Coastal erosion hazards
Coastal erosion is a natural process that is generally only of concern when threatening human habitation or development. The extensive urbanisation of Auckland, a region with 3100 km of coastline, has resulted in exposure to hazardous coastal erosion and accretion processes and both 'soft' and 'hard' shoreline and cliff erosion.
'Soft shorelines' refer to sandy beaches and dunes made up of unconsolidated or very weakly consolidated materials. Sandy beaches in the Auckland region experience periods of accretion (net sediment accumulation) and erosion (net sediment decrease). On beaches experiencing a sustained period of erosion, sediment loss results from waves, currents and wind removing sediment from the beach faster than it is replaced.
Erosion and instability also happens from natural and human factors such as cliff height and modification. Geology is the main factor that affects the extent of coastal erosion in Auckland. Volcanic coasts are formed of much harder rock than sedimentary coasts and less likely to form bedding, joint and plane failures. Greywacke rocks exposed along the southeast coastlines are softer and contain rock defects making these cliffs more vulnerable to instability than those of volcanic rock.
Impacts of coastal erosion
Coastal erosion can pose a risk to residential developments, roads, lifeline utilities and coastal structures. Soft shorelines bordering the Manukau and Waitemata Harbours are particularly vulnerable. Sea walls are particularly vulnerable and have been modified by seawalls in many places in attempts to control erosion. The sandy beaches around Omaha and Orewa have developments very close to or on dune deposits meaning that soft shoreline erosion is a possibility.
Specific impacts of coastal erosion in Auckland include:
- danger to life in the case of sudden onset landslide events
- structural damage or destruction of buildings and infrastructure
- damage or destruction of lifeline infrastructure such as water, sewage and gas pipes and roads
- loss of land, resulting in coastal cliffs or shorelines retreating closer to other buildings
- land instability at neighbouring slopes and properties
- loss of beach amenity due to cliff collapse or sea wall construction.
Coastal inundation hazards
Coastal inundation, often resulting from storm surge, occurs in Auckland and can cause significant disruption to low-lying coastal areas. Coastal flooding is commonly associated with severe storm events and is often the result of a combination of factors including:
- strong onshore winds
- low barometric pressure
- high astronomical tides
- wave set-up
- wave run-up.
Impacts of coastal inundation hazards
Impacts of coastal inundation may include:
- isolated coastal communities
- damage to properties and critical infrastructure from flooding and waves
- evacuation of some coastal areas
- corrosion of electrical devices and other metal objects
- salinisation of flooded land affecting agriculture
- secondary hazards such as land instability and possible fire.