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Natural Hazards: Coastal erosion and inundation

Coastal inundation

The coast is a narrow stretch between the land and the sea. The sea, land and air constantly change its shape and form.

Our coast is shaped by many forces - the sea, the wind, the structure of the rock and earth along different parts of the coast, and even people and their activities. Coastal landscapes are formed by a combination of erosion, transportation and deposition processes.

Coastal erosion

Coastal erosion is a complex natural process, where waves wear away and break up the edge of the land along the coast. The stronger the wave the more erosion it will cause. Waves are created by wind blowing over the surface of the sea.

The size of a wave depends on:

  • Length of time the wind has been blowing
  • Strength of the wind
  • 'Fetch' - how far the wind has travelled

When a wave breaks, water is washed up the beach - this is called the swash. The water then runs back down the beach - this is called the backwash. The strength of the swash and backwash determines the type of wave created. There are two types of waves - constructive and destructive waves.

Constructive waves break on the shore and deposit material, building up beaches. They occur when the swash is stronger than the backwash.

Destructive waves are created in storm conditions. They are created from large, powerful waves when the wind is strong and has been blowing for a long time. These waves occur when the backwash is stronger than the swash, the waves are said to be destructive.

Destructive waves erode the coastline to create our beaches, bays and harbours in four ways:

  1. Hydraulic action - Air becomes trapped in cracks on a cliff face. When a wave breaks, the trapped air is compressed which weakens the cliff and causes erosion.
  2. Abrasion - Bits of rock and sand in waves grind down cliff surfaces like sandpaper and rocks become smaller and rounder
  3. Attrition - Waves smash rocks and pebbles on the shore into each other, and they break and become smoother.
  4. Solution - Acids and salt in the sea dissolving the minerals in the rock.

Coastal erosion becomes a hazard where people's homes, properties, road or services are threatened by loss of land.

Coastal inundation

Inundation is the flooding of coastal areas during storms. The flooding results from a combination of sea storms and high tides, which will drive high, strong waves onshore onto normally dry, but low lying coastal land.

Sea water can damage homes, property and even contaminate coastal aquifer water supplies.

Getting in touch

East Coast Lab
159 Dalton Street, Private Bag 6006 Napier, 4142
P: 06 835 9200
E:info@eastcoastlab.org.nz

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