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. For example, geological and geotechnical conditions are uncertain at the location of the new spillway channels that are currently assumed in the preliminary concept arrangement. Similarly, there is uncertainty regarding the integrity of the east-west aligned ridgeline at the south-eastern edge of the existing dam fill, which would be relied upon to support additional dam fill. ∂ Timeframes and cost: , The major scale of the project is reflected in a programme from investigation through to commissioning of approximately five to eight years and a preliminary construction cost estimate of $117 to 135 million (excluding GST). The appraisal of raising Mangatangi Dam was based solely on physical works immediately at the dam. Realising the increased yield will likely require upgrading downstream treatment and transmission infrastructure, which will entail additional impacts, risks, and challenges. The challenges involved in raising other dams are likely to be broadly similar, i.e. loss of significant areas of terrestrial habitat on the storage lake margins, major earthworks, retrofitting an existing structure built to historical design standards, flood and dam safety risk during construction, replacement and damage to dam safety monitoring instrumentation, geotechnical risks, challenges in construction, significant consenting risks, and risk of interruption to water supply provided by existing dam levels. Raising an existing dam would also not introduce any new geographical / source diversity in terms of security of supply and would be affected by the same weather patterns that periodically reduce the yield of the other existing storage lakes. The increase in yield from raising an existing dam falls short of the minimum threshold of interest for the projected demand under consideration. Moreover, consideration of the practicalities of raising Mangatangi Dam as an example has demonstrated that raising an existing dam is a complex, risky and major project in terms of capital cost, timeframes for development, technical challenges and viability, environmental impacts, and consenting challenges.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry Report on option to increase water supply by raising existing dams 1 Introduction

1.1 Background Watercare Services Limited (“Watercare”) is a lifeline utility providing water and wastewater services to a population of 1.7 million people in Auckland. Its services are vital for life, keep people safe and help communities to flourish. More specifically, Watercare is the council-controlled organisation of Auckland Council responsible for municipal water supply within Auckland, and the provider of bulk water supply services to Pokeno and Tuakau in the Waikato District5. Watercare supplies approximately 440,000 cubic metres of water per day (“m3/day”) on average across the year, derived from a range of sources and treated to the Ministry of Health Drinking Water Standards for New Zealand 2005 (revised 2018). Watercare’s three main water supply sources are:6 • Water storage lakes in the Hūnua and Waitākere ranges; • A groundwater aquifer in Onehunga; and • The Waikato River. The exact proportion supplied from each source varies daily, depending on a range of factors including the levels in the storage lakes, forecast rainfall, treatment plant capacity, and maintenance requirements. In December 2013, Watercare applied to the Waikato Regional Council (“WRC”) for resource consents to authorise abstracting an additional 200,000 m3/day (net) of water from the Waikato River, a new water intake structure and discharges from a new water treatment plant. Since that time, Watercare’s water take application (and the associated applications) have been on hold while the WRC processes and determines other applications to take water from the Waikato River Catchment that were lodged before Watercare’s application. During the period from late 2019 through to mid-2020, the Auckland region experienced one of the most extreme drought events in modern times with rainfall for the period between January and May 2020 being approximately 30% of what would normally be expected for that period. At Watercare’s recommendation, in May 2020 Auckland Council imposed water use restrictions in Auckland for the first time since the early 1990s. Watercare also took additional steps to improve security of supply during the drought by exercising emergency powers under section 330 of the Resource Management Act 1991 (RMA),7 and by re-establishing supply from previously decommissioned sources.8 While the above steps have been taken to ensure Auckland’s short-term water supply requirements are met, the focus has now turned to the future. Watercare’s focus remains planning for water demand over the long term by securing sustainably sourced water to achieve: ∂ Certainty of supply in up to a 1:100-year drought with 15% residual dam storage; and ∂ Certainty of supply to meet the peak demand. On 30 June 2020, after considering advice provided by the Environmental Protection Authority, the Minister for Environment issued a direction under section 142(2) of the RMA to call in Watercare’s

5 Under a bulk supply agreement with Waikato District. 6 Watercare also operates individual water supplies from various sources including groundwater and surface water for several other communities such as Muriwai, Algies Bay, Snells Beach, Bombay, Waiuku, Warkworth, Helensville and Wellsford. 7 Reduced environmental flows from the Waitakere, Wairoa and Cosseys Storage Lakes, and a short term take from the Waikato River. 8 Groundwater bores at Pukekohe and the Hays Creek Storage Lake in Papakura.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 1 Report on option to increase water supply by raising existing dams 2013 application and refer the matter to a Board of Inquiry to determine the application. The Minister recognised Watercare’s application as a proposal of national significance. Given the passage of time since the 2013 application was lodged, Watercare has updated the application to address a range of matters including updates to population and demand assessments, changes to the policy framework within which the application is to be considered, consultation that has taken place, reassessment of potential water supply sources and intake options, and updated assessments of environmental effects including the effect that granting Watercare’s application would have on the allocation available to other users. The updated application will be heard by the Board of Inquiry. The most significant revision to the 2013 application, resulting directly from Watercare’s ongoing engagement with Waikato-Tainui is a reduction in the volume of the proposed water take from 200,000 m3/day (net) to 150,000 m3/day (net). This reduction reflects Waikato-Tainui’s special relationship with the Waikato River as outlined in the Waikato-Tainui Raupatu Claims (Waikato River) Settlement Act 2010. It recognises Waikato-Tainui’s relationship with the Waikato River and its respect for the River lies at the heart of Waikato-Tainui’s spiritual and physical wellbeing, tribal identity and culture. Watercare currently holds three resource consents authorising the abstraction of water from the Waikato River adjacent to the Waikato Water Treatment Plant (“Waikato WTP”) near Tuakau as follows: a Resource consent 960089.01.04 authorising a net take rate of up to 150,000 m3/day at any time of the year. b Resource consent 141825.01.01 (referred to as the “Seasonal Water Take” consent) authorising a net take rate of up to: , 100,000 m3/day during the period 1 May to 30 September (inclusive); and , 100,000 m3/day during the period 1 October to 30 April (inclusive) when the 7-day rolling average flow of the Waikato River at Rangiriri exceeds 330.03 m3/second. c Resource consent 142090.01.01 (referred to as the “Hamilton City Council Water Allocation” consent), authorising a net take rate of up to 25,000 m3/day (or such lesser volume as determined by Hamilton City Council as being available for any given day) during the period 1 October to 30 April (inclusive). This is a short-term consent till 1 May 2023. In the event that the consent sought through the Board of Inquiry process is granted for the volume sought, Watercare proposes that its Seasonal Water Take consent and Hamilton City Council Water Allocation consent would be surrendered. Watercare’s combined take from the Waikato River under its existing resource consent 960089.01.04, and the new water take consent sought through the Board of Inquiry would not exceed a net year round take volume of 300,000 m3/day.

1.2 This report This report presents an assessment of the potential to increase water supply system yield of Watercare’s storage lakes by raising the existing dams. This contributes towards the alternative options assessment, which supports Watercare’s updated application to be considered by the Board of Inquiry. The subject assessment involved two key parts: 1 The potential increase in supply system yield across ten existing storage lakes has been assessed; and 2 The practicality of raising dams has been assessed for an example case, Mangatangi Dam. Although, the approach to raising dams will vary in detail for other dams, the challenges are

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 2 Report on option to increase water supply by raising existing dams likely to be broadly similar, i.e. loss of significant areas of terrestrial habitat on the storage lake margins, major earthworks, retrofitting an existing structure built to historical design standards, flood and dam safety risk during construction, significant consenting risks, and risk of interruption to water supply provided by existing dam levels. This report is an amalgamated and updated version of two previous reports: ∂ “Effect on potential of raising dams”, T+T May 2015, project number 29133.220; and ∂ “Practicality, impacts and benefits of raising Mangatangi Dam”, T+T December 2016, project number 29133.221. As part of the current scope, we have undertaken a relatively high-level review of the previous investigations to confirm/refine the earlier conclusions based on up-to-date information. The updated assessments are presented in this combined report.

