Marine erosion, transport, longshore drift and deposition

Overview

Hydraulic action at the coast, solution at the coast and four ways in which the sea transports sediment.

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 hydraulic action at the coast?

   Model answer

   Erosion by the sheer force of moving water. A breaking wave hits the cliff face with great pressure, and as it does so it forces water and air into joints, bedding planes and cracks in the rock. The trapped air is compressed by the incoming water and then expands explosively as the wave withdraws, and the repeated pressure and release, sometimes called cavitation or wave quarrying, widens the fissures and eventually prises blocks of rock out of the face. It is most effective on well-jointed and bedded rock and in storm conditions.

2. 2

   What is abrasion, and why is it usually the most effective erosion process on a rocky coast?

   Model answer

   Abrasion, also called corrasion, is the wearing away of the cliff and the shore platform by the load the waves carry. Sand, shingle and boulders picked up by the waves are hurled against the rock face and dragged back and forth across the platform, acting like a saw or sandpaper. It is usually the most effective process because it uses the wave's energy through hard tools rather than water alone, and it works most intensely in the zone between high and low tide where breaking waves and available sediment coincide, which is why notches form at that level.

3. 3

   What is attrition, and why does it not erode the coast?

   Model answer

   Attrition is the wearing down of the sediment itself: rock fragments carried by the waves collide with one another and with the sea bed, so their edges are chipped and rounded and they become progressively smaller and smoother. It does not erode the coastline because the material being worn away is the load, not the shore. Its importance is indirect: it produces the rounded pebbles and eventually the sand that make up beaches, and it explains why beach material tends to become finer and rounder in the direction of longshore drift.

4. 4

   What is solution at the coast, and where does it matter most?

   Model answer

   Solution, or corrosion, is the chemical dissolving of rock minerals by sea water. It matters most on coasts made of limestone and chalk, where the calcium carbonate is dissolved by weakly acidic water, and it is a significant reason why cave systems, widened joints and pitted surfaces develop on those coasts. It also acts on the cement binding some sandstones. Unlike the other processes it is not dependent on wave energy, so it continues in sheltered conditions and at all levels reached by sea spray, not just at the wave break.

5. 5

   What factors control how quickly a cliff is eroded?

   Model answer

   Wave energy, determined by fetch, wind strength and exposure, and by whether the coast is a headland where refraction concentrates energy. Rock resistance, meaning both hardness and, just as importantly, the density of joints, faults and bedding planes, since a well-jointed hard rock can retreat faster than a weak but massive one. The dip of the strata, since beds dipping seaward allow blocks to slide out while beds dipping landward hold together. The presence and width of a beach, which absorbs wave energy before it reaches the cliff foot, so removing a beach accelerates cliff retreat. Tidal range, which controls how long each part of the cliff is under attack. And subaerial processes acting on the cliff face, which weaken it from above while the sea undercuts it from below.

6. 6

   Why should subaerial processes be included in an explanation of cliff retreat?

   Model answer

   Because marine erosion attacks only the base of the cliff, yet whole cliff faces retreat. Weathering by frost, salt crystallisation, wetting and drying and chemical action loosens the rock of the face, and rainwater percolating into the cliff adds weight and raises pore water pressure, lubricating potential slip surfaces. The result is mass movement, in the form of rockfall, slumping, mudflow and landslide, which delivers material from the upper cliff to its base. Marine erosion then removes that debris, so the sea and the subaerial processes work together: the sea undercuts and clears away, the land-based processes bring the face down.

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