Hard Engineering Seawalls How it is designed to overcome the problem Sea walls are usually built along the front of cliffs, often to protect settlements. A seawall is a structure constructed parallel to the coastline that shelters the shore from wave action. This structure has many different designs; it can be used to protect a cliff from wave attack and improve slope stability and it can also dissipate wave energy on sandy coasts. Challenges /Limitations /Disadvantages However, a seawall may not protect a coast from erosion in the long run. As waves break against the seawall, the curved sea walls reflect the energy of the waves which is redirected downwards, to the base of the seawall, resulting in a strong backwash. The backwash wears away the base of the seawall, causing it to weaken and eventually collapse. Hence, seawalls have to be carefully maintained. the probable collapse of the seawall may also affect access to the beach by people and tourists. It is expensive to build seawalls. In England, it can cost 1 million pounds ($3 million) to build 1 kilometre stretch of seawall.
Advantages A seawall provides excellent defence where wave energy is high, reassures the public and has a long life span. It protects the base of cliffs, land and buildings against erosion. and can prevent coastal flooding in some areas. Breakwaters How it is designed to overcome the problem An offshore breakwater is a structure that parallels the shore (in the nearshore zone) and serves as a wave absorber. It reduces wave energy in its lee and creates a salient or tombolo behind the structure that influences longshore transport of sediment. More recently, most offshore breakwaters have been of the submerged type; they become multipurpose artificial reefs where fish habitats develop and enhance surf breaking for water sport activities. These structures are appropriate for all coastlines. Challenges /Limitations /Disadvantages They are unable to provide complete protection as they still leave areas of the coast unprotected. The unprotected areas will be prone to erosion. Even though this form of defence is intended to give only partial protection to the shoreline the impacts on shoreline processes, intertidal habitats and landscape will still be high, and may be unacceptable in environmentally sensitive areas. Erosion in the lee of the gaps may well continue for several years after construction while a new beach planshape develops.
On frontages affected by longshore transport the breakwaters may reduce drift rates, resulting in the erosion of downdrift stretches of coast, but helping to stabilise the updrift shore. As the structures may be separated from the shore at peak water levels they are potentially hazardous to anyone using them as a perch and becoming stranded as the tide rises, particularly if there are also heavy seas. Where the nearshore waters tend to be silty the breakwaters may encourage lee-side deposition of mud leading to both unwanted odours and unsafe beach areas. Other lee side deposits may include sea weed and jetsam from ships (plastic containers, nets, rope, etc) Wave induced currents around the ends of breakwaters can be locally strong and a danger to beach users.
Advantages In Singapore, breakwaters are built along the beaches at East Coast Park, as well as Siloso beach on Sentosa Island. The cost of each breakwater is estimated to be around $1 million.
Groynes How it is designed to overcome the problem In some places, to protect a beach from erosion, people have built groynes along the beach. Groyne structures, usually made of wood, concrete, or piles of large rocks, which run perpendicular to the shoreline. They extend from the beach towards the sea, usually at right angles to the sea. They are generally constructed in groups called groyne fields. A groyne creates and maintains a healthy beach on its updrift side, which in turn provides protection to the land behind. It acts...
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