Coastal

Coastal erosion

Parts of our coast are changing at a rapid rate – through coastal sediments being either eroded (material lost) or accreted (material gained). This is driven by a mix of processes such as varying sediment supply, rising sea levels, the predominant wave climate and human coastal modification (e.g., sea walls and breakwaters). Predicting rates of coastal change is complex but we can develop models that represent coastal processes, including looking back at past changes from old aerial photographs to help determine the most likely future shoreline position.

Coastal flooding (coastal inundation)

Flooding in coastal areas can result from several factors, often acting together:

• Land subsidence – compaction of sediment over time and tectonic subsidence which occurs as part of natural earthquake cycles are examples of subsidence causing relative sea level rise on the West Coast.
• High tides – Spring tides and king tides can flood low-lying land
• Storm surges – Caused by low pressure systems and strong onshore winds
• Sea level rise – Driven by climate change through ice melt and thermal expansion; increases the frequency and severity of flood damage from storm surges and high tide levels.

For an overview of how current storm surge and future sea level rise may impact coastal communities check out NIWA’s coastal flooding interactive map here.

More information on the changing coastline of Aotearoa including an interactive web-map is available here.

Coastal risk on the West Coast

Mapping areas at risk of coastal inundation or erosion involves determining what parts of the land are at risk of being flooded in different-sized storms, and what rate coastal erosion is occurring in at-risk communities. Following national coastal hazard guidance (more information here) we use the following methodology:

• Look back at past changes from old aerial photographs, carefully compare shorelines and develop models to determine the most likely future shoreline position
• Calculate the height of a 1% annual exceedance probability (AEP) storm surge as measured at sea, without including run-up of waves on the beach, which can be significant. We are currently in the process of gathering coastal data which will allow this to be modeled in the future. 1% AEP means an event with a 1% chance of happening in any given year (and is also referred to as a 1 in 100-year event).
• Determine areas most at risk of inundation based on elevation from a digital model built using LiDAR mapping of the coast (more on West Coast LiDAR). Repeat surveys help us to identify areas at risk and monitor changes regularly.
• For future planning purposes, the models have been rerun using scenarios of higher sea-levels to map all areas likely to be at risk in coming decades (see TTPP coastal hazard overlays to see if your property may be at risk)