach fill cross-section, Corpus Christi Beach, Corpus Christi, Texas.
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From page 240... ...
=12 mail 1 1 0 20 40 60 80 100 120 DISTANCE (m) FIGURE D-20 Veneer beach fill cross-section, Key West, Florida.
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From page 241... ...
The bottom elevation seaward of the trench is only about -0.3 m, so the constructed trench actually serves as a bathing basin. The project design anticipated a loss of about 15,000 m3/year of veneer sand, of which approximately 7,500 m3 was expected to be lost offshore to the trench while the other 7,500 m3 would be lost by alongshore transport from the project area.
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From page 242... ...
_ +2.3 m to + 2.4 m NGVD 0 5 10 15 20 DISTANCE (m) 25 30 FIGURE D-21 Veneer beach fill cross-section, Grand Isle, Louisiana.
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From page 243... ...
Coastal Engineering 10: 193 -210. Bruun, P
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From page 244... ...
Pp. 1319-1333 in Proceedings of the 14th Coastal Engineering Conference, New York: American Society of Civil Engineers.
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From page 245... ...
Pp. 3319-3330 in Proceedings of the 23rd International Coastal Engineering Conference, Vol.
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From page 246... ...
In: Proceedings of the 24th Coastal Engineering Conference. New York: American Society of Civil Engi neers.
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From page 247... ...
Pp. 3357-3370 in Proceedings of the 23rd International Coastal Engineering Conference, Vol.
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From page 248... ...
Pp. 1818-1838 in Proceedings of the 16th Coastal Engineering Conference.
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From page 249... ...
1989a. Water Level and Wave Heights for Coastal Engineering Design.
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From page 250... ...
Pp. 1451-1463 in Proceedings of the 13th Coastal Engineering Conference.
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