STORMWATER FLOOD REPORT

Transcription

1 OREWA COASTAL RETIREMENT RESORT 25 ANNALISE PLACE OREWA for COASTAL PROPERTIES LTD STORMWATER FLOOD REPORT Job No March 2016

2 Orewa Coastal Retirement Resort Page 1 of 9 CONTENTS Item Description Page 1.0 Introduction Site Location Stormwater Catchment Area Flooding Overland Flow Path Coastal Inundation Recommendations 8

3 Orewa Coastal Retirement Resort Page 2 of INTRODUCTION This report has been prepared as part of a Land Use Consent for the proposal to construct a new multi-level retirement resort on the site located at 25 Annalise Place, Orewa. The proposed development will consist of a six level building that will comprise 122 retirement units, along with other associated amenities. Since the subject site is identified on the Auckland Council s GIS Flood Hazard Mapping as being located just within the 100-year flood zone, a flood report is required to assess the potential for flooding on the site and to provide a minimum finished floor level for habitable buildings that will be constructed on the site. In addition, the site is also located within a Coastal Inundation Zone, identified within the Auckland Council Unitary Plan. This report compares the required freeboard levels for the 100-year flood and the potential sea-level rise, and freeboard requirements outlined in the Auckland Council Unitary Plan. Also the Auckland Council Unitary Plan requires that any development within flood-sensitive or flood-prone areas has no adverse effects on the surrounding properties or residents. 2.0 SITE LOCATION The site is located within the suburb of Orewa, North Auckland, set back from the Orewa Beach, as shown in Figure 1 below. Auckland Council GIS Flood Hazard Mapping has identified that the site is located just within the 100-year flood plain. In addition, the site has been classified within a 1m seal-level rise zone for coastal inundation under the Auckland Council Unitary Plan. It is proposed to construct a six level retirement building on the site located at 25 Annalise Place, Orewa. The property consists of approximately 1.15ha of mainly flay land, located adjacent to the Nukumea Stream.

4 Orewa Coastal Retirement Resort Page 3 of 9 25 Annalise Place, Orewa Figure 1 Location Plan 3.0 STORMWATER CATCHMENT AREA The site is located in the lower northern area of Orewa Beach and is bound to along its northern boundary by the Nukumea Stream. The Nukumea Stream flows to the east and discharges to the sea at the northern end of Orewa Beach. Current zoning of the catchment shows that the area has a mix of medium to high intensity residential properties. The general topography is very flat, with a slight fall to the north towards the Nukumea Stream. Auckland Council s GIS has identified two Overland Flow Paths on the site. Refer to Figure 2 for the location of the Overland Flow Paths identified in Auckland Council s GIS.

5 Orewa Coastal Retirement Resort Page 4 of 9 Figure 2 Overland Flow Paths 4.0 FLOODING The site is near level with only a 1m (approximate) level difference across the main part of the property. The site is located on the boundary of the 100-year ARI flood plain generated on the Auckland Council GIS viewer, which follows the path of the Nukumea Stream. A Stormwater Catchment Management Study and Management Plan for the Nukumea Stream Catchment was completed by Babbage Consultants Ltd dated June In this report, the subject site falls within the Puriri Park Sub-catchment area. For the Puriri Catchment the report has calculated a flood level (ponding level) of RL 3.73m for the 100 year storm event and recommends a minimum habitable floor level of RL 4.30m. Based on these levels, the area of the site that floods along the Nukumea Stream boundary is shown below:

6 Orewa Coastal Retirement Resort Page 5 of 9 Figure Year Flood Area As seen in Figure 3 above, the proposed new retirement building will encroach into the existing 100 year flood area in the north-western corner of the proposed new retirement Building. The encroachment will reduce the total cross-section area of the Nukumea Stream 100 year Flood area from approximately 73m² to 72m² as shown in Figure 4 below. Figure 4 Nukumea Steam Cross-Section The proposed new retirement Building will reduce the total flood area for the 100 year storm event by approximately 1m² (1.4%). This is considered only minor and could be mitigated by some minor re-contouring of the ground immediately adjacent to the north-west corner of the building.