1.3 Scope The scope of the assessment was agreed with Watercare (T+T letter dated 21 October 2020) and comprised the following steps: ∂ Meeting with Watercare to clarify detailed scope and information requirements for related reporting on Watercare’s updated application to take water from the Waikato River, and to identify updated information available from Watercare relevant to review. ∂ Yield: , Review previous yield assessments for nine Watercare storage lakes, five in the Waitakere ranges and four in the Hunuas, for dams raised by 1 m, 2 m, 5 m and 10 m. , Update yield assessments, as required based on additional information available, and add Hays Creek Dam storage lake into the investigation. , Additionally, assess yield for 15 m raising of Mangatangi and Upper Mangatawhiri Dams. ∂ Raising Mangatangi Dam: , Review earlier work and confirm additional information requirements necessary to update the investigation. , Initial workshop with Watercare to obtain feedback in relation to previous reporting, to identify updated information on the dam that could affect practicality of dam raising, and to confirm assumptions and detailed scope. , Review the previous concept for raising Mangatangi Dam, including: o The level of dam raising that appears technically feasible. o The increased extent of inundation. o The likely physical works that would be required at the dam. , Review previous approximate order of cost for works at Mangatangi Dam based on the confirmed and agreed assumptions, and update including provision for cost escalation. , Review previously identified fatal flaws or major risks (including cost) for rebuild / raising Mangatangi Dam relating to topography, geology, and engineering-related constraints. , Review the previous estimate of the increase in peak and average flow released from Mangatangi Dam so that others can assess the implications for downstream treatment and transmission infrastructure.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 3 Report on option to increase water supply by raising existing dams , Review the previous estimate of the duration that Mangatangi storage lake would need to be out of service or partially de-watered (if needed) during dam raising construction, and provide an initial indication on the likely impact on yield if partially de-watered. , Regulatory review and identification of potential consent requirements for dam raising activities, in relation to the Regional Plan and National Policy Statement – Freshwater, and take into account the District Plan, and including commentary on any major risks or constraints from a consenting perspective. , Review the previous estimate of the indicative timeframe for construction. , Review the previous comparison of the increase in yield with growth in demand over the consenting and construction period. ∂ Reporting.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 4 Report on option to increase water supply by raising existing dams 2 Increased yield due to raising existing dams

2.1 Introduction T+T has assessed the potential increase in water supply system yield of Watercare’s storage lakes by raising the existing dams. Specifically, raising existing dams was assumed to comprise raising dam crest and spillway levels, and thus raising storage lake full supply levels, or Normal Top Water Levels (NTWL). The assessment used spreadsheet analysis of storage lake drawdown based on revised storage lake surface area and storage volume data for the raised dams, and using catchment runoff flows generated from the Integrated Source Management Model (ISMM) for each of the catchments. The yields for ten Watercare storage lakes were investigated, being: ∂ In the Hunua Ranges: , Cosseys , Wairoa , Upper Mangatawhiri , Mangatangi , Hays Creek ∂ In the Waitakere Ranges: , Upper Nihotupu , Lower Nihotupu , Upper Huia , Lower Huia , Waitakere. The results of the assessment are presented in the following sections.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 5 Report on option to increase water supply by raising existing dams 2.2 Methodology For a range of raised dam heights, the 100 year yield with 15% residual storage for each storage lake was determined. This corresponds broadly to Watercare’s current level of service agreed with Auckland Council. For each storage lake, the 100 year yield was assessed for four increased storage levels, being + 1 m, + 2 m, + 5 m and + 10 m above the present existing NTWL. A further + 15 m case was considered for Mangatangi and Upper Mangatawhiri dams. The detailed tasks included: ∂ Extension of the respective storage rating curves for each storage lake, based on information from the LiDAR survey information of the storage lake catchments from Auckland Council. ∂ Identification of the lowest point on the storage lake catchment boundary to determine at what level above the present dam a higher storage lake would spill out of the catchment and assuming that the existing freeboard between NTWL and dam crest level is preserved. This step in the assessment provided an indication of the maximum dam raising without the requirement for a saddle dam. ∂ Modelling of storage lake drawdown using the storage lake catchment inflows based on ISMM modelling of historical and synthetic rainfall, 1036 years data in total. Evaporation and direct rainfall on the storage lake was included in the assessment. ∂ For each increased dam height and NTWL, storage lake drawdown was modelled based on a number of mean daily demands over the 1,036 year data period using spreadsheet analysis, and assuming: , Mean daily demand factored for each month to vary during the year. , Environmental compensation releases to supply residual flows downstream, as required by consents for the respective storage lakes. Noting that individual storage lake storage levels were used for the each of the Waitakere storage lakes in this analysis, whereas the consent conditions relating to these storage lakes are based on total system storage with storage lakes operated conjunctively. ∂ For each mean daily demand, the frequency of storage drawdown was determined through the historical sequence by identifying the independent events in which the storage lake levels are reduced below the 15 % storage threshold. ∂ The yield for the storage lake was identified from interpolation between demand cases to determine the demand with 100 year frequency that corresponds to the 15 % storage capacity of the storage lake at the increased NTWL.

2.3 Raised dam configurations For the Mangatangi and Upper Mangatawhiri storage lakes, contours at +15 m and + 10 m above the existing NTWL (spillway height) were generated. For each of the other storage lakes, a contour at + 10 m above the existing NTWL was generated. ∂ The contours were enclosed as a polygon and buffered by 10 m (in plan) so that they were slightly larger and intersected with the terrain layer. ∂ A Digital Elevation Model (DEM) was created from this polygon, and the existing spillway was assigned to it so that it is a flat surface at the existing NTWL. ∂ The DEM was offset by the possible changes in spillway height (1, 2, 5, 10 or 15 m). The increased storage lake volume was calculated (using Global Mapper tools) based on the volume enclosed between the DEM at the raised and existing NTWL and the terrain layer of the catchment surface.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 6 Report on option to increase water supply by raising existing dams The + 15 m and + 10 m surface areas were measured from the LiDAR data. However, the surface areas at other levels were interpolated / extrapolated from the surface area at the + 10 m level (from Global Mapper) and the existing NTWL (from the ISMM model databases). The calculated surface areas and volumes for the raised dams are summarised in Tables A.1 to A.3 in Appendix A. It is noted that: ∂ Waitakere storage lake would need an additional saddle dam for increase in NTWL greater than 1 m; and ∂ Wairoa storage lake has a spillway separate to the dam structure which would need to be raised in addition to the dam embankment.

2.4 Yield modelling results The results of the yield modelling of the ten storage lakes for various increases in NTWL are summarised in Table 2.1 and Table 2.2 following. Table 2.1 and Table 2.2 indicate that the potential increase in yield (based on raising NTWL by 10 m) may be up to 65 % for the Waitakere storage lake, with other storage lakes less than this. In practical terms, a 10 m increase in storage level realises an increase in yield of less than 10,000 m3/day at each of the storage lakes, except Upper Mangatawhiri and Mangatangi. The potential increase at the latter two is 12,600 m3/day and 13,550 m3/day respectively. As part of the assessment of alternative water resources that could be available to meet the future water demand for Auckland’s drinking water supply, Watercare has adopted a minimum yield requirement of 20,000 m3/day as a lower threshold for screening options. The increased yields due to raising Upper Mangatawhiri and Mangatangi by 15 m comes the closest to meeting this minimum yield requirement, but still fall short. It is noted that: ∂ The analyses are based on present consent conditions for residual flow and release requirements from the storage lakes. There is potential for these to be increased through a consent process, which would reduce yield. ∂ The Lower Nihotupu and Lower Huia results are based on outflow from the Upper dams at existing NTWL, i.e. not raised in unison with the Lower dams. ∂ Thus, the yield of the Lower storage lakes is likely to be reduced if the Upper dams were raised and more water was captured upstream. ∂ The total of the aggregated increase in yield if all storage lake spill levels were raised by up to 15 m is potentially up to 73 MLD (i.e. 73,000 m3/day). ∂ This potential increase is based on analysis of each of the raised storage lakes being operated independently, i.e. no conjunctive use as part of an integrated system. The aggregated potential increase in yield is summarised in the following Table 2.3.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 7 Report on option to increase water supply by raising existing dams Table 2.1: Raised Hunua dams: Potential increase in yield (m3/day)

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 8 Report on option to increase water supply by raising existing dams Table 2.2: Raised Waitakere dams: Potential increase in yield (m3/day)

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 9 Report on option to increase water supply by raising existing dams Table 2.3: Raised Watercare dams: Aggregated potential increase in yield (m3/day)

Note: “+ 15 m” values based on raising Mangatangi and Upper Mangatawhiri Dams by 15 m, and all other dams by 10 m.

The yields of the individual storage lakes have been assessed for Watercare previously (T+T, 2007, 2010, and 2014). Storage lake operation for those analyses was based on the historical rainfall data set only, i.e. from 1848. Comparison of the results of this investigation (using 1,036 years of synthetic and historical rainfall data) with those from previous yield analyses in 2014 using 165 years historical rainfall data are summarised in Table 2.4 below.

Table 2.4: Watercare dams yield analysis: Comparison of rainfall datasets (m3/day)

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 10 Report on option to increase water supply by raising existing dams The comparative results above are generally consistent, recognising the respective sample sizes for the analyses. The aggregated yield of the storage lakes for the 1,036 year analysis is 313,520 m3/day. The comparative value for the 165 year historical record only is 311,750 m3/day.

2.5 Summary Based on the analyses, the potential aggregated increase in yield if all storage lake NTWLs (spill levels) were to be raised by up to 15 m is up to 73 MLD (i.e. 73,000 m3/day). The stand-alone yields of the storage lakes increased by between 9 % and 53 % for the Hunua storage lakes, and by between 18 % and 65 % for the Waitakere storage lakes. In practical terms, a 15 m rise in NTWL for the two largest storage lakes only realises an increase in yield of no more than 17,850 m3/day for each storage lake. For eight of the ten storage lakes, the increase in yield for a 10 m rise in NTWL is less than 10,000 m3/day. The results indicate that there is a diminishing benefit in terms of yield as the dam height increases, shown particularly for the Mangatangi and Upper Mangatangi storage lakes, i.e. the increased yield between +10 m and + 15 m is less than the yield benefit between + 5 m and + 10 m. This is related to the water resource limits of the catchments, i.e. how much water is available. In practical terms, at some point, increasing dam height and storage lake volume no longer increases yield, because there is no more water available. The potential yield increases are based on analysis of each of the storage lakes being operated independently, i.e. no conjunctive use as part of an integrated system. The analyses are also based on present consent conditions for residual flow and release requirements from the storage lakes. The increase in storage lake volume and capture of water for supply to the Auckland region will result in some degree of change in the downstream flow regime. A specific assessment of environmental effects of changes to the flow regime would require further analysis. Moreover, that specific assessment could potentially result in changes to existing environmental flow and release requirements, which may change the yield from what has been assessed here. Comparison of the results of this investigation (using 1,036 years of synthetic and historical rainfall data) with those from previous yield analyses using 165 years historical rainfall data are generally consistent. The aggregated yield of the storage lakes for the 1,036 year analysis is 313,520 m3/day, and 311,750 m3/day for the 165 year historical record only.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 11 Report on option to increase water supply by raising existing dams 3 Preliminary assessment of practicality of raising Mangatangi Dam

3.1 Introduction An initial concept for raising the dam has been developed to indicate the rough order of magnitude of physical works. The primary benefit of raising the dam would be increased yield as described in Section 2, albeit the increase in yield was found to be relatively small. This section of the report provides a brief overview of the likely technical challenges / fatal flaws, approximate order of cost, indicative timeframes for development, high level planning review, and information that can be used by Watercare to estimate the requirements for associated upgrades to wider treatment and transmission infrastructure. The assessment has been based on a desktop review of selected drawings, reports, published databases and informed judgement rather than detailed analysis and calculations. The assessment has also drawn on Watercare’s knowledge of its assets through interactive meetings and review of draft concepts and assumptions prior to finalisation, rather than an exhaustive review of information.

