Waterfronts for Waves
Jetties are a shore perpendicular hard structure, normally placed adjacent to tidal inlets to control inlet migration, and to minimize sediment deposition within the inlet. A jetty is an armored structure that extends out into the water and is intended to protect a navigation channel or marina. On open seacoasts, a jetty is structure extending into a body of water to direct and confine the stream or tidal flow to a selected CHANNEL, or to prevent shoaling. Jetties are built at the mouth of a river or entrance to a bay to help deepen and stabilize a channel and facilitate navigation. A jetty is structure usually projecting out into the sea at the mouth of a river for the purpose of protecting a navigational channel, a harbor or to influence water currents. Jetties are typically comprised of large riprap. A jetty is a dike-like structure extending from the bank out into the water. The dike or jetty protects the bank from erosion. One good aspect of a jetty is that it traps sand on one side, but on other side of a jetty there is an increase of erosion because it has stopped the littoral drift. The beach grows on one side, but erodes on other side. A row of jetties will often create a beach that looks like a set of saw blades. Materials and design can cause the jetty to be complex and expensive.
A jetty spur is a relatively short structure extending at an angle from the main jetty axis that protects a navigation channel. Spurs may also act as a breakwater and provide wave height reduction along the local beach and the structure itself. The spur diverts sediment that may shoal in the channel back towards the beach, where it can nourish the beach. Spurs are usually constructed of rock rubble similar to the connecting jetty. If longshore sediment transport is primarily unidirectional, sediment will deposit on the upcoast side of the inlet, and the downdrift beach will erode. For this case, spur jetties can provide a protected area within which land- or water-based equipment can dredge and bypass the accumulated sediment. Spurs are typically nearly perpendicular to the jetty, but may also be constructed at some angle with respect to the jetty up to about 45 deg.
Groins are shore perpendicular structures, used to maintain updrift beaches or to restrict longshore sediment transport. Permeable groins are becoming popular, and may negate some of the negative effects of impermeable groins. A groin is barrier-type structure that extends from shore and is similar to, but much smaller than a jetty. Groins are generally aligned perpendicular to shore for the purpose of protecting shoreline and adjacent upland by influencing movement of water and/or transport of materials.
Shore parallel structures include breakwater, seawalls, bulkheads and revetments. These structures are designed to protect coastal property not the beach fronting the property. They may be built above the high water mark, and they are relatively inexpensive.
A breakwater is an offshore structure that is usually aligned parallel to shore and provides protection from waves. A breakwater is within the meaning of "jetties" and "sea walls". An offshore breakwater is a breakwater built towards the seaward limit of the littoral, parallel (or nearly parallel) to the shore. A mole is a a massive solid-filled structure (generally revetted) of earth, masonry or large stone, used especially as a breakwater or to enclose an anchorage or harbor.
A revetment is a protective structure placed along slopes, constructed of a sturdy material such as stone. A revetment is a facing of erosion resistant material, such as stone or concrete, that is built to protect a scarp, embankment, or other shoreline feature against erosion. The major components of a revetment are the armor layer, filter, and toe. The armor layer provides the basic protection against wave action, while the filter layer supports the armor, provides for the passage of water through the structure, and prevents the underlying soil from being washed through the armor. Toe protection prevents displacement of the seaward edge of the revetment.
Certain types of structures such as riprap revetments have fewer initial impacts than other hard structures, since construction normally requires significantly less excavation than, for example, a seawall. Permanent impacts of revetments however, are similar to those of seawalls, and the footprint of the revetment is typically larger. Riprap is a layer of various-sized rocks used to protect a bank from erosion. Riprap is effective because the rock can adjust to the contours of the bank and vegetation can grow among the rocks to provide habitat for wildlife in and above the water. Riprap is easy to install and repair, has a natural appearance, and does not harm the environment. Riprap works well with a combination of soil bioengineering techniques used up the slope. Class D or E revetment stone, preferably broken limestone, dolomite, or quartzite, are available from most quarries. Rough, angular surfaces and variety of sizes will allow the rock to fit together tightly to form a dense barrier.
A seawall is a barrier, usually vertical walls, between the land and water that protect from wave erosion. A seawall is a massive structure that is designed primarily to resist wave action along high value coastal property. Seawalls may be either gravity- or pile-supported structures. Common construction materials are either concrete or stone. Seawalls can have a variety of face shapes. Concrete Seawalls are often pile-supported with sheetpile cutoff walls at the toe to prevent undermining. Additional rock toe protection may also be used. The seaward face may be stepped, vertical, or recurved. Rubble-Mound Seawalls are designed like breakwaters using a rock size that will be stable against the design wave.
