Southshore residential red zone: Summary of Technical Information as at September Area ID: 1. Purpose of this document

Transcription

1 Southshore residential red zone: Summary of Technical Information as at September 2016 Area ID: 1 Purpose of this document The following information is a summary of the local environmental values, conditions and potential hazards. The information has been gathered using the best available technical data held by a number of agencies and organisations. It does not identify or assess land use options or the feasibility of land uses. The information in this document does not, and should not, be interpreted as pre-determining, recommending or making any decisions about future uses. No decisions about the future use of this land have been made. Overview of Southshore residential red zone (Area 1) Area 1 is located along the western margin of Southshore and faces the Ihutai/Avon-Heathcote Estuary. It is approximately 15 hectares in area and is typically flat and low lying. The western margin of the area is bounded by the eastern shore of the Ihutai/Avon-Heathcote Estuary while the residential properties adjoin the area to the east. Area 1 is highly susceptible to a number of natural hazards, which can potentially occur either in isolation or collectively leading to a number of adverse, cascading effects. The risk from many of these hazards will increase over time due to the accelerating effects of sea level rise. Location Map 1

2 Natural Environment and Cultural Setting and Features Natural Environment Waterways Groundwater depth below surface m = metres Coastal Marine Area Public Facilities Reserves Mana Whenua / Ngāi Tahu Interests Area 1 adjoins the Ihutai/Avon Heathcote Estuary and Environs Significant Ecological Site identified as Sites of Ecological Significance (Site ID no. SES/LP/14) in Chapter 9 (Natural and Cultural Heritage) Appendix of the notified version of the proposed Christchurch Replacement District Plan. This site is significant because the estuary 'is an originally rare ecosystem that contains indigenous vegetation communities that have been greatly reduced within the Low Plains Ecological District, and is also of local, national and international importance in terms of it supporting a representative assemblage of indigenous and migratory birdlife, including 23 threatened species'. The area is also adjacent to the Christchurch Coastal Strip (Site ID no. SES/LP/6), although there is green zone land between the sub-area and this SES. This site is significant because it 'supports both remnant and planted indigenous plant communities that are representative of the Low Plains Ecological District, and supports several species of flora and fauna that are either nationally threatened, at risk or uncommon'. The site is adjacent to the Ihutai/Avon Heathcote Estuary. Median groundwater depth is typically between 0.9m and 1.7m below surface level at present. Median groundwater depth with 1m sea level rise is shallower than 0.5m across much of the area, and shallower than 1m across the remainder. Immediately adjoining the proposed Coastal Marine Area to the west and south, except where there is legal, unformed road between the residential red zone and the coast. No public facilities mapped in Area 1. There is a playground at the eastern end of Plover Street. Whilst there are no reserves within Area 1, it has a close relationship to the reserve land which follows the coastline of the Spit. This includes the Spit Reserve (which is a Scenic Reserve under the Reserves Act) Southshore Beach Park (a regional park), South New Brighton Park and the Ihutai/Avon Heathcote Estuary Regional Park. This area is recognised to have strong cultural significance with the connection to Ihutai and the former Māori Reserve. The whole of Area 1 falls within a culturally significant zone specifically covering Southshore as well as being part of a wider Ngāi Tūāhuriri takiwā area of interest.it is also part of a Statutory Acknowledgement Area - a statutory recognition defined under the Ngāi Tahu Claims Settlement Act 1998 and applied to areas of cultural, spiritual, historical and traditional association. Mana Whenua and Ngāi Tahu strongly recognise the relationship of this area with Te Ao Tūroa (natural environment), ecological values and the water. This area also has significant Mahinga Kai values. 2

