Methods - how we monitor
We estimate the position of the dune toe from beach profile data, which is collected at 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, using measuring tapes, surveying poles and/or GPS equipment as described in the measurement technique section below.
Beach profile data reported in this indicator is from the following sites:
|Location||Number of sites|
|Tairua (Ocean Beach)||2|
|Wharekaho (Simpson's Beach)||2|
|Whitianga (Buffalo Beach)||5|
Data history and monitoring frequency
In the past, data collection frequency has been irregular. Monitoring at most 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 at some sites 3-monthly, and 6-monthly or annually at other sites. Additional surveys are occasionally undertaken following major storms or periods of severe erosion.
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 1.8m 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 some beach profiles using differential GPS.
How the raw data is stored
The location of each survey start point or benchmark has been located using differential GPS and is stored on Waikato Regional Council's geographical information system (GIS) database.
Raw data are currently stored in a specialised database and data analysis application (“BPAT”). Data can also be exported in text format for use in other packages such as Excel.
Beach profile data is analysed using BPAT, which provides data on horizontal beach movement at any given elevation.
BPAT is able to extract the horizontal location of any chosen elevation in the beach profile data. Due to variations in the shape of the different beaches, an elevation to represent the toe of dune was selected for each beach, 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 one of the earliest surveys.
The measurement uncertainty is in the order of +/- 0.01m vertically and +/- 0.05m horizontally.
Each beach profile only represents the shoreline position at that particular location on a beach at a single moment in time. In reality, 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 somewhate subjective as actual elevation of the dune toe at any site alters due to changes in beach shape following erosion and accretion.
Climatic cycles can drive shoreline fluctuations over even longer periods of time than the 34 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.