Review of the Wairau River Sustainable Flow Regime

REPORT NO. 2505

REVIEW OF THE WAIRAU RIVER SUSTAINABLE FLOW REGIME

CAWTHRON INSTITUTE | REPORT NO. 2505 MAY 2014

REVIEW OF THE WAIRAU RIVER SUSTAINABLE FLOW REGIME

JOE HAY, JOHN HAYES

Marlborough District Council

CAWTHRON INSTITUTE 98 Halifax Street East, Nelson 7010 | Private Bag 2, Nelson 7042 | New Zealand Ph. +64 3 548 2319 | Fax. +64 3 546 9464 www.cawthron.org.nz

REVIEWED BY: APPROVED FOR RELEASE BY: Rasmus Gabrielsson Roger Young

ISSUE DATE: 30 May 2014 RECOMMENDED CITATION: Hay J, Hayes J NE 2014. Review of the Wairau River sustainable flow regime. Prepared for Marlborough District Council. Cawthron Report No. 2505. 45 p. © COPYRIGHT: Cawthron Institute. This publication may be reproduced in whole or in part without further permission of the Cawthron Institute, provided that the author and Cawthron Institute are properly acknowledged.

CAWTHRON INSTITUTE | REPORT NO. 2505 MAY 2014

EXECUTIVE SUMMARY

Specification of flow regimes to maintain proper functioning of river ecosystems and related in-stream values require:

 A minimum flow to fulfil water quality and habitat requirements  Allocation limits, or flow sharing rules, to maintain ecologically relevant flow variability and avoid long periods of flat-lining of the minimum flows. Flow variability at a variety of scales is required for maintenance of channel form, sediment and periphyton flushing, benthic invertebrate productivity, fish and bird feeding opportunities, fish migration, and fishing opportunities. In respect of fish migrations, floods and freshes are critical for allowing salmon passage that often is naturally limited at low flows (e.g. ≤ mean annual low flow; MALF) and therefore may not be provided for by the minimum flow.

The Wairau River is a complex system from a flow management perspective. Natural flow variability is exacerbated by existing hydro-power generation, which often produces short duration flow fluctuations associated with peaks in generation demand (‘hydro-peaking’). As well as potential adverse environmental effects, this hydro-peaking causes difficulties for water allocation and flow management. Also, there is significant loss of flow to ground in the lower section of the river (~8 m3/s between the Waihopai confluence and Selmes Road).

Developing appropriate management objectives to reflect community aspirations is the critical first step in setting appropriate flow regimes. This should involve the identification of in-stream and out-of-stream values, and deciding on in-stream values that are to be maintained. This process should include identifying ‘critical values’, with the understanding that by providing sufficient flow to sustain the most flow-sensitive, important value (species, life stage, or recreational activity), the other significant values will also be sustained. This provides the basis for assessing the likely flow regime requirements to achieve the desired management objectives.

Management objectives were a key point of contention throughout the Trustpower Wairau Hydro-electric Power Scheme hearing process. Having clearly defined management objectives in the plan could alleviate much potential controversy at consent hearings. We consider that, appropriate environmental in-stream management objectives for the Wairau River are the maintenance of:  Black-fronted tern and black-billed gull populations

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 Dwarf galaxias population  Brown trout and salmon fishery  Aquatic macroinvertebrate communities and productivity to support the other in-stream management objectives listed above.

Of the ecological values in the Wairau River adult brown trout are the flow critical value because their habitat has among the highest flow requirements and trout are highly valued. Benthic invertebrates are also a flow critical value because invertebrates provide the main food base for trout and other fish, and birds, and they have even higher flow requirements than trout. Passage requirements for adult salmon will also be flow critical. A flow regime set for these ‘critical’ values should be adequate for other ecological values. With regard to minimum flow setting, the lower reaches of the Wairau could arguably be viewed as critical, on the basis of the:

 Relatively high trout abundance, as assessed by drift diving  Relatively high level of fishing effort (~50 % of fishing effort)  Known flow loss to groundwater in this section of the river (~8 m3/s) exacerbating flow reductions during periods of low flow  Location relative to abstraction pressure, i.e. the lower reaches are subject of the cumulative impacts of water abstractions upstream.

The lower Wairau River would arguably warrant a 90% level of habitat retention for adult brown trout. This would align with precedents set by other regional councils.

The Wratts Road two dimensional (2-D) hydraulic-habitat model was developed as part of Trustpower’s investigations for its proposed Wairau Hydro-electric Power Scheme. We used its predictions to estimate that a minimum flow of 10.4 m3/s would retain 90% of the adult brown trout habitat available at the MALF in the lower reaches, based on MALF of 13.4 m3/s at Tuamarina. A minimum flow of 8.4 m3/s would retain 80% habitat.

We suggest that effective minimum flow monitoring requires that a ‘virtual’ flow monitoring site is established immediately downstream of the Branch Scheme tailrace. This would be based on the flow monitoring layout required by the consent conditions for Trustpower’s Wairau Hydro-electric Power Scheme. An upstream monitoring site should make it a straightforward matter to advise abstractors of their entitlement. However, the issues of hydro-peaking and where on the flow fluctuation to implement shut-offs would still remain to be addressed. We suggest that MDC should also retain a downstream monitoring site (e.g. Tuamarina) to help assess compliance. At the same time, continue to move toward gathering more real-time metering data from abstractions, to provide a more direct measure of compliance. Until the recommended upstream monitoring site is established, the Tuamarina monitoring site will have to continue to be used to trigger abstraction restrictions.

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Reducing hydro-peaking would alleviate potential adverse ecological effects, improve efficiency of abstraction, and aid management of abstraction. This could be done through construction of flow regulation ponds. Alternatively, targeted abstraction (including flow harvesting) during the hydro-peaking peaks could reduce flow fluctuations.

If hydro-peaking remains, we recommend using the daily minimum flow as the trigger flow for shut-off / step-down of abstraction. This is based on the premise that the minimum water level effectively defines the area of productive habitat. Alternatively, the daily average flow could continue be used to trigger abstraction restrictions. But the minimum flow could be increased by half the expected maximum daily hydro-peaking range (i.e. by about 2 m3/s at Tuamarina), in recognition that the instantaneous flow will have already dropped below the minimum before a shut-off would be triggered based on the daily average flow.

The current WARMP includes a 2:1 flow-sharing provision (i.e. one share of water in the river for every two shares taken by abstractors) for maintaining some flow variability. However, this has not been implemented to date due to logistical problems associated with managing abstraction from a downstream monitoring site combined with hydro-peaking flow fluctuations. As well as being difficult to implement, the current 2:1 flow sharing rule is not equitable (i.e. it favours abstractors), and is unlikely to substantially mitigate large potential impacts on benthic productivity of the large total allocation allowed in the current WARMP.

Alternative options to the current flow sharing rule include:

 Keep the 15 m3/s allocation of B class water as in the existing Plan, but with a block-by- block flow-sharing arrangement above the minimum flow up to full allocation, using a more equitable 1:1 flow sharing ratio. This would see full B class abstraction implemented at about 401 m3/s, (the flow that retains about 90% of benthic invertebrate habitat relative to the median flow).  Reduce the B class allocation to about 50% of MALF (i.e. ~6.7 m3/s), which would still provide for slightly more than existing abstraction from the Wairau. This option would substantially reduce the future impact of abstraction on mid-to-low range flows that potentially support benthic invertebrate production, thus alleviating the requirement for flow sharing.

We consider that a higher minimum flow for supplementary allocation (introducing a gap between B and C class allocation) should also be considered. Without a gap, the B and C class allocation effectively become a single large allocation block, with potentially large adverse effects on the aquatic ecosystem (especially benthic habitat and trout and salmon passage). The median flow is widely used as a minimum flow for supplementary “flow harvesting” allocation by other councils. Alternatively, seasonal median flows could be

1 ~10 m3/s minimum flow, then 15 m3/s allocation plus a 15 m3/s flow share for the river = 40 m3/s. Could step the flow sharing in 1 m3/s increments (i.e. 1 m3/s for abstraction then 1 m3/s for the river).

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considered for C class minimum flow triggers. This would allow more efficient use of water, while mitigating for effects on habitat and trout and salmon passage and to a small extent also on potentially productive benthic habitat. It would also be worthwhile considering either flow sharing above the C class minimum flow, or a limit (cap) on C class allocation to avoid significant reductions of flushing flows, and possibly encouraging off-season abstraction to storage.

Further investigation may be required into methods to predict flow at the recommended upstream monitoring site, immediately downstream of the Branch Scheme outfall, and into the pros and cons of alternative allocation regimes.

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TABLE OF CONTENTS

1. INTRODUCTION ...... 1 2. CURRENT SUSTAINABLE FLOW REGIME ...... 1 3. NATIONAL POLICY STATEMENT FOR FRESHWATER ...... 3 4. ECOLOGICAL FLOW REGIME ASSESSMENT ...... 4 4.1. Key hydrological features of flow regimes for sustaining river ecosystems and in-stream values ...... 4 4.2. A recommended in-stream flow assessment framework ...... 6 4.3. Minimum flow and allocation limits ...... 9 5. IN-STREAM VALUES IDENTIFIED FOR THE WAIRAU ...... 15 5.1. Critical values and critical habitat ...... 16 5.2. Management objectives and levels of maintenance ...... 17 6. FLOW IMPLICATIONS OF TRUSTPOWER WAIRAU HYDRO-ELECTRIC POWER SCHEME DECISION ...... 21 7. POSSIBLE MINIMUM FLOW AND ALLOCATION REGIME FOR THE LOWER WAIRAU RIVER ...... 24 8. CONSEQUENCES OF HYDRO-PEAKING FLOW FLUCTUATIONS ...... 28 8.1. Existing Branch Hydro-electric Power Scheme ...... 28 8.2. Abstraction shut-off level: flow minima versus daily or rolling average ...... 30 8.3. Location of flow monitoring ...... 33 8.4. Proposed Wairau Hydro-electric Power Scheme ...... 35 9. FLOW SHARING ...... 36 10. OUTSTANDING ISSUES AND RECOMMENDATIONS ...... 40 10.1. Issues ...... 40 10.2. Recommendations ...... 40 11. ACKNOWLEDGEMENTS ...... 42 12. REFERENCES ...... 43

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LIST OF FIGURES

Figure 1. Illustrative hydrograph showing a minimum flow condition and key variable flow features with their physical and ecological function...... 4 Figure 2. Framework for in-stream flow assessment, based on the Ministry for the Environment (1998) Flow Guidelines for in-stream values, with adaptations from Hayes & Jowett (2004) and the National Environmental Standards (NES; Beca 2008)...... 8 Figure 3. Illustrative hydrograph showing a minimum flow and the flow slice taken by the allocation limit...... 9 Figure 4. Illustrative hydrograph showing effect of run-of-river abstraction with relatively large allocation rate on key flow features...... 10 Figure 5. Derivation of minimum flow based on retention of a proportion of available habitat at: a) the habitat optimum, or b) the mean annual low flow (MALF), whichever occurs at the lower flow...... 11 Figure 6. Predicted habitat response for adult brown trout, Deleatidium mayfly and invertebrate food producing in the Wratts Road reach of the Wairau River, based on a 2-D hydraulic-habitat model developed by Hudson et al. (2005)...... 25 Figure 7. Wairau River schematic longitudinal flow profile showing daily maximum and minimum flows resulting from Branch Scheme generation hydro-peaking during low flow conditions...... 28 Figure 8. Comparison of alternative methods for averaging flow data to trigger abstraction restrictions for the Wairau River, based on data from March 2013 at Tuamarina...... 31

LIST OF TABLES

Table 1. Suggested significance ranking of critical values and levels of habitat retention from Jowett and Hayes (2004)...... 12 Table 2. Monthly minimum flows (m3/s) stipulated in consent conditions for Trustpower’s proposed Wairau River Hydro-electric Power Scheme...... 21

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1. INTRODUCTION

Marlborough District Council (MDC) is currently working through the process of reviewing its resource management planning. This report was commissioned to investigate issues relating to the difficulties MDC faces in establishing appropriate flow monitoring points on the Wairau River to implement the flow sharing and rationing provisions set out in the current Wairau Awatere Resource Management Plan (WARMP) and achieve a relevant and meaningful environmental flow regime over the length of the Wairau River. The review draws on the hydrological and ecological information presented at hearings for Trustpower’s hydro-electric power schemes on the Branch and Wairau Rivers, and the outcome of these processes (in terms of consent conditions). Due recognition is given to the complex hydrological regime and the implications of this in protecting the critical in-stream values, particularly the fisheries values.