3.2 Concept design

3.2.1 Approach For the purposes of the current assessment, we have assumed that the option to raise the existing Mangatangi Dam would be undertaken: ∂ In accordance with the New Zealand Dam Safety Guidelines (NZSOLD 2015); and ∂ Without reducing the level of protection against dam safety risks in any areas where the level of protection is currently greater than required by NZSOLD 2015. In line with our discussions with Watercare, we have also assumed that investigation and remediation of dam safety deficiencies would most likely be brought forward and addressed as part of the dam raising works.

3.2.2 Extent of dam raising and estimated benefit in terms of increased yield The extent of dam raising has been selected by balancing several considerations, including: ∂ Topography; ∂ Increased inundation of native forest and walking tracks; ∂ Technical complexity, scale and cost of dam raising; and ∂ Maximising the increase in yield. The increased extent of inundation is shown in Figure 3.1. The configuration and increase in yield for the assumed extent of dam raising is summarised in Table 3.1. A more detailed view of the physical works immediately at the dam is included in Appendix B.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 12 Report on option to increase water supply by raising existing dams Figure 3.1: Increased extent of inundation with NTWL raised 15 m

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 13 Report on option to increase water supply by raising existing dams Table 3.1: Extent of dam raising and increased yield

Element Existing Layout Modified Layout Dam crest level 200.84 mRL 216 mRL (15 m approximate increase) Normal Top Water Level (NTWL) 196.23 mRL 211.4 mRL (15 m approximate increase) Live storage volume at NTWL 35.3 million m3 69.4 million m3 (34.1 million m3 increase) Freeboard at NTWL 4.61 m 4.6 m Estimated yield (average) 97,450 m3/day 115,300 m3/day (17,850 m3 /dayincrease)

As indicated in Table 3.1 and Section 2.4, we estimate an increase in yield of 17,850 m3/day (average) for the assumed extent of dam raising. Raising Mangatangi Dam by 15 m would comprise a major civil engineering project of a similar scale to the original construction of the dam (comparable quantity of earthworks). Moreover, there would be additional complexity introduced by retrofitting an existing structure, while very likely still maintaining some degree of operational capability during construction, i.e. not dewatering the dam and continuing to take water. The scale and complexity of the project would be reflected in the cost of the raising works, scope of investigation and design required, consenting and construction scope and timeframes, technical challenges and environmental impacts, which are discussed further in the remainder of Section 3. The increased extent of inundation at the NTWL extends back to the Rata Ridge Track and Mangatangi Ridge Track. The water level appears to be below the level of Rata Ridge track, but this would need to be confirmed. Water levels would also be higher than NTWL during many flood events. The raised water level may affect the Mangatangi Ridge Track stream crossing. A section of the vehicle track to Workman trig would also need to be realigned above the raised storage lake level. The realigned track is likely to sidle along steeper terrain than the current alignment. Stability of the cut slopes for the track would require further investigation and design and may prove to be geotechnically challenging. The site topography does not favour raising the dam more than 15 m. The south-eastern edge of the existing dam fill is confined by an east-west aligned ridgeline. If the dam is raised more than 15 m, fill would need to be placed within the storage lake on the upstream face of the dam to prevent spill into the next gully to the south of the ridgeline. The assumed dam raising concept avoids placing fill on the upstream face to (1) minimise the quantity of fill given the curvature of the embankment, and (2) to avoid dewatering and associated implications for meeting demand. No saddle dams are required to contain the storage lake at the revised crest level of 216 mRL. As already noted, the scale of the dam raising project is major. However, the increase in yield of 17,850 m3/day (average) falls short of the minimum threshold9 of 20,000 m3/day adopted for screening options in the alternative options assessment process for the Waikato River take application. As noted above, a greater extent of raising to increase yield further would not fit as well with the site topography and would intrude further into native forest upstream.

9 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.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 14 Report on option to increase water supply by raising existing dams 3.2.3 Physical works at the dam

3.2.3.1 Overview of works The preliminary conceptual arrangement for raising Mangatangi Dam is shown in Appendix B. Environmental effects of the physical works are addressed separately in Section 3.6. In broad terms, the physical works involve raising the dam crest and raising the NTWL by raising the spillways. However, in more specific detail, this requires: ∂ Clearing trees and vegetation from the area of increased inundation; ∂ Establishing borrows, haul roads, temporary stockpiles and spoil disposals; ∂ Relocating permanent access roads above the right abutment; ∂ Relocating the vehicle track to the Workman trig; ∂ Extending the existing spillway conduit, scour conduit, valve tower ventilation and emergency access and constructing a new energy dissipation structure10 at the modified toe of the dam; ∂ Stripping the existing embankment, cutting into the embankment where necessary to tie into existing drainage and core zones and provide a suitable surface for compacting fill, placing and compacting fill on the downstream face and dam crest to raise the dam to the new level, re- grassing and reinstating surface drainage and the wave wall on the dam crest; ∂ Excavating a new open channel primary spillway and auxiliary spillway above the left abutment; ∂ Concrete-lining the primary spillway and constructing a concrete energy dissipation structure at the toe of the primary spillway; ∂ Constructing a bridge across the new spillways to provide access for maintenance; ∂ Strengthening and extending the valve tower; ∂ Replacing the foot bridge to the valve tower with a new foot bridge at a higher elevation; ∂ Relocating the debris boom at a higher elevation; ∂ Decommissioning the bellmouth spillway and auxiliary fuse plug spillway; and ∂ Relocating / reinstating the boat ramp, standby generator, aeration compressor and operational buildings, rain gauge, public lookout and toilets, as well as dam monitoring instrumentation. Benefits of the preliminary conceptual arrangement include: ∂ Replacing the current bellmouth and fuse plug spillways with open channel spillways above the left abutment, which are considered fundamentally more robust and reliable; ∂ Reducing the slope of the cut batter above the new spillway channels to improve slope stability and reduce the risk of slip debris blocking the spillways, noting that further investigation and design would be required to confirm this benefit; and ∂ Raising the level of the top of the core zone above flood levels, noting that the existing core zone appears to only extend to NTWL i.e. floods would rise above the top of the core zone. More detailed investigation and concept development could potentially identify additional elements required to raise Mangatangi Dam. Further consideration would be needed to determine any changes to the hydropower generation arrangement, noting there will be additional head available for generation. We have also assumed that the existing embankment, core and drainage zones,

10 The new energy dissipation structure at the modified toe of the dam would be used to manage flood flows while constructing the new spillway and dam raising earthworks, noting that the existing bellmouth spillway would not be decommissioned until after these works were in place.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 15 Report on option to increase water supply by raising existing dams existing scour conduit and valves would not need to be significantly modified to suit the raised arrangement, but this would need to be confirmed by detailed assessment beyond the scope of the current exercise. There is unavoidable uncertainty in many areas given the complexity of raising the dam and the preliminary nature of a screening-level assessment. In areas of uncertainty, we have erred on the side of assuming that raising Mangatangi Dam is technically feasible to avoid eliminating the option prematurely. More detailed work could change the fundamental approach to raising the dam (i.e. introduce the need for dewatering the storage lake or require rebuilding more of the embankment) or even prove that raising the dam is not technically and / or practically feasible.

