Sand and gravel beaches are built from naturally mobile sediments, which are constantly reworked and transported by waves and currents. This makes beaches naturally changeable systems, with erosion and rebuilding of the dunes and shoreline occuring over periods of years and even decades, in response to individual weather events, seasonal conditions and climatic cycles.
Unfortunately, people love to live close to beaches and coastal areas, and sometimes very little space has been left between the sea and high value residential and commercial assets. Natural shoreline changes can threaten coastal development and infrastructure. Engineering works that are built to protect coastal development often degrade the values of the beach that are valued by local communities.
Shoreline change data is critically important in the understanding the current and future management of our beaches. In order to manage existing development and to plan new development to avoid coastal erosion hazard, we must understand the extent of this natural (and in some cases human-influenced) shoreline change. Climate change may also to affect the way our beaches behave due to rising sea level and potential changes to the frequency and intensity of climate cycles. Regular shoreline monitoring provides valuable information for future managers when evaluating the effects of climate change for managing development.
The Waikato Regional Council collects beach profile data at over 60 sites on the eastern coastline of the Coromandel Peninsula, and at Ngarunui Beach at Raglan. This monitoring programme has provided data on changes in the position of the dune at Coromandel beaches since 1979. This data is summarised in the shoreline change indicator. The Council also supports video beach monitoring at Tairua and Ngarunui beaches.
The key purpose for shoreline monitoring is to understand the way that the shoreline changes over both short and long timeframes. The goal is to determine the width of coastal land that is (and will be) vulnerable to future shoreline fluctuations and sea level rise. This information is used to guide the placement of new coastal development and infrastructure, and the management of existing private and public assets near the coast.
Shoreline change data is frequently used by District Councils, researchers and consultants. It can be used to estimate short-term erosion and to monitor and understand dune restoration outcomes. Beach profile data can provide beach volume measurements, which can be used to better understand coastal processes.
Results from this monitoring programme show that our sandy Coromandel beaches can fluctuate by up to 30 m, and even more close to the mouths of estuaries or rivers. These changes occur over periods of decades, and are linked not just to individual storms, but to larger climate cycles that influence weather patterns over periods of decades. Patterns of erosion or beach building can therefore continue for 10-20 years or more before reversing. Despite these sometimes dramatic changes, our records don't show that any of our Coromandel beaches are experiencing a long term trend for erosion. This may change in the future with projected sea level rise.
Many Coromandel beaches experienced severe erosion in the late 1970s, culminating in a storm in 1978 that caused widespread damage. During the 1980s and 1990s, many beaches built forward, creating wide dune buffers between houses and the sea. Most or all of this buffer has been eroded at many sites since the late 1990s. While there are these general patterns of change, each beach responds in a different way to any given storm event or weather conditions.