2. CURRENT SUSTAINABLE FLOW REGIME

The current Sustainable Flow Regime (SFR) is prescribed by rules in the Wairau / Awatere Resource Management Plan (WARMP) Vol 2, Chapter 27, Rules under 27.1.1. The Plan provides for a three-tiered system of water allocation, with the three classes of water permit having progressively decreasing security of supply. Class A has the highest security of supply, expected to be subject to restrictions only one week every five years. Class B has lower security of supply and may generally be allocated only after all A class water has been allocated. It is expected to be fully available 80% of the time, partially available 18% of the time and completely unavailable 2% of the time. Flow harvesting is provided for through C class permits.

The minimum flows in the Wairau River stipulated under the SFR are 8 m3/s at Tuamarina and 14 m3/s at the Narrows. These two flows are approximately equivalent due to inputs from tributaries and relatively consistent losses to groundwater of about 8 m3/s (actually thought to vary between about 6 m3/s and 12 m3/s, but usually close to 8 m3/s — pers. comm. Val Wadsworth, MDC hydrologist) downstream of the Waihopai confluence. There is also a trigger level of 9 m3/s stipulated at Wash Bridge, which is intended to achieve the SFR minimum flow at the Narrows. The SFR also includes a total allocation cap for B class water from the Wairau of 15 m3/s when flow at Tuamarina is above 30 m3/s, progressively reducing to 0 m3/s when flow is 8 m3/s at the Tuamarina recorder, and/or 9 m3/s at the Wash Bridge (with the latter provision applying to the section of the Wairau between the Branch and Waihopai confluences only). The progressive reduction in abstraction with reduced flow is intended to be implemented on a 2: 1 flow-sharing basis, with one unit of water left in the river for every two units of water abstracted above the minimum flow. However, this flow-sharing provision has not been implemented to date, due to the

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impracticality of executing this type of regime in a situation with multiple abstractors, a downstream monitoring location, and fluctuating flows caused by the Branch Scheme hydro-peaking. Instead MDC have used the average flow from the previous day to assess whether abstraction should be shut-off. When the daily average flow at Tuamarina breaches the SFR minimum flow of 8 m3/s, abstraction is shut-off the following day.

The WARMP also provides for C class water abstraction from the Wairau River, which is 67% of any flow in excess of 30 m3/s at Tuamarina, with no upper limit. However, C class water permits may only be drawn to supply a storage reservoir, or recharge groundwater or generate electricity.

No A class water is available from the Wairau River (although there is A class allocation from the Waihopai). This is because the relatively large and consistent losses of flow to groundwater in the lower reaches (~8 m3/s between the Waihopai confluence and Selmes Road) were effectively seen as equivalent to an uncontrolled A class take (Wadsworth, MDC hearing evidence, paragraph 68). However, the B class abstraction in the Wairau has effectively acted like an A class, albeit with a lower security of supply, since the intended flow sharing arrangement has not be implemented to date.

Takes of up to 5% of the instantaneous flow and abstracting up to 10 m3/day are also provided for as permitted activities for stock watering and domestic purposes.

Seasonal minimum (SFR) flows for the lower Branch River are 1 m3/s and 1.5 m3/s from 1 May to 31 December and 1 January to 30 April, respectively.

The Southern Valley Irrigation Scheme (SVIS) is one of the larger consented takes from the Wairau River. It is managed by MDC and as well as providing irrigation water it is used to maintain flows in Gibson Creek, a tributary of the Opawa River. On an informal basis it is also often operated to mitigate the effect of abstraction during low flow periods, by voluntarily reducing the rate of abstraction as the minimum flow is approached, thus making more water available to maintain in-stream values or for other abstractors.

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3. NATIONAL POLICY STATEMENT FOR FRESHWATER

The revised WARMP should give effect to the National Policy Statement for Freshwater Management (NPSFM) which came into force on 1 July 2011. The NPSFM signalled a new direction for the management of freshwater resources in New Zealand. It requires regional councils to establish freshwater objectives for their freshwater bodies and to set allocation limits in terms of water quantity and quality. While the current WARMP includes allocation limits, it could be argued that in practice the implementation of the current WARMP on the lower Wairau is not in the spirit of the NPSFM. Although an allocation limit for B class permits (15 m3/s) is stated for flows exceeding 30 m3/s, there is no limit on C class water above 30 m3/s. B class abstractors have largely been unrestricted (below 30 m3/s) down to the minimum flow. This is because the current level of abstraction is only about 40 % of the allowable B class allocation and the 2:1 flow sharing rule has been too difficult to implement. The 2:1 flow-sharing rule stipulated in the WARMP for C class water would also be difficult to manage. If the current approach to implementing the WARMP was to continue under full B and C class allocation, then most of the available flow above the minimum (8 m3/s at Tuamarina) could theoretically be taken. This is except up to the 7 m3/s, which is the difference between the 15 m3/s B class allocation able to be taken below 30 m3/s, and the 8 m3/s minimum flows. Consequently, because the WARMP is already written and implemented, there is insufficient protection of key hydrological features of the flow regime and components of the river ecosystem.

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4. ECOLOGICAL FLOW REGIME ASSESSMENT

4.1. Key hydrological features of flow regimes for sustaining river ecosystems and in-stream values

Scientific research in New Zealand has identified several ecologically important components of flow regimes. While there is still insufficient ecological understanding to precisely quantify the likely ecological response of a given reduction in flow, existing knowledge provides a starting point for assessing flow regime changes and useful rules-of-thumb regarding aspects of flow regimes that need to be maintained.

The hydrograph in Figure 1 shows key flow features and their physical and ecological function. Also depicted is an example of a minimum flow condition of 1 m3/s, c.f. a natural mean annual low flow (MALF) 1.15 m3/s. Large floods, the size of the annual flood or larger, are important for maintaining the channel form and clearing terrestrial vegetation from the flood fairway. These are likely to be in the order of the mean annual maximum flow, with flows of more than about ten times the mean flow or 40% of the mean annual maximum flow beginning to move a substantial portion of the river bed (Clausen & Plew 2004).

Moderate size floods (freshes) about 3–6 times the median flow (Biggs & Close 1989; Clausen & Biggs 1997) are also important for regularly flushing periphytion and fine sediment from the river bed. Maintaining the quality of benthic invertebrate habitat is the main ecological benefits of this process.

Figure 1. Illustrative hydrograph showing a minimum flow condition (1 m³/s) and key variable flow features with their physical and ecological function. The blue-shaded area represents that part of the hydrograph that potentially provides habitat for algal and benthic invertebrate production (following flood disturbance and resetting of communities).

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Lower flows, including the minimum flow, are of course important for maintaining fish and benthic invertebrate habitat. The MALF has been identified as an ecologically relevant flow statistic for trout populations and native fish species, at least where the amount of suitable habitat declines through the MALF (Jowett 1990, 1992, Jowett et al. 2008). This is generally the case in small or braided rivers. Jowett (1990, 1992) found that the quality of in-stream habitat (HSI, habitat suitability index predicted by hydraulic-habitat modelling) for adult brown trout at the MALF was correlated with adult brown trout abundance in New Zealand rivers. The MALF is indicative of the low flows likely to be experienced during the generation cycles of trout and provides an index of the minimum flow that can be expected from year to year. The lowest flow that a river falls to each year sets the lower limit to physical space available for adult trout, although the duration of low flow is also relevant. This annual limit to living space potentially sets a limit to the average numbers of trout. This concept is intuitively sensible to anyone who has spent a lot of time looking for trout in rivers. Rivers that fall to very low flows each year hold few trout, while those that sustain high low flows hold a lot of trout.

It seems reasonable that the MALF should be similarly relevant to native fish species with generation cycles longer than one year, at least in situations where habitat declines toward the MALF. If the minimum flow restricts habitat for any species, there is potential for a detrimental effect on that population. Research in the Waipara River in North Canterbury, where native fish habitat is limited at low flow, showed that the detrimental effect on fish numbers increased with reduced magnitude and increasing duration of low flow (Jowett et al. 2008). Research on the Onekaka River in Golden Bay also showed similar findings. When habitat availability was reduced by flow reduction, abundance of native fish species responded in accord with predicted changes in habitat availability in both direction and magnitude (i.e. eels and kōaro habitat was reduced and these species declined in abundance, while redfin bully habitat increased and so did their numbers; Jowett et al. 2008).

In contrast to long-lived species such as trout, some aquatic invertebrates have more than one cohort per year, and in New Zealand generally have asynchronous life- cycles (i.e. a range of different life stages are likely to be present at any given time). This allows them to rapidly repopulate areas following disturbance (e.g. by drift from tributaries and from other rivers by winged dispersal) (Williams & Hynes 1976; Scarsbrook 2000). Re-colonisation of some river beds by benthic invertebrates following disturbance has been reported to occur within 4–10 weeks (Sagar 1983; Scrimgeour et al. 1988). In other words, benthic invertebrates can respond relatively quickly to available habitat conditions, so their populations respond to more frequent limiting events (e.g. floods or low flows that occur over the time-scale of months). Flow variability influences the community structure of benthic invertebrates (Booker et al. 2014) and flow recessions following floods may also be important for contributing to benthic production. The latter point is illustrated by the blue-shaded area in Figure 1, which represents that part of the hydrograph that potentially provides habitat for

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periphyton and benthic invertebrate production (following flood disturbance and resetting of communities). However, the degree to which flow recessions will support significant benthic production depends on their duration. The most important habitat for benthic production is that which stays wet for longest, providing the current is not so great as to frequently move the bed or sandblast periphyton and invertebrates from the surface of stones. In rivers with very frequent flooding, the average duration of flow recessions may be too short for flows above the baseflow to wet margin habitat long enough to substantially contribute to benthic production, and so the base flow largely governs the amount of productive benthic habitat.

Because invertebrates colonise available habitat fairly rapidly (in the order of weeks to months), typical flows, in the mid to low flow range, are relevant for benthic invertebrate (and periphyton) production. The median flow (or seasonal median flows) is often viewed as providing an approximation of the typical habitat conditions experienced, and able to be utilised, by benthic invertebrates (Jowett 1992). This in turn may help define carrying capacity for fish and bird populations that feed on invertebrates. Jowett (1992) found that the quality of invertebrate food-producing habitat (HSI defined by Waters (1976) general invertebrate habitat suitability criteria) at the median flow was correlated with trout abundance (Jowett 1990, 1992).

