Why Is The Risk Of Coastal Flooding Increasing?

Erosion

On coastlines erosion is subdivided into four main types: 

•         Hydraulic Action
•         Corrasion (Abrasion)
•         Attrition
•         Solution (Corrosion)

Hydraulic Action

Hydraulic action is the force of water; as well as the impact of simply being hit by a wave this also includes the effects of trapped pockets of air.

As waves hit cracks, trapped air causes high pressure which can break them open. Even if it does not do this then it may strike a cliff face with a crack or small cave. The pockets of air are compressed or squeezed by the wave creating high pressure (such as blowing up a balloon) which is then released suddenly (such as popping the balloon).

Cracks become fissures which become small caves which become larger caves. 

Caves can work their way deep into the rock following lines of weaknesses such as cracks or faults.

In headlands (rocky areas sticking out into the sea and surrounded by water and waves) these caves can be eroded through from each side to join up and form dramatic arches. 

Eventually the roof of an arch may be weathered and eroded, and collapse leaving an isolated pillar of rock called a stack.

Corrasion

Corrasion is the name for erosion in which the water or waves uses rocks and stones as tools to increase erosion. In many modern textbooks the term abrasion is also used to mean the same thing.

Corrasion/abrasion from rocks and stones carried by waves is added to the hydraulic actions of the waves as they further hit the shore line. This is often seen as a notch at the base of cliffs as they are undermined by the sea which can cause them to collapse.

As the cliff is eroded back, at low tide the waves can roll rocks and stones along to create a wide flat platform with a gentle gradient or angle; this is called a wave cut platform and can be seen here on the Glamorgan Coastline.

In the pictures you can see such notches along the Glamorgan Coastline and attempts to fill them in with bricks and cement to protect an important clifftop lighthouse. 

Attrition

Attrition is where the rocks and stones moved by waves and water are broken up, rounded and smoothed. Overtime rocks on a coastline will get smaller, rounder and smoother until can they even appear to look polished.

Corrosion

Corrosion is the name for erosion in which the water or waves chemically dissolves rocks and stones; sometimes the term solution is also used to mean the same thing. Some types ofrocks such as limestone can dissolve if there is acid in the water. Just like sugar dissolves in warm water.

Transportation

The most important process of transportation is caused by longshore drift.

Longshore drift is caused when waves hit the shoreline at an angle; in this picture 1 is the beach and number two 2 is the sea. The longshore drift is number 3 and is caused by the wind direction which is number 4, causing crests of waves to form at an angle of 90° to the wind direction (clearly wind directions change - the most common wind direction or prevailing wind direction usually sets up the direction of longshore drift). As the waves hit the beach at an angle the swash moves material up the beach at this angle as is shown in number 5. The backwash usually moves material straight back down the beach due to gravity. Through this process material moves in a zigzag pattern along a beach.

Deposition 

Eventually the material which has been eroded and transported will be deposited. The longshore drift current may encounter an obstacle such as a rocky headland. When this happens the transported material may move offshore and be deposited as banks or bars which may be visible at low tide. This material may also be swept away by offshore currents and moved many hundreds of km before being deposited. However the material may move into a more sheltered section of the coastline where it may start to be deposited as a beach.

In higher energy locations especially those facing a long fetch a beach will normally be a very flat (low profile) sand beach backed by a bank of large cobble stones. This type of beach is called a storm beach and Newgale is a very good example.

Lower energy sheltered beaches will tend to have a slightly higher angle/profile of sand and will often be backed by sand dunes. This can be seen in this photograph of Three Cliffs Bay on the Gower Peninsular.

Photo from Wikipedia by Author; Roger Davis used under CC BY

Sometimes longshore drift will carry on when a beach reaches deep water where the coastline changes direction. When this happens the longshore drift will drop its sandy load into the deep water and a strange beach will grow out into the sea surrounded on three sides by water. This type of beach is called a spit; they are quite common in Wales. This example is Fairbourne Spit also known as Penrhyn Point in Gwynedd which is located at the mouth of the River Mawddach where it reaches Cardigan Bay.