WHAT IS COASTAL EROSION?

COASTAL EROSION

Coastal erosion (also known as coastal retreat) is the loss of coastal land due to removal of sediments or rock from around the shoreline. Coastal erosion can occur quickly during a single storm event or slowly were small amounts of sediment are transported away from the beach year after year. Shorelines which experience ongoing coastal erosion recede landward and are often referred to as retreating shorelines. Accreting shorelines are the opposite of this and describe shorelines which are moving oceanward due to the deposition of sand in the area.  The main drivers for coastal erosion are waves, currents and inadequate sediment supply from adjacent beaches.

STORM EROSION

Increased water levels associated with storm activity due to waves, wind and changes in atmospheric pressure. Larger waves associated with storm events lead to more water being moved shoreward than under typical conditions this increase in water volume near the shoreline results in an increased water level along the coastline. Winds can increase water levels at the shoreline if the wind direction is blowing onshore as the force of the wind causes water to pile up against the shoreline. Changes in barometric pressure can influence the water level with periods of low barometric pressure associated with higher sea levels as there is not as much atmospheric pressure pushing down on the ocean.

LONG TERM EROSION

Long term erosion refers to prolonged shoreline changes which are predominantly dictated by sediment supply to and from adjacent beaches. This sediment supply is determined by the longshore sediment transport regime which refers to sediment transport along the beach parallel to the shoreline. Longshore sediment transport typically results in slower changes to the beach system and is usually associated with more permanent long term changes to the beach system.

Longshore sediment transport is driven by longshore currents which are created by waves. The angle of the wave and its energy as it enters the nearshore dictates the speed and size of these current along the beach. The larger the wave and the greater the angle the faster the current speed. The key factor in determining how the shoreline responds to longshore sediment transport is to do with the supply of sediment in the beach system. If there is sufficient sediment supply from adjacent beaches the beach stays stable. When there isn’t enough sediment coming from adjacent beaches then erosion starts to occur. A typical example of this is the erosion caused on the leeward side of a groyne due to the structure physically blocking the longshore sediment transport along the beach.

SHORELINE RETREAT DUE TO SEA LEVEL RISE

There is a unique mechanism which takes place when sea levels rise and its associated with the change in depths along the beach profile resulting from increased sea levels. As the sea level rises the depths across the beach profile increase, this allows more wave energy to penetrate shore-ward. This results in increased erosion near the shoreline and the offshore movement of sand (cross-shore sediment transport). This sand is then deposited on the lower sections of the beach profile increasing the bed level until the depths are equal to those that existed prior to the sea level rise increment. The reduction in depth associated with the offshore movement of sediment results in a reduction in wave energy at the shoreline back to the equilibrium level (for that wave climate) that occurred before the sea level rise. The result is the landward migration of the shoreline which is commonly referred to as coastal erosion or coastal retreat. The recession resulting from sea level rise can be in the order of 100 times the sea level rise increment. For example 50cm of sea level rise could potentially result in 50m of landward retreat.

CASE STUDY: WESTERN AUSTRALIA

In the Western Australia state planning policy 2.6 if a property is identified to be at risk under the councils “Coastal Hazard Risk Management & Adaptation Planning” the following is then required to be placed on the property title informing any prospective buyers of its risk:

“VULNERABLE COASTAL AREA –This lot is located in a area likely to be subject to coastal erosion and/or inundation over the next 100 years.”

As always this is completed on old (2007) sea level rise data which will be required to be updated in the near future increasing the size of the Vulnerable Coastal Area. The policy also states that if new information becomes available the coastal hazard assessment needs to be reviewed when the information could “significantly modify the understanding of coastal hazards”.