A recent analysis suggests that flow variability appears to be an important factor influencing community structure for both migratory and non-migratory fishes in New Zealand (Crow et al. 2013). While low flow was found to be an important explanatory variable for community structure, flow variability was substantially more influential than the effects of low flow, particularly for non-migratory fishes (Crow et al. 2013); essentially non-migratory species are less likely to occur in rivers with frequent floods. Flow variability may also provide a stimulus for fish migrations. Flows in the order of 2–4 times the median or preceding baseflow have been associated with movement of several fish species in New Zealand (Snelder et al. 2011). The distance that can be covered by migrating fish during a flow event is obviously related to the duration of elevated flows. Consequently, if water abstraction causes more rapid flow recessions it may curtail migration opportunity.

4.2. A recommended in-stream flow assessment framework

Developing appropriate management objectives to reflect community aspirations is the critical first step in setting appropriate flow regimes (Ministry for the Environment Flow Guidelines 1998, Jowett & Hayes 2004, Biggs et al. 2008). Developing appropriate management objectives should involve the identification of in-stream and out-of-stream values, and deciding on in-stream values that are to be maintained. This provides the basis for assessing the likely flow regime requirements to achieve the desired management objectives.

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In their ‘Flow guidelines for in-stream values’ reports MfE (1998) suggest a framework approach for assessing and managing flow dependent in-stream values. Figure 2 is an adaptation of that framework taking account of concepts suggested by Jowett & Hayes (2004) and recommendations in the National Environmental Standards (NES; Beca 2008). The adaptations are:

1. The identification of ‘critical values’ as part of determining the management objective. The concept of critical values is that by providing sufficient flow to sustain the most flow-sensitive, important value (species, life stage, or recreational activity) the other significant values will also be sustained (Jowett & Hayes 2004). Candidates for critical value status might include flow-sensitive rare or endangered species, or species with high fishery value. In cases where habitat analysis is used to inform flow management decisions, identifying critical flow- dependent in-stream values circumvents the complexities of interpreting a range of different species’ predicted habitat2 versus flow curves independently: 2. Assigning the level of maintenance of the critical flow-related factors that will ensure that the critical values identified in the management objective are maintained, according to the significance of these values (discussed further in Section 4.3). Note the distinction between critical values, which are what the flow management seeks to maintain (e.g. a population of a flow-demanding endangered fish), and critical factors — which are the aspects of the river or flow regime that need to be managed to ensure the critical values are maintained (e.g. habitat for the endangered species of fish): 3. Considering the amount of water allocation before deciding on the appropriate technical assessment methods to be used. This step can save considerable effort being expended needlessly, where out-of-stream demand is low and is unlikely to increase substantially. In such cases the abstraction is unlikely to have a significant effect on the in-stream values and a conservative minimum flow (say, the natural MALF) is unlikely to impinge significantly on security of supply for abstractors. Therefore, a low risk approach with minimal data requirements (setting a conservative minimum flow based on historical flow data, for example) may be justified, circumventing the need for expensive and time consuming in- stream flow assessment techniques. Consideration should be given to both current and probable future demand.

2 The predictions of hydraulic-habitat models are usually expressed as WUA (weighted usable area) or HSI (habitat suitability index) for chosen species or life-stages, with the predictions plotted against flow.

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Identify out-of-stream Identify and assess values of water the significance of resource instream values

Identify instream values that are to be sustained Determine the Instream Management Objective Identify the Critical Instream Values

Identify the Critical Factors to maintain instream values

Assign level of maintenance according to significance of instream values

Consider level of water allocation (degree of hydrological alteration)

Select appropriate technical method(s) according to significance of instream values and degree of hydrological alteration

Apply technical assessment methods

Flow regime requirements

MONITOR: Does the flow regime

No requirement sustain the Instream Management Objective

Figure 2. Framework for in-stream flow assessment, based on the Ministry for the Environment (1998) Flow Guidelines for in-stream values, with adaptations from Hayes & Jowett (2004) and the National Environmental Standards (NES; Beca 2008).

It is worth noting that monitoring following the implementation of flow-regime management is likely to detect only large reductions in fish or invertebrate populations, given the high degree of natural variability in most New Zealand rivers. Even then the detection of change depends in large part on the existence of quantitative data documenting the state of the system over a reasonable duration prior to the implementation of flow management. However, this does not diminish the need to monitor the response of in-stream values to flow management.

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4.3. Minimum flow and allocation limits

The Ministry for the Environment Flow Guidelines (1998) state that there are two critical parameters of a flow regime that need to be prescribed for sustaining in-stream values that are dependent on proper functioning of river ecosystems. These are:  a minimum flow to fulfil water quality and habitat requirements  flow variability.

Minimum flows are usually required to maintain in-stream habitat but they are also intended to meet minimum water quality requirements of in-stream life. As discussed above, provision of flow variability at a variety of scales is required for maintenance of channel form, sediment and periphyton flushing, benthic invertebrate productivity, fish and bird feeding opportunities, and fishing opportunities. Flow variability can be managed with allocation limits or flow sharing rules to maintain some floods and freshes for flushing, and perhaps some natural flow recession and especially to avoid long periods of flat-lining of the minimum flow.

The hydrological effect of a run-of-river flow allocation is illustrated in Figure 3. By removing the allocated flow (white band) the blue sections of the hydrograph above the allocation limit fall down onto the blue section below the minimum flow. The result is that sections of the hydrograph display flat-lining at the minimum flow. Increasing the allocation rate increases the frequency of flat-lining at the minimum flow with potential adverse consequences on benthic production, including the invertebrate food supply for fish and birds.

Figure 3. Illustrative hydrograph showing a minimum flow and the flow slice taken by the allocation limit. By removing the allocated flow the blue sections of the hydrograph above the allocation limit fall down onto the blue section below the minimum flow. The result is that sections of the hydrograph display flat-lining at the minimum flow.

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Figure 4 is the same hydrograph as in Figure 3, with the same minimum flow (1 m3/s), but showing the effect of run-of-river abstraction with relatively large allocation rate of 3 m3/s (2.6 x MALF) on the key flow features. Natural flows are represented by the blue line and flows after abstraction by the green line. The blue-shaded area represents that part of the hydrograph that potentially provides habitat for algal and benthic invertebrate production (following flood disturbance and resetting of communities).

Figure 4. Illustrative hydrograph showing effect of run-of-river abstraction with relatively large allocation rate (2.6 × MALF) on key flow features. Natural flows are represented by the blue line and flows after abstraction by the green line. Allocation = 3 m³/s, MALF = 1.15 m³/s minimum flow = 1 m³/s ). The blue-shaded area represents that part of the hydrograph that potentially provides habitat for algal and benthic invertebrate production (following flood disturbance and resetting of communities).

Both the minimum flow and allocation rate are important for sustaining aquatic life and dependent values. The minimum flow should provide for at least ‘minimum’ annual, or seasonal, habitat and water quality requirements of the target in-stream values (e.g., fish species). Living space for fish is more likely to be limiting at the minimum flow, and with fish concentrated in the remaining habitat, there is the potential for increased competition and predation risk — which may translate to lower growth and survival. Of course all of these potential effects will worsen if flow is drawn below the minimum, and will be exacerbated the longer low flows are sustained.

In some situations deciding upon an ecologically defensible minimum flow, based on in-stream habitat modelling results, can be straightforward; such as when optimum habitat occurs below or at the ecologically relevant flow (Figure 5a). Traditionally the minimum flow is set either at the optimum (resulting in an increase in habitat) or at the breakpoint in the habitat × flow relationship. This is where habitat is often represented by weighted usable area (WUA) — the habitat index calculated in hydraulic-habitat

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modelling (Jowett et al. 2008). The breakpoint is the point of greatest rate of change in the WUA × flow curve and can be justified on the basis that higher flows offer diminishing benefits for in-stream habitat. However, there is no scientific evidence that the breakpoint is correlated with biological response.

Unfortunately, there are many situations where setting minimum flow is not nearly so straightforward. For instance, in small streams or braided rivers optimum habitat for flow-demanding fish such as trout and torrentfish occurs above the MALF and declines monotonically through the flow range under negotiation (Figure 5b). Therefore, there is no clearly identifiable point at which in-stream conditions become good or bad. Rather the habitat simply gets worse as flow falls below the optimal value (although the rate of habitat change may vary with flow). In this case, minimum flows necessarily have to be negotiated on the basis of incremental (or percentage) changes in habitat and decision making can be facilitated by deciding upon levels of habitat maintenance.

Figure 5. Derivation of minimum flow based on retention of a proportion (90% in this case) of available habitat (weighted usable area; WUA) at: a) the habitat optimum, or b) the mean annual low flow (MALF), whichever occurs at the lower flow. This is as recommended by Jowett and Hayes (2004).

The level of habitat retention is arbitrary, since scientific knowledge of the response of river ecosystems, and fish populations in particular, is insufficient to confidently identify levels of habitat below which ecological impacts will occur. Jowett and Hayes (2004) recognised that, in practice, the choice of a habitat retention level is based more on risk management than ecological science. The risk of ecological impact increases the more habitat is reduced. When in-stream resource values are factored into the decision-making process, then the greater the resource value, the less risk is acceptable. With this in mind, Jowett and Hayes (2004) suggested that water managers could consider varying the percent habitat retention level (Table 1) depending on the value of in-stream and out-of-stream resources (i.e. highly valued in-stream resources warrant a higher level of habitat retention than low valued in-

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stream resources). This concept has been adopted by several regional councils in their flow-setting process (e.g. Greater Wellington, Hawkes Bay, Horizons, Environment Southland).

Table 1. Suggested significance ranking (from highest [1] to lowest [5]) of critical values and levels of habitat retention from Jowett and Hayes (2004).

Significance % habitat Critical value Fishery quality ranking retention Large adult trout — perennial fishery High 1 90 Diadromous galaxiid High 1 90 Non-diadromous galaxiid - 2 80 Trout spawning / juvenile rearing High 3 70 Large adult trout — perennial fishery Low 3 70 Diadromous galaxiid Low 3 70 Trout spawning / juvenile rearing Low 5 60 Redfin / common bully - 5 60

Ideally, the categories and levels of habitat retention ought to be set in consultation with the community and stakeholders to reflect the level of risk that is deemed acceptable given a certain perceived level of value. However, as discussed by Jowett and Hayes (2004), the suggested levels of habitat retention in Table 1 are conservative, in that they are unlikely to be proportional to a population response. Theoretically, a change in available habitat will only result in a population change when all available habitat is in use (Orth 1987). Since a range of factors other than habitat, especially flood size and frequency, can influence species abundance, population densities are likely to be at less than maximum levels in many cases, although as habitat is reduced there must ultimately come a point where habitat becomes limiting. That being the case, and speaking very broadly, a habitat retention level of, say 90%, would maintain existing population levels, whereas retention levels of 50% might result in an effect on populations, especially where densities were high.

As discussed in Section 4.1, the MALF is viewed as an ecologically relevant flow statistic for trout populations and native fish species (Jowett 1990, 1992, Jowett et al. 2008). On this basis, minimum flows are commonly set at the MALF or a proportion of it, usually based on percentage habitat retention and risk assessment (with fish habitat retention referenced to the MALF or flow at which habitat is optimum if that occurs a lower flow).

