REPORT Water Source Options Assessment for Metropolitan Supply: Report on option to increase water supply by raising existing dams Prepared for Watercare Services Ltd Prepared by Tonkin & Taylor Ltd Date December 2020 Job Number 29133.222.v3 Tonkin & Taylor Ltd December 2020 Report on option to increase water supply by raising existing dams Job No: 29133.222.v3 Watercare Services Ltd Document Control Title: Report on option to increase water supply by raising existing dams Date Version Description Prepared by: Reviewed Authorised by: by: 16/11/2020 1 First issue of combined version DMK/TB/RC TB/DAB/KB TB of two previous reports 27/11/2020 2 Final draft DMK/TB/RC/KB TB TB 8/12/2020 3 Final issue DMK/TB/KB TB TB Distribution: Watercare Services Ltd 1 PDF copy Tonkin & Taylor Ltd (FILE) 1 PDF copy Table of contents 1 Introduction 1 1.1 Background 1 1.2 This report 2 1.3 Scope 3 2 Increased yield due to raising existing dams 5 2.1 Introduction 5 2.2 Methodology 6 2.3 Raised dam configurations 6 2.4 Yield modelling results 7 2.5 Summary 11 3 Preliminary assessment of practicality of raising Mangatangi Dam 12 3.1 Introduction 12 3.2 Concept design 12 3.2.1 Approach 12 3.2.2 Extent of dam raising and estimated benefit in terms of increased yield 12 3.2.3 Physical works at the dam 15 3.3 Technical challenges and possible fatal flaws 18 3.4 Approximate order of cost 20 3.4.1 Approach 20 3.4.2 Exclusions 21 3.4.3 Estimated capital cost 22 3.5 Indicative programme for development 22 3.6 High-level planning review 24 3.6.1 Introduction 24 3.6.2 Statutory context 24 3.6.3 Key stakeholders 28 3.7 Requirement for upgrades to wider infrastructure 28 3.8 Relevance of assessment for other dams 29 3.9 Key conclusions 30 3.9.1 Positive attributes 30 3.9.2 Negative attributes 31 3.9.3 Consentability 32 3.9.4 Overall conclusion 33 4 Applicability 34 Appendix A : Yield Modelling Results Appendix B : Dam Raising Arrangement Appendix C : Summary of existing environmental values and potential environmental effects of dam raising Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry Report on option to increase water supply by raising existing dams Executive summary Tonkin + Taylor (T+T) was engaged by Watercare Services Ltd (Watercare) to investigate the potential to increase water supply to the Auckland region by raising its existing dams1 so as to increase the yield of its storage lakes2. The investigation involved two key parts: a Assessment of the potential increase in yield for ten of Watercare’s existing storage lakes. b Appraisal of the practicality of raising these dams by way of an example case, Mangatangi Dam. Although raising other dams would vary in detail, the challenges are likely to be broadly similar in nature as discussed further below. The key conclusions from part (a), the assessment of increase in yield, are: ∂ The estimated yield increase provided by raising any one of the ten existing dams falls short of the minimum screening threshold3 of 20,000 m3/day that has been adopted in the broader alternative options assessment process that supports the Waikato River take application. ∂ The estimated increases in yield were 1,835 to 13,550 m3/day for raising each of the ten dams by 10 m. The largest increases were for Mangatangi Dam and Upper Mangatawhiri Dam, so consideration was given to raising these two dams further with the aim of exceeding the screening threshold. ∂ The estimated increase in yield was 15,600 m3/day for raising Upper Mangatawhiri Dam by 15 m, and 17,850 m3/day for raising Mangatangi Dam by 15 m. Both options still fall short of screening threshold of 20,000 m3/day. ∂ The aggregated increase in yield if all ten dams were raised by 10 to 15 m is 73,000 m3/day. Although this exceeds the screening threshold, raising each dam is a complex, major project in its own right as described further below. The key conclusions from part (b), the appraisal of raising Mangatangi Dam, are: ∂ Major project: , Raising Mangatangi Dam would be a major civil engineering project of a similar scale as the original construction of the dam. By way of illustration, approximately the same volume of earthworks is required to raise the dam by 15 m, as was required for the original construction of the whole dam4. ∂ Environmental impacts: , Raising the dam, and thus the storage lake levels, would inundate an additional margin of native broadleaf / podocarp / Kauri forest which supports ecologically significant habitats, with possible presence of threatened / endangered species of plants and animals. Furthermore, an additional section of Mangatangi Stream, would be inundated at the upstream end of the storage lake with associated impacts on aquatic ecology. , Construction and operation of the raised dam would result in significant environmental effects, particularly on indigenous biodiversity and habitats and on freshwater systems. 