New Plymouth Coastal Erosion Assessment (Appendix 5 of Section 32: Coastal Environment) Technical Report
- Author
- Tonkin &; Taylor
- Year
- 2019
- Publisher / Organisation
- New Plymouth District Council
- Month
- July
- Pages
- 49
- Keywords
- erosion, New Plymouth, assessment
- Summary
- The Onaero shoreline is a north-facing cliff shoreline, located approximately 25 km east from New Plymouth. The shoreline has had a long history of coastal erosion with multiple hazard assessments have been completed in the past. New Plymouth District Council is currently reviewing its District Plan which needs to be prepared in accordance with the New Zealand Coastal Policy Statement 2010 (NZCPS), so areas at risk of coastal hazards need to be identified. The erosion hazard provisions in the Operative District Plan are some 30 years old and since that time understanding of processes has advanced, data availability has increased, and official guidance including the need to predict the effects of future climate change scenarios must be incorporated. As a result of a first-pass, region-wide erosion hazard assessment completed in early 2018, the Onaero shoreline was identified as a high risk area. Tonkin & Taylor Ltd were commissioned by New Plymouth District Council to undertake a detailed, second-pass coastal erosion hazard assessment for the Onaero shoreline so that it is suitable for inclusion in the Proposed District Plan. The detailed, second-pass erosion hazard lines were defined using a probabilistic approach which combines standard and well-tested methods. The approach is based on a stochastic method of combining erosion parameter distributions to allow for inherent variance and uncertainty. Results provide a range of potential erosion hazard distances for current and future timeframes (e.g. 2080 and 2130) including sea level rise scenarios. Results show that the current erosion hazard is greatest in cells B and E where the cliff are highest. Within these cells the current ASCE is up to -25 m. The current erosion hazard is lowest along the sections where protection structures exist. For the erosion hazard to remain low the structures have to be maintained and remain effective. The future ASCE are greatest within cells A and E due to the large historic erosion rates. Within cells A and E the cliff toe is likely to erode -60 to -70 m by 2130 based on the RCP8.5H+ P50% scenario. With no sea level rise (SLR) the erosion of the future cliff toe ranges from 16 to 56 m by 2130. When accounting for maximum SLR (RCP8.5H+) the erosion of the future cliff toe ranges from 29 to 99 m by 2130. There is rock revetment in several sections along the shoreline. The erosion hazard lines indicate that, without the revetment, the seaward extent of Onaero Beach Road, including multiple residential dwellings, would be at risk to future erosion without any SLR and the extent at risk increases with increasing SLR scenarios.