However, there is no compelling ecological reason why the minimum flow should be set at, or lower than, the MALF. Alternatively, a minimum flow could be set to provide for more than ‘minimum’, or even optimal, habitat conditions for the target value. A

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reason for setting the minimum flow higher than the MALF could be to mitigate potential adverse effects of a high allocation rate.

Once the minimum flow is set, consideration needs to be given to an appropriate flow allocation limit, or flow sharing rules, to maintain the key features of natural flow variability and avoid prolonged periods of flat-lining at the minimum flow. Setting the minimum low at the MALF, or less, in the absence of appropriate allocation limits risks adversely affecting benthic production and the food supply for fish and birds. As discussed in Section 4.1, the median flow (or seasonal medians) can be viewed as providing an approximation of the typical habitat conditions able to be utilised to support benthic invertebrate production.

Maintenance of invertebrate production (which fish depend on for food) is arguably more dependent on allocation limits or flow sharing rules, which ensure that the median flow is not substantially reduced by abstraction, than on the minimum flow per se.

Water managers need to understand the interplay between the minimum flow and allocation limit. The risk of adverse effects increases with decreasing minimum flow, increasing duration of minimum flow, and increasing allocation volume. The pros and cons of higher or lower minimum flows can be interpreted with respect to the following principles:

 A higher minimum flow results in higher levels of habitat retention, reducing the risk that the minimum flow will adversely affect the critical in-stream values and dependent fisheries and mahinga kai3.  On the other hand a higher minimum flow decreases the security of supply of water to abstractors, assuming the same allocation rate.  A lower minimum flow increases the risk that critical in-stream values will be adversely affected and so consideration should be given to reducing the allocation rate to offset this risk.

The last point above highlights the interplay between the minimum flow and allocation rate in influencing effects of flow alteration on river ecosystems.

There are various methods of deriving allocation limits in conjunction with a minimum flow. One method, which has been used by Horizons Regional Council (e.g. Roygard & Carlyon 2004; Hurndell et al. 2007), quantifies the expected increase in the frequency and duration of occurrence of the minimum flow in response to different total allocation volume scenarios. A possible alternative method, outlined in Jowett

3 Garden, cultivation, food-gathering places.

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and Hayes (2004, Section 6. Total allocation), involves trading off the magnitude of the minimum flow against the total allocation volume.

The frequency of occurrence and duration of the minimum flow will affect the surety of supply for abstractors (through abstraction restrictions), but also has the potential to have ecological effects, as discussed above. The method employed by Horizons Regional Council would lend itself well to community consultation, whereby stakeholders could negotiate the frequency and duration of minimum flow occurrence that they deem acceptable, on the basis of relative in-stream values and out-of-stream water uses (including requirements for surety of supply).

The discussion above focuses on in-stream flow requirements. Consideration should also be given to whether hydrological alteration of rivers will affect connectivity of rivers with riparian wetlands, springs and groundwater (Beca 2008).

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5. IN-STREAM VALUES IDENTIFIED FOR THE WAIRAU

The in-stream ecological and fishery values currently recognised in the WARMP generally align well with those identified and discussed through the hearing process for Trustpower’s Wairau Hydro-electric Power Scheme, for the relevant section of the river (i.e. WARMP Volume 1 Appendix A values for the section of the Wairau between Wash Bridge and State Highway 1).

In the initial round of caucusing during the consent hearing, agreement was reached that Trustpower experts had “… selected appropriate indicator measures and organisms to describe the effects of the Scheme in terms of the maintenance of aquatic values in the residual river. These indicators include:

1. Wetted area (related to benthic production points 4 and 5) 2. Brown trout 3. Dwarf galaxias 4. Deleatidium mayfly 5. Food production 6. Black fronted terns — not directly discussed by aquatic experts, but provides part of the rationale for the discussion of other indicators.” (Hayes, Environment Court evidence, paragraph 4.27).

Significant ecological values of the Wairau River (with respect to Sections 6(c) and 7(h) of the Resource Management Act (RMA)4, were summarised in the evidence of Hayes (paragraph 5.3), as supporting:

 A population of the ‘at risk’ native fish – dwarf galaxias (Galaxias divergens) — where ‘at risk’ means ‘declining’.  A major, and internationally significant, population of the nationally endangered black-fronted tern (Sterna albostriata).  A population of nationally endangered black-billed gulls (Larus bulleri).  A wetland of international importance because it supports more than 1% of the population of an endangered species, i.e. blackbilled gull and black fronted tern (IUCN or RAMSAR Convention criteria).  A brown trout fishery of national importance at the catchment level and at least of regional importance in the proposed diversion reach.

4 Section 6 Matters of national importance, (c) the protection of areas of significant indigenous vegetation and significant habitats of indigenous fauna. Section 7 (h) the protection of the habitat of trout and salmon.

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By way of comparison, the WARMP lists the following ecosystem related values provided by the section of the Wairau River between Wash Bridge and SH1:

 River corridor for fish and birds  Fish passage and habitat  Trout and salmon spawning  Habitat for several species of water birds  Fishing for trout, salmon and eels  Native biodiversity (including indigenous fishes).

Although Chinook salmon and eel fishery values were not explicitly included in the list of indicator values agreed during caucusing, these are nonetheless recognised as important values of the river. For example, the salmon fishery was classified as regionally important in the evidence of Mr Deans (paragraph 3.1).

An obvious point of difference between the agreed list of values and those already listed in the WARMP is the inclusion of food production in the former. Here food production referred to productivity of benthic macroinvertebrate communities in respect of food for fish and birds. As discussed in Sections 4.1 and 4.3 this is a critical consideration in setting an ecological flow regime. Given the importance of invertebrate productivity to support other listed values it should be included in the list of recognised values in Appendix A to the WARMP.

With regard to the native fish values of the catchment, Appendix A to the WARMP notes ‘Threatened fish’ refers to species listed in the Department of Conservation publication “Setting priorities for the Conservation of New Zealand’s Threatened Plants and Animals”. The conservation status of New Zealand’s native fish has recently been updated (see Goodman et al. 2014). Classifications within the Plan will require updating as part of the plan review process to align with most recent threat classification rankings.

5.1. Critical values and critical habitat

As discussed in Section 4.2 identification of ‘critical values’ ought to be part of determining in-stream management objectives. The concept of critical values is that by providing sufficient flow to sustain the most flow-sensitive, important value (species, life stage, or recreational activity) the other significant values will also be sustained (Jowett & Hayes 2004).

Of the ecological values in the Wairau River adult brown trout and benthic invertebrate habitat have the highest flow requirements combined with the highest values (Hayes

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evidence para 5.9). The NIWA Section 92 technical review confirmed this (Norton et al. 2005, p 7), stating: “…our reviewers identify that, in terms of aquatic values, fish food production and adult brown trout habitat have the highest flow requirements and would therefore be the in-stream value most affected by reduced flows in the Wairau mainstem”. Flows set for these values (i.e. food production and adult brown trout) should be adequate for other ecological values. Passage requirements for adult salmon will also be flow critical.

The brown trout fishery extends throughout the river (as summarised in the evidence of Mr Deans). The most important reaches are in the lower river (downstream of The Narrows) on the basis of drift dive counts and highest angler usage, and in the upper reach (above Wash Bridge) on the basis of quality of its backcountry fishery. About half of the fishing effort is in lower river, below SH6, due to proximity to Blenheim, accessibility, navigability by boats and relatively high fish numbers in the lower single channel river (Deans evidence). The upper Wairau River (above Wash Bridge, or Goulter Confluence) is regarded as a headwater fishery in a near pristine river, with relatively few, but also trophy-sized fish (Deans evidence). In the opinion of Mr Deans, the section between should be viewed as an integral component of the fishery because it contributes to maintaining resilience of the total trout population and diversity of angling opportunity in the catchment. The middle river is intermediate in its features and receives less but still significant use, providing challenging fishing in a braided river environment (Deans evidence).

With regard to minimum flow setting, the lower reaches of the Wairau could arguably be viewed as critical on the basis of:

 The relatively high trout abundance, as assessed by drift diving  The relatively high level of fishing effort (~50% of fishing effort)  The known flow loss to groundwater in this section of the river (~8 m3/s) exacerbating flow reductions during periods of low flow upstream.

5.2. Management objectives and levels of maintenance

Including clearly-defined management objectives and desired levels of maintenance in the Plan, to reflect community desires, would be beneficial. It may alleviate potential disagreement on a case by case basis at consent hearings. Environment Southland provides an example of how this could be done. Critical in-stream values for particular surface water management units and required levels of habitat maintenance are defined in an appendix to the regional water plan (Appendix I to the Southland Regional Water Plan), along with specification of technical assessment methods to be applied to flow setting, depending on the level of allocation pressure.

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The Trustpower Wairau hearings illustrate the contention and lengthy legal process that can arise when management objectives and maintenance levels (see Section 4.2) are not clearly defined. Despite reaching agreement on key ecological values for the Wairau, appropriate management objectives for these values and particularly appropriate levels of maintenance remained the main point of contention between those providing in-stream ecological evidence at the Trustpower hearings. This was recognised by Hayes (evidence paragraph 4.28) “While we have agreed on the appropriate indicator measures and organisms, we could not reach agreement on the management objectives or a flow regime to achieve these objectives. In my opinion the flow regime proposed by Trustpower is unlikely to sufficiently fulfil habitat requirements to maintain ecosystem productivity in the residual reach, because it does not maintain sufficient mid-range flow variability to support food production and feeding opportunities for fish.”

Those providing evidence on behalf of Fish & Game focused on maintaining abundance and productivity of populations, as exemplified by Hayes’ recommended management objectives (paragraph 5.6) “…in my opinion, appropriate in-stream management objectives for the Wairau River are the maintenance of:

 the black-fronted tern and black billed gull populations  the dwarf galaxias population  the brown trout and salmon fishery  aquatic macroinvertebrate communities and productivity.”

Aquatic macroinvertebrates were included in the list of in-stream management objectives because they are critical to life supporting capacity, especially in the context of sustaining the productive capacity of the ecosystem for maintaining the other in-stream management objectives. In this context it is important to maintain diversity, density and overall abundance.

By contrast Trustpower’s experts focused on maintaining similar ecological structure and or function, as summarised in evidence by Ryder (paragraph 6.28) “…the emphasis that myself and colleagues adopted for developing ecological management objectives in relation to the proposed Scheme has been one of maintaining the general functioning of the existing river ecosystem through a flow regime that reflects seasonal flow patterns and maintains the braided nature of the river…”. This difference in focus for defining management objectives translated into a difference in opinion regarding adequate levels of habitat maintenance and associated flow requirements. This is not surprising since flow regime requirements are essentially dictated by the management objectives identified.

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The approach of maintaining the general functioning of the existing river ecosystem was apparently accepted by the consents hearing commissioners and Environment Court since Trustpower’s proposed flow regime was adopted in the consent conditions. However, we still hold the view that this is not sufficient for sustaining fisheries values (e.g., trout, salmon and eels), since abundance and productivity, rather than just presence or absence, are fundamental to maintaining these values. An analogy is a farm. The presence of a community of pasture grasses alone does not underpin an economically viable farming operation. Rather it is the maintenance of the grass community and total production (i.e. dry matter produced per year) that is important. It is the biomass of grass produced per hectare multiplied by the area of farm (total hectares) that underpins the productivity and viability of the farm. Likewise, in a river it is the total area of benthic habitat that remains wet for long enough to support benthic production that underpins productivity. This wetted area is continually changing in a river in response to varying flows; but the most relevant flow range for supporting benthic productivity is from low flows to about the median.

