Coastal Erosion

Coastal erosion hazards on the shoreline are generated by either wind or wave action. In each case, there occurs a net loss or transport of coastal sediments from the immediate area, resulting in erosion or retreat of the beach profile.

Wind Erosion

Wind erosion occurs generally on beaches and dune systems exposed to persistent or strong winds, where unconsolidated or sandy soils are dry and lack sufficient vegetation cover to stabilise the land. In the Tasman District, wind erosion is experienced on isolated active dune systems along parts of the west coast north of Kahurangi Point and is most prevalent on Farewell Spit. Persistent westerly and southwest winds blow sand off the beach during low tide, which then forms or adds to a dune that is itself blown inland, often smothering established vegetation.

Wind erosion also occurs at a lesser scale on smaller, more sparsely vegetated parts of sandspit systems that are dry at high tide. Sandspits at Onahau, Parapara, Pohara and Wainui Bay in Golden Bay and Motueka sandspit in Tasman Bay exhibit wind erosion effects from time to time.

Wind erosion also occurs on sandy beaches exposed to strong onshore winds. Kaiteriteri beach, for example, experiences onshore winds that periodically blows beach sand inland onto the adjacent road.

Wave Erosion

Wave action on shorelines comprised of unconsolidated or poorly cohesive materials can result in reworking of the interface between land and sea. Stability of natural shorelines in both the short and long term is a function of shoreline vegetation type and resilience, the energy of the prevailing wave climate, the frequency and persistence of larger wave events and the sediment balance within a particular area. Coastal erosion is principally caused by and results from wave action on shorelines where sediment volumes transported along or onto that shoreline is less than the sediment volumes removed from or taken off that shoreline by wave action.

Wave erosion can occur over various time scales, from short term or event to annual, decadal and multi decadal periods. It is possible that a storm event may cause significant erosion to occur, but recovery then occurs after that event has passed that restores the shoreline profile to the pre-event condition. This is uncommon in the district but can be observed on beaches in the Abel Tasman National Park, which are relatively enclosed systems. However, monitoring of the Golden Bay and Tasman Bay coastline through visual observations, periodic survey of coastal cross sections and comparison of series of aerial photographs shows that more than 70% of the shoreline in our district is subject to some degree of longer term, persistent erosion. There are also areas of longer term significant persistent erosion, including west of Rangihaeata Head in Golden Bay and along the Ruby Bay - Mapua shoreline. In these areas, longer term erosion rates have exceeded 1m per year.