Hard engineering strategies

Overview

Hard engineering at the coast, disadvantages of a sea wall and terminal groyne syndrome.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is hard engineering at the coast?

   Model answer

   The use of built structures, typically of concrete, rock, steel or timber, to resist the sea directly, by absorbing or reflecting wave energy, by trapping sediment, or by holding the shoreline in a fixed position. It works against natural processes rather than with them, is usually expensive to build and to maintain, and is generally chosen where high-value property or infrastructure has to be protected and the shoreline cannot be allowed to move.

2. 2

   How does a sea wall work, and what are its advantages?

   Model answer

   A sea wall is a substantial barrier, usually of concrete, built along the shoreline at the base of the land it protects, designed either to reflect wave energy back out to sea or, in more modern curved and stepped designs, to absorb it and to throw overtopping water back seawards with a recurved crest. Its advantages are that it gives a high degree of protection against both erosion and flooding, it halts erosion of the land behind for as long as it stands and is maintained, it has a long design life, it often incorporates a promenade which is valuable for tourism and access, and it reassures residents, which matters politically. It is generally the option chosen where a town, a main road or a railway is directly at risk.

3. 3

   What are the disadvantages of a sea wall?

   Model answer

   It is the most expensive of the common defences to build, typically several thousand pounds per metre, and it carries a permanent maintenance liability since it is continually attacked and will eventually need replacing. By reflecting wave energy downwards it causes scour at its own base, which undermines the foundations and lowers the beach in front of it, so the wall progressively destroys the beach that would otherwise have protected it. It is visually intrusive on a natural coast and can deter the tourists it is meant to serve. It cuts the land off from the beach, both for access and ecologically, and where the shore would naturally retreat it squeezes intertidal habitat out of existence. And by fixing the shoreline it prevents the cliff behind from eroding and supplying sediment to the coast downdrift.

4. 4

   What is a revetment, and how does it differ from a sea wall?

   Model answer

   A revetment is a sloping structure placed against the base of a cliff or the seaward face of a bank, made of open timber slats, concrete or interlocking blocks. Its slope and its open construction are the point: instead of presenting a vertical face that reflects waves, it allows water to run up it and pass partly through it, dissipating the energy gradually and letting sediment settle behind it. Compared with a sea wall it is considerably cheaper, causes far less scour because it absorbs rather than reflects, and is less visually dominant, but it offers less protection against severe storms and overtopping, has a shorter life, and timber versions in particular need regular repair.

5. 5

   What is rip-rap, and what are its strengths and weaknesses?

   Model answer

   Rip-rap, also called rock armour, is a mound of large, resistant boulders piled at the foot of a cliff or sea wall. The gaps between the boulders and the irregular surface break up and dissipate wave energy as water passes through and around them, so the force reaching the structure or cliff behind is much reduced. It is comparatively cheap and quick to install, can be topped up rather than replaced, and is easy to repair. Its weaknesses are that the rock is often not local, so it looks out of place and imported stone raises the cost and the carbon footprint of the scheme, the boulders can be displaced and rearranged by severe storms and need periodic reworking, and the uneven surface is hazardous to walk on and restricts access to the beach.

6. 6

   What are gabions, and when are they used?

   Model answer

   Gabions are wire mesh cages filled with rocks or shingle, stacked to form a wall or a sloping face at the back of a beach or against a cliff foot. Water passes into the spaces between the stones, so wave energy is absorbed rather than reflected, and vegetation can eventually colonise them and soften their appearance. They are much the cheapest of the hard options, use locally available stone and can be built quickly by hand without heavy plant, which makes them attractive for small-scale schemes and for lower-budget situations. Their weakness is durability: the wire corrodes in salt water and abrades against the stones, typically failing within five to ten years, after which the cage bursts, the rock spills out and the broken wire is both ugly and dangerous.

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