New Plymouth District Plan Review: Coastal Management (Appendix 4 to Section 32: Coastal Environment) Technical Report
- Author
- Tonkin &; Taylor
- Year
- 2016
- Publisher / Organisation
- New Plymouth District Council
- Month
- November
- Keywords
- New Plymouth, management
- Summary
- New Plymouth District Council (NPDC) have initiated a review of the New Plymouth District Plan with Strategic Coastal Management being an area of focus. To assist in this review, Tonkin & Taylor Ltd. (T+T) has been engaged to identify low lying areas on the New Plymouth District coast where the impacts of coastal storm surge and sea level rise will be evident. This assessment focusses particularly on areas where there is existing urban development. The New Plymouth coast is susceptible to storm driven coastal inundation hazard resulting from a combination of high tide, storm surge, variation of sea level and wave set-up. This hazard may increase in the future due to projected increase in sea level rise associated with global climate change. Previous assessment by Tate (2013) provided useful information on potentially vulnerable areas. However, the approach of summing individual components (known as a ‘building block approach’) is conservative, particularly when low probability components are combined. Further, some very high sea level rise scenarios (up to 2m) were adopted. The derived levels are therefore likely to over-predict potential coastal inundation extents when considering timeframes over the next 100 years and we do not recommend adopting these levels for planning purposes. However, the information presented in Tate (2013) on the effects of such flooding (land area, number and value of houses and people affected at different levels) could still be used together with updated extreme sea level information and additional information on social and environmental effects to assess resultant risk. By considering key drivers of elevated coastal water levels along the New Plymouth coastline using a less conservative, probabilistic framework, extreme static water levels have been defined for 1yr, 10yr and 100yr annual recurrent interval (ARI) events. The levels have been defined for both present day sea levels and for future sea levels at 2065 (50 years) and 2115 (100 years) timeframes based on a range of sea level rise scenarios ranging from extrapolation of past rates to various IPCC future emission scenarios. The extents of inundation have been mapped for areas with available topographic data (Oakura to Waitara, and Onaero and Urenui) for a range of elevated coastal water levels using a connected bathtub tool. Using this approach areas are flooded only where they connect to the coastal water body. This provides more realistic flooding extents by accounting for natural and human influenced (e.g. stopbanks) topography. The extents of inundation at Waitara have been assessed both including and excluding the recently constructed flood protection stopbanks. Due to the resolution of available topography (0.5m contours), inundation extents have been mapped at 0.25 m intervals and extreme values have been rounded up to the nearest 0.25m to determine the corresponding extent. Areas not covered by available topographic data should use the closest extreme static water level (see Table 3.2) together with local survey information to determine inundation levels and extents at site Based on the assessed extreme static water levels and maps showing potential inundation extents (Appendix B), the majority of the urban areas along the New Plymouth District coast are unlikely to be inundated during the present day. Two exceptions to this are around Puke Ariki Landing and northern Brougham St in New Plymouth, where flood water would flow back up the Huatoki Stream Outlet and onto adjacent land in a 10yr ARI, or greater, event. Parts of Waitara would likewise flood at events greater than 10yr ARI without the protection of stopbanks. These banks prevent sea inundation of adjacent inland urban areas. For the future time frames (2065 and 2115) widespread inundation of urban areas remains unlikely except for around the Puki Ariki Landing and northern Brougham St, parts of Motorua industrial area and large areas along the Waitara River banks if stopbanks do not remain effective. Furthermore, it is likely that some areas adjacent to streams (lower Waiwhakaiho River, lower Waiongana Stream, Huatoki Stream, Onaero River and Urenui River) and behind foredunes (Fitzroy, Waiongana and Marine Reserve at Waitara) may be inundated. Bell Block and Onaero both have limited exposure to coastal flooding under the adopted sea level rise scenarios and coastal inundation is unlikely to occur for the assessed water levels.