There are two issues related to flood size and frequency that complicate the argument around the need to provide for higher flows than the MALF (or minimum flow) to maintain the production of benthic invertebrates. Both issues were the subject of debate between experts in the Trustpower hearings. The first is whether the flood frequency allows sufficient time for margin habitat wetted by flood recessions to contribute significantly to benthic production in addition to the production supported by the MALF. At the time of the Trustpower hearings there was no cost effective means of determining this, but now there is. A new model, BITHABSIM (Benthic Invertebrate Time series Habitat Simulation) can be applied to a hydrograph and invertebrate WUA – flow relationship to estimate the retention of productive invertebrate habitat by a modified flow regime relative to the natural flow regime (Olsen et al. 2013). However, a data input requirement is a relationship between bed disturbance and flood flows but Hudson did not include flood flow modelling in his 2-dimensional hydraulic-habitat modelling results for the Trustpower Wairau Hydro-electric Power Scheme hearings. It is unlikely that his models could be used to model flood flows since his topographical measurements “…extended from about 30–50 cm above the area inundated at the highest modelling flow (65 cm)” (Hudson et al. 2005). So in order to run BITHABSIM on the Wairau River additional hydraulic modelling would be required.

The second issue is whether fish (and birds) are food limited, such that a reduction in the area of productive invertebrate habitat would adversely affect their abundance or growth. In braided rivers flood size and frequency is the key factor potentially suppressing abundance of fish. The same applies to river birds, through nest destruction, although bird experts believe avian and probably mammalian predation is the major source of mortality. The issue of whether the area of productive invertebrate habitat limits the growth and abundance of fish in the Wairau, and other New Zealand rivers, remains unresolved owing to lack of research. An environmentally cautious management response to this information vacuum is to assume that habitat and food

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is limiting when WUA — flow relationships are interpreted. In our experience, this has been the approach followed by commissioners in most recent flow management hearings in which habitat modelling has been used to inform flow decisions in New Zealand. The Trustpower Wairau hearings were exceptions.

In order to make provision for potentially productive invertebrate habitat above the MALF a portion of the flows between the MALF and seasonal median flows would need to be retained in the river. This could be accomplished (as discussed in Sections 4.3 and 9).either through:

 reduced allocation  increased minimum flows  some form of flow sharing.

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6. FLOW IMPLICATIONS OF TRUSTPOWER WAIRAU HYDRO- ELECTRIC POWER SCHEME DECISION

Consent conditions outlined in the Environment Court decision on Trustpower’s proposed Wairau Hydro-electric Power Scheme5 stipulate flow regime requirements for the residual river reach and controls on fluctuating flows downstream of the Scheme (See consent conditions 173–188). Conditions require that abstraction must cease when:

 flow at the Wash Bridge exceeds 200 m3/s  flow at Wash Bridge is below 10 m3/s  flow immediately below PS5 is below 14 m3/s.

In addition, minimum flow provisions are stipulated at three locations. The two main minimum flow locations are immediately below the intake and immediately upstream of the outfall from the last power station on the proposed canal (power station 5 [PS5]). The third location is below the Goulter River confluence and is intended to also include Branch River residual / spill flow and discharge from the existing Branch power station tailrace.

Seasonally varying minimum flows are stipulated immediately below the intake (to be calculated based on the flow at Wash Bridge minus the abstraction) and immediately upstream of PS5 (Table 2) with the caveat that flows are allowed to naturally fall below these minima.

Table 2. Monthly minimum flows (m3/s) stipulated in consent conditions for Trustpower’s proposed Wairau River Hydro-electric Power Scheme.

Months Minimum flow below Minimum flow above PS5 intake Jan–Jul 10 15 Aug 12 17 Sep 15 20 Oct–Nov 20 25 Dec 15 20

An additional flow sharing regime is also stipulated between 1 October and 31 January, applying to flows measured immediately below the Goulter River confluence (condition 177). This condition applies whenever flow at the Wash Bridge exceeds the

5 Available from http://www.marlborough.govt.nz/Services/Resource-Consents/Trustpower-Decision.aspx (accessed on 5/2/14).

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seasonal minima shown in Table 2 by 5 m3/s or more. Essentially it provides for the first 5 m3/s above the seasonal minimum to be abstracted, then a 1/3 share of flows above this level to be retained in the river. However, since the measurement point for this flow sharing is downstream of the Branch Scheme power station tailrace, all or part of the 1/3 flow share for the river can be abstracted at the intake and used to generate power through the Branch Scheme power station before being returned to the river.

The flow regime resulting from these conditions would cause the median flow at the intake to be reduced by 59.3%, from 25.3 m3/s to 10.3 m3/s. However, the proportional flow reduction attenuates downstream due to tributary inflows, especially downstream of the Goulter confluence. Immediately upstream of the outfall the median flow will be reduced by 43.1% (from 54.3 m3/s to 30.9 m3/s).

Conditions to control flow fluctuations below PS5 include:

 A direct control on the maximum levels of flow fluctuation allowed during low flow periods, such that when flow at Tuamarina is ≤ 15.5 m3/s, the magnitude of flow fluctuation must not be greater than ±10% around the 24 hour rolling average flow.  A control on the proportion of canal flows that can be retained behind PS5, such that when the 24 hour rolling average inflow to PS5 is > 5 m3/s, the discharge from this power station must not fall below 50% of the 24 hour rolling average inflow.  A control on the allowable rate of change in the discharge from PS5, a maximum ramping rate of 20 m3/s per hour.

The effect of these flow conditions is that flow fluctuations between Marchburn and Tuamarina relative to those under the existing Branch Scheme will be increased for flows > 30 m3/s at Tuamarina, but be decreased for flows below 20 m3/s (Ryder evidence, paragraph 12.7). According to Mitchell (evidence paragraph 3.22b), during critical irrigation / low flow periods (< 15.5 m3/s at Tuamarina), PS5 flow fluctuations will be in the order of half what they are currently due to the operation of the existing Branch Scheme (i.e. current fluctuations are approximately ± 20% around the 24 hour rolling average flow (paragraph 6.12).

During maximum hydro-peaking, water level fluctuations at Tuamarina are expected to be in the order of 100–150 mm (Mitchell evidence appendix paragraph 11.21) corresponding to a fluctuation in wetted width of approximately 24 m, with total width varying between 102 m and 130 m (Hayes evidence paragraph 7.54). By contrast, the existing Branch Scheme typically induces water level fluctuations at Tuamarina of 50– 120 mm (Mitchell evidence appendix paragraph 11.22).

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By way of comparison, the Branch Scheme has a consented take of 29 m3/s, a sustained maximum take during normal operation of 20 m3/s, a maximum generation capacity of 20 m3/s, and no ramping restrictions (Mitchell evidence paragraph 6.6). When inflows to the Branch Scheme are less than 20 m3/s (~80% of the time), the Scheme is operated to maximise generation during morning and evening peak power usage periods (Mitchell evidence paragraph 6.10; discussed below).

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7. POSSIBLE MINIMUM FLOW AND ALLOCATION REGIME FOR THE LOWER WAIRAU RIVER

The lower reach (downstream the Waihopai confluence) is arguably the critical reach for flow setting, owing to its high in-stream values and the fact that it experiences the lowest flows due to losses to ground combined with the cumulative effect of abstraction upstream.

As part of investigations for Trustpower’s proposed Wairau Hydro-electric Power Scheme, Hudson et al. (2005) developed a 2-dimensional (2-D) hydraulic-habitat model for a reach in the vicinity of Wratts Road, approximately 7 km upstream of Tuamarina. This model was largely excluded during the hearing process because it fell outside the section of the river directly affected by the proposed diversion, with most discussion focusing on three other modelled reaches (Argyle, Hillersden, Marchburn). There was much discussion during the hearings about the reliability of the habitat models developed by Hudson et al. (2005), with disagreement regarding the adequacy of calibration and validation in particular. Ultimately, Mr Jowett was called upon to review the models and give an opinion on their adequacy. Some aspects of the models were adjusted by Dr Hudson in an attempt to improve performance and he provided some of the underlying data to Mr Jowett to help with his assessment. Mr Jowett concluded that the Argyle and Hillersden models appeared to overestimate water level, suggesting problems with calibration (Jowett evidence addendum paragraph 11). However, he considered that general statements about the response of habitat to flow could be made on the basis of the Marchburn model (paragraph 55). This is despite some reservations with the model performance (e.g. “…possible errors of up to 3 m3/s in main channel flows in the Marchburn reach and poor channel definition in much of the main channels”, paragraph 46). The Wratts Road model was not subjected to this additional work and scrutiny. Consequently, the accuracy of predictions from this model remains unknown. Nevertheless, in the absence of other information we have used the predictions of Hudson’s Wratts Road model to help inform a minimum flow for the lower reach based on habitat retention relative to flow statistics6 (MALF and median flow) (c.f. concepts discussed in Section 4). The relatively high level of uncertainty regarding the accuracy of the modelling predictions on which this analysis was based should be borne in mind when interpreting the results discussed below.

Figure 6 shows the predicted habitat — flow response for adult brown trout, Deleatidium mayfly7 and invertebrate food producing in the Wratts Road reach.

6 Ideally natural flow statistics (MALF and median) should be used for habitat retention analysis. However, as these were not readily available we used non-naturalised MALF and median flows here. This means that minimum flows estimated from our analysis actually retain a slightly lower percentage of habitat relative to the natural MALF. The error will be small because the flow record is long relative to the period over which significant abstraction has been occurring. 7 In Hudson’s Wairau hydraulic-habitat models Deleatidium habitat suitability criteria were originally applied without the influence of depth and substrate composition was set to a uniform value throughout each reach, i.e.

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Habitat for adult brown trout is predicted to peak at about 20 m3/s and to decline fairly linearly with flow reduction below about 15 m3/s. Habitat for the two invertebrate suitability criteria continues to increase with flow across the modelled flow range.

Figure 6. Predicted habitat (WUA) response for adult brown trout, Deleatidium mayfly and invertebrate food producing in the Wratts Road reach of the Wairau River, based on a 2- D hydraulic-habitat model developed by Hudson et al. (2005).

The lower Wairau River would arguably warrant a 90% level of habitat retention for trout (Figure 6) and according to Hudson’s Wratts Road model this would be delivered by a minimum flow of 10.4 m3/s, based on a MALF of 13.4 m3/s at Tuamarina. A minimum flow of 8.4 m3/s would provide for 80% habitat retention. This compares with the existing 8 m3/s minimum flow at Tuamarina in the WARMP. As mentioned in Section 4.3, this approach of calculating minimum flows based on percentage habitat retention has been widely used by regional councils in other areas (e.g. Environment Southland and Horizons, Hawkes Bay, Greater Wellington regional councils).

The Trustpower Wairau Hydro-electric Power Scheme conditions include seasonally varying minimum flows. However, they were necessary to mitigate the hydrological and ecological effects of very large allocation (350% of MALF). Where allocation is low–moderate there is no need for seasonally varying minimum flows. The rationale for this view is that the summer minimum flow will be the key annual limit to space for fish so higher minimum flows at other times of the year will not provide additional benefit. However, seasonally higher minimum flows will provide more habitat for benthic production which can be cropped by fish and birds. Alternatively, a single

the habitat predictions for Deleatidium were based solely on water velocity suitability. This may bias against the value of deep-fast water as habitat and potentially slightly underestimate flow requirements for Deleatidium.

