Investigation of a large offshore sandbar at Raglan, New Zealand: Impacts on surfing amenity

摘要

Raglan is a small coastal town situated on the west coast of the North Island, New Zealand with "world class" surfing breaks. These generate significant income for the community through tourism (local and international) and surfing related industries, e.g. surf shops, surfing school. Raglan has a large-scale headland with a rock armoured shoreline that provides consistent "peeling" surfing waves. In 2004 surfers noticed that the waves at "Indicators," a surfing wave on the headland, were not breaking during certain tides. This was a very uncommon occurrence and created news throughout the surfing community, with articles appearing in New Zealand surfing magazines. An investigation was therefore instigated to study the reasons for the variation in shoaling of the waves, with the most likely scenario being a large sandbar that had appeared offshore at the headland. Data was recorded using a series of quarterly bathymetric surveys from "Outsides" (farthest out break at the headland) to Whale Bay. Aerial and oblique photographs were also undertaken to visually confirm the location of the breaking waves at the headland. The study was aimed at measuring and tracking the movement of this large sand deposit at the headland. The results have provided valuable scientific insight into the temporal and spatial variation of the sand "slug" in this environment, which appears not previously to be well documented in the scientific literature. Morphological comparison of the sequential surveys and numerical modelling was undertaken to assess the migration of the sand bar and the impacts on the surfing breaks, respectively. The results of the bathymetric surveys confirm that a large sandbar was located offshore of Indicators in 2004-2005, which subsequently moved east and dispersed through 2006. The numerical modelling supports the observed impacts on wave patterns at the Raglan surfing breaks and the changes that occurred as the pulse of sand moved east and dispersed.