Coastal defences

[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

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.

For more information see: Groynes.

[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.

See also: Seawall.

[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.

NB Culvert, screen and outfall manual, (CIRIA C786) published by CIRIA in 2019, defines revetment as: ‘Works to protect the bed or banks of a channel against erosion, typically constructed from stone or concrete blocks.’

[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.

For more information see: Breakwater.

[edit] Gabions

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.

They are defined by Culvert, screen and outfall manual, (CIRIA C786) published by CIRIA in 2019 as a: 'Wire or plastic mesh container filled with stones to protect against scour or (to) form a retaining wall. Available as cuboids, mattresses or tubes.'

[edit] Related articles on Designing Buildings

• Berm.
• Caisson.
• Coastal change management area.
• Delta Works sea defences.
• Difference between cofferdams and caissons.
• Dredging.
• Dyke.
• Engineers and hurricanes.
• Flood.
• Flood and water management act.
• Flood defences.
• Flood risk.
• Floodwater.
• Groynes.
• Land-sea interface.
• Levee.
• Pile wall.
• Planning for floods.
• Scour.
• Seawall.
• Storm surge.
• Temporary flood defences.
• Three pieces of infrastructure that have saved lives.
• Underwater foundations.
• Water engineering.

[edit] External resources

• ‘Introduction to Civil Engineering Construction’ (3rd ed.), HOLMES, R., The College of Estate Management (1995)
• SE-Coastal Group - Coastal Defences