Shoreline change

The measurement uncertainty varies depending on the method used. The error associated with the current surveying method (differential GPS) is in the order of +/- 0.01 m vertically and +/- 0.05m horizontally.

Each beach profile only represents the shoreline position at that specific location on a beach at a single moment in time. Beach shape and changes can vary considerably along the beach, and over time. At most beaches, results from several beach profile sites spread along the beach have been analysed to provide an overall picture of shoreline change, but it is still not possible to accurately record all alongshore variations with beach profile data.

The method of estimating dune toe location is somewhat subjective as actual elevation of the dune toe at any site alters over time due to changes in beach shape following erosion and accretion.

Climatic cycles can drive shoreline fluctuations over even longer periods of time than the 40-year beach profile record. To understand the full range of shoreline change, beach profile data must be considered against longer term less frequent data sources such as aerial photographs and cadastral surveys.

Beach profile data is reported here from 46 sites over 17 beaches on the eastern coast of the Coromandel Peninsula. Beach profiles measure the shape of the beach from the dune to the waterline, or to wading depth. Profiles begin at fixed points in the dune, with several points spread out along the beach to measure and pick up changes in beach shape, we collect the data from the same fixed starting points each time. The position of the dune toe is extracted from the beach profile data for this indicator.

Monitoring sites

Beach profile data reported in this indicator is from the following beaches:

• Whiritoa
• Whangamata
• Onemana
• Opoutere
• Pauanui
• Tairua
• Hot Water Beach
• Hahei
• Cooks Beach
• Maramaratotara
• Buffalo Beach (Whitianga)
• Wharekaho (Simpsons) Beach
• Opito Bay
• Kuaotunu (East and West)
• Rings Beach
• Matarangi
• Whangapoua

Data history and monitoring frequency

Data collection has been irregular in the past. Monitoring at some sites began in 1979 and was repeated in 1981. During the 1980s and early 1990s, beach profile data was collected only occasionally. Since 2002, data has been collected 3-monthly at some sites, and 6-monthly or annually at other sites. Additional surveys are occasionally undertaken following major storms or periods of severe erosion. In recent years (2018 onwards), monitoring has ceased at some sites.

Measurement technique

The early profiles completed by the University of Waikato (and Hauraki Catchment Board) were surveyed using standard level and staff techniques. The accuracy was estimated at +/-0.01m in the vertical with distances by Stadia or tape +/-0.5m. These techniques have been outlined by Dell (1981).
Until the end of 1995, profiles continued to be surveyed by Waikato Regional Council using standard survey methods and instruments.
Since 1996, Waikato Regional Council has adopted the Emery Pole method of profiling as the standard method of field measurement. This technique involves measuring the change in height between two 1.8 m graduated poles by sighting to a fixed point (usually the horizon) over short distances (3 to 5 metres) measured by a tape measure. More recently, Waikato Regional Council has surveyed beach profiles using differential GPS.

How the raw data is stored

The location of each survey start-point or benchmark is recorded using differential GPS and is stored on Waikato Regional Council's geographical information system (GIS) database.
Raw data is currently stored in a specialised beach monitoring database and data analysis application (“BPAT”), data can also be exported in text format.

Beach profile data is analysed using BPAT, which provides data on horizontal beach movement at any given elevation. Due to variations in the shape of the different beaches, an elevation to represent the toe of dune was selected for each beach profile site, after analysis of field books and profile shapes from all dates. The location of the dune toe was then plotted over time as a change relative to the earliest survey when data was collected at all profiles.

The graphs below show horizontal shoreline changes at Coromandel east coast beaches since beach profile records began. The beach profile record shows that the position of the dune usually fluctuates by 10-15 m and occasionally up to 25 m. Shoreline fluctuations can be even larger near stream or estuary entrances, where the toe of dune can move by up to 100 m. In some cases, hard materials underneath the sand limit the extent of erosion that can occur. Site specific influences on shoreline change are summarised in the figure captions below.

The dune at Whiritoa has fluctuated by just over 10 m since 1979, with no overall trend for erosion or accretion. Sometimes one end of the beach erodes while the other end accretes. This is common at pocket beaches on the Coromandel Peninsula, and is sometimes called “beach rotation”, as it changes the orientation of the beach slightly. This suggests sand is being moved along the beach from one end to the other due to changing conditions. Other times erosion and accretion occurs consistently along the whole beach.

This graph shows data from Whangamata Ocean beach (CCS56 - Surf Club) and Otahu Beach (CCS57 - Island View and CCS58 - South). The data shows shoreline fluctuations of 10-15 m. Looking at the beach profile data, there appears to be a possible trend for accretion. However, old aerial photograph records show that this build-up is part of a longer-term cycle that lasts many decades.

This graph shows changes in the position of the dune at Onemana Beach since 1981. The data shows that the shoreline has fluctuated by 10-15 m since 1996. The last data collection at Onemana Beach was in March 2018. Due to low erosion risk this beach is no longer surveyed by the Waikato Regional Council on a regular basis but will be periodically surveyed to check the shoreline position and ensure that the erosion risk remains low.

At Opoutere, measurements are taken at four beach profile sites, from CCS41 (north) to CCS44 (south). The data shows there can be erosion of up to 10 m during a storm event, but this recovers over time. The shoreline at the north end of the beach has accreted significantly since records began. Other parts of the beach appear to be simply fluctuating. Due to low erosion risk this beach is no longer surveyed by the Waikato Regional Council on a regular basis but will be periodically surveyed to check the shoreline position and ensure that the erosion risk remains low.