3.2.3.2 Earthworks and borrows We have assumed that the dam would be raised by placing fill on the dam crest and downstream face. Placing fill on the upstream face is considered to be undesirable because: ∂ It would likely necessitate dewatering of the storage lake, which is undesirable for meeting ongoing demand and would entail technical challenges as discussed further in Section 3.3. ∂ After drawdown, clean up and drying of the face materials would be needed to provide an appropriate surface for new dam fill to be placed. Strippings have been placed against the upstream heel of the dam as part of its original construction and this would need to be removed before placing new structural fill. ∂ It would increase flood risk to the dam raising works during construction. For the purposes of the current assessment, noting that site visits and site investigations have been excluded from our scope, we have assumed that the materials available for dam raising would be similar to the fill materials used for the original construction. Accordingly, we have also retained the same batter slopes and bench arrangements, although these assumptions could be revised with more detailed investigation and analysis. Based on the experience gleaned from preliminary research of the original dam construction, we have assumed that the general fill, rip rap and core material would be sourced from nearby borrows in greywacke / argillite type rock and soils, although sources of core material in particular are more uncertain. We have assumed that drainage materials would likely be imported from off site. At least 30% of the general fill, rip rap and core material required for dam raising was assumed to be provided from the excavation for the new primary and auxiliary spillway channels above the left abutment and from flattening the batter slope above these channels. This area is understood to have been previously used as a borrow site during the original dam construction. The remainder of the general fill, rip rap and core material required was assumed to be provided by: ∂ Enlarging the spillway excavation and flattening the slope above the channels even further to provide additional material; and / or ∂ From the slopes above the storage lake in the valley upstream of the dam, taking advantage of borrows and haul roads from the original construction where practical (noting that some of the original borrows were below storage lake level); and / or ∂ From establishing new borrows downstream of the dam. Appropriate borrows would need to be confirmed by geotechnical investigations, laboratory testing and design work. Borrows and haul roads upstream and downstream of the dam would likely be located in steep country. Availability of, and access to these borrows, is considered to be a major uncertainty in our assessment. In terms of zoning of the raised portion of the embankment, we have assumed a sloping core that ties into the top of the existing core, which would be extended above flood levels. High-density

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 16 Report on option to increase water supply by raising existing dams polyethylene could be considered further as a supplementary or alternative seepage control measure to a core zone, but further investigation would be needed to confirm acceptability and is a refinement beyond the scope of the current study. We have assumed a sloping chimney drain immediately downstream of the new sloping core that ties into the existing vertical chimney drain. A further drainage blanket could also potentially be considered at the interface between the existing and new fill. The interface blanket would provide a back-up measure should seepage bypass the existing vertical chimney drain. Our assumed approach relies heavily on the geometry of the existing zones and suitability of the existing dam fill materials based on the selected drawings and construction records that have been considered.

3.2.3.3 Spillways It may be possible to strengthen the existing bellmouth spillway so that it can be raised 15 m, though this is very uncertain and detailed hydraulic and structural analysis would be needed in conjunction with site investigations and testing. Consideration would also need to be given to managing the flood risk during construction. For the purposes of the current assessment, we have conservatively assumed that the bellmouth would be decommissioned and that a new concrete-lined open channel spillway controlled by an ogee crest would be constructed above the left abutment as a primary spillway. A new energy dissipation structure would be needed at the toe of the spillway. We consider that this approach also replaces a less reliable bellmouth type spillway with the preferred failsafe open channel overflow spillway which has the effect of enhancing the facilities’ dam safety profile. We have further assumed that the auxiliary fuse plug spillway would be replaced with an open channel excavated in rock as a new auxiliary spillway to the true left of the new primary spillway. The slope above the auxiliary spillway would be laid back to improve slope stability and mitigate the risk of slip debris blocking the spillways11. A run-out area and access bench could also be provided at the toe of the slope. A bridge over the spillways has been assumed to provide access for maintenance. Again, we consider that this approach replaces a less reliable fuse type spillway with the preferred failsafe open channel overflow spillway which has the effect of enhancing the facilities’ dam safety profile. We also considered right abutment options for the spillways. The right abutment options would require less excavation, but this was not considered a significant benefit since the excavated material was assumed to be used for fill to raise the dam. The left abutment spillway options were preferred in terms of dam safety since the east-west aligned ridgeline along the south-eastern edge of the main dam fill isolates and directs spillway flows away from the main embankment fill. It also allows concerns regarding existing slope stability to be addressed positively as part of the overall raising project. The primary spillway would operate relatively frequently, possibly up to an event in the order of a 200 year return period flood. For events larger than the 200 year return period flood, ranging up to the Probable Maximum Flood12 (PMF), both the primary and auxiliary spillway would operate in

11 Exchange of information with Watercare’s Dam Safety Team indicates that the weathered materials on top of the ridge have been experiencing ongoing movement since the existing auxiliary spillway was originally excavated. The measures described above (flattening the slope and allowing a run-out area) are proposed to improve stability. However, these measures are conceptual and would require detailed investigations and analysis. Further stages of investigation could conclude that the slope cannot practicably be stabilised or that an independent spillway (away from the unstable slope) is required for resilience. We note that the current arrangement has two independent systems, each capable of managing flows up to the PMF, and proposed works would aim to maintain or improve the current level of dam safety resilience. 12 The Probable Maximum Flood is the theoretical largest flood resulting from a combination of the most severe meteorological and hydrologic conditions that could conceivably occur in a given area.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 17 Report on option to increase water supply by raising existing dams combination and some repairable damage would likely result but would be designed to avoid an uncontrolled release. As noted above, the open channel arrangements are considered fundamentally more robust and reliable than the bellmouth and fuse plug arrangements. Watercare currently has no plans to upgrade or change the bellmouth and fuse plug spillways and it would only be considered if there was a major dam modification like raising of the dam. The existing and modified spillway arrangements are compared in Table 3.2 following. The peak water level, freeboard and peak discharge shown in Table 3.2 for the PMF with the existing layout is based on “Watercare PMF Study” (Opus 2008). The freeboard and peak discharge indicated in Table 3.2 for both the existing and modified layouts assumed conservatively that the storage lake level was at the crest of the primary spillway at the start of the PMF event.

Table 3.2: Existing and modified spillway arrangements

Element Existing Layout Modified Layout Primary spillway Bellmouth spillway with crest at Concrete-lined channel controlled by ogee 196.23 mRL crest at 211.4 mRL Auxiliary spillway Fuse plug spillway with fuse at Open channel cut in rock at 212.6 mRL 199.7 mRL Dam crest level 200.84 mRL 216 mRL Peak water level in 198.42 mRL 213.6 mRL approx. PMF (primary spillway operating only) (primary and auxiliary spillways operating) Freeboard in PMF 2.42 m 2.4 m approx. Peak discharge in PMF 307.55 m3/s 160 m3/s approx. (substantial attenuation provided by additional storage at raised storage lake level)

3.2.3.4 Valve tower and bridge The preliminary assessment only considered limited information on the valve tower (mainly reinforcement drawings). We have assumed that the valve tower could be extended but would need to be strengthened. Ideally, the strengthening works could be undertaken from within the valve tower without taking the valve tower out of operation, but this is an assumption that would need to be confirmed by more detailed investigation and further consideration. The strengthening works could involve such measures as installing ground anchors and / or fixing steel plates to the inside face of the tower. A replacement pedestrian bridge to the tower would also be required at a higher elevation.

3.3 Technical challenges and possible fatal flaws Technical challenges and risks relating to topography, geology, and engineering considerations for raising Mangatangi Dam include: ∂ Increased consequence of a hypothetical dam breach due to the increased height of the dam and increased volume stored, relevant to Watercare as dam owner and operator, as well as affected stakeholders. ∂ 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

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 18 Report on option to increase water supply by raising existing dams 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. ∂ Limited nearby space for spoil disposal and temporary stockpiles resulting in slow construction and limited space for erosion and sediment control. ∂ Significant uncertainty regarding borrow materials and batter slopes, potentially mitigated by more detailed geotechnical investigation. Based on the original construction records, core material in particular may be scarce. Any new borrows and haul roads upstream or downstream of the dam are likely to be located 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. As noted above, a section of the permanent vehicle access track to Workman trig would need to be realigned above the raised NTWL, which will shift the track to steeper terrain that may be challenging in terms of geotechnical stability. ∂ Uncertainty regarding the geological and geotechnical conditions at the location of the new spillway channels, potentially mitigated by more detailed geotechnical investigation. ∂ Uncertainty regarding the integrity of the east-west aligned ridgeline at the south-eastern edge of the existing dam fill, and capacity to support additional dam fill, potentially mitigated by more detailed geotechnical investigation. ∂ Some of the existing dam monitoring instrumentation is likely to be replaced due to the works. Surveillance is the “front line of defence” for dam safety, and monitoring instrumentation is a key part of the surveillance process. Ongoing evaluation of dam safety based on monitoring instrumentation is highly dependent on whether new readings are consistent with expected behaviours from historical trends. Therefore, disruption of the continuity of monitoring instrumentation is undesirable for dam safety. ∂ The overall cost of raising the dam is expected to be significant, as discussed further in Section 3.4. As previously noted, there are several key assumptions we have made in areas of uncertainty to progress this current assessment which, if revised, could change the fundamental approach to raising the dam or even prove that raising the dam is not technically and/or practically feasible, for example: ∂ We have assumed only minor modification to the existing embankment, sufficient to tie into existing embankment zones and provide a suitable surface for placing fill material. However, this depends on the following: , That the filter and drainage arrangements and materials of the existing embankment do not need to be modified for the raised dam arrangement. , That it is acceptable to have a sloping core that ties into the top of the existing core. More detailed assessment of the potential for cracking under seismic loading would be required. If more detailed assessment indicates that the filter and drainage zones of the existing dam require modification or it is necessary to provide a more gradual geometric transition from the existing core zone to the new core zone, this could potentially involve rebuilding a large part of the existing embankment.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 19 Report on option to increase water supply by raising existing dams ∂ We have assumed the existing valve tower can be strengthened and extended without being taken out of service. If the valve tower has to be taken out of service during strengthening, as is quite possible, this would have direct implications for operation. If a new tower is required because it is not possible to strengthen the existing tower, this would constitute further major works and would involve additional cost and necessitate dewatering and the associated issues with dewatering described below. ∂ 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. This assumption could potentially be impractical, e.g. based on more detailed consideration of construction sequencing and construction flood risk, or because the existing valve tower has to be replaced rather than strengthened, or greater modification of the existing embankment is required, or the only source of borrow material proves to be within the storage lake. Dewatering would be undesirable for several reasons: , Implications for operation and ongoing ability to supply water during the construction period (depending on the scale of dewatering), potentially requiring other projects that provide additional yield to be brought forward to compensate. , Water quality with low levels of water and construction activities may require treatment (settling ponds and filters) before water can be sent to Ardmore Water Treatment Plant. , Providing enough diversion capacity to keep the storage lake dewatered may be problematic given the original diversion system has been blocked off with mass concrete plugs and the existing scour outlet has limited capacity.