1 For the current report, a “dam” is defined as an artificial barrier and its appurtenant structures that is constructed to hold back water to form a storage lake. 2 For the current report, a “storage lake” is defined as the raw water stored upstream of the dam. For the wider application to take water from the Waikato River, Watercare is referring to raw water storages as “storage lakes”, and treated water storages as “reservoirs”. 3 The screening threshold reflects the necessary scale of increased yield required to make significant gains against the forecast deficit in water supply. Refer “Outage, headroom and the supply demand balance” (Beca/T+T 2020) for further information on the forecast deficit. 4 If the dam cross section is envisaged roughly as a triangle, the increase in height and volume is conceptually added to the base of the triangle. Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry Report on option to increase water supply by raising existing dams Notwithstanding any other measures to avoid, minimise and remedy effects, we consider substantial offsets and/or compensation would still be required. ∂ Consentability: , The project will be subject to significant scrutiny and likely opposition through any consent process. We also note the application of the effects management hierarchy required by the Freshwater National Policy Statement together with the requirement to demonstrate a functional need to locate in a particular environment, and also to consider alternatives under the RMA, pose significant consenting risks. As such, gaining resource consent approval for the works would be very challenging. ∂ Dam safety: , The risk of a breach or dam failure due to a flood could potentially be worsened during construction. This would be mitigated to the extent practicable by careful construction sequencing, defensive measures, and risk management. , The consequences of a dam failure to people, property, and the environment downstream would be permanently worsened due to the increased height and water volume that could hypothetically be released by a breach. , The likely replacement and possible damage to some of the existing dam monitoring instrumentation during the works is also undesirable for ongoing dam safety management, which depends on continuous historical records and trends. ∂ Security and continuity of water supply provided by the existing storage lake: , We have assumed that the dam raising works can be undertaken without dewatering the storage lake and without taking the storage lake out of service. However, this is based on several critical assumptions, such as that the existing valve tower can be raised and strengthened while in operation, and that earthworks to raise the dam can be constructed on the downstream face and crest with minimal intrusion into the existing embankment. , There is a risk to water quality during construction with low levels of water and construction activities that may require treatment (settling ponds and filters) before water can be sent to Ardmore Water Treatment Plant. ∂ Technical challenges: , Complexity introduced by retrofitting an existing and operational asset, i.e. tying in with the existing core and filter zones in the dam, working within a constrained space to strengthen the valve tower, avoiding contamination risks to the operational storage lake, and managing flood risk during construction while also aiming to avoid dewatering and impacts on operation. , Uncertainty regarding the condition of existing structures and what might be required to remediate any deficiencies, i.e. structural and seismic capacity of the valve tower, and the potential need to take the valve tower out of operation with implications for water supply during construction. , Borrow areas were extensively mined during the original construction, so suitable material is likely to be further away or potentially already exhausted. Some of the original borrows are now inundated by the storage lake. , Limited nearby space for spoil disposal and temporary stockpiles which will constrain the speed of construction and extend the period of disruption and risk. , Limited space for erosion and sediment control. , Significant uncertainty regarding borrow materials and batter slopes, potentially mitigated by more detailed geotechnical investigation. Any new borrows and haul roads Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry Report on option to increase water supply by raising existing dams Watercare Services Ltd upstream or downstream of the dam are likely to be on steep slopes, which has implications for cost, haul times, erosion and sediment control, and risk of slips. Consenting requirements for the borrows and access may be prohibitive, especially where located in native bush. , Geotechnical risk and uncertainty potentially mitigated by more detailed investigation.