Beach Nourishment and Protection (1995) / Chapter Skim
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F Project Construction and Sediment Sources, Transfer, and Placement
F Project Construction and Sediment Sources, Transfer, and Placement
Pages 267-293
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From page 267... ...
Offshore Sources Over the past decade, the primary source of sand for beach nourishment has been "offshore" deposits on the continental shelf. One of the earliest beach nourishment projects using sand from offshore deposits was at Coney Island, New York, where over 1.3 million m3 of sand dredged from the seabed not closer than 500 m from shore was placed on the beach during 1922-1923; (Farley, 1923, Domurat, 1987; Dornhelm, 1995~.
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From page 268... ...
At the same time, the coastal processes that deposited these materials have shifted landward as sea level rose. Because the closure depth for measurable sand movement is well inshore of relict sand, offshore borrow sites tend to fill in with finegrained material that is not suitable as beach fill.
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From page 269... ...
But sand was obtained from offshore borrow areas because use of the ebb-tide shoal has been objected to because the shoal is the source of sand for the downdrift beach along Assateague Island, which has suffered extensive erosion since jetties were constructed at the Ocean City inlet. Beach Sources Littoral Drift In some instances, accretional downdrift beaches have served as sources of sand for beach nourishment projects by "backpassing." An interesting example is the nourishment on Sandy Hook, New Jersey (Nordstrom et al., 19793.
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From page 270... ...
Sand bypassing systems are also in operation at South Lake Worth Inlet in Florida, at the Indian River Inlet on the Delaware coast (see Figure F- 1) , and at other locations on the Atlantic coast (USAGE, 1991, 1994~.
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From page 271... ...
In the case of barrier islands, dune systems are fundamental to the natural stability of the islands themselves. Thus, dune sand is not normally a primary source of beach fill material, although recoverable dune sand moved landward by overwash during major storms has sometimes been relocated back to beach areas to restore some measure of natural protection.
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From page 272... ...
provide quantitative comparisons of the volumes of sand supplied from nourishment projects to California beaches and the sand volumes derived from natural sources. In the 60 years prior to 1987, Herron estimates that within the 390 km of coast between Santa Barbara and the Mexican border some 70 million m3 of nourishment sand has been the byproduct of projects in coastal areas, such as excavations for harbors, power plants, sewage treatment plants, and highways.
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From page 273... ...
Colitic aragonite sands, for example, have been imported from the Bahamas for a nourishment project on Fisher Island, Florida, immediately south of Miami Beach (Bodge and Olsen, 1992~. The potential use of colitic sands for beach nourishment was initially explored in the 1960s, when laboratory wavetank tests were undertaken to establish the properties of beaches composed of that sediment (Cunningham, 1966; Monroe, 1969~.
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From page 274... ...
These placements suggest that nonindigenous materials can be used successfully in lieu of native sediments for beach fill purposes. TRANSPORT AND PLACEMENT Bridging the gap between the investigation and analysis of potential borrow sites and the design parameters attendant to the configuration of a renourished beach requires a basic understanding of dredging equipment, processes, capabilities, and limitations.
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From page 275... ...
Varying resources among contractors establish degrees of cost advantage or disadvantage. The ability to work offshore or, to meet high production capabilities, ownership of certain equipment such as hopper dredges or certified dredges, and the financial resources to bond high-cost projects are all factors that tend to narrow the field of participants in large nourishment projects with offshore sources.
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From page 276... ...
It is connected to the floating section with a ballpoint connection on some type of barge or flotation arrangement. The purpose of the floating segment is to allow flexibility to the dredge in movement and to allow disconnection from the pipeline in cases where the dredge must be taken to safe harbor to escape bad weather conditions or for major repairs.
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From page 277... ...
This dredge differs from a cutter-suction dredge in that it is a freetraveling vessel that is either a ship or a tug-propelled barge that sails back and forth over the area of the borrow site and that trails one or two arms on which are mounted dragheads that loosen the sand and deliver it to the suction pipe, which then loads the slurry into the hopper of the vessel. In order to deliver the sand to the beach, the hopper dredge must either (1)
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From page 278... ...
These dredges are usually more susceptible to adverse sea conditions than are hopper dredges. Cutter dredges can work in water depths of less than 5 m.
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From page 279... ...
The typical 30-tn.-diameter cutter-suction dredge used on offshore projects in the United States has the capability to dredge from borrow sites in water depths ranging from 5 to 20 m. Hopper dredges are used in water depths of 7 to 15 m at full capacity.