3.4 Approximate order of cost

3.4.1 Approach A preliminary estimate of construction cost has been developed based on the initial concept described in Section 3.2.3 and presented in Appendix B. Only capital costs associated with physical works immediately at the dam have been considered. Costs to upgrade the treatment and transmission infrastructure have not been included. Costs to upgrade hydropower infrastructure have also not been included. As noted in Section 3.3, there are several key assumptions that we have made in areas of uncertainty to progress the assessment, which if revised could change the fundamental approach to raising the dam and thus could also result in substantial changes in construction cost. For example, if it proves necessary to dewater the storage lake, or substantially modify the existing embankment, or replace rather than strengthen and raise the existing valve tower, these departures from current assumptions could involve significant changes in cost. Another area where there is potential for cost increase is storage lake clearing, which is currently assumed to be undertaken using handheld tools and access by the existing lake, which may not be practical.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 20 Report on option to increase water supply by raising existing dams Table 3.3: Assumed contingency and allowances

Item Allowance Preliminary and general (P&G) 25% Consultant services (detailed design) 9% Project management 2% Commissioning support 0.5% Management, supervision, and quality assurance (MSQA) 9% during construction, client fees and consent monitoring Contingency 30% to 50%

3.4.2 Exclusions The following items have not been included in the cost estimates: ∂ Costs to upgrade treatment and transmission infrastructure; ∂ Costs to upgrade hydropower infrastructure; ∂ Relocation of existing services; ∂ Land value and / or purchase or reallocation; ∂ Subsidies, compensation, or purchase of commercial operations (if any); ∂ Environmental offsets and compliance; ∂ Government taxes; ∂ Legal costs; ∂ Financing costs; ∂ Consenting and approvals; ∂ Insurance; ∂ Construction cost volatility due to changes in costs of commodities subject to currency exchange fluctuations or world demand such as steel and fuel; the degree of demand for relevant potential contractors in the market at the time of bidding; and ∂ Operational and maintenance costs.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 21 Report on option to increase water supply by raising existing dams 3.4.3 Estimated capital cost Table 3.4 presents an estimate of capital costs of the physical works immediately at the dam. The cost estimate is in terms of Q2 2020 terms. This high-level estimate provides an indication of cost only for comparison to other alternative water resource options and should not be used for budgeting purposes.

Table 3.4: Capital cost for physical works immediately at the dama

Item Capital costb Access $ 2,500,000 Storage lake $ 1,000,000 Dam embankment $ 45,000,000 Spillways $ 7,000,000 Valve tower and scour conduit $ 4,000,000 Miscellaneous $ 500,000 P&G (25%) $ 15,000,000 Subtotal direct costs $ 75,000,000 Consultant services (9%) $ 6,500,000 Project management (2%) $ 1,500,000 Commissioning support (0.5%) $ 500,000 Management, supervision, and quality assurance (MSQA), client fees and consent monitoring (9%) $ 6,500,000 Subtotal indirect costs $ 15,000,000 Subtotal direct and indirect costs $ 90,000,000 Contingency (30% to 50%) $ 27,000,000 to $ 45,000,000 Total $ 117 to 135 million a. Realising the increased yield associated with raising Mangatangi Dam will likely require upgrading downstream treatment and transmission infrastructure, which is not accounted for in the capital cost above. b. All cost estimates above exclude GST.

3.5 Indicative programme for development The following construction sequencing has been assumed: i The first task on the critical path is expected to comprise extending the existing spillway conduit, scour conduit, and constructing a new energy dissipation structure at the modified toe of the dam. This task essentially involves getting the diversion in place to provide continuity of environmental and spillway flows during construction. The task would likely be timed to coincide with when the storage lake is drawn down in the course of normal operation to minimise flood risk to the works. ii Establishing borrows, haul roads, stockpiles and spoil disposals could potentially be completed in parallel with (i). iii Following (i) and (ii), the dam raising earthworks and excavation of new spillway channels could be completed in parallel. The dam raising earthworks would involve cutting into the dam crest to tie into the existing chimney drain and core zones below NTWL. To manage flooding risk, the dam raising earthworks further downstream could be raised above the level of the existing dam crest before cutting down to the chimney drain and core zone. iv Strengthening and extending the valve tower and replacing the foot bridge to the tower could proceed in parallel with (i), (ii) and (iii).

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 22 Report on option to increase water supply by raising existing dams v Once the new spillway channels are constructed and most of the dam crest has been raised, the current auxiliary fuse plug spillway would be decommissioned. The fuse plug may need to be decommissioned in sections to avoid lowering the effective level of containment of the storage lake. Ideally the decommissioning would be timed to coincide with when the storage lake is drawn down in the course of normal operation to minimise flood risk. vi Once the new spillway channels are constructed, all of the dam crest has been raised to the new level, and the valve tower and foot bridge have been raised, the existing bellmouth spillway would be decommissioned. As for the preliminary conceptual arrangement for dam raising, the sequencing described above depends on key assumptions in areas of uncertainty and has the potential to change fundamentally with more detailed assessment. Table 3.5 provides an indicative programme for raising Mangatangi Dam, including investigation and design, consenting through to construction. We have assumed for the present assessment that the storage lake would not need to be dewatered, and would remain in operation throughout the construction period. In practice this may be very difficult to achieve without any dewatering. However, if more detailed assessment identifies that dewatering of the storage lake is required to enable construction to raise the dam, clearly the stored water volume would be reduced. Operation of the storage lake at a lower water level effectively reduces the size of the storage lake and there would be a reduction in yield. The actual reduction would depend on the top water level that could be maintained in the storage lake during the construction period. Depending on the scale of the reduction, this could potentially require that additional projects to provide yield are brought forward to compensate.

Table 3.5: Indicative programme for option of raising Mangatangi Dam

Item Stage Indicative estimated duration 1 Investigation and design, including external peer 1 year review and possibly early contractor involvement. Environmental investigations, AEE and pre-application stakeholder engagement 2 Resource consent application and consent 1 to 2 years* process and post-lodgement consultation 3 Building consent** 2 months 4 Construction 2 to 3 years for earthworks (assuming 15,000 - 20,000 m3/week), plus an additional 1 to 2 years for non-earthworks elements Overall 5 to 8 years *Any appeals to the Environment Court (or higher) would extend this timeframe. **The Building Consent Authority has 20 working days to process an application for a building consent. The estimated duration in Table 3.5 allows for pre-lodgement discussion as well as requests for information that “stop the clock”.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 23 Report on option to increase water supply by raising existing dams 3.6 High-level planning review

3.6.1 Introduction The assessment below provides a high-level planning review of the option of raising the level of the Mangatangi Dam. We have previously reported on this in 2016 and the assessment below includes and builds on this assessment. We have focused in particular on the direction set at a national level which has changed appreciably since this time, as well as the key direction set at a regional level13. The works required to raise the level of the Mangatangi Dam and to dam, take and use surface water will require numerous resource consents at both a regional and district level. This assessment does not identify all the consents that may be required but rather includes commentary on major risks or constraints from a consenting perspective. We also note that the planning assessment below focuses on raising the dam and associated works. However, in order to realise the increase in yield it may also be necessary to upgrade wider infrastructure, such as treatment and transmission infrastructure. A planning review in relation to wider infrastructure requirements is outside the scope of this report.

3.6.2 Statutory context

3.6.2.1 Freshwater NPS The National Policy Statement for Freshwater Management 2020 (NPS - Freshwater) came into force on 3 September 2020. Key requirements of the Freshwater NPS include14: ∂ Manage freshwater in a way that ‘gives effect’ to Te Mana o te Wai. ∂ Improve degraded water bodies, and maintain or improve all other water bodies using bottom lines defined in the Freshwater NPS. ∂ An expanded national objectives framework including new attributes aimed specifically at providing for ecosystem health. ∂ Avoid any further loss or degradation of wetlands and streams (Policy 7 requires that ‘The loss of river extent and values is avoided to the extent practicable’). ∂ Identify and work towards target outcomes for fish abundance, diversity and passage and address in-stream barriers to fish passage over time. Importantly, expectations and direction regarding tangata whenua involvement in freshwater management and decision making has been strengthened in the Freshwater NPS. The new hierarchy also places the health and wellbeing of water bodies and freshwater ecosystems first, before the essential needs of people, which is in turn prioritised over other uses. Whereas previously, decision makers undertook a balancing exercise in terms of Part 2 of the Resource Management Act 1991 (RMA), this has been supplanted by the hierarchy established through the Freshwater NPS. The Freshwater NPS also establishes an effects management hierarchy which requires that adverse effects are avoided where practicable, and then (in order of preference): minimised, remedied, offset, and compensated for. If aquatic compensation is not appropriate, then the activity itself is to be avoided. Within a resource consenting context, the focus on avoiding adverse effects in the first instance is important, particularly in light of the direction set by the King Salmon and Davidson decisions15.