7 Orewa Coastal Retirement Resort Page 6 of 9 Based on the recommendations of the Stormwater Catchment Management Study and Management Plan for the Nukumea Stream Catchment completed by Babbage Consultants Ltd, any new habitable building constructed on the site should have a minimum finished floor level of RL 4.3m. We note that the proposed Retirement Building has a proposed habitable finished floor level of RL 5.5m, which is 1.2m higher than the minimum level established in the Catchment Management Plan. We also note that a non-habitable parking basement is proposed as part of the Retirement Building. The basement area will have a minimum finished floor level of RL 2.5m, which is 1.23m below the expected 100 year flood level. Since this space is nonhabitable, the proposed finished floor level is allowed under the Building Act. However we would recommend that the basement is designed as an impermeable structure and that all entries to the basement (i.e. access stairs and vehicle ramps) are located above the minimum finished floor level established by the Catchment Management Plan (i.e. RL 4.30m). We also note that it is also proposed to relocated the historic Rose Cottage that is currently located centrally on the site, to a new location along in the north-eastern corner of the site. The proposed new site for Cottage is within the 100 year flood area, as shown in Figure 3 above. Since the Cottage will be located within the flood area, we recommend that a minimum finished floor level of RL 4.3m is adopted and that the building is constructed on piles that will allow the 100 year flood to flow under the building. 5.0 OVERLAND FLOW PATHS As noted above, Auckland Council s GIS has identified two Overland Flow Paths are located on the site. One commences near the end of Annalise Place and then runs to the north and discharges into the Nukumea Stream. The other overland flow path is shown along the southeastern and eastern boundary of the site. It is proposed to divert the overland flow path that crosses the site from the end of Annalise Place, by forming a new overland flow path that will be constructed along the south-western

8 Orewa Coastal Retirement Resort Page 7 of 9 and western boundaries of the site. The overland flow path will discharged directly to the Nukumea Stream. The proposed overland flow path is shown in Figure 5 below. The overland flow path shown along the south-eastern and eastern boundaries of the site is assessed as being only minor. There is no notable upstream stormwater catchment so the actual flows will be minimal. The proposed development of the site will still allow any overland flow to run along the southern and eastern boundaries as currently indicated. Figure 5 Proposed Drainage Plan showing the location of the Overland Flow Path 6.0 COASTAL INUNDATION The Proposed Auckland Unitary Plan GIS Map shows that the site is within the coastal inundation zone. In the Proposed Auckland Unitary Plan text (30 September 2013), it states in Section 5.12 Natural Hazards:

9 Orewa Coastal Retirement Resort Page 8 of 9..finished floor levels of new dwellings located in coastal inundation areas to be above the mapped 1% AEP coastal storm tide event plus a 1m projected sea level rise. Auckland Council has provided clarification on the 1% AEP coastal storm levels for estuaries on the east coast of Auckland in a letter to Council from NIWA titled, Revision of coastalstorm inundation elevations for small east-coast estuaries in the Auckland Region, dated 6 th March, For the Orewa area, the Reduced Level (RL) provided is 2m. This means that the minimum floor level, when 1m sea-level rise is added, is RL 3m. As noted in Section 4.0 above, the minimum finished floor level for habitable buildings located on the site is RL 4.3m, which is greater than the inundation minimum level and therefore the governing criteria for this site. 7.0 RECOMMENDATIONS Based on our review of the Catchment Management Plan, the coastal inundation information provided by Auckland Council and the design of the proposed new Retirement Buildings, we recommend the following: A) All new habitable buildings constructed on the site should have a minimum finished floor level of RL 4.30m. B) The proposed basement of the Retirement Building should be designed as an impermeable structure and all entries to the basement should be located above RL 4.30m. C) The proposed relocated Rose Cottage should have a minimum finished floor level of RL 4.30m and should be designed on a pile foundation that allows flood flows that will allow the 100 year flood to flow under the building.

10 Orewa Coastal Retirement Resort Page 9 of 9 D) No fill should be placed along the northern boundary below the existing RL 3.73m contour. E) Minor re-contouring of the ground immediately adjacent to the north-west corner of the proposed Retirement Building should be completed to mitigate the reduction of flood volume in this area. F) A new Overland Flow Path should be construed along the south-western and western boundaries of the site to divert any overland flows from Annalise Place around the proposed new retirement building. Prepared By: Damian Crampton NZCE (Civil), GIPENZ SPL Consultants Ltd