3 Archaeology and Heritage Special or Other Features Chapter 9 (Natural and Cultural Heritage) of the notified version of the Proposed Christchurch Replacement District Plan: No listed features. NZ Archaeological Association: There are six listed archaeological sites within the boundary of Area 1, with several more listed sites and one known archaeological area in the immediate vicinity. The sites all relate to Māori occupation and more specifically are described as containing archaeological remnants of horticulture and middens. The area is known to be of archaeological significance and accordingly, there is potential for items or sites of archaeological value to be discovered. A legal, unformed road is present in some places along the estuary edge adjacent to the red zone properties. Hazards Coastal Hazards Coastal inundation Coastal Erosion Tsunami SLR = Sea Level Rise assumed to be 0.4m over the next 50 years and 1m over the next 100 years. All of Area 1 will be subject to coastal inundation in a one in 50-year storm taking into account sea level rise within the next 50 years. Area 1A is slightly higher in this location (this is assumed to be fill) and will be less affected by coastal inundation. Areas 1B and 1C will be susceptible to daily tidal influence, due to sea level rise, within the next 50 years. It is noted that this is an area of active wave erosion and wave set-up (compared with areas further up-stream where the erosion may be more related to the changing water level in the river channel and salinity as a result of the tidal influence). Private property owners along the western side of the residential red zone have in the past filled the land and constructed retaining walls along the estuary edge of varying permanence. The existing average ground elevation above perigean high tide level is 0.9m in Area 1A, 0.6m in Area 1B and 0.4m in Area 1C. Most of the land is within ±0.2m of these average levels. This area has been identified as being within an evacuation sector, given its risk from future possible tsunamis (most likely being one generated off the coast of South America or Alaska). The area has also been mapped as being in a tsunami inundation zone. ECan has tsunami factual and emergency evacuation information located at: 3

4 Flooding Geotechnical Earthquake induced Liquefaction, lateral spreading Subsidence (consolidation) Contamination LLUR Listed Land Use Register Flooding has historically occurred in this locality due to the very low land levels and ineffective drainage after significant rain events. Under the proposed Christchurch Replacement District Plan, all Area 1 falls within the Flood Management Area (200 year flood event). Areas 1B and 1C fall within the High Flood Hazard Management Area (more than 1m depth in 500 year flood). Depth of flooding for 200 year flood event with 1m SLR may be over 1m across much of the area. For further information on flood hazards refer to the: Flooding and District Plan pages on the Regenerate Christchurch webpage. The entire area was subject to considerable geotechnical issues associated with the Canterbury Earthquake Sequence. Major lateral spreading has occurred throughout the area due to the thin and weak surface crust. As such there is a large potential for lateral spreading in any future earthquake events. The propensity for liquefaction induced lateral spreading will increase as the thin crust decreases in depth due to an expected rise in groundwater (due to sea level rise). Areas underlain by fine cohesive sediments, especially where there is a high organic content, will be subject to subsidence. For further information on geotechnical issues in this area refer to the following geotechnical summary: Area 1A = Minor ground contamination constraints exist. Rocking Horse Road Landfill noted on the LLUR however the actual landfill area is smaller than mapped. Landfill contents are known to be relatively inert from post-earthquake investigations in this locality. Area 1A is noted on Christchurch City Council shallow fill records. Review of borelogs within the vicinity shows evidence of waste in the fill on some adjacent properties (Mermaid Place and south end of Rocking Horse Road). There are no bore logs available for this specific area. Areas 1B and 1C = No known pre-earthquake ground contamination concerns. Age of dwellings means that soil contamination from asbestos is possible. Infrastructure Wastewater Water supply Stormwater management A wastewater main travels along the boundary of Area 1 on Estuary Road, with a further short wastewater main at the Pumping Station on Rocking Horse Road (approximately at the mid-point of the Area 1B). This is to be replaced by a pressure sewer system. The water supply main similarly, follows the alignment of Estuary Road, north of Ebbtide Street. There is a stormwater main on Estuary Road, with several stormwater mains crossing the area to discharge into the Estuary between Caspian and Plover Streets, and at the very southern end from Pūkeko Place. Stormwater discharge points are not effectively draining after significant rain events, causing ponding elsewhere. 4

5 Transport This Area is accessed from either Estuary Road or Marine Parade and then from Rockinghorse Road. Estuary Road, at the northern end of Area 1, is classified as a Collector Road under the new Christchurch District Plan. 5