13 Refer to Appendix C for a summary of existing environmental values and potential environmental effects of dam raising. 14 Farming activities are also a significant focus of both the Freshwater NPS and NES but are not relevant here and therefore not canvassed in this assessment. 15 Environmental Defence Society Incorporated v The New Zealand King Salmon Company Limited & Ors [2014] NZSC 38; and R J Davidson Family Trust v Marlborough District Council [2018] NZCA 316.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 24 Report on option to increase water supply by raising existing dams Subpart 3(1) of the Freshwater NPS includes a specific requirement that every regional council includes the following policy (or words to the same effect) in its regional plan(s): “The loss of river extent and values is avoided, unless the council is satisfied: (a) that there is a functional need for the activity in that location; and (b) the effects of the activity are managed by applying the effects management hierarchy.” The project would therefore need to demonstrate a ‘functional need’. While the existing dam provides a baseline in terms of functional need (i.e. raising of the existing dam can only occur in that environment), the assessment would also look more broadly at the functional need in terms of alternative water supply options. Further we note that Clause 6(1)(a) of Schedule 4 of the RMA requires a description of alternative locations or methods for undertaking the activity (and similarly an assessment of alternatives would be required for any designation process). Considering the significant effects on indigenous biodiversity and freshwater systems including the loss of approximately 2 km of river habitat, the requirement to demonstrate a functional need under the Freshwater NPS, and also to consider alternatives under the RMA, pose significant hurdles to gaining consents. In terms of the effects management hierarchy, subpart 3.24(3) of the Freshwater NPS requires that the regional council changes its regional plan to ensure that an application is not granted unless the applicant has demonstrated how each step in the hierarchy will be applied to any loss of extent of values of the river. Any application will therefore need to demonstrate how effects will be avoided in the first instance – we expect this would likely extend to a consideration of alternative supply options, before the other steps in the hierarchy are applied. Notwithstanding any other measures to avoid, minimise and remedy effects, we consider substantial offsets and/or compensation would still be required. Regional councils are required to implement the Freshwater NPS as soon as reasonably practicable and to include specific provisions in their regional plans relating to establishing Freshwater Management Units and assigning values, setting environmental flows, allocation (take limits to achieve environmental flows), monitoring and fish passage. In terms of allocation limits and environmental flows, as already noted in Section 2.5, the increase in storage lake volume and capture of water for supply to the Auckland region would also result in some degree of change in the downstream flow regime. A specific assessment of environmental effects of changes to the flow regime would require further analysis. We also consider the requirements regarding the environmental flow regime are likely to increase under the Freshwater NPS (which prioritises the health and wellbeing of water bodies and freshwater ecosystems). The above high-level assessment suggests that the Freshwater NPS and how it is then implemented through regional plans has significant implications for any future resource consent application to raise the level of the Mangatangi Dam. We note that the Freshwater NPS will also have implications for the existing dam management regime including environmental compensation flows.

3.6.2.2 Freshwater NES The National Environment Standards for Freshwater 2020 (Freshwater NES) came into force concurrently with the Freshwater NPS. Amongst other matters, the Freshwater NES has a particular focus on river and wetland protection, and includes restrictions on: ∂ Vegetation removal, earthworks/land disturbance, drainage or taking, damming or diverting water from in or around a natural wetland; ∂ Reclamation of rivers; and ∂ Certain structures that can impede fish passage.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 25 Report on option to increase water supply by raising existing dams Any works for the construction of ‘specified infrastructure16’ within proximity to a natural wetland will require resource consent under the Freshwater NES. This is likely to be assessed overall as a discretionary activity (Regulations 45 and 47).

3.6.2.3 Other national direction and key considerations A significant and broad ranging review of the RMA has recently been completed with the review panel recommending major changes to New Zealand’s resource management legislative framework. The Government has confirmed it intends to repeal and replace the RMA. While the nature and timing of these reforms is currently unknown, this is likely to result in a different consenting framework to the existing regime. The draft National Policy Statement on Indigenous Biodiversity (NPS-IB) sets out objectives and policies to manage natural and physical resources so as to maintain indigenous biological diversity (‘biodiversity’) under the RMA. Policy 6 sets out the direction for managing effects within a significant natural area and Policy 7 recognises the need to provide for social, cultural, and economic wellbeing in the management of significant natural areas. The draft NPS-IB also sets out the direction in relation to biodiversity offsetting and compensation that a future NPS may take. This national policy statement is currently in a draft form and has no statutory weight. Due to the uncertainty in the final content of this document we have not considered it in any detail except to note that the current direction would indicate further investigations and hurdles beyond those already identified in this report.

3.6.2.4 Waikato RPS The Mangatangi Dam is located in the Hunua Regional Park within the jurisdiction of the Waikato Regional Council. The Waikato Regional Policy Statement (Waikato RPS) provides an overview of the resource management issues in the region. Along with a suite of environmental objectives and policies, the Waikato RPS provides direction for decision makers to recognise and protect the value and long-term benefits of regionally significant infrastructure (Objective 3.12, Policy 6.6). Further, Policy 6.7 specifically provides for the ‘ongoing operation, maintenance, upgrading and development of the Mangatangi Dam and associated infrastructure to provide for the justified and reasonably foreseeable needs of current and future generations’.

3.6.2.5 Waikato Regional Plan The Waikato Regional Plan (WRP) is currently subject to Plan Change One – Healthy Rivers/Wai Ora17 and is also under review in combination with the Waikato Regional Coastal Plan with the view to creating the Waikato Regional Resource Management Plan. This review is also likely to incorporate the Freshwater NPS and NES. The WRP first became operative over 10-years ago (in 2007). We expect it to change significantly through the review process currently underway (in particular with respect to water allocation in the Mangatawhiri and Mangatangi Rivers which at present are “split”18 between WRP provisions and provisions in the Auckland Council Regional Plan: Air, Land and Water (ALW Plan)). Existing ongoing consents are in place for the damming, take and discharge and are due to expire in 2035. These consents would need to be replaced or a change to conditions of consent sought to allow for the raising of the Mangatangi Dam and increased yield. A change to consent conditions is

16 Definition includes ‘infrastructure that delivers a service operated by a lifeline utility (as defined in the Civil Defence Emergency Management Act 2002)’. 17 PC One – Healthy Rivers/Wai Ora is primarily regarding water quality in the Waikato and Waipa Rivers rather than allocation and therefore is not considered further. 18 At the time that the WRP was made operative, the parts of the Mangatangi and Mangatawhiri Rivers containing the Watercare storage lakes were included in the Auckland Region and were not directly regulated by the WRP.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 26 Report on option to increase water supply by raising existing dams assessed as a discretionary activity, whereas new consents are assessed as per the rules in the plan. In any case, the scale of the change proposed is likely to lead WRC to conclude that an alteration of conditions is beyond the scope of the original application and that a new suite of consents would be necessary. A preliminary review suggests the damming and take of water are likely to be assessed as discretionary activities under the relevant rules in the WRP. However, a suite of consents would also be required for earthworks, vegetation removal, works in the bed of a watercourse, discharges, etc. Therefore, we cannot preclude a non-complying consent trigger, in which case the application would be bundled and assessed overall as a non-complying activity. A non-complying activity may only proceed to consideration under s104 of the RMA if the consent authority is satisfied that one of the limbs of s104D of the RMA are satisfied. That is: ∂ The adverse effects of the activity on the environment will be minor (s104D(1)(a)); or ∂ The application is not contrary to the objectives and policies of a proposed plan and/or plan (s104D(1)(b)). An application would not be able to meet the first ‘gateway test’ above (i.e. adverse effects must be no more than minor) and therefore would need to meet the second gateway test i.e. not contrary to the regional plan objectives and policies. The key to progressing an argument that the application is not contrary to the policy direction of the relevant plan(s) will be determining the emphasis placed on protecting significant indigenous biodiversity values, freshwater resources and wetlands, vegetation and habitats; avoiding adverse effects on them; and developing mitigation proposals that address and respond to the effects. The provisions that recognise the regional significance of the existing water supply infrastructure and the municipal water supply will also be factored into this assessment. Unless an application can pass one of the s104D gateway tests, it would not be possible for the Councils to grant consent. Even if the application was considered to pass the second gateway test, this does not mean that consent is granted but rather that the application can then proceed to consideration under Section 104 of the RMA, and can be granted or declined pursuant to Section 104B.

3.6.2.6 Waikato District Plan The Waikato District Plan (WDP) is also under review with the Proposed Waikato District Plan (PWDP) being notified in July 2018. Hearings on the PWDP are currently underway. Land associated with the proposal is zoned ‘Forest Conversation’ in the WDP and is subject to Watercare designation 91(E) and Auckland Council designation 100 (Watercare designation O6 and Auckland Council designation T1 in the PWDP). There are no conditions attached to either of these designations. Under the PWDP the area is identified as a Significant Natural Area through an overlay that had immediate legal effect from when the Plan was notified. Land use consent is not required for activities undertaken in accordance with a designation and instead, an outline plan is required in accordance with section 176A of the RMA. However, the nature and scale of the works required to raise the level of the dam are unlikely to fit within the purpose or extent of the designation and therefore land use consents under the WRP, or an alteration to designation or new designation in accordance with s181 or s168 of the RMA, would be required. An assessment of alternatives and biodiversity offset requirements are also likely to be triggered.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 27 Report on option to increase water supply by raising existing dams 3.6.3 Key stakeholders

3.6.3.1 Auckland Council The land is part of the wider Hunua Regional Park, which is owned by Auckland Council and administered in accordance with the provisions of the Auckland Council Regional Parks Management Plan 2010. Watercare leases the land from Auckland Council and is provided a 20 m buffer zone in which Watercare has exclusive right and use. Beyond this 20 m zone, the land is co- shared with Auckland Council and would require negotiation to operate in and use this land.

