| Introduction | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 1 Sensitivity: General 1 Introduction OceanaGold New Zealand Ltd (OGNZL) owns and operates both open pit and underground mines in the provincial town of Waihi, New Zealand. The main area of current mining occurs within the Martha Underground Mine located mostly under the Martha open pit. Since the 1980s, exploration work has been conducted on an area of Department of Conservation (DOC) land in the Coromandel Peninsula, about 10 km north west of the existing processing plant at Waihi. OGNZL is in the process of securing resource consent for further development at their Waihi Mine site. This development includes a new underground mine, Wharekirauponga Underground (WUG) to the north of Waihi. OGNZL has purchased a farm property at the end of Willows Road, Waihi. This 200-hectare parcel of land shares a boundary along its northern and western extents with the DOC estate. Should the project proceed, a portal, tunnel and mine infrastructure are planned to be located on this property. The WUG mine will operate by utilising support facilities located at the Surface Facilities Area (SFA) and by storing excess development rock on the farm for future backfilling purposes. Ore will be transported to the existing Waihi processing plant hence no requirement is envisaged for a processing plant at the new mine site. This report covers the proposed stormwater management for the Willows Farm site, excluding the Willows Rock Stack (WRS) and associated drainage features. The WRS drainage and surface runoff management is covered by the GHD Water Management Study and Golder Geotechnical Report due to the potential chemical properties of the surface runoff. These reports should be read in conjunction with this stormwater management report. 2 Stormwater Philosophy The objective of the proposed stormwater systems is to mitigate potential effects of the site development on the stormwater discharge from the site in its current state and to keep the ‘clean’ runoff separated from the development stormwater runoff. The temporary willows rock stack (WRS) is managed under a different stormwater philosophy (refer GHD Water Management Study). The concept design for the development stormwater has been based on the following criteria and assumptions: ● Waikato stormwater runoff modelling guideline (TR2020/06), Waikato Regional Council. ● Waikato stormwater management guideline (TR2020/07, updated May 2020), Waikato Regional Council ● Regional Infrastructure Technical Specifications (RITS), Waikato local Authority Shared Services ● Rainfall data was obtained from HIRDS (June 2021) and uses the RCP 8.5 data for climate change to 2050. Climate change to 2050 has been selected as this is a temporary site and is not expected to in operation for more than 30-years. ● Stormwater runoff from developed areas (excluding the WRS) will drain via pipes and swales to the SFA detention pond where practical. For these areas, swales will provide the primary source of water quality treatment for surface runoff, with extended detention at the SFA pond to provide some additional treatment. ● The water management study undertaken by GHD (Waihi North Project – Water Management Studies, 2021) has recommended that a 24-hour storm duration be adopted for detention design criteria, as peak flows in the Mataura Stream are reached earlier than this duration based on GHD’s assessment of the Mataura catchment and time of concentration.
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