This indicator measures shoreline change (erosion and accretion) since 1979 at a number of sandy beaches in the Waikato region.
Sandy beaches often targetted for housing development and used intensively by the public for recreation. Sandy beaches are naturally prone to changes in shoreline position. These movements occur in response to major storms and changes in climate patterns over longer time periods.
Fluctuations in the position of the shore can create risk to houses and infrastructure that are too close to the sea. Engineered structures placed on the beach to protect at-risk assets often degrade the natural and recreational values of the beach and can put nearby property at risk. By understanding of the extent of natural beach change, councils can guide existing and new developments on these coasts.
Reducing, and where possible avoiding coastal hazard risk is a key focus of the New Zealand Coastal Policy Statement (NZCPS) and of regional and district council plans. Waikato Regional Council has a responsibility to provide information to district councils about coastal hazards and hazard areas. Beach profile information is one tool that can be used to understand which areas of coastline are at risk from erosion due to natural changes in the shoreline.
Coastal erosion and shoreline change is a natural process, and most coastal erosion seen on Waikato beaches will naturally rebuild over time. Coastal erosion is not an environmental problem in itself, but it becomes a management problem when development is placed too close to the beach and is threatened by these natural shoreline changes.
The location of the mean high water spring mark (MHWS) changes quickly and dramatically to changing water level and wave conditions. Processes of erosion and rebuilding occur quickly in this part of the beach. The dune is only exposed to wave action during storms or after a long period of erosion, so is more stable and slower to erode and recover. This indicator reports changes in the location of the dune, which provides the best illustration of the type beach changes that can threaten coastal development.
Changes measured in the position of the dune toe show that Coromandel east coast beaches have undergone periods of erosion and beach building since 1979. Despite these changes, there is little evidence of long term erosion or accretionary trends for the beaches presented here. Much of this erosion occurred between 1995 and 2000, and this erosion has not recovered.
Individual storms can cause the dune to erode by several metres over periods of hours or days. This erosion generally recovers naturally in the following months. A trend for erosion or beach building can also occur over several decades, usually in response to climatic cycles such as ENSO and IPO (learn more here (external link) ). These changes are cyclic and also are naturally reversed. While widespread long term (permanent) shoreline change has not yet been recorded on Waikato beaches, but could occur in response to accelerated sea level rise and/or climate change.
During the early 1990s, there was a period during which most beaches were accreting, or growing toward the sea. After about 1996, and in particular during the last few years of the 1990s, many of the beaches eroded, some quite severely. Many beaches reached their most eroded state between 2000 and 2003. Most beaches then recovered gradually until further severe erosion occured at some locations in 2008. Some beaches have since recovered, while others still have not completely rebuilt since the erosion in the late 1990s.
The results of this study show that beaches don’t all respond in the same way to any one storm. In particular, the northern beaches (north of the Kuaotunu peninsula) respond differently to those on the rest of the Coromandel coastline. The severity of erosion depends on the location and orientation of the beach in relation to the characteristics of the storm or storms.
It is recommended that the data should be repeated for Coromandel beaches every three years
Coastal Earth Scientist - Science and Strategy Directorate