types of coastal structure mjay

8/2/2019 Types of Coastal Structure Mjay

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Types of Coastal structure:

1-Seawalls:Seawalls are usually massive, vertical structures used to protect

backshore areas from heavy wave action, and in lower wave energy

environments, to separate land from water. They can be

constructed using a range of materials, the most common being

poured concrete, steel sheet pile, concrete blocks, gabions, and

timber cribs.

There are various types of seawalls.

Curved

Curved Seawall

Usually massive structures, constructed with poured concrete,

curved seawalls have a slight concave curve built into their face that

is designed to deflect incoming wave energy up and away from the

bottom, thereby reducing scour at the base of the wall.

Gravity

Gravity seawalls rely on the weight of the materials they are

constructed of to provide the required stability against wave

action. They require strong foundation soils to adequately support
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their weight, and they normally do not sufficiently penetrate the soil

to develop reliable passive resisting forces on the offshore side.

Therefore, they depend primarily on shearing resistance along the

base of the structure to support the applied loads.

Steel Sheet Pile Walls

Steel Sheet Pile Seawall

This type of seawall consists of thin, interlocking sheet piles driven

deeply into the ground. In most cases, the sheet pile is backfilled and

anchored to the bluff/bank face using tie-back structures. Drainage

openings are also utilized to provide drainage from the backshore

area.

Concrete Walls

Vertical walls made from poured concrete, or from the vertical

placement of concrete blocks.


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Rock Walls

Rock Seawall.

A type of gravity seawall that is composed of vertically placed units

of heavy armorstone or other similar natural rock material.


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2-Bulkheads:

These are vertical retaining walls to hold or

prevent the soil from sliding seaward. Their

main purpose is to reduce land erosion and

loss to the sea, not to mitigate coastal

flooding and wave damage. For eroding

bluffs and cliffs, by retaining soil at the toe,

they increase stability by protecting the toefrom undercutting. Bulkheads are either

cantilevered or anchored sheet piles or

gravity structures such as rock-filled timber cribs and gabions,

concrete blocks, or armorstone units.

Cantilever Bulkheads

Cantilever bulkheads derive their support from ground penetration;

therefore, the effective embedment length must be sufficient to

prevent overturning. Toe scour results in a loss of embedment length

and could threaten the stability of such structures.

Anchored or Tie-Backed Bulkheads

Anchored or tie-backed bulkheads are similar to cantilevered

bulkheads except they gain additional support from anchors

embedded on the landward side or from structural piles placed at a

batter on the seaward side. For anchored bulkheads, corrosion

protection at the connectors is particularly important to prevent

corrosive failures.

Bulkhead VerticalRetaining Wall.


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Gravity Bulkheads

Gravity structures eliminate the expense of pile driving and can often

be used where subsurface conditions support their weight or

bedrock is too close to the surface to allow pile driving. They require

strong foundation soils to adequately support their weight, and they

normally do not sufficiently penetrate the soil to develop reliable

passive resisting forces on the offshore side. Therefore, they depend

primarily on shearing resistance along the base of the structure to

support the applied loads. Gravity bulkheads also cannot prevent

rotational slides in materials where the failure surface passesbeneath the structure.

3-Revetments:

Revetments are a cover or facing of erosion

resistant material placed directly on an existing

slope, embankment or dike to protect the area

from waves and strong currents. They are

usually built to preserve the existing uses of the

shoreline and to protect the slope. Like

seawalls, revetments armor and protect the

land behind them. They may be either

watertight, covering the slope completely, or

porous, to allow water to filter through after

the wave energy has been dissipated.

Most revetments do not significantly interfere

with transport of littoral drift. They do not

redirect wave energy to vulnerable

Revetment Structure

Revetment drawing

illustrates

components.
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unprotected areas, although beaches in front of steep revetments

are prone to erosion. Materials eroded from the slope before

construction of a revetment may have nourished a neighboring area,

however. Accelerated erosion there after the revetment is built canbe controlled with a beach-building or beach-protecting structure

such as a groin or a breakwater.

Revetments are commonly constructed using armorstone (high wave

energy environments), or rip-rap stone (lower wave energy

environments), in combination with smaller stone and geotextile

fabrics to act as a filter and underlayer. The filter and underlayersupport the armor, yet allow for passage of water through the

structure. Toe protection prevents undercutting and provides

support for all the above layer materials. A typical design section is

shown in the figure to the right.

Other construction materials have also been used to form the armor

layer including gabions, poured concrete (usually in stepped fashion),pre-cast concrete blocks, and grout filled bags.


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4-Dikes & levees:

Dikes are typically earth structures

(dams) that keep elevated water levels

from flooding interior lowlands. In open

coast areas, dikes that separate low

lying areas from open water are often

constructed with a revetment or similar

armor layer on the open water side toprotect the dike from wave action and

erosion. A typical design of such a

structure is found in the diagram.

Levee and

Flood Control

Structure

Dike Figure


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5-Breakwaters:

Breakwaters are generally shore-parallel

structures that reduce the amount of wave

energy reaching the protected area. They are

similar to natural bars, reefs or nearshore

islands and are designed to dissipate wave

energy. The reduction in wave energy slows

the littoral drift, produces sedimentdeposition and a shoreline bulge or "salient"

feature in the sheltered area behind the

breakwater. Some longshore sediment

transport may continue along the coast

behind the nearshore breakwater.

There are various types of breakwaters. Theseinclude:

Headland Breakwaters

A series of breakwaters constructed in an

"attached" fashion to the shoreline and

angled in the direction of predominant wave

approach such that the shoreline behind the

features evolves into a natural "crenulate" or

log spiral embayment (see figure at left).