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minimum flow with reduced allocation will achieve the same result because abstraction will not be sufficiently great to draw the river down to the minimum flow during seasons with higher median flow (e.g. spring in the Wairau River).

Allocation limits (for B and C class water) or flow sharing are important to retain key features of flow variability for maintaining channel morphology, controlling terrestrial vegetation, flushing periphyton and fine sediment, benthic invertebrate productivity, and fish migrations. In respect of fish migrations, floods and freshes are critical for allowing salmon passage which often is naturally limited at low flows (e.g. ≤ MALF) and therefore may not be provided for by the minimum flow. Salmon passage is highly likely to be limited at flows at and below the MALF upstream of the Waihopai confluence. The 2-D modelling by Hudson et al. (2005), undertaken for the Trustpower Wairau Hydro-electric Power Scheme hearings was unable to cover sufficient length of river to include critical shallow bars that restrict adult salmon passage at low flows, so it will have given a misleading impression of adequacy of salmon passage. Shallow bars (with maximum depths less than the critical depth for salmon passage; 25 cm) are relatively frequent in mid-summer / autumn, when salmon are in the river (pers. obs. J. Hayes, Cawthron Institute).

The proposed NES for Ecological Flows and Water Levels (MfE 2008) suggested default limits that provide a starting point for considering flow allocation rates. The proposed interim limits for rivers with a mean flow greater than 5 m3/s are a minimum flow of 80% of MALF and total allocation of 50% of MALF (MfE 2008). By contrast, the allowable B class allocation in the WARMP (15 m3/s) is 112% of MALF. On top of that unlimited C class allocation is allowed with 2:1 flow sharing available at flows above 30 m3/s.

Invertebrate habitat retention can be referenced to the median flow, which is indicative of the typical flows supporting invertebrate production (as discussed in Section 4). According to Hudson’s Wratts Road habitat model, a flow of ~32–45 m3/s would be required to maintain 90% of food producing and Deleatidium habitat relative to that at the median flow (~60.7 m3/s at Tuamarina). Alternatively, 26–31 m3/s will provide 80% of habitat retention.

This suggests that abstraction below 30 m3/s has the potential to substantially adversely affect benthic invertebrate habitat. However, the speed of typical flow recessions in the Wairau in late summer is such that flow falls to 20 m3/s within about 10 days following a flood. This is too short a time for newly wetted margins to contribute significantly to benthic production so the effects of allocation drawing flow prematurely down to this level will be minor. It takes 20 days or more for flow to fall to 15 m3/s following a late summer flood in the Wairau. Channel wetted by flows of 15 m3/s or less will therefore contribute more significantly to benthic production, given typical benthic invertebrate re-colonisation rates (see Section 4.1). Early summer flow recessions are longer so flows above 15 m3/s, especially between 15 m3/s and 20

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m3/s, will also benefit benthic production then. A point to bear in mind is that the effect of allocation on productive benthic habitat is a continuum. Because the best habitat for benthic invertebrates is that which remains wet for longest, the effect of allocation on reducing productive habitat is greatest for flows near the minimum. The effect will then decrease for higher flows through to about 30 m3/s in summer, but probably higher flow in spring — early summer when base flows are higher.

Full allocation of B class water under the current WARMP will have a substantial adverse effect on potentially productive benthic invertebrate habitat above the minimum flow. For example, if 15 m3/s (full B class allocation) were abstracted above a minimum flow of 8 m3/s then food producing (general invertebrate) habitat would be reduced by about 50% and Deleatidium habitat by about 36 %, based on Hudson et al.’s (2005) Wratt Road habitat modelling predictions. If the 2:1 flow sharing rule were applied this effect would be reduced by one third.

Given the magnitude of the potential effect of current allowable flow allocation on benthic invertebrate habitat (the basis for in-stream production for fishes and birds), there is justification for reducing the B class8 allocation in the WARMP — at least for flows less than 20 m3/s. There appears to be scope to do so without adversely affecting existing water users since less than half of B class water allowable in the WARMP has currently been allocated. Moreover, reducing allocation would remove the need for flow sharing which has been too difficult to implement (see Sections 2 and 9). A B class allocation of 6.7 m3/s, slightly more than current total abstraction, would be consistent with the default suggested in the NES for large rivers, i.e. 50 % of MALF. In addition to reducing adverse effects on in-stream benthic production, and fish passage, reduced allocation will also avoid a reduction in security of supply to existing abstractors that would otherwise occur as more of the current allowable allocation is consented.

However, in the absence of flow sharing a gap between B and C class allocation should also be considered to retain a proportion of ecologically relevant mid-range flow variability, especially that providing for trout and salmon passage (see Section 9). Without a gap the B and C class allocation effectively become a single large allocation block.

8 Note: the discussion of possible changes to B class allocation limits and minimum flows in this report assumes that the status quo zero A class allocation from the Wairau River remains unchanged.

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8. CONSEQUENCES OF HYDRO-PEAKING FLOW FLUCTUATIONS

8.1. Existing Branch Hydro-electric Power Scheme

During periods of moderate flows, Branch River water is held back in Lake Argyle overnight, and then used to generate during the day. At lower flows when continuous daytime generation is not possible, a twice daily generation regime to meet the morning and evening peaks is usually operated. In addition, at these flows power generation follows a weekly pattern, with more water being held back in the weekends, due to lower demand, allowing higher levels of generation during the week (Wadsworth, MDC hearing evidence, paragraph 72.1). The typical effect of this hydro- peaking on minimum and maximum flows experienced downstream are shown in Figure 7. The flow fluctuations are largest immediately below the Branch Scheme tailrace outfall and attenuate downstream. Discharges of 10–15 m3/s from the Branch Scheme typically result in fluctuations of 2–4 m3/s at Tuamarina (over 70 km downstream, c.f. Proposed Scheme outfall ~32 km from Tuamarina) (Wadsworth, MDC hearing evidence, paragraph 87).

Figure 7. Wairau River schematic longitudinal flow profile showing daily maximum and minimum flows resulting from Branch Scheme generation hydro-peaking during low flow conditions. Operation during low flow periods typically involves two hydro-peaking cycles per day.

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Flow fluctuations of the type associated with hydro-peaking have recognised adverse ecological consequences, including fish stranding and reducing productive habitat. Young et al. (2011) reviewed literature on fluctuating flow effects on fish. They found that fish stranding is more common in rivers with relatively low lateral gradient (like the Wairau), than in steep sided rivers. As well as being beach stranded (i.e. left high and dry), which is generally fatal, fish can become stranded in isolated patches of water, such as when side channels become disconnected from the mainstem. While fish may survive in such isolated patches of water for relatively long periods if conditions are favourable, they can be exposed to increased predation risk, as well as heat stress and depleted dissolved oxygen, with potential impacts on growth, condition, and survival. Although mortality / loss rates may be relatively low for individual stranding events, the cumulative effects of repeated events may be of greater consequence. For example, in a North American river estimated loss of salmon fry per drawdown was 1.5%, with a total loss of 59% of fry population over one season (Young et al. 2011). Little research has been undertaken on short term stranding losses in New Zealand rivers, and none on long-term losses. Other negative effects of fluctuating flows on fish include: downstream displacement, dewatering of spawning sites, disruption of migration, and reduced food availability. Hydro-peaking can influence the daily pattern of invertebrate drift and foraging by drift feeding salmonids — with more drift and fish feeding more actively when the flow is increasing (Miller & Judson 2014).

Frequent (daily to weekly) flow variations commonly experienced downstream of hydropower stations create a ‘varial’ zone that is repeatedly wetted and dried as water levels rise and fall (discussed in Olsen evidence paragraph 5.38). With frequent flow fluctuations, this zone will not sustain immobile plant and invertebrate species (Beca 2008). The density, biomass and species diversity of benthic macroinvertebrates are negatively affected by frequent flow fluctuations (Young et al. 2011). While mobile species such as fish, and probably some invertebrate species, can make use of recently inundated areas of river bed, a varial zone that is wetted and dried at more frequent intervals than a week is unproductive and can be regarded as lost habitat (Beca 2008). Thus, the minimum flow in a hydro-peaking cycle (the blue line in Figure 7) will define the lowest extent of continuously wetted (and therefore productive) habitat. The loss of productive habitat in the varial zone may affect fish through reduced food supply (Young et al. 2011). This potential adverse effect on reducing the area of productive benthic habitat needs to be put in context of natural flow fluctuation and in particular the frequency of natural floods / freshes and the rate of natural flow recession. During periods of frequent floods the natural variability in flow and water level will far exceed that of hydro-peaking, and will naturally limit benthic production in the river margins. However, the Wairau River experiences long periods of flow recession and low flow during summer, when irrigation demand is high. It is these periods when hydro-peaking will have an adverse effect on benthic production — and summer is also the time when fish have highest food requirements owing to temperature controlling their metabolism. Whether this potential effect translates to a real adverse effect on fish growth and abundance is unknown. It depends on whether

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fish growth and abundance is limited by the quantity of invertebrates, which is related to the total area of available invertebrate habitat (see Section 5.2). This was a hotly debated topic in the Trustpower hearings over the Wairau Hydro-electric Power proposal and could not be resolved owing to lack of research on the Wairau and other New Zealand rivers.

As well as probable ecological effects, the flow fluctuations caused by hydro-peaking have caused difficulties for MDC managing abstraction in the reaches below the Branch Hydro-electric Power Scheme. During low flow periods, hydro-peaking can cause flow to fluctuate above and below the minimum flow at Tuamarina within a day, making it difficult to decide whether and when to shut-off abstractors (discussed further below). Also since the flow peaks and troughs have already passed the reach where most abstraction occurs by the time they are recorded at Tuamarina, management action triggered from this point would come too late to avoid adverse effects upstream or to allow additional abstraction when flow peaks are passing through.

8.2. Abstraction shut-off level: flow minima versus daily or rolling average

As already mentioned the 2:1 flow sharing regime specified in the WAMRP has not been implemented due to the difficulties of managing the fluctuating flows from the Branch Scheme hydro-peaking and having to use a flow recording point that is downstream of abstractions. Instead MDC have used the average flow on a given day to assess whether abstraction is to be shut-off the following day. This approach has been criticised by the Nelson-Marlborough Fish & Game Council because it means that flow will have already dropped below the minimum flow for all or part of a day, or potentially for part of several days, before abstraction is ceased. The problem is exacerbated by the flow fluctuations caused by hydro-peaking. As flow approaches the 8 m3/s shut-off level for B class consents, the river typically fluctuates 1 to 2 m3/s either side of the daily average. Although flow may drop below the minimum threshold for part of the hydro-peaking cycle, the average flow over the period may remain above the threshold for abstraction to cease, due to the high flow phase of the hydro- peaking cycle. There has been discussion and an investigation into using a moving average flow, rather than daily average flow to alleviate this concern (Wadsworth 2013a). Four alternative approaches are demonstrated in Figure 8. Essentially, the longer the averaging period the less frequently abstraction restrictions would have to be switched on and off.

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Figure 8. Comparison of alternative methods for averaging flow data to trigger abstraction restrictions for the Wairau River, based on data from March 2013 at Tuamarina. The broken horizontal lines indicate whether abstraction would be shut-off (blank sections) or not under each scenario, with the colour of these lines matching those in the hydrograph above. Figure adapted from Wadsworth 2013a.

