Coastal management
From Wikipedia, the free encyclopedia
Coastal management or coastal defence is used throughout the world for many different purposes, but predominantly to reduce coastal erosion and flooding. There are many techniques of coastal management but they all fall into two main categories, "hard" and "soft" engineering. Hard engineering is the more traditional engineering response to erosion and involves the construction of structures which stop wave energy reaching the shore, or absorb and reflect the energy. These have often caused problems themselves, such as increasing erosion elsewhere, and soft engineering techniques have become more popular because of this. These techniques involve promoting natural systems such as beaches and salt marshes which protect the coast, and are usually cheaper to construct and maintain than hard engineering techniques, and may be self-sustaining.
In some jurisdictions the terms sea defence and coastal protection are used to mean, respectively, defence against flooding and erosion. The term coastal defence is the more traditional term, but coastal management has become more popular as the field has expanded to include techniques that allow erosion to claim land.
The following, is a catalogue of relevant techniques that could be employed as coastal mangment techniques. Please remember that the costs given are very rough estimates made during 2005, based around UK Pound sterling.
Contents |
[edit] Hard Engineering Techniques
[edit] Groynes
Groynes are wooden, concrete and/or rock barriers or walls at right angles to the sea. Beach material builds up on the updrift side, where littoral drift is predominantly in one direction, creating a wider and a more plentiful beach, therefore enhancing the protection for the coast because the sand material filters and absorbs the wave energy. However, there is a corresponding loss of beach material on the downdrift side, requiring that another groyne be built there. Moreover, groynes do not protect the beach against storm-driven waves and if placed too close together will create currents, which will carry sand material offshore.
Groynes are extremely cost-effective coastal defense measures, requiring little maintenance, and are one of the most common coastal defense structures. However, groynes are increasingly viewed as detrimental to the aesthetics of the coastline, and face strong opposition in many coastal communities.
Many experts consider groynes to be a "soft" solution to coastal erosion because of the enhancement of the existing beach.
Cost – est. at £200,000 per groyne, £60 per metre
[edit] Sea walls
Walls, usually of concrete and/or stone, built at the base of a cliff or beach, or used to protect a settlement (from eroding). Seawalls aim to resist and reflect the energy of the waves back out to sea, and for this purpose are often curved which also deflects sediment. The sea walls should be parallel to the water edge. Because the wall greatly absorbs the energy instead of reflecting, it greatly erodes and destroys the seawall structure; therefore, major maintenance will be needed within 10 years of being built. Additionally, the seawall will suffer a 98% damage over 30 year period. Furthermore, sometimes the reflected wave or energy helps the rapid depletion of the attached beach. Sea walls are probably the second most traditional method used in coastal management.
Cost – est. at £800 to £5,000 per m2
[edit] Revetments
Wooden slatted or upright blockades, built parallel to the sea on the coast, usually towards the back of the beach to protect the cliff. The most basic revetments consist of timber slants with a possible rock infill. Waves brake against the revetments, which dissipate and absorb the energy. The cliff base is protected by the beach material held behind the barriers. They may be 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 longshore drift. Since the wall greatly absorbs the energy instead of reflecting, it erodes and destroys the revetment structure; therefore, major maintenance will be needed within a moderately time of being built, this will be greatly determined by the material the structure was built with and the quality of the product.
Cost – Confirmed by material used; est. £2000 - £1,000,000. Average £1000 per meter built.
[edit] Rip Rap
Large rocks are piled or placed at the foot of cliffs, which are placed with native stones of the beach. Generally used in areas prone to erosion and thus absorb the wave energy and hold beach material.
Cost – est. around £300 per m
[edit] Gabions
Boulders and rocks are wired into mesh cages and usually placed in front of areas vulnerable to heavy to moderate erosion: sometimes at cliffs edges or jag out at a right angle to the beach like a large groyne. When the seawater breaks on the gabion, the water drains through leaving sediment, also the rocks and boulders absorb a moderate amount of the wave energy.
Cost – est. £11 per m
[edit] Offshore breakwater
- Main article: Breakwater
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. This leads to wider beaches, which absorb the reduced wave energy, protecting cliff and settlements behind. The Dollos which was invented by a South African engineer in East London has replaced the use of enormous concrete blocks because the dollos is much more resistant to wave action and requires less concrete to produce a superior result.
Wire cages filled with crushed stone used to reduce erosion.
Cost – est. £1,950 per m. Water depth may increase the cost.
[edit] Cliff stabilisation
Prevents cliff destabilising by draining off excess rainwater to reduce water-logging: terracing planting and wiring hold the cliffs in place.
Cost – Unknown, as would have to be assessed on a cliff to cliff basis
[edit] Soft engineering techniques
[edit] Beach nourishment
Beach nourishment or replenishment is one of the most popular soft engineering techniques of coastal defence management schemes. This involves importing alien sand off the beach and piling it on top of the existing sand. The imported sand must be of a similar quality to the existing beach material so it can integrate with the natural processes occurring there, without causing any adverse effects. Beach nourishment can be used along side the groyne schemes. The scheme requires constant maintenance: 1 to 10 year life before first major recharge.
