Some forecasters are predicting an increase in global temperatures, due to global warming.Over the past 25 years an increase of 0.2 degrees Celsius per decade has been observed.
This is likely to cause global sea levels to rise yet further with sea levels currently rising around 3 mm per year. A rise in sea levels will also see an increase in frequency and magnitude of storm events.
When these two factors are combined, they will have the effect of focusing wave energy closer to our shores. This will lead to increased rates of coastal erosion in areas where cliffs are composed of soft rocks.
The lithology of a coastline depends on how quickly a coastline is eroded. Hard rocks such as Gabbro are resistant to weathering and erosion. A coastline made up of granite, such as Land’s End will change slowly. Soft rocks, like Limestone are more susceptible to weathering and erosion so a coastline made up of chalk, for example the Dorset coastline, will change relatively quickly.
If we were to look at a coastline from above and observe the geology of the area, we’d be able to see that the rock type changes as you approach the coastline and the different types of rock are arranged in bands. The position and angle of these bands in relation to the coastline indicates either a concordant or discordant coastline.
Concordant coasts have alternating layers of hard and soft rock running parallel to the coast. The layers of hard rock act as protection to the softer rock that lays behind, helping to preventing erosion.
If the softer rock is exposed, mainly caused by the hard rock being breached, a cove can form.
A discordant coastline, a coastline of alternating layers of hard and soft rock perpendicular to the coast, can cause soft rock to become exposed and therefore eroded faster than hard rock. This differential creates headlands and bays along discordant coastlines.
At Aldbrough, on the Holderness coast in North Yorkshire, the cliffs are compiled of soft till; a mixture of clay, silt, sand, gravel, cobbles and boulders deposited by a glacier.
These cliffs are being actively eroded by wave attack at the base. Erosion rates are estimated to be around 1.5 metres per year.
Wave erosion at the base removes the support for the upper section of the cliff. This causes the cliff to become unstable and fail, most commonly as a rotational landslide.
Rocks can often form in layers of different types, often known as beds. These beds can be subject to tectonic forces that tilt them, causing them to dip at an angle. Depending on the angle at which the beds sit effects how they are eroded and the profile of the resulting cliffs.
Horizontal beds produce steep cliffs with notches, where any differential erosion has taken place.
Near vertical beds (with a dip of ~90˚) also produce steep cliffs but differential erosion is less prevalent in these structures. Beds that dip seaward produce gentler cliffs but are less stable because loose material can slide down the bedding planes in mass movements.
Landward dipping beds tend to produce more stable & steeper cliffs.
As coastal erosion becomes an ever increasing concern, it’s important for councils and governments to start managing coastlines in order to protect them from increasing coastal erosion and flooding due to altering sea levels. Coastal management is vital to protect homes and local business from being damaged or destroyed.
When managing the coastline, there are four key approaches to consider:
1. Hold the line - Existing coastal defences are maintained but no new defences are added.
2. Advance the line - New defences are built further out to sea in an attempt to reduce the stress on current defences and possibly extend the coastline.
3. Retreat the line - Move people out of danger zones and let mother nature take her natural course.
4. Do nothing - Deal with the effects of flooding and erosion after the event, as they happen or simply ignore them. This is generally what happens in unmanaged areas with little or nothing of value to protect.
When planning and building coastal defences there are 2 techniques to consider; hard engineering or soft engineering.
Hard engineering techniques include building sea walls, probably the most obvious of defence methods. These giant walls can span an entire coastline. These attempt to reduce erosion and prevent flooding in the process. Modern sea walls have a curved structure that reflects waves back into incoming waves, breaking them up toreduce their energy, further reducing erosion.
Further examples of hard engineering techniques include:
Gabions- quite simply bundles of rocks in a metal mesh. They’re placed at the base of a cliff in an attempt to reduce the impact of waves on the cliff and prevent the cliff from being undercut.
Revetments- concrete (or in some cases wooden) structures that are built along the base of a cliff. They’re slanted and act as a barrier against waves, not too dissimilar to a sea wall. The revetments absorb the energy of the waves, preventing the cliffs from being eroded.
Riprap- simply rocks and stones that have been put against the base of a cliff. They’re similar to gabions in their purpose but they aren’t bound together in a mesh structure.
Breakwaters- offshore concrete walls that break incoming waves out at sea so that their erosive power is reduced to next to nothing when they reach the coast. Breakwaters are effective but can be easily destroyed during storms and they don’t look particularly nice.
Big, retractable walls built across estuaries- can be used as a floodgate to prevent storm surges. They’re hugely effective but they’re also hugely expensive.
When looking to prevent the effects of coastal erosion,we sometimes have the option to take a much softer approach.
These softer engineering techniques include:
Groynes – A soft technique used to prevent erosion. They’re low lying wooden walls that extend out to sea. The idea of groynes is to capture sand that moves down the beach via long shore drift and help build up a larger section of beach in front of an area that’s experiencing coastal erosion.
Beach nourishment- The technique of adding sand and shingle to a beach in order to make it wider. This increases the distance a wave has to travel to reach the cliffs and so the wave loses more energy and has less erosive power. The sand and shingle has to be obtained from elsewhere, normally from dredging.
Land management - often used to help protect and rebuild dunes. Sand dunes act as a good barrier against coastal flooding and erosion and can be exploited as a natural defence against the sea. However in order to do so, the dunes must be left relatively undisturbed so boardwalks are constructed with sections of sand dune systems marked as out of bounds to the general public in order to reduce the erosion of the dunes by human use.
Marshland- can be used to break up the waves and reduce their speed, reducing the waves erosive power. The marshlands also limit the area that waves can reach, further preventing flooding. The marshlands can be created by encouraging the growth of marshland vegetation such as glass worts.
Beach stabilisation - The goal of beach stabilisation is the same as beach nourishment; to widen the beach and dissipate as much wave energy as possible before it reaches the cliffs. Beach stabilisation involves planting dead trees in the sand to stabilise and lower the profile of the beach while widening the beach.
A detailed survey of the composition of the land and the past and future effects of erosion and flooding should be undertaken before remedial and preventative works are planned or commissioned.
