Coastal erosion and inundation

Coastal inundation

Processes causing storm surge.

The terms inundation and flooding are often used interchangeably. Inundation tends to be used to describe extensive and overwhelming amounts of water.

Coastal inundation is when normally dry, low-lying coastal land is flooded by the sea. It is primarily caused by severe weather events along the coast, estuaries and adjoining rivers.

Storm surge is one of the main causes of coastal inundation. A storm surge is a rise in water level from low pressure weather systems, over and above the predicted tide height, caused by a severe storm.

These severe storm events bring strong wind and heavy rain. The wind drives large waves and heavy rain raises water levels in rivers and streams.

The worst flooding occurs when larger-than-normal tides (‘king tides’) and storms occur at the same time.

Other types of flooding

Coastal inundation is one type of flooding that Christchurch and Banks Peninsula settlements are susceptible to.

Other types of flooding include:

  • Fluvial flooding occurs when streams and rivers are at capacity and water flows over the banks
  • Pluvial flooding occurs when the stormwater drainage system cannot cope with extremely heavy rain.

Coastal inundation and climate change

The sea level is rising due to climate change and this needs to be taken into consideration when assessing the risk of future coastal inundation. Land-use planning in coastal areas needs to respond to these increasing risks.

Projected potential sea level rise.

Sea-level rise
Sea-level rise is a relatively slow process which will permanently raise the overall level of the sea. Sea-level rise is caused by the expansion of the oceans caused by warmer water temperatures and the melting of the Antarctica and Greenland ice sheets and glaciers.

This means that the coast as we know it today will move inland and therefore coastal processes such as high tides and storms will reach further inland too. Areas that may be only inundated occasionally now are likely to be flooded more often in future.

Historic sea-level rise in New Zealand has averaged 1.7 +/- 0.1mm per year, with Christchurch and Banks Peninsula showing a slightly higher rate of 1.9 +/- 0.6mm per year. The rate of sea-level is not expected to remain constant, it is predicted to increase significantly from about 2065. This is because the rate at which the ice sheets melt and the rate of thermal expansion is expected to increase significantly from around this time.

Coastal erosion

Shoreline retreat.

Coastal erosion occurs when the sea wears away the land and removes beach sediments, dunes or coastal cliffs. It is a natural and ongoing process. Over time most shorelines vary between periods of erosion and accretion (deposition or building-up).

Shoreline change with sea-level rise.

Coastal erosion and climate change

The sea level is rising due to climate change and this needs to be taken into consideration when assessing the risk of future coastal erosion. Land-use planning in coastal areas needs to respond to these increasing risks.

The sea level is rising, through the expansion of the oceans caused by warmer water temperatures and the melting of the Antarctica and Greenland ice sheets and glaciers. This means that the coast as we know it today will move inland and therefore coastal processes such as high tides and storms will reach further inland too.

Historic sea-level rise in New Zealand has averaged 1.7 +/- 0.1mm per year, with Christchurch and Banks Peninsula showing a slightly higher rate of 1.9 +/- 0.6mm per year. The rate of sea-level is not expected to remain constant, it is predicted to increase significantly from about 2065. This is because the rate at which the ice sheets melt and the rate of thermal expansion is expected to increase significantly from around this time.

Our open coast in Christchurch is currently accreting with periods of erosion from storm events. However, the long-term trend is of a shoreline moving further inland, and this is the trend we need to plan for. With climate change, along with sea-level rise, periods of storminess are expected to be more frequent resulting in significant erosion of the open coast.

The South Brighton spit is a dynamic and unstable landform and has undergone large changes in shoreline positions in the past. Predictions are that sea-level rise will cause the spit to become even more unstable.

Tsunami

Another coastal hazard is tsunami - a sustained elevated sea-level that can flood coastal areas.