Headland

Breakwaters

Headland

Breakwater

Design

Single Breakwater

Single

Breakwater

Design
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Detached Breakwaters

As the name implies, these are breakwaters that are constructed

away from the shoreline, usually a slight distance offshore (as in the

photograph above). They are detached" from the shoreline, and are

designed to promote beach deposition on their leeside.

Single Breakwater

Single breakwaters may be attached or detached depending on what

they are being designed to protect. A single detached breakwater

may protect a small section of shoreline. A single attached

breakwater, may be a long structure designed to shelter marinas or

harbors from wave action (see photo at right).

System Breakwaters

This usually refers to two or more detached, offshore breakwaters

constructed along an extensive length of shoreline.

Breakwaters are typically constructed in high wave energy

environments using large armorstone, or pre-cast concrete units or

blocks. In lower wave-energy environments, grout-filled fabric bags,

gabions and other proprietary units have been utilized. Typicalbreakwater design is similar to that of a revetment, with a core or

filter layer of smaller stone, overlain by the armoring layer of

armorstone or pre-cast concrete units (see figure below).


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6-Groins:

Groins are the oldest and most common shore-

connected, beach stabilization structure. They

are structures that extend, fingerlike,

perpendicularly or nearly right angles from the

shore (see photo at right), and are relatively

short when compared to navigation jetties at

tidal inlets. Usually constructed in groups calledgroin fields, their primary purpose is to trap and

retain sand, nourishing the beach compartments between them.

Groins initially interrupt the longshore transport of littoral drift. They

are most effective where longshore transport is predominantly in

one direction, and where their action will not cause unacceptable

erosion of the downdrift shore. When a well designed groin field fillsto capacity with sand, longshore transport continues at about the

same rate as before the groins were built, and a stable beach is

maintained.

Modern coastal engineering practice is to combine beach

nourishment with groin construction to permit sand to immediately

begin to bypass the groin field system. At the end of the sedimentcell, terminal groins may be used to anchor the beach and limit the

removal of sand into a navigational channel or onto an ebb-tidal

shoal at tidal inlets.

There are a variety of different types and configurations of groins.

These include:

erial View of

Groin

pplication


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Normal or Straight Groin

The standard shore perpendicular construction

Angled or Inclined Groin

A standard groin set at a slight angle to the shoreline

Single Groin

A solitary groin, usually used to anchor or build a small, site specific,

sand accumulation. In some cases may serve as a "terminal" groin to

anchor a nourished beach.

Groin System or FieldNumerous groins constructed along a significant reach of shoreline

Forms of Groin

Geometry

Notched or Zig-Zag Groin

A groin with small notches or compartments that help anchor sand

and sediment.

Permeable Groins

Groins with gaps in them that allow currents and sediment to pass

through them, thereby maintaining some of the natural littoral drift.

Adjustable Groins

Groins that can be moved landward or seaward accordingly with

long-term rises or falls in water elevation.


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T, L, or Y Shaped Groins

Groins with small attachments on their seaward end that are

designed to better trap and anchor sediment trapped by the groin

structure.

Typical Groin

Design Profile

Groins have been constructed from a wide range of materialsincluding armorstone, pre-cast concrete units or blocks, rock-filled

timber cribs and gabions, steel sheet pile, timber sheet pile, and

grout filled bags and tubes.

Groin Conceptual

Designs


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6-Sills/Perched beaches:

Construction of a low retaining sill to trap sand

results in what is known as a "perched beach,"

one that is elevated above its original level.

Perched beaches have many of the same

qualities as natural beaches, and the submerged

sill does not intrude on the view of the

waterfront. Perched beaches are appropriateerosion control measures where a beach is

desired and sand loss is too rapid for convenient

or economical replacement. They can also be

used to create a new beach for recreation and shore protection.

Construction materials and design considerations for perched beach

sills are generally similar to those for fixed breakwaters. For example,the sill can be constructed of a range of materials including

armorstone, grout-filled bags, or sheetpiling (see figure at right). The

most important difference is that in order to effectively retain sand,

sills must either have filter material on the landward side or be

constructed of tongue- and-groove interlocking elements. Also, the

location of the sill should be indicated in some way so that beach

users do not step off the perched beach into unexpectedly deep

water.

The sand for a perched beach may be trapped by the sill after being

carried inshore by the normal wave action, or it may be transported

from another site as beach fill. Trapping of sand could deplete

adjacent beaches that world have received these deposits in the

absence of the sill. Fill from other sand sources can be used to limit

Construction of a

low retaining sill

to trap sandresults in a

pearched beach


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this effect on neighboring beaches and make the perched beaches

available for protection and enjoyment sooner than wave-carried fill.

7-Jetties and piers:

Jetties are shore-normal stone structures

commonly used for training navigation channels

and stabilizing inlets. In this case, jetties are used

for controlling the navigation depth of channels.

Jetties prevent intrusion of long-shore sediment

transport. Jetties also constrain the steady flow

causing higher flow velocities and thereby

scouring the channel to a depth required for safe

navigation. Pier structures are sometimes

referred to as jetties. Piers are usually pile-

supported structures with wooden or steel decksand are used for pedestrian access, fishing,

research, and supporting commercial

establishments.

Repairs to north

etty tip,

Yaquina, OregonInlet (2000).


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References:

http://chl.erdc.usace.army.mil/
http://chl.erdc.usace.army.mil/http://chl.erdc.usace.army.mil/

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