Estimating patterns and rates of coastal cliff retreat around Tauranga Harbour
Garae, C. P. (2015). Estimating patterns and rates of coastal cliff retreat around Tauranga Harbour (Thesis, Master of Science (Research) (MSc(Research))). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/9501
Permanent Research Commons link: https://hdl.handle.net/10289/9501
Cliffs are common around the shores of Tauranga Harbour, including in the urban areas. The existence of cliffs indicates a potential hazard to people living close to the cliff edge, as they imply both a tendency for the upper surfaces to erode, and for the debris to fall onto lower areas. To identify the relative risk around the harbour shoreline, it is useful to determine the rates and patterns of shoreline erosion. This study initially determined the shoreline changes between 1943, 1982 and 2011 using a combination of aerial photograph analysis using Digital Shoreline Analysis System (DSAS), airborne LiDAR data and Laser scan data. Long term rate of retreat was determined for the study area with the use of digitised aerial photographs and DSAS applications. Cliff locations are often difficult to determine from aerial photographs due to shadows and vegetation obscuring the ground surface. Hence, LiDAR data were used to initially identify the cliff edge positions in 2011. These results were compared to the 2011 aerial photograph to assess how easily the cliff edge can be identified with the photographs. The inferred cliff edge from the 2011 aerial photograph was digitised in GIS, and the corresponding cliff edge for 1943 was also determined and digitised using stereopairs to aid cliff edge identification. The 1982 aerial photograph was scanned, georeferenced and shoreline digitised. Using the DSAS software, the rates were computed. However, this was also compared to other methods using the intersection points generated from DSAS. Laser scan surveys were conducted to determine surface volume changes to acquire short term rate of retreat at the cliff face. This analysis involved surveys carried out within a period from May 2014, July 2014 – November 2014. The data collected was compared to earlier laser scan data from September 2012 – July 2013 and LiDAR data from September 2011. Several approaches were used to determine the best estimate of rate of retreat from the DSAS analyses. It was identified that the buffer end-point rate produced the best rate cliff retreat in the study area. The range of rate of retreat using this method was -0.2 ± 0.16 to 0.07 ± 0.16 m.y-1. It was also identified that the Matua Subgroup had the highest rate of erosion within the study area located at South west Matakana Island. The rate was lowest at East Pahoia Peninsula. Although laser scans were conducted only over a short period of time, comparing the dataset to earlier laser scan data a range of rates were obtained -0.01 to 0.02 m.y-1. Although rates were determined for both methods the errors were large and need to be considered.
University of Waikato
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