However, from an ecological perspective using the flow minimum in a hydro-peaking cycle as the shut-off trigger level is more relevant than the daily average or a moving average. This is because the minimum water level in the cycle will define the lowest extent of continuously wetted (and therefore productive) habitat. As discussed above, habitat that is repeatedly wetted and dried during flow fluctuations is effectively lost to invertebrate and periphyton production unless the periods of drying are sufficiently short to avoid desiccation. Conversely, the high flow part of the hydro-peaking cycle could be viewed as ‘wasted flow’, in that it confers no tangible ecological benefit (since it simply wets the varial zone which has been regularly desiccated).

While using the hydro-peaking minima to trigger abstraction shut-off is more defensible from an ecological perspective, there are two obvious potential disadvantages with this approach. First the flow minima may occur at times that are inconvenient to achieve or enforce shut-off compliance (e.g. in the middle of the night). This could be addressed either by predicting the minimum flow on a given day by extrapolating from the minima of previous days, or simply using the flow minimum from the preceding day as the shut-off trigger, in much the same way as the daily average flow has been used to date. The latter approach would still mean that flow would have been less than the minimum flow for part of the day before abstraction

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would be shut-off. However, the period below the minimum flow before shut-off is likely to be much shorter. This method could be adapted to avoid missing abstraction during the rising limb of a flood by including a switch from an upstream recorder site (e.g. Dip Flat), taking expected travel time into account.

The second problem with using the hydro-peaking flow minima as an abstraction shut- off trigger is that it would potentially be inefficient, in that during the high flow phase of the hydro-peaking cycle flow would be above the shut-off trigger for part of the day but would be conferring no real environmental benefit. Also, it is likely to be seen as unfair for downstream water users to be shut-off simply as a result of the exercise of Trustpower’s Branch Hydro-electric Power Scheme consent when they would not have been under normal flow conditions without hydro-peaking. One way to address this would be to allow targeted abstraction of the fluctuating flow peaks (discussed further below).

An alternative approach, suggested by Nelson Marlborough Fish & Game, is to continue with either the daily average or a rolling average as the flow trigger but to increase the minimum flow in recognition that instantaneous flow would actually fall below the average for part of the hydro-peaking cycles. Raising the minimum flow by half the expected maximum daily hydro-peaking flow range (i.e. 2 m3/s at Tuamarina) would essentially result in a similar increase in the incidence of abstraction restrictions as using the hydro-peaking minima as the abstraction shut-off with the existing minimum flow. A buffer (or safety margin) of this sort on the minimum flow would also help to address the fact that the response of flow to abstraction shut-off can be delayed, particularly for groundwater and gallery type takes (pers. comm. Val Wadsworth, MDC hydrologist).

Moving the monitoring point for abstraction shut-off upstream of the Branch Scheme outfall would theoretically avoid the problem of when in the hydro-peaking cycle to implement rationing / shut-off takes. Since the hydro-peaking would occur downstream of the monitoring site the shut-off trigger flow (management flow) at the site would not be affected by the hydro-peaking. However, the flow fluctuations would continue to occur below the monitoring point, i.e. it would not alleviate the environmental impacts. Also, the high flow released during hydro-peaking would effectively become unavailable for abstraction with this approach, which is inefficient from water users standpoint.

Flow regulation ponds on the outfalls from the Branch power scheme and the proposed Wairau scheme would offer improved flow management efficiency and better protection for in-stream life (i.e. by reducing flow fluctuation). While this is not a requirement of consent conditions for either of these schemes, MDC could still consider constructing regulation ponds as part of its river works programme. This would have the advantage of mitigating the adverse environmental effects of hydro- peaking and reducing the likelihood of artificially induced breaches of the minimum

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flow for abstractors downstream, while potentially also saving staff time in monitoring compliance in a complex system.

The option of targeting abstraction to harvest the flow peaks in the hydro-peaking cycle may have merit. These ought to be predictable in timing and approximate magnitude, given the period of peak electricity demand is fairly consistent from day-to- day. This may simply require timers to be placed on the power supply of irrigation pumps with a known rate of take capacity. However, the efficacy of run of the river takes to remove water from these flow peaks may be constrained by the irrigation application rates, but presumably this could be addressed by including storage in some abstractions. For example, the existing Council run Southern Valleys Irrigation Scheme (SVIS) intake is consented to take up to 2.7 m3/s. If this take could be operated to target hydro-peaking peaks it alone could theoretically skim the tops off the 2–4 m3/s flow fluctuation observed at Tuamarina as a result of discharges of 10– 15 m3/s from the Branch Scheme. This may require taking at a higher rate during the peaks and a reduced rate during the rest of the day. Without storage this type of temporally targeted abstraction may be impractical, although the prospect of increased availability of water for abstraction may provide sufficient incentive to make construction of storage worthwhile. However, even if the SVIS could be operated in this manner hydro-peaking effects would remain upstream; these are most extreme in the reach immediately below the existing Branch Scheme.

8.3. Location of flow monitoring

A flow monitoring site upstream of abstraction pressure would be desirable for flow management and triggering abstraction shut-off. As stated by Wadsworth (2013b ‘Barriers to Rationing’ discussion paper), an upstream monitoring site would make rationing relatively straightforward. The main advantage is that flow would be quantified before it is influenced by abstraction. This ought to make it a simple matter to advise abstractors of their entitlement, notwithstanding dealing with the issue of hydro-peaking and where on the daily flow fluctuation to implement shut-offs.

Ideally, a monitoring site would be located a short distance below the Branch Scheme outfall and provide real-time flow data. Travel time from this site to the main abstraction reach (~4–8 hours) would provide some scope for warning when downstream takes should be reduced or shut down. It could also identify when a hydro-peaking flow pulse was coming down the river and might give abstractors time to target abstraction to the higher part of the hydro-peaking cycle (as discussed above). Some obvious questions follow:

1. How to monitor flow at the chosen location, since it is probably impractical to maintain a rateable cross-section in the river in this reach. It is likely to require

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creation of a ‘virtual’ monitoring site through calculation based on real-time data from Wash Bridge, the Branch and Goulter rivers and Branch Scheme outfall to give flow in the Wairau. We note that this is essentially the flow monitoring lay out required by consent conditions for Trustpower’s Wairau Hydro-electric Power Scheme (Condition 178), but this monitoring layout could be implemented before Trustpower’s scheme is constructed. 2. Whether the travel time (4–8 hours) for a flow pulse to reach the locations of water takes downstream would be long enough to give sufficient advanced warning of when these takes should be reduced or shut-off, particularly if changes occur overnight? However, as discussed above, hydro-peaking pulses ought to be reasonably predictable in timing and approximate magnitude. If the travel time is insufficient to notify abstractors in time for them to alter their takes, then flow fluctuations would need to be predicted based on the likely power generation pattern. 3. Are abstractors able to incrementally alter their rate of abstraction if some form of incremental rationing / flow sharing was to be implemented (i.e. are takes either on or off, or can they be adjusted incrementally)? Apparently most existing abstractors are fairly constrained in their ability to vary their rate of abstraction, since it is dictated by the water application rates of their irrigation systems (e.g. centre pivots, drip irrigation of vines) (pers. comm. Val Wadsworth, MDC hydrologist). Potential solutions to this limitation could include varying the duration of irrigation through time based rostering, or building in on-site storage so that water does not have to be used at the same rate as it is abstracted.

A downstream monitoring site (e.g. Tuamarina) is potentially still useful to help assess compliance. However, once flow has reached this location it is no longer available for abstraction upstream. Real-time metering data from abstractions would theoretically provide a more direct measure of compliance, with the advantage that unused allocation would be made available to downstream abstractors (which would allow for temporally trade-able water permits). As more abstractors move towards telemetered monitoring of takes the information could feed into a system similar to the “Water Matters” web tool developed by Horizons Regional Council9. We understand that MDC are working towards more telemetry-based information retrieval, and currently have over 200 of out approximately 1,700 consents metered (pers. comm. Val Wadsworth, MDC hydrologist). Notwithstanding potential logistics and cost issues, increased use of telemetry is ultimately likely to improve water management. Priority could be given to telemetering takes in the Wairau catchment, given the high in- stream and out-of-stream values at stake.

With respect to Trustpower’s proposed Wairau Hydro-electric Power Scheme, the required locations of monitoring sites are clearly laid out in consent conditions

9 http://www.horizons.govt.nz/managing-environment/resource-management/water/watermatters/watermatters- overview/

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(conditions 178,182–184). These include a flow monitoring site immediately downstream of the discharge from PS5 (condition 182c), which will provide an appropriate upstream monitoring site for abstraction further down the catchment. Obviously the reduced travel time from this location to abstraction will reduce the warning period for changes to allowable abstraction rates and shut-offs.

As an interim measure, until appropriate upstream monitoring sites and flow relationships10 can be developed, the Tuamarina site will have to be used as the main flow management monitoring point.

8.4. Proposed Wairau Hydro-electric Power Scheme

As discussed in Section 6, flow fluctuations caused by Trustpower’s proposed Wairau Hydro-electric Power Scheme for the reach between Marchburn and Tuamarina, relative to those under the existing Branch Scheme, will be increased for flows > 30 m3/s at Tuamarina, but decreased for flows below 20 m3/s (Ryder evidence, paragraph 12.7). During critical irrigation / low flow periods (< 15.5m3/s at Tuamarina), PS5 flow fluctuations will be in the order of half what they are currently due to the operation of the existing Branch Scheme (Mitchell evidence, paragraph 3.22b). The effect of the existing Branch Scheme generation is to cause flow fluctuation in the order of ± 20 percent about the rolling 24-hr average flow at Tuamarina (Mitchell evidence, paragraph 6.12). However, consent condition 185 for the Wairau Scheme stipulates that when flow at Tuamarina is ≤ 15.5 m3/s flows must not fluctuate more than ±10% around the 24-hr rolling average flow.

This will reduce the magnitude of flow pulses that would potentially be wasted if the hydro-peaking minima were to be used as the flow trigger for abstraction shut-off. However, the proximity of the outfall to the main abstraction reach would also reduce the travel time and hence forewarning if abstraction was to be targeted to the hydro- peaking high points.

10 We understand that existing flow relationships available for predicting flow at the recommended upstream monitoring site, immediately below the Branch Scheme tailrace, have unacceptable margins of error for use in flow management (pers. comm. Val Wadsworth, MDC hydrologist).

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9. FLOW SHARING

Flow sharing is one mechanism for retaining some of the natural flow variability and, importantly, retaining the portion of flow recessions that support invertebrate production (see Section 4.1). The flow-sharing provisions in the existing WARMP were introduced to address concerns raised by the Department of Conservation and Nelson-Marlborough Fish & Game Council about the potential for large allocation of B and C class water to flat-line flow at the minimum flow (Wadsworth, MDC hearing evidence, paragraph 69). These concerns were warranted given the high degree of hydrological alteration that would occur under full allocation allowed for in the Plan, and potential effects extend to reduction in flushing and channel forming flows. The B class allocation block of 15 m3/s represents approximately 112 % of the MALF at Tuamarina (13.4 m3/s). By way of comparison, the National Environmental Standard on Ecological Flows and Water Levels (NES) suggests that; “Abstraction of more than 40% of MALF, or any flow alteration using impoundments would be considered a high degree of hydrological alteration – irrespective of region or source of flow” (Beca 2008, p13). The degree of potential hydrological alteration is higher still when C class allocation is added to B class allocation; especially given that there is no gap between the B and C class trigger flows (i.e. the trigger flow at which full B class abstraction is allowed is the same flow at which C class takes are allowed to commence).