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From page 280... ...
Two projects covering the same 13-km reach of beach fill showed a complete reversal of plant choices by two different contractors owing to equipment availability and borrow site location and type. An early state-funded project in the northerly sections used a hopper dredge to pump from the north borrow site and a large cutter-suction dredge and booster pump to nourish the southerly section.
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From page 281... ...
The contractor's job is to accept the designated borrow site limits, excavate the available material, and place it within the design template on the beach. Unless otherwise indicated, special processes, such as screening or separation of oversized material or washing of fines, are not considered part of this process.
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From page 282... ...
The manner in which a hopper dredge transits over the area from which it borrows the material dictates that shoal depths of the borrow site are limited to approximately 0.6 to 1 m below the loaded draft of the equipment being utilized. The shoal depths also apply to the sailing route chosen between the borrow site and the bottom placement or pumpout site.
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From page 283... ...
, some operational depth for maneuvering or operation of attendant plant is required. Very shallow borrow sites are restrictive to cutter- suction and hopper dredges, while very deep ones may exceed excavation depth limits and pump constraints.
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From page 284... ...
For dredging sites in rivers or bays that are relatively protected, the weather will have little effect except for cessations caused by short squalls or shutdowns caused by major hurricane events or flooding. The offshore borrow sites, however, will be subject to periods of reduced productivity as well as complete stoppages because of the effects of sea state and wind.
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From page 285... ...
These storms may also destroy work already in place, which may or may not be accepted depending on the contractual arrangements. Borrow Site Considerations Typical designs for borrow site use establish limits to excavation both horizontally and vertically.
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From page 286... ...
Contractual Constraints Project Schedule Requirements The schedule for requirements on a beach nourishment project takes into account the protection offered (or recreation afforded) by the existing beach, construction interferences with the public during high-use periods, weather impacts on the cost of operations, impacts on the environment, and political timing with regard to funding cycles.
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From page 287... ...
Dredging in most cases, and certainly in the case of offshore borrow sites, is accomplished by large individual pieces of equipment that each cost millions or tens of millions of dollars. Coupled with this high capital investment is a relatively low yearly use, which may be on the order of 6 to 8 months for all types of navigation channel maintenance and construction dredging and generally 6 months or less for beach nourishment projects.
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From page 288... ...
Ability to Mine Deeper Sand Deposits and Deliver Farther Larger equipment supporting longer dredge ladders or remote active dragheads will be developed as borrowing of farther offshore deposits becomes economical. Higher-head pumping systems that use more sophisticated booster control will enable delivery of sand from farther offshore borrow sites.
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From page 289... ...
Project Quality Control Having discussed the multitude of steps necessary to bring a beach nourishment project to construction, the owner must ensure proper construction technique with the following quality control measures: · detailed pre- and postfill surveys, with sufficient extensions past closure depth; · daily samples of fill material and grain-size distribution analysis; · records of borrow site excavation coverage on a daily basis and calculation of gross quantities removed; detailed calculations of fill volume within and without pay tolerances; and · records of contractor equipment used, hours worked, payrolls, and fuel consumption. Alternative Construction Concepts As additions to the presently considered beach nourishment concepts and techniques, the following ideas may have some merit.
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From page 290... ...
Storm Emergency Fleet Based loosely on other federal programs for private hopper dredges, it may be desirable that certain contract requirements be preprocessed for a core of emergency response equipment to facilitate protective rebuilding after natural disasters have decimated beaches and dune systems, and left people and property at risk. Sand Bypass Systems The use of sand bypassing systems was described in general terms in Chapter 4.
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From page 291... ...
. lStlCS; · calculations and observations of alongshore transport of sand; · cross-shore movement of sand by waves and tidal currents; · estimates of sand transport into the entrance or harbor, ebb-tide shoal, and external sand trap if one is a part of the project; · loss of sand to the offshore caused by structures; · sand budget, areal and temporal, based on calculations and observations of accretion at nearby structures, such as groins or jetties; storm surge climate; calculation of wave, water level, and sand movement during severe storms to evaluate system component safety; identification and mapping of habitats; effect of system on biological communities; effect of pumping and deposition of sand on biological communities, on other uses, and on public safety; and · calculation of downdrift changes with time for several scenarios of sand budget and placement schedules.
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From page 292... ...
1958. Behavior of Beach Fill and Borrow Area at Harrison County, Mississippi.
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From page 293... ...
1963. Behavior of offshore borrow zones in beach fill operations.
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