Methods - how we monitor
Information for this indicator is collected at 26 settlements on the Coromandel Peninsula:
|East Coast||West Coast|
|Cooks Beach||Otautu Bay|
|Kuaotunu East Beach||Te Puru|
|Kuaotunu West Beach||Waikawau|
|Tairua Ocean Beach|
|Whitianga – Buffalo|
|Whitianga – Ohuka|
The data for dwellings are collected from 2003, 2007 and 2012 aerial photographs. These data will be updated approximately every 5 years, as new aerial photographs are taken. Data for properties is collated from the Thames-Coromandel District Council properties database.
The dwellings data have been taken from aerial photographs taken in 2003, 2007 and 2012. Archived property data was collated for the same years.
Waikato Regional Council's GIS was used to overlay properties with the coastal development setbacks. Locations of dwellings were also marked individually from aerial photographs to allow analysis of the number of dwellings within these hazard zones.
- Development setback recommendations were overlapped on top of the Core Record System (CRS) in the GIS.
- The number of properties that intercept the two coastal development setback lines (CCEL and FCPL) were identified.
- The portion of each property overlapped by the CCEL and FCPL was also established. From this, the significance of hazard to the property was defined as minor (<10%), moderate (10 - 30%) or major (>30%).
- Where the proportion of a property within a setback zone was minor (<10% of total property area), this effect in practical terms (the way that the hazard will affect the use of the property) was considered to be relatively insignificant. These properties were therefore removed from graphed data for the purposes of this indicator.
- Using registered aerial photographs (2003, 2007 and 2012), the most seaward point of each beachfront dwelling was marked. The overlap of these marks with the CCEL and FCEL were then recorded to give figures on the extent of risk to dwellings.
- Results are obtained for individual location (28 coastal settlements) and summarised for eastern and western Coromandel Peninsula. It is important to note here that there is currently no FCPL defined for the western coast of the Coromandel. Therefore reported figures for this coast relate only to the CCEL. Properties at risk under current conditions are included in the total values for the long term risk area (FCPL).
This indicator utilised existing Coastal Development Setback Recommendations and Review of Primary Development Setback at Selected Beaches. The current coastal erosion line (CCEL) is a zone generally between 10 and 45 metres wide. It represents the width of coastal margin at risk from coastal erosion associated with current processes of natural shoreline change. The future coastal protection line (FCPL) is from Focus (2012) and is a zone between 30 and 75 metres wide that represents the coastal margin likely to be affected by coastal erosion as a result of projected sea level rise over the next 100 years.
Properties that are only just ‘touched’ by the setback lines will be tagged as ‘affected’, which may not be true in practical terms. This has been accounted for by measuring and ranking the extent of the overlap of the property area with the setback zones. Where the overlap is less than 10 per cent, the affect is recorded as minor on that property and these properties were removed from the graphed reporting. This data is retained in the Excel indicator data provided here. This indicator therefore reports the number of properties where at least 10% of the total property area is within the hazard zone.
Not all dwellings within the coastal development setbacks are at equal risk. The level of risk depends on the exact location of the dwelling. For example, dwellings near the seaward boundary of the setback zones will be at a very low risk from erosion, while others further seaward could be at high risk.
There are small safety factors included in the coastal development setbacks, which reflect uncertainty and data limitations, and provide for a small buffer of land between a dwelling and the sea following the worst expected erosion. Therefore some dwellings that lie only just inside the hazard area may be actually at very low risk of erosion, but could be at risk from instability or unforeseen extreme events.
There is no setback to provide for the effect of sea level rise on beaches on the western coast of the Coromandel Peninsula. Therefore the figures given here for long term risk underestimate the true risk to property on this coast. While erosion risk is likely to increase with sea level rise, the most significant increase in risk to coastal development on the western Coromandel Peninsula would be from coastal flooding.
This indicator will be updated as new aerial photography becomes available, and will use the most up to date version of the Properties Database at the time of analysis (or as relates to the date of the photography.
Quality control procedures
The dataset was checked for accuracy.
Positional data loaded into GIS was checked for errors in map reference records.