The dune toe at Pauanui has fluctuated by up to 20 m since 1995. Profile site CCS38/1 is at the north end of the beach closest to the estuary entrance, and experiences greater shoreline fluctuations than elsewhere on the beach. The shoreline in this area has built seaward by nearly 25 m in the last 20 years. At the southern end of the beach (CCS39/2) there has been slow erosion over the period of this record, thought this may simply be part of a longer-term cycle.

The shoreline has fluctuated by approximately 15 m at the southern end of Tairua Beach (CCS36), but only 7 m at the northern end (CCS37). Tairua Ocean Beach sometimes undergoes “beach rotation”, when one end of the beach erodes while the other end accretes. This occurs when sand is moved along the beach in response to changing conditions. The overall average beach change is relatively small.

At Hahei, the dune toe has fluctuated by 10-15 m since 1979. There is no clear long-term trend for shoreline change. In some areas at Hahei, rock underneath the dunes limits the extent of coastal erosion.

At Cooks Beach, shoreline fluctuations of over 30 m have occurred at sites towards the eastern end of the beach. At these sites (CCS31 and CCS 31/2) there has been a trend for erosion since the mid-1990s. However, if we look further back to the late 1970s, we know that the shoreline was in a relatively eroded state then, in a very similar location to the present day. The erosion we are seeing at Cooks Beach may therefore be part of a natural cycle that occurs over several decades. This is also the area of the beach that is closest to the Purangi estuary entrance and is therefore naturally dynamic. Results are further complicated by recent beach scraping works in this area.

There have only been relatively small changes (<15 m) at the northern and central areas of the beach. Shoreline change in some areas near the south-eastern end of the beach (CCS31/1) have been largely prevented by shoreline armouring structures.

At Maramaratotara Beach, the dune toe has fluctuated by approximately 10 m over the last 30 years. Coastal hazard studies have identified hard, erosion resistant materials underneath the sandy beach, which will probably prevent erosion from occurring far beyond the current position. There has been almost no change in the location of the shoreline since 2008.

The shoreline at Ohuka Beach (CCS24) has been influenced by human activities, including stormwater drainage and more recently by the construction of a geotextile seawall. At the north end of Buffalo Beach (CCS25), severe erosion occurred in the winter of 2000, partly due to end effects from the adjacent seawall. The shoreline in this area was then artificially rebuilt and seawalls have prevented any significant erosion since. The shoreline has been able to fluctuate more naturally further south at CCS25/1, though this area has experienced erosion partly caused by effects of the seawall. This site is located north of Taputapuatea Stream (“Mother Brown’s creek”) and has experienced over 25 m of erosion since the late 1990s. Historic aerial photographs taken in the 1940s show this shoreline has fluctuated significantly over periods of many decades. While the shoreline at Buffalo Beach has not recovered since the erosion in the late 1990s and in 2000, the beach has been relatively stable at most sites in the last 20 years, with fluctuations of no more than 10 m. There is no beach profile monitoring site located at the southern end of the beach close to the Harbour entrance, but other records show us that there has been significant accretion over the last 40 years.

Central and northern areas of Wharekaho Beach have been very stable since 1981, with shoreline fluctuations of only 4 m over this time. At the more exposed southern end of the beach, the dunes have fluctuated by up to 10 m since 1981, but there is no clear trend for long term change.

At Opito Bay, hard materials are present underneath the sand in central areas of the beach, preventing significant change in shoreline position. The beach profile sites graphed here are the north-western and south-eastern ends of the Bay, where there are sand dunes. In most areas, the dune toe at Opito Bay has fluctuated by less than 10 m (usually less than 5 m). Major accretion occurred between 1980 and 2000 at CCS49 (Skippers Road) due to movement of the stream position on the beach.

At Kuaotunu East, shoreline change of up to 25 m has occurred in the last 20 years. Storms in 2006 and 2008 caused significant erosion, which is now gradually recovering; although sections of the beach are still eroded compared to when monitoring began. Profile CCS21 is at the eastern end of the beach, and has experienced significant erosion since records began, most likely linked to changes in the stream mouth. Considerable recovery has occurred since 2010 and this appears to be continuing.

The shoreline at Kuaotunu West has fluctuated by just 5-7 m over the last 38 years. The overall pattern indicates possible ongoing erosion. Like Kuaotunu East, this beach was impacted quite severely by the 2006 and 2008 erosion events. Due to low erosion risk this beach is no longer surveyed by the Waikato Regional Council on a regular basis but will be periodically surveyed to check the shoreline position and ensure that the erosion risk remains low.

Rings Beach is very stable, and the dune toe has fluctuated by only about 2 m over the 40-year beach profile record. Due to low erosion risk this beach is no longer surveyed by the Waikato Regional Council on a regular basis but will be periodically surveyed to check the shoreline position and ensure that the erosion risk remains low.

There have been two periods of severe erosion at Matarangi Beach, in 1997 and in 2008. Accretion has occurred between these times. Overall, the dune toe has fluctuated by up to 30 m over the data record.

The dune toe at Whangapoua has fluctuated by just over 10 m over the 41 yr beach profile record. The beach was most severely eroded in the late 1990s and again in 2008 (central and southern parts of the beach). Steady beach recovery occurs in between these erosion events. Further erosion occurred at all sites in 2014. These records are somewhat complicated in recent years by the beach scraping and dune works undertaken, particularly in central and southern sections of the beach.