3.6.3.2 Mana whenua Iwi and hapu groups that whakapapa to the area in particular will hold a strong interest in the proposal. Extensive consultation would be required to determine cultural values and the level of impact that the proposal would have on those values. In addition, as noted above, the Freshwater NPS anticipates comprehensive engagement with tangata whenua and their input into the management and decision-making process. There are also extensive statutory acknowledgement areas within the Waikato region as identified in the Waikato RPS, some of which may apply to the Mangatangi Dam and environs.

3.6.3.3 Other stakeholders In terms of stakeholders more broadly, we consider there will be significant interest from a wide range of parties, including individuals, other potential water users such as farmers, community groups, local and national environmental groups including Forest & Bird and Environmental Defence Society, the Department of Conservation, Fish & Game, etc. We would expect there to be considerable opposition to the proposal, with a large body of submissions on the applications anticipated, and a high likelihood of appeals to the Environment Court (and beyond) should consents be granted.

3.7 Requirement for upgrades to wider infrastructure In order to realise the increase in yield provided by dam raising, it may also be necessary to upgrade wider infrastructure, such as treatment and transmission infrastructure. T+T has estimated the increase in peak and average flow released from Mangatangi storage lake associated with the potential increase in yield. This information is presented below to provide Watercare with a basis for understanding the likely implications for wider infrastructure. To determine the potential increase in yield for this assessment, the raised Mangatangi storage lake was modelled as a stand-alone source. The modelled peak flow from the storage lake was 131,100 m3/day, corresponding to highest demand in February. We note that the storage lake would actually be operated conjunctively as part of Watercare’s multi-source system. Thus, the peak flow (and ultimate yield available) would depend on connecting infrastructure to the Ardmore Water Treatment Plant and the capacity of the plant itself. These need to be considered together with the other sources supplying the plant, i.e. the Upper Mangatawhiri, Cosseys and Wairoa storage lakes. The present capacity of the connecting infrastructure from the dam to the treatment plant is approximately 125,000 m3/day. To enable operation of the larger Mangatangi storage lake to maximise yield (i.e. up to 115,300 m3/day for the dam raised by 15 m, refer Table 2.1) the downstream capacity would need to be upgraded to at least (131,100 m3/day) for the presently expected demand patterns through the year. However, to provide for operational flexibility of the conjunctive headworks system it is likely that the downstream capacity should be even greater.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 28 Report on option to increase water supply by raising existing dams 3.8 Relevance of assessment for other dams The process and challenges involved in raising Watercare’s other dams would vary in detail but are likely to involve the following broad aspects: ∂ Loss of significant area of terrestrial habitat on the storage lake margins, ∂ Major earthworks, ∂ Retrofitting an existing structure built to historical design standards, ∂ Flood and dam safety risk during construction, ∂ Geotechnical risks, ∂ Challenges in construction, such as for construction site access, ∂ Replacement and damage to dam safety monitoring instrumentation, ∂ Significant consenting risks, and ∂ Risk of interruption to the water supply provided by existing dam levels. In addition to the broad aspects above, exchange of information with Watercare’s Dam Safety Team (via interactive meetings and review of draft outputs) identified several example challenges for specific dams as listed below. The list below is preliminary and not exhaustive. We have not reviewed drawings, reports, and published databases for these dams. Example challenges for specific dams: ∂ Cosseys , Construction traffic impacts on nearby horse riding, camping, tramping, and picnicking areas were noted. ∂ Wairoa , Tying extended core and filter zones into the existing zones of the dam embankment is expected to be challenging because of seismic risks and increased hydraulic loading. , Some degree of dewatering is likely required to manage flood risk while extending the core zone, possibly taking 50% of the storage volume out of service. ∂ Upper Mangatawhiri , If the dam were raised, it is likely that filter and drainage zones would need to be installed. This would likely require some degree of dewatering and reduction in available storage volume during construction. , Fledgling Kokako populations were noted in this catchment. ∂ Hays Creek , Raising this dam by 2m or more is expected to flood an extensive length of Creighton Road, including the intersection with Jones Road, cut off private access, as well as flooding neighbouring property. , There is uncertainty regarding how a historical landslide in the vicinity of Creighton Road would respond to raised lake levels. ∂ Upper Nihotupu , Concrete would likely need to be brought in from off-site, which would need to be considered in terms of access and traffic impacts. , Access to the dam is constrained for large-scale construction works (currently limited to a 2m3 concrete truck). , An existing waterfall is located nearby, which will constrain the spatial extent of raising works, particularly with respect to siting a relocated valve chamber and plunge pool.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 29 Report on option to increase water supply by raising existing dams ∂ Lower Nihotupu , The spillway would likely need to be replaced and a new energy dissipater constructed to manage the increased energy of the flows due to raising the dam. , Borrow materials for filter and drainage zones are unlikely to be sourced locally and would likely need to be brought in from off-site. ∂ Upper Huia , Access to the dam is constrained for large-scale construction works (currently inaccessible to concrete trucks). Improving access would likely involve enlarging a road tunnel. , There is uncertainty regarding how the abutment rock would respond to increased hydraulic loading associated with raising the NTWL. Physical works to address the risk could potentially be difficult, noting the existing cut off walls already extend 40m into the abutment rock. ∂ Lower Huia , Increased hydraulic loading because of raising the NTWL would likely require installing new filter and drainage zones, which would necessitate dewatering and potential interruption to the existing water supply from this dam. , The existing pump station would likely need to be demolished and a new pump station constructed downstream. , Borrow materials for shoulder and core zone material are expected to be limited. ∂ Waitakere , Concrete would likely need to be brought in from off-site, which would need to be considered in terms of access and traffic impacts. , An existing waterfall is located nearby, which will constrain the spatial extent of raising works. , There is uncertainty regarding how the foundation below the dam would respond to increased lake level, and consequences for dam stability and waterfall regression. , Existing seepage around the right abutment would need to be addressed if hydraulic loading were increased due to raising the NTWL.

3.9 Key conclusions

3.9.1 Positive attributes Raising the Mangatangi Dam and NTWL by 15 m was estimated to increase average yield from 97,450 m3/day (existing layout) to 115,300 m3/day (raised layout), i.e. an increase of 17,850 m3/day. The primary benefit provided by dam raising is the increase in yield outlined above. Another advantage of this option may be that water quality is better than many of the other options being considered in the broader alternative options assessment for Watercare’s application to the Board of Inquiry. Further, existing treatment and transmission infrastructure would be used though some upgrade to this infrastructure is likely to be required. There may also be some secondary benefits (still to be investigated), such as increased head for hydropower generation, increased attenuation of floods due to additional storage, improved slope stability above the left abutment, a fundamentally more robust open channel spillway arrangement (and decommissioning of the existing bellmouth spillway), and a reduced seepage and internal erosion risk with the core zone raised above flood levels.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 30 Report on option to increase water supply by raising existing dams 3.9.2 Negative attributes Raising Mangatangi Dam would not introduce any new geographical / source diversity in terms of security of supply, and would be affected by the same weather patterns that periodically reduce the yield of the other southern storage lakes. The increase in yield falls short of the minimum screening threshold19 of 20,000 m3/day that has been adopted for the broader alternative options assessment process that supports the Waikato River take application. An unavoidable disadvantage of raising Mangatangi Dam is the increased consequence of a hypothetical dam breach due to the increased height of the dam and increased volume of water stored, relevant to Watercare as dam owner and operator as well as affected stakeholders. 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. The dam raising would comprise a major civil engineering project of a similar scale to the original construction of the dam (comparable quantity of earthworks) with additional complexity introduced by retrofitting an existing structure. The programme for investigation and design, consenting and construction could be approximately 5 to 8 years. A preliminary estimate of the capital cost of the physical works is in the order of $ 117 to 135 million excluding GST, which excludes upgrades to treatment and transmission infrastructure and operation and maintenance. Technical challenges include complexity associated with 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 and above water to strengthen and raise 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. There is also limited nearby space for spoil disposal and temporary stockpiles. Any new borrows and haul roads upstream or downstream of the dam are likely to be located on steep slopes and in an established bushland catchment environment, which has implications for: ∂ Cost; ∂ Haul times; ∂ Erosion and sediment control; ∂ Terrestrial ecology; ∂ Amenity; ∂ Cultural values; and ∂ Risk of slips. There is uncertainty regarding many aspects, recognising that some aspects could possibly be mitigated by further investigation. For example, there is uncertainty regarding: ∂ Suitability of existing structures to integrate with dam raising works; ∂ Proximity and suitability of borrow sources; ∂ Geological and geotechnical conditions at the location of new spillway channels; ∂ Suitability of the east-west aligned ridgeline at the south-eastern edge of the existing dam fill to support additional dam fill;

19 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.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 31 Report on option to increase water supply by raising existing dams ∂ Geometric constraints on extending the core zone including its ability to withstand cracking in earthquakes; ∂ Condition and capacity of the existing filter and drainage zones; ∂ Potential to raise the dam without substantial modification to the existing embankment; ∂ Potential to strengthen and extend the existing valve tower without it being taken out of service; and ∂ Potential to raise the dam without dewatering or taking the storage lake out of service. Construction and operation of the raised dam would result in significant environmental effects, particularly on existing habitats and associated ecological values which will be difficult to mitigate. Construction works will restrict public access to the local area and could result in reduced downstream water quality due to sediment runoff from earthworks and storage lake margins. The raised dam and storage lake will result in further alteration to the downstream hydrology of the Mangatangi Stream. In the context of the statutory provisions in the Freshwater NPS, regional and district plans and the significant environmental effects of the dam raising, gaining resource consent approval for the works would be very challenging (refer below). Key stakeholders would include Coromandel, Hauraki and Waikato River Iwi, Auckland Council Parks, local Hunua communities, Department of Conservation, Forest & Bird, and Fish and Game. We have assumed that the storage lake would not need to be dewatered, and would remain in operation throughout the construction period. However, more detailed investigation outside the current scope could change this assumption, which, depending on the scale of dewatering and interruption of operation, could potentially require that additional projects to provide yield are brought forward to compensate during the period of having the storage lake out of service.