Cost – est. £5,000-£200,000 per 100 m, plus control structures, ongoing management and minor works
[edit] Sand dune stabilisation
"Vegetation encourages dune growth by trapping and stabilising blown sand. Transplanting marram grass to the face of eroded dunes will enhance the natural development of yellow dunes above the limit of direct wave attack. Sand couchgrass or lyme grass can be transplanted to encourage the growth of new foredunes along the toe of existing dunes, as these species are tolerant to occasional inundation by seawater. Planting grasses from seed can be undertaken but will not normally be successful in the very active foredune environment. These natural dune grasses act to reduce wind speeds across the surface, thereby trapping and holding sand. They grow both vertically and horizontally as the sand accumulates." - Adaptation from: © Scottish Natural Heritage - www.snh.org.uk
Sand dunes trap sand and beach material washed and blown up, the rate of erosion is slowed and an effective flood barrier is created. Footpaths will also have to be introduced to stop trampling. Sand dunes stabilisation is economical, environmentally friendly, does not disrupt the coastline further on, creates natural habitats for animals and plants and is not regarded as unattractive. However, for successful dunes to be placed it must be thoroughly researched before hand and will take a long time to establish. These are present at Hengistbury Head.
Cost – Unknown: research will have to be carried out beforehand, very rough est. of £20 per m2
[edit] Beach Drainage (Beachface Dewatering)
Beach drainage or ‘beach face dewatering’ involves the localized lowering of the water table beneath and parallel to the beach face. This has been demonstrated to cause accretion of sand above the installed drainage system. Sand is in continual movement on a wet beach face due to wave and tidal action in the ‘swash’ zone. Under specific conditions, beach drainage systems can halt beach erosion and promote sand accretion by adjusting the dynamic equilibrium that exists on sand beaches.
The accretion or erosion of a beach is influenced by a number of hydrodynamic forces in a beach surf zone. The effects and interaction of these sediment transport mechanisms have been studied since the 1940s. It is well understood that lowering the water table in granular soils improves their stability and eliminates the tendency for them to move (ie, ‘well-pointing’). A number of theories have been proposed to explain the empirical evidence for sand deposition from beach drainage (ie, backwash reduction, seepage reduction, liquefaction reduction). These and other theories continue to be studied around the world expanding our understanding of the complex interplay of forces that can be seen working on the beach face.
It is evident that lowering the water table under the beach eliminates buoyancy factors and reduces the lubricating effect between the grains, restoring the frictional characteristics of the sand. Percolation of 'swash water' into the beach means less backwash energy, which encourages suspended sand to settle out on the beach face.
This is achieved by installing a drainage system in the beach that lowers the beach face water table, intercepting the flow of swash, tidal and inland ground water. Collection pipes are buried in the beach parallel to the coastline to create an unsaturated zone beneath the beach face. This unsaturated zone is achieved by draining the seawater away by gravity to a collector sump and pumping station. The sump and buried pumping station can be located at the back of the beach, where they are not readily visible. A typical pumping station might consist of two submersible electric pumps located in a buried concrete chamber. The only visible feature of the system may be the pump station control panel that regulates and monitors the pumps, sends data and receives control signals.
A useful side effect of the system is that the collected seawater is very pure because of the sand filtration effect. It may be discharged back to sea but can also be used to oxygenate stagnant inland lagoons /marinas or used as feed for heat pumps, desalination plants, land-based aquaculture, aquariums or seawater swimming pools.
Beach drainage systems have been installed in many locations around the world to halt and reverse erosion trends in sand beaches. Twenty four beach drainage systems have been installed since 1981 in Denmark, USA, UK, Japan, Spain, Sweden, France, Italy and Malaysia.
Costs
The costs of installation and operation per meter of shoreline protection will vary from project to project due to the following cost sensitive factors, • system length (non-linear cost elements) • pump flow rates (sand permeability, power costs) • soil conditions (presence of rock or impermeable strata) • discharge arrangement /filtered seawater utilization • drainage design, materials selection & installation methods • geographical considerations (location logistics) • regional economic considerations (local capabilities /costs) • study requirements /consent process.
The costs associated with a beach drainage system are generally considerably lower than hard engineered structures. They also compare very favorably with beach nourishment projects, particularly when long-term project economics are considered (nourishment projects often have a limited life or a program of re-nourishment).
[edit] Managed retreat
Managed retreat is an alternative to constructing or maintaining engineering solutions. The earliest managed retreat in the UK was an area of 0.8 ha at Northey Island in Essex, that was flooded in 1991. This was followed by Tollesbury and Orplands in Essex, where the sea walls were breached in 1995. The technique is used when the land adjacent to the sea is low in value. A decision is made to allow the land to erode and flood, creating new sea, inter-tidal and salt-marsh habitats. This process may continue over many years and natural stabilization will occur.
Cost – The main cost is generally the purchase of land to be flooded. Housings compensation for relocation of residents may be needed. Any other human made structure which will be engulfed by the sea may need to be safely dismantled to prevent sea pollution. In some cases, a retaining wall or bund must be constructed inland in order to protect land beyond the area to be flooded, although such structures can generally be lower than would be needed on the existing coast. Monitoring of the evolution of the flooded area is another cost. Costs may be lowest if existing defenses are left to fail naturally, but often the realignment project will be more actively managed, for example by creating an artificial breach in existing defences to allow the sea in in a controlled fashion at a particular place, or by pre-forming drainage chanels for created salt-marsh.
[edit] See also
[edit] External links
 |
Wikimedia Commons has media related to: |
By Adam Nolan
