Last edited 15 Nov 2016

Coastal defences

Sea wall.jpg

Contents

[edit] Introduction

Despite occupying less than 15% of the Earth’s land surface, coastal zones accommodate more than 40% of the world’s population. Historically, this is due to the increased commercial and industrial potential of areas that are near the coast, such as shipping, fishing and tourism industries.

Coastal defences are a key part of coastal management, in which the land-sea boundary is protected from flooding and erosion, categorised as hard engineering and soft engineering.

[edit] Soft engineering

Soft engineering can be a more sustainable, long-term and potentially more cost-effective approach to coastal defence, working with natural processes to protect the shoreline.

Techniques include:

  • Beach replenishment: This involves the importing of beach-grade sediments to ‘top up’ beaches.
  • Sand dune management: This can include constructing footpaths, ladders and boardwalks to prevent degradation of the beach by humans.
  • Beach drainage: Lowering the water table locally beneath the beach face, causing sand accretion above the drainage system.

[edit] Hard engineering

Hard engineering can be more expensive, and is sometimes less durable and be more intrusive than soft engineering. Hard engineering can also cause issues elsewhere, simply moving the problem along the coast.

Some common examples of hard engineering solutions are set out below:

[edit] Groynes

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Groynes are walls of concrete, stone or timber that extend out from beaches, acting as barriers. They protect or retain beach material and slow losses through long-shore drift. Steel sheet piling may also be used, but it must be suitably capped and backed with concrete. It is important that the piling penetrates to a depth that will prevent wave action from underscoring the structure.

Despite having a positive local effect, groynes can cause ‘sediment starvation’ which shifts the erosion further down the coastline.

[edit] Sea wall

Sea walls are structures that usually incorporate into a promenade, and are built to limit erosion caused by wave attack. They can be made from materials such as; timber, steel, masonry blocks, precast concrete units and in situ concrete. They are commonly 3-5 m (10-16 ft) high and curved to reflect back the energy of the waves and prevent wave overtopping.

[edit] Revetments

Revetments are onshore sloped structures used as an alternative to sea walls to reduce the landward migration of beaches. They limit the energy of the waves as they break and so reduce their erosive power. They can be constructed using stepped concrete, stone or asphalt and should be designed to have a sufficiently high crest to avoid wave overtopping. In their most basic form, they can be constructed using timber with a rock infill.

[edit] Breakwaters

Also known as ‘moles’ are constructed in outer harbour areas to dampen heavy waves and allow vessels to enter and exit with less swell. They can be sloped or vertical and are typically constructed from concrete blocks, rock fill or a combination of both depending on site-specific conditions such as water depth, range of tides, and foundation conditions.

If they are constructed using blockwork, the marine bed may need to be dredged and a concrete bed laid for the foundation. The blocks can then be lowered by cranes operated from pontoons, perhaps with divers to help position the blocks.

Rubble or rock-fill breakwaters generally use heavy stones ranging from 1-5 tonnes. These can be transported and placed by bottom-opening or side-tipping barges.

[edit] Gabions

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Gabions are steel mesh cages that are filled with rocks, concrete and sometimes aggregate, and used to stabilise vulnerable areas, by absorbing wave energy. They can project out at right angles from the coastline like groynes, or can be constructed as retained walls, battered or stepped back rather than being stacked vertically. The strength of the wire used to tie the cages together is the critical factor. Galvanised steel wire is commonly used, but stainless steel and PVC-coated wire can also be used.

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