3.9.3 Consentability Consentability of the project will be determined by a broad range of factors, including the relevant provisions of the statutory documents outlined in Section 3.6, the scale and significance of effects and measures to address these effects, and the interests and involvement of stakeholders. We have not addressed these matters in detail here as they go beyond the scope of this report. We have however, made broad observations on these matters in Section 3.6 above based on our experience. As highlighted above, any resource consent application to raise the level of the Mangatangi Dam is likely to be assessed overall as a discretionary or non-complying activity. A non-complying activity may only proceed to consideration under s104 of the RMA if the consent authority is satisfied that one of the gateway tests of s104D of the RMA is satisfied. Even if the application was considered to pass one of the gateway tests, it would still be subject to consideration under Section 104 of the RMA and can be granted or declined. Decision makers must now also apply the hierarchy established under the Freshwater NPS which places the well-being of the waterbody above that of the essential needs of people and also applies the mitigation hierarchy which prioritises avoiding adverse effects in the first instance. Determining a construction and operating regime that gives effect to Te Mana o te Wai, and recognises and responds to the policy provisions of the regional and district planning documents, would be critical to being able to obtain the necessary statutory approvals to raise the Mangatangi Dam. Ultimately, gaining the necessary RMA approvals will be a key determinant of the feasibility of the proposal. Recent dam projects in other parts of the country point to the likely scrutiny that this proposal would be subject to, and while the proposal does not involve a new dam, the associated effects on indigenous biodiversity and vegetation and freshwater systems will be significant. Notwithstanding any other measures to avoid, minimise and remedy effects, we consider substantial offsets and/or compensation would still be required.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 32 Report on option to increase water supply by raising existing dams In summary, in light of the scale and significance of potential effects on the environment, the application of the effects management hierarchy required by the Freshwater NPS 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 hurdles to gaining consents. Based on our high level review and without detailed investigation, we therefore consider that raising the level of the Mangatangi Dam would be a challenging consent process with significant risks regarding consentability.

3.9.4 Overall conclusion The modest increase in yield and secondary benefits provided by raising Mangatangi Dam are considered to be outweighed by the risk of interruption or reduction in the water supply provided by existing dam levels, environmental impacts, cultural impacts, consenting risks, technical challenges and risks, substantial cost, and long timeframes for development of the option. The increase in yield provided by raising Watercare’s other existing dams is less than for Mangatangi Dam. Moreover, although raising other dams would vary in detail, the challenges involved in raising other dams are likely to be broadly similar, i.e. ∂ Loss of significant areas of terrestrial habitat on the storage lake margins, ∂ Major earthworks, ∂ Retrofitting an existing structure built to historical design standards, ∂ Flood and dam safety risk during construction, ∂ Replacement and damage to dam safety monitoring instrumentation, ∂ Geotechnical risks, ∂ Challenges in construction, ∂ Significant consenting risks, and ∂ Risk of interruption to water supply provided by existing storage lake levels.

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 33 Report on option to increase water supply by raising existing dams 4 Applicability

This report has been prepared for the exclusive use of our client Watercare Services Ltd, with respect to the particular brief given to us and it may not be relied upon in other contexts or for any other purpose, or by any person other than our client, without our prior written agreement. We understand and agree that our client will submit this report as part of an application for resource consent and that Waikato Regional Council as the consenting authority will use this report for the purpose of assessing that application. The construction rates utilised for this high level cost estimate are based on assumed design concepts, estimated quantities and a combination of recently submitted tender rates for similar projects within the regional area along with the latest available rates from QV Cost Builder database (formerly Rawlinsons). Consequently, a significant margin of uncertainty exists on the cost estimate and the contingency we have allowed should be considered as part of the cost rather than a potential add on. No allowance has been included for cost escalation beyond May 2020. COVID-19 impacts: The significance and extent of COVID-19 impacts on construction costs is uncertain at this time but likely to impact both labour and materials rates. We have not made any attempt to allow for the impact of COVID-19 in this estimate and recommend you seek specialist economic advice on what budgetary allowances you should make for escalation and changed construction costs post COVID-19.

Tonkin & Taylor Ltd

Report prepared by: Authorised for Tonkin & Taylor Ltd by:

...... …...... …...... Dewi Knappstein Tom Bassett

PROJECT MANAGER PROJECT DIRECTOR

DMK/TB t:\auckland\projects\29133\29133.2220\workingmaterial\4.0 reporting\29133.dam raising report v03.20201208-tracked changes.docx

Tonkin & Taylor Ltd Waikato River Water Take and Discharge Proposal – Board of Inquiry 34 Report on option to increase water supply by raising existing dams Appendix A: Yield Modelling Results

Table A.1: Raised Hunua dams: Increase in surface area (ha)

Table A.2: Raised Waitakere dams: Increase in surface area (ha)

Table A.3: Raised Watercare dams: Increase in storage volume (m3) Appendix B: Dam Raising Arrangement

∂ Initial Concept Layout for Mangatangi Dam 105 Carlton Gore Road, Newmarket, Auckland

T:\Auckland\CAD\29133\CAD\FIGS\29133.221-F01.dwg Layout1 19/09/2016 10:39:38 a.m. 10:39:38 19/09/2016 Layout1 T:\Auckland\CAD\29133\CAD\FIGS\29133.221-F01.dwg www.tonkintaylor.co.nz Appendix C: Summary of existing environmental values and potential environmental effects of dam raising

Flora and Fauna: ∂ Adjacent land covered in native broadleaf / podocarp / Kauri forest which supports ecologically significant habitats, with possible presence of threatened / endangered species of plants and animals. Identified as important for vegetation biodiversity in the Waikato Regional Plan online maps. ∂ Storage lake and Mangatangi Stream (upstream of dam) support some four native fish species, with stream downstream of dam supporting eight native species. The dam restricts migratory passage of some native fish species. Cultural and Archaeological sites of significance: ∂ No sites of significance are recorded in either the WDP or on Archsite3. The wider area has a history of Maori occupation and use, and a number of iwi have strong cultural associations with the area and claim mana whenua status. There is a provision in the Mangatangi Dam resource consent for a higher flow event to coincide with the annual poukai at Mangatangi Marae. Landscape values: ∂ The PWDP identifies the Hunua Ranges area as an outstanding natural feature. Recreational values: ∂ Little recreational use or access to the storage lake or immediate storage lake margins is known. Various walking tracks are present in the Hunua Regional Park. The Rata Ridge Track and Mangatangi Ridge Track have sections that are in proximity to and cross over the Konini Stream, which is a tributary of the upper Mangatangi Stream. Workman’s Track has an access point from the eastern margin of the dam embankment. ∂ Trout fishing has been permitted from the dam face but is not known to be widely utilised. ∂ Some public access into Hunua Regional Park via Workman Road access and some viewing of storage lake and dam by public. Potential construction effects: ∂ Loss of significant area of terrestrial habitats on storage lake margin, with some likely to hold high ecological values and support threatened or endangered species. ∂ Reduced downstream water quality due to sediment runoff from disturbed surfaces and storage lake margins. ∂ Potential cultural / spiritual effects on Maori values. ∂ Reduced landscape and natural character values. ∂ Restrictions on recreational and public access to dam face and construction area, potentially impacting on recreational use of Workman’s Track. ∂ Possible impact on trout habitat and recreational fishing values. ∂ Construction traffic effects on Workman Road and other local roads. ∂ Risks associated with hazardous substances management (fuels etc) at construction sites. Potential permanent ongoing effects: ∂ Loss of significant area of terrestrial habitats on storage lake margin, with some likely to hold high ecological values and support threatened or endangered species. ∂ Loss (due to storage lake inundation) of about 2 km of river habitat of Mangatangi Stream above extent of existing storage lake. ∂ Provision of additional storage lake habitat. ∂ Effects on downstream hydrology of Mangatangi Stream, including reduced flood flows and potential for reduced low flows (mitigated by consented residual flow requirements). ∂ Possible inundation of sections of Rata Ridge Track and Mangatangi Ridge Track by upstream extent of storage lake, possibly requiring re-routing of tracks. ∂ Potential cultural / spiritual effects on Maori values. ∂ Altered landscape and natural character values.