A bulkhead is used as a retainer, providing protection and stabilizing the land that it supports. Bulkheads are retaining walls whose primary purpose is to hold or prevent the backfill from sliding while providing protection against light-to-moderate wave action. They are used to protect eroding bluffs by retaining soil at the toe, thereby increasing stability, or by protecting the toe from erosion and undercutting. They are also used for reclamation projects, where a fill is needed seaward of the existing shore, and for marinas and other structures where deep water is needed directly at the shore. Bulkheads are either cantilevered or anchored sheetpiling or gravity structures such as rock-filled timber cribbing.
Vertical structures in particular can deflect wave energy causing increased erosion and altering natural habitat in front of the structure. Seawalls reflect the energy of the waves away from the coast and slow down erosion. They have been erected to protect the ocean-view homes and hotels that are perched atop sea cliffs and along beaches. Seawalls that limit erosion also cut sand supply. So putting in a seawall will, for a short period of time, lessen the amount of erosion, but the result is that sediment is no longer there to be taken to the beaches. The beaches receive part of their sand supply from cliff sides. And as the seawall slows down the erosion of cliff sides, the beaches are losing an important source of their sediments.
A vertical, relatively narrow seawall will take up a very small amount of beach, but a riprap revetment, to be stable, may have to extend 20 to 40 feet seaward from the foot of the bluff - in some cases completely covering the usable beach area. Where such a structure is built along a shoreline that is undergoing long-term net erosion, the effect will be the gradual loss of beach in front of the structure as the shoreline migrates landward beyond it. Private structures may be temporarily saved, but the public beach is lost.
Most protection structures to date have been justified for "coastal protection." But "beach erosion" and "shoreline erosion" are not the same thing. Seawalls and revetments are not built to protect public beaches; they are built to protect property and structures on dunes, bluffs, or cliffs. In practice, a seawall has never been built to save a beach.
Waterfront Flood Control
The term "embankment" is used to refer to the roadway, railroad, and other manmade embankments along the shores of a reservoir, waterway or harbor. It may also be used to refer to the main dam embankments that form part of a earth-filled or rock-filled dam structure. The two principal types of embankment dams are earth and rock-fill dams, depending on the predominant fill material used.
A weir is a barrier set across a stream that blocks the passage of fish but allows water to pass. A weir is a calibrated structure used to relate water-level (i.e., head) to flow (i.e., discharge) in an open channel. An open channel is a channel at atmospheric pressure where the only force acting on the water is gravity. A weir consists of a bulkhead, notch, crest, and gage. Water flow over a weir creates a weir pond, head, nappe, and flow contractions.
Levees are usually constructed of rolled (compacted) earth fill. In some cases, internal drainage or underseepage treatment is incorporated into the levee. When landscaping and planting are provided on the existing levee, the internal blanket drain and/or toe drain will have to be extended. Designs for levees, except those to be located in agricultural and similar sparsely inhabited areas, shall meet the landscape planting criteria outlined in paragraph 3-1a. During design, landscape planting will also be considered for levees in the following areas: at pumping installations in public view, at public road crossings, near residences, and at other areas where planting could protect or restore the existing environmental values. Plantings will normally be located outside the limits of the basic structure.
Since Urban levees are highly visible to large numbers of people, planting may be included for the total length of levees constructed in urban areas. Top soil and planting can be used for restoration of borrow and waste areas created during construction of levees. Although rural or agricultural levees are seen by relatively few people, environmental considerations should be included in the design. Planting should be considered for the following areas: at pumping installations in public view, at public road crossings, near residences, and at other areas where planting could protect or restore the existing environmental values. Planting and regrading appropriate for restoration should be considered for borrow and waste areas. Where opportunities exist, creation of higher value environments should be considered. Sand levees will be stabilized with native grass species.
Floodwalls are generally used in those urban areas where land or materials required for levee construction are not economically available. These walls are subject to hydraulic forces on one side, which may be resisted by little or no earth loading forces on the other side. Although there are several types of floodwalls, the two most common are the inverted T-type reinforced concrete wall and the cantilever Itype sheet piling wall. Landscape planting should be included in the floodwall design, particularly for those walls that encroach upon or change existing scenic values, e.g., where the wall becomes a barrier along a street or near dwellings, parks, and commercial or industrial developments. Planting should also be considered for floodwalls constructed in areas adjacent to open tracts of land where it can be determined that development will occur during the early stages of the project life.
The Inverted T-type reinforced concrete wall structure may have a toe drainage system to check and control piping and boils, control seepage as a result of roofing where piles are used, and control uplift pressures. These drainage systems must be protected from the invasion of roots, which could clog the drainage system. A vegetation- and root-free zone will be established at the top outside edge of the toe drains and at the landside face of wall joints when planting is included in the design. The possibility of eventual loosening and eroding of wall joint seals is a serious consideration in the design of planting at floodwalls. Wall joints must be protected against possible root penetration and resultant damage to the wall.
Cantilever I-type sheet piling floodwall landscape planting should be designed similarly to that for the T-type wall. Vegetation- and root-free zones should be established for the structure, similar to those for T-type walls.
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