Coastal Defense: Hard Stabilization

Strategies Stabilization techniques
Hard Stabilization Soft Stabilization



Hard Stabilization techniques include the following:

i) Construction of groynes:

Groynes are wooden, concrete and/or rock barriers or walls perpendicular to the sea.

groyne: figure 1

groyne: figure 2A view of Groynes in Sitges (Spain)

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(ii) Construction of Seawalls:

Seawall is a structure which is designed to protect a shoreline from flooding and erosion.  Seawalls (usually of masonry, concrete or rock) are built at the base of a cliff or at the back of aa seawall beach, or used to protect a settlement against erosion or flooding.

Essentially, a seawall acts as a layer between the vulnerable coastline and the ocean. Wave action can beat at the seawall without eroding the coast, although the seawall itself will eventually break down and require repair or replacement.

Seawalls also help to insulate communities from flooding, although high waves can still breach most seawalls. A seawall can also provide a space for recreation, since the top of a seawall is often flat, allowing people to walk on it or to fish from it.

In some cases, a seawall will be constructed on shore to break high waves which might otherwise damage structures and roads on the shore. In these instances, several lower barriers may precede the seawall, to help break up the wave energy before the waves hit the wall.

Other sea walls are built in the water right next to a shoreline, as is the case on many islands. Some nations also build seawalls in the open water to act as flood barriers and to dissipate waves before they reach the shore, encouraging them to break more gently.

Many seawalls are curved, allowing waves to break against them while reducing their energy. Others are straight, and they are designed to bring waves to a standstill.

It is not uncommon to bury low mounds of rubble in front of a seawall to assist with the goal of reducing the power of waves as they hit the shore. In the case of a seawall which is meant to resist flooding, the seawall may be quite high so that it can cope with storm surge.

Traditional seawalls are built from large rocks, pebbles, and other rubble; some remains of ancient seawalls built in this style can be seen in many communities. A modern seawall can be made from these materials, although it may also integrate concrete, metal bars, and other tools to make the seawall more sturdy.

Cheaper seawalls may be made from wood and plastic, which provide some protection although they can potentially fail in heavy weather.

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(iii) Construction of revetments:

Revetment: schematic diagram 1A revetment is a type of structure that is built along an embankment, shoreline or steep facing slope in order to protect it against erosion generated by wave or current action.

A revetment is like a common sea wall in that it is designed to maintain the location and shape of a shoreline. Instead of standing vertically like a sea wall, however, a revetment “coats” the shoreline with protective material.

Revetment: schematic diagram 2

Starting from the shoreline and extending on to shore, a revetment will usually have a filter cloth. A filter cloth allows groundwater to drain through the structure without creating erosion channels that could weaken the revetment. revetment: picture

On top of the filter cloth different grades of stone are layered. The base layer of stone, called “core stone“, is smaller to allow for settling, while the outermost layer of stone, called “armor stone” is larger and heavier.

The filter cloth and core stone provide a stable base to build upon, and the armor stone protects the revetment (and thus the shoreline) from wave and ice action.

The layers of stone extend above and below the shoreline surface, with the top extending above the “high water level”, the highest anticipated water level based on historical data, preventing waves from going over the revetment.

Likewise, the bottom of the revetment extends below water level. This “toe reinforcement” prevents waves from removing sand at the revetment base and thus undermining the structure.

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(iv) Use of Rock Armour:

Large rocks are piled or placed at the foot of dunes or cliffs, which are placed with native stones of the beach: — also known as rip rap, rubble or shot rock.

The rock used will typically be a very hard rock armour: pictureigneous rock that is exceedingly resistant to erosion.

When rock is placed on the beach it is arranged so that large void spaces are left. When waves hit the rock they are only partially reflected, and can flow round the rocks, expending their energy less quickly which reduces scour.

Another advantage of using rock is that if the rocks are moved during heavy storms it is a simple procedure to replace them in the correct position. (Figure above shows piling of rock armour, groynes can also be seen).

Rock armour is often used to protect the base of seawalls and prevents direct attack and damage of the wall. Additionally the rock provides support for the wall, improving its stability.

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(v) Use of Gabions:

Boulders and rocks are wired into meshgabion : picture cages and usually placed in front of areas vulnerable to heavy to moderate erosion.

Gabions are normally used on eroding cliff faces where their purpose is two fold.

Firstly they provide stability to the cliff face, preventing failure, and secondly they permit water to drain away that might otherwise contribute to landsliding.

Gabions are relatively cheap and easy to construct but they are not suitable for use in locations exposed to wave action because they are too flexible and are easily destroyed.

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(vi) Construction of Offshore breakwater:

Breakwaters are structures constructed breakwater: pictureon coasts as part of coastal defence or to protect an anchorage from the effects of weather and longshore drift.

Offshore breakwaters, also called bulkheads, reduce the intensity of wave action in inshore waters and thereby reduce coastal erosion. They are constructed some distance away from the coast or built with one end linked to the coast.

The breakwaters may be small structures, placed one to three hundred feet offshore in relatively shallow water, designed to protect a gently sloping beach.

Breakwaters may be either fixed or floating: the choice depends on normal water depth and tidal range. (figure on the right shows 3 out of 4 breakwaters Forming Portland Harbour, England)

When oncoming waves hit these breakwaters, their erosive power is concentrated on these structures some distance away from the coast. In this way, there is an area of slack water behind the breakwaters. Deposition occurring in these waters and beaches can be built up or extended in these waters.

However, nearby unprotected sections of the beaches do not receive fresh supplies of eroded sediments and may gradually shrink due to erosion.

Enormous concrete blocks and natural boulders are sunk offshore to alter wave direction and to filter the energy of waves and tides. The waves break further offshore and therefore reduce their erosive power.

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References:
from standard technical literature & various websites including:
http://www.wisegeek.com, http://www.japantimes.co.jp,
http://www.egr.msu.edu, http://content.answers.com,
http://dcm2.enr.state.nc.us, http://www.northnorfolk.org,
http://www.english-nature.org.uk, http://www.havant.gov.uk,
http://www.coastalplanning.net, http://www.herveybay.qld.gov.au,
http://www.unesco.org, http://www.globalsecurity.org,
http://jrscience.wcp.muohio.edu & http://www.wikipedia.org

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