Te Tai o Poutini – Coastal Processes

The Project

Coastal compartments

We’re using the coastal compartment approach to underpin improved coastal management and adaptation based on the extent of coastal systems and coastal processes. This approach has been used internationally to determine how coastal processes move sediments along pathways to/from offshore sources, and from rivers to beaches, dunes and into /out of estuaries and hapua (river mouth lagoons).

With a hierarchy of coastal compartments, we can take a more holistic view of coastal hazards and risks that considers sea level rise within the context of landscape change and geological inheritance. We want to be able to:

• assess how landscapes change due to climate change and sea-level rise, such as coastal erosion
• accommodate the pre-existing, natural structure, shape, and geological make-up of a landscape. For example, areas of naturally hard rock not so prone to erosion versus areas of naturally soft sediments that may be prone to erosion

While independent of local government boundaries or iwi or hapū rohe, we expect the coastal compartment approach will encourage greater collaboration at both scientific and community levels.

The Southern Alps and Alpine Fault

These two are the proverbial double-whammy in geological terms.

It is impossible to consider this narrow strip of relatively flat land along with West Coast without awareness of the Southern Alps rising rapidly to the southeast and the Alpine Fault which runs along the base of the mountains.

The Alpine Fault last ruptured in an M (magnitude) 8 earthquake c1717AD. The probability of the next major earthquake occurring within the next 50 years is estimated at about 75% (Howarth et al. 2021).

Rivers

Several high-volume rivers drain the western Southern Alps, transporting a large proportion of the 5-10m/year of rain that falls on this region out to the Tasman Sea. Erosion rates are high, and the rivers carry heavy sediment loads. Fine sediment is largely transported into the Tasman Sea while coarse bedload is stored within the alluvial plains. Large quantities of gravel are transported during severe rainstorms, causing landslides and raising river levels.

Since 1982 NZTA and NIWA data shows that the riverbed at the SH6 bridge has risen about six metres. A massive amount of rain in late March 2019 led to the bridge being washed away in flooding.