The 2:1 flow sharing rule in the current WARMP leaves one share of water in the river for every two shares taken by abstractors. This could be challenged as unfairly favouring abstraction at the expense of in-stream values, especially given that the combined B class and C class allocation is large. While there is no scientific evidence available that would justify one level of sharing over another, 1:1 sharing between in- stream and out-of-stream uses is inherently equitable and the less flow taken means less hydrological effect and potentially less environmental effect.

However, as already mentioned, the flow sharing-provision in the WARMP has not actually been implemented due to logistical problems associated with managing abstraction from a downstream monitoring site combined with hydro-peaking flow fluctuations. The lack of flow sharing to date has probably not resulted in significant adverse effects since the level of consented abstraction remains relatively moderate (i.e. ~40% of the total a B class allocation block of 15 m3/s, or slightly less than half the MALF at Tuamarina).

Instantaneous flow sharing is likely to be impractical to implement without a large advance in technology use. And experience elsewhere has shown that ensuring compliance with flow sharing policy can be difficult where there are multiple abstractors (pers. comm. Andrew Parrish, Environment Canterbury). Compliance may require real time (i.e. telemetered) information on river flow and abstraction rates, including the individual shares of the total volume available for abstraction to be calculated and adjusted in real time. It may also require automated, incremental

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control over intakes. Although the river flow information is likely to be readily available, real time data on abstraction rates for all takes is still likely to be some way off and abstractors are generally not able to continuously and incrementally alter their rate of take.

Alternative options to instantaneous flow sharing that would help maintain mid-range flow variability, and in particular mid-to-low range flows potentially supporting invertebrate production, include:

1. A higher minimum flow. This would retain a larger proportion of the potentially most productive habitat (i.e. that which remains wet for longest during flow recessions). This concept was suggested by Dr Hayes in his Environment Court evidence (paragraph 7.45), and Mr Jowett’s Consents Hearing evidence (paragraph 58), on Trustpower’s Wairau Hydro-electric Power Scheme Proposal. Both experts suggested a summer minimum flow of 20 m3/s below the proposed intake. A higher minimum flow would provide better mitigation for the most important block of flows close to the MALF that sustains invertebrate and trout feeding habitat for the longest periods. Mr Jowett considered that increasing “the summer minimum flow to about 20 cumecs, [would achieve] the same biological benefits as [1:1] flow sharing, while allowing more water for generation than would be the case with flow sharing”. While a higher minimum flow for the reach affected by Trustpower’s consented Wairau Hydro-electric Power Scheme is probably not feasible, given that consent has been granted, the minimum flow for abstraction in the critical lower reach could potentially be increased (as discussed in Section 7). However, the consequences for security of supply to abstractors would need to be investigated. 2. A reduction in allocation limit would leave more flow in the river. As suggested in Section 7, the total allowable B class allocation could be reduced to the default suggested in the NES for large rivers, i.e. 50% of MALF, yielding a total allocation of 6.7 m3/s (c.f. the 15 m3/s total allowable abstraction in the existing WARMP). This would still provide for slightly more than currently allocated from the Wairau River (~6 m3/s), while substantially reducing the potential impact of abstraction on mid-to-low range flows. 3. Operating flow sharing over a longer time step. For example, sharing the daily average flow rather than the instantaneous flow would alleviate some of the issues around compliance, while still maintaining a share of additional flow in the river. However, it would require prediction of average flow at least a day in advance in order to give abstractors forewarning of their allowable take. 4. Some form of incremental step down in abstraction. As an example, Otago Regional Council’s Water Plan provides a block by block flow sharing for its equivalent of B class allocation. This policy (Policy 6.4.9) provides for block by block 1:1 flow sharing above the primary allocation block, up to the mean flow, with the increments for each allocation block defined based on the MALF. So for a

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river with a MALF > 1 m3/s the allocation blocks step up in 1 m3/s increments, with the first 500 l/s in each block left in the river and the next 500 l/s available for abstraction. A block by block approach like this ought to be easier to manage than instantaneous flow sharing, but would still provide a similar benefit in terms of retention of potentially productive flow close to the minimum flow. 5. A bulk allocation above the minimum flow with flow sharing above this management flow (i.e. minimum flow + bulk allocation quantum), as stipulated in condition 177 for Trustpower’s Wairau Hydro-electric Power Scheme. However, this approach does not avoid the issues associated with flow sharing. As discussed in Hayes’ Environment Court evidence (paragraph 7.51) “conceptually the problem with this approach is that the section of the hydrograph that is removed as the bulk allocation, is the very part that is most useful for maintaining invertebrate productivity (i.e. the part immediately above the minimum flow, …). Consequently, the gains of this approach in terms of habitat retention are likely to be minimal, unless the bulk allocation is very modest.”

A gap between B and C class allocation would also be worthwhile considering, at least in the absence of flow sharing. As mentioned in Section 7, without a gap B and C class allocation effectively becomes a single large allocation block. Hay and Kitson (2013) conducted a review of supplementary water allocation and flow harvesting policy among regional councils and unitary authorities in New Zealand, as well as the potential environmental impacts of abstraction to storage. The majority of Councils that have explicitly addressed supplementary allocation (flow harvesting) in policy tend to have a minimum flow threshold for abstraction and usually also either a cap on allocation or provision for flow sharing above this threshold (Hay & Kitson 2013). The most commonly used minimum flow for supplementary allocations is the median flow (Hay & Kitson 2013). As discussed above, the median flow is often viewed as providing an approximation of habitat conditions typically experienced by benthic invertebrates (Jowett 1992) — the food base for fish and birds. A pragmatic alternative would be to prescribe seasonal C class minimum flows based on seasonal median flows. This would allow more efficient use of water while helping to mitigate effects on trout and salmon passage and to a small extent also on potentially productive benthic habitat.

A summer C class minimum flow would be in the order of 30–45 m3/s. If this were coupled with a 10.4 m3/s minimum flow (for A and B class takes) and reduced B class allocation of 6.7 m3/s (as suggested in Section 7) then there would be a gap of at least 12.9 m3/s (i.e. 30 − (10.4 + 6.7) = 12.9).

Another important consideration is to avoid significant reductions in flushing flows (and channel forming flows) through excessive C class abstraction. Policy could explicitly prescribe the level of change to flushing flow frequency and magnitude that would be allowed (as is being considered by Greater Wellington Regional Council (Hay & Kitson 2013). Also it may be worthwhile considering opting for a simple cap on

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supplementary (i.e. C class) allocation or perhaps a block by block flow sharing arrangement (as used by Otago Regional Council for their lower flow supplementary allocation) to avoid the problems associated with flow sharing.

Several councils have policies that encourage off-season abstraction to storage during the winter, either explicitly or implicitly (Hay & Kitson 2013). This concept has merit since demand for other uses is often lower during this period.

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10. OUTSTANDING ISSUES AND RECOMMENDATIONS

10.1. Issues

Additional information pertinent to flow management in the Wairau River came to light during the finalisation of this report, including challenges associated with predicting flows at the recommended upstream monitoring site with sufficient precision to be useful for flow management, and proposed alternative allocation regimes. However, these were not able to be addressed within the scope of the existing report. We recommend that these points, and any other issues arising subsequently, should be addressed in future analyses.

10.2. Recommendations

1. Management objectives. These were a key point of contention throughout the Trustpower Wairau HEPS hearing process. Having clearly defined management objectives in the Plan could reduce controversy at consent hearings. We consider that appropriate environmental in-stream management objectives for the Wairau River are the:

 maintenance of the black-fronted tern and black billed gull populations;  maintenance of the dwarf galaxias population;  maintenance of the brown trout and salmon fishery;  maintenance of aquatic macroinvertebrate communities; and  benthic invertebrate productivity to support the other in-stream management objectives listed above.

2. Flow critical values. Of the ecological values in the Wairau River adult brown trout and benthic invertebrate habitat have the highest flow requirements combined with the highest values. Flows set for these values (i.e. food production and adult brown trout) should be adequate for other ecological values. With regard to minimum flow setting, the lower reaches of the Wairau could arguably be viewed as critical on the basis of:

 the relatively high trout abundance, as assessed by drift diving  the relatively high level of fishing effort (~50 % of fishing effort)  the known flow loss to groundwater in this section of the river (~8 m3/s) exacerbating flow reductions during periods of low flow  the location relative to abstraction pressure, i.e. the lower reaches are subject of the cumulative impacts of water abstractions upstream.

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3. Habitat retention levels. The lower Wairau River would arguably warrant a 90% level of habitat retention for trout, based on precedents set by other regional councils.

4. Prospective minimum flow for the lower reach (at Tuamarina). Based on the Wratts Road 2-D hydraulic-habitat model developed by Hudson et al. (2005), a minimum flow of 10.4 m3/s would retain 90% of the adult brown trout habitat available at the MALF, based on MALF of 13.4 m3/s at Tuamarina. A minimum flow of 8.4 m3/s would provide for 80% habitat retention.

5. Minimum flow monitoring locations. Create a ‘virtual’ flow monitoring site immediately downstream of the Branch Scheme tailrace, based on the flow monitoring lay out required by consent condition 178 for Trustpower’s Wairau Hydro- electric Power Scheme. An upstream monitoring site ought to make it a straightforward matter to advise abstractors of their entitlement, notwithstanding dealing with the issue of hydro-peaking and where on the daily flow fluctuation to implement shut-offs. Retain a downstream monitoring site (e.g. Tuamarina) to help assess compliance. However, continue to move toward gathering more real-time metering data from abstractions to provide a more direct measure of compliance. However, we understand that existing flow relationships for predicting flow at the recommended upstream monitoring site, immediately below the Branch Scheme tailrace, have unacceptable margins of error for use in flow management (pers. comm. Val Wadsworth, MDC hydrologist). As an interim measure, until appropriate upstream monitoring sites and flow relationships can be developed, the Tuamarina site will have to continue to be used as the main flow management monitoring point.

6. Reduce hydro-peaking impacts. Consider flow regulation ponds to reduce potential adverse ecological effects, improve efficiency of abstraction, and aid management of abstraction. Alternatively, target the hydro-peaking peaks for abstraction (including flow harvesting).

7. Abstraction shut-off trigger. If hydro-peaking remains, use the daily minimum flow as the cut-off / step-down trigger, based on the premise that the minimum water level effectively defines the area of productive habitat for benthic invertebrates (the food base for fish and birds). Alternatively, continue to use the daily average flow to trigger abstraction shut-off, but increase the minimum flow (by half the expected maximum daily flow fluctuation, i.e. 2 m3/s at Tuamarina) in recognition that the instantaneous flow will actually fall below the average for part of the hydro-peaking cycles. A buffer (or safety margin) of this sort on the mimimum flow would also help to address the fact that the response of flow to abstraction shut-off can be delayed, particularly for groundwater and gallery type takes (pers. comm. Val Wadsworth, MDC hydrologist).

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8. Limitations of flow sharing rule in the current WARMP. As well as being difficult to implement, the current 2:1 flow sharing rule is not equitable, and unlikely to substantially mitigate large potential impacts of the large total allