SpencerHolmes engineers - surveyors - planners

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1 Preliminary Stormwater Design Mary Potter Hospice Mein Street Newtown Wellington SpencerHolmes engineers - surveyors - planners PO Box 588 Level 6, 8 Willis Street Wellington New Zealand Phone admin@spencerholmes.co.nz Brief Spencer Holmes Limited has been commissioned by Mary Potter Hospice to undertake a preliminary stormwater design for the proposed development of the Mary Potter Hospice, 48 Mein Street, and the neighbouring sites to the east (58, 60, 62 Mein Street) in Newtown, Wellington. Limitation of Report This report has been prepared for the use of Mary Potter Hospice for resource consent application purposes, and any reliance on this report by third parties will be at their own risk. The drainage assessment is based on an inspection of the site and surrounds by Spencer Holmes Limited. No geotechnical investigations have been undertaken. The assessment and design is in accordance with section E1 Surface Water of the New Zealand Building Code (NZBC) and the Regional Standard for Water Services (November 2012) for the Wellington region. Site Description The existing Mary Potter Hospice site at 48 to 50 Mein Street has vehicle access that runs through the property one way and exits at 30 Owen Street to the east of the hospice at a lower level. The proposal is to construct a proposed building on the neighbouring sites at 58, 60, and 62 Mein Street behind the existing residential buildings, to provide carparking under this building, and alter the access to the site and carparking from Mein Street. The area of the carparking area adjacent to Mein Street that was grass and garden is to be hard surfaced is approximately 220m 2. This area is collected by sumps and discharged to the stormwater main in Mein Street. The area of the proposed building is 420m 2, and the area of carpark that extends to the east beyond the building footprint is 100m 2. The area is primarily grassed at present which falls to the south east, and any overland flow falls down the existing driveway to be collected by sumps, which discharge to the drainage in Owen Street. Aerial photographs of the site which show the area of proposed alteration are shown in Appendix 1.

2 Mary Potter Hospice, Mein Street, Newtown, Wellington Page 2 Stormwater Design This report describes the preliminary design to demonstrate that with specific attenuated stormwater design, the stormwater runoff can be controlled to within the limits of the level that occurs from the existing site at present, therefore mitigating the proposed development s stormwater run-off to within acceptable levels. This concept is described by Wellington Water as stormwater neutrality for the site. The preliminary design includes consideration of the site topography, soil type and availability of public stormwater infrastructure, and is undertaken in accordance with the requirements of the New Zealand Building Code and the design guidance of the Wellington Regional Standard for Water Services. The soakage rate for the ground of the site would be low from inspection of cut batters and previous knowledge of the local ground conditions, and we would not consider that soakage is appropriate for stormwater disposal for this site. The existing stormwater is shown on the Wellington City Council webmap drainage plan in Appendix 2. The site currently discharges to the stormwater network in Mein Street and Owen Street. The connection in Mein Street is currently a 225mm diameter earthenware pipe that terminates at a manhole in the footpath outside 42 Mein Street. This manhole has a 225mm diameter connection that appears to run to the garden area immediately to the east of the existing driveway entrance of the hospice. We would expect there to be a buried manhole at this location, and this coincides with the private drainage that falls down the driveway on the webmap plan. The connection in Owen Street is a 225mm diameter pipe discharging to a 675mm diameter concrete pipe in Owen Street installed in We have not been able to confirm this connection size. The existing quantities for stormwater neutrality are as follows (10% AEP, 20 minute rainfall); Carpark 220m 2 Existing (C=0.35) Proposed (C=0.90) 1.0 litre/s 2.8 litre/s Building and Carpark 520m 2 Existing (C=0.35) 2.2 litre/s Proposed (C=0.90) 6.6 litre/s The proposed stormwater includes for the climate change factor, wheras the existing quantity does not. Proposed Stormwater Attenuation and Disposal It is proposed that the increase in stormwater runoff from the site is mitigated by the installation of stormwater attenuation on the site in order to create stormwater neutrality for the site. The attenuation tanks would discharge water at the same or lesser rate than the existing surfacing,

3 Mary Potter Hospice, Mein Street, Newtown, Wellington Page 3 however for a longer period of time. This effectively reduces the peak rate from the site and enables the public stormwater network to convey the stormwater over a longer period when the system has suitable capacity. It is proposed that the two separate areas of development would discharge to the same catchment as existing, that is to the two separate stormwater connections for the site to Mein Street and Owen Street. Mein Street The proposed increase in carpark area adjacent to Mein Street would discharge stormwater direct via a sump to the existing manhole on site. Due to the levels on site, this stormwater is unable to be attenuated as there is insufficient fall to enable a tank to be installed. In order to create neutrality for this stormwater connection, it is proposed that the downpipes from approximately 250m 2 of roof (that have an existing discharge to this catchment) are connected to a tank of 5,000 litres capacity, and this tank discharges with a restricted flow of less than 1.0 litre per second to the stormwater system. Owen Street The proposed building and carpark behind the residences on Mein Street would discharge stormwater direct to a tank, probably located under the carparking to the east of the building. The capacity of the tank would be 10,000 litres capacity. This tank would discharge with a restricted flow of less than 2.0 litre per second to the stormwater system. Conclusion After consideration to the site topography, soil types and availability of public stormwater infrastructure, the proposed stormwater attenuation tanks and connections combine to control the stormwater runoff from the development to the site in a sustainable manner that meets the requirements of the New Zealand Building Code and the design guidance of the Wellington Regional Standard for Water Services. Attenuation runoff designed not to exceed the existing stormwater discharge from the site will ensure the downstream stormwater network from the site is alleviated to lesser runoff flows than existed prior to the proposed development, meeting the Wellington Water requirement for stormwater neutrality. Report prepared by: Spencer Holmes Limited Jon Devine Director CPEng

4 APPENDIX 1: SITE PHOTOGRAPHS

5 Aerial site photograph, with Mein Street to the north of the site, and Owen Street to the east

6 Proposed Carpark alteration Proposed Building Aerial site photograph, showing the carpark alteration adjacent to Mein Street, and the proposed building location to the south of the three residences at 58, 60, and 62 Mein Street

7 APPENDIX 2: DRAINAGE PLAN

8 Wellington City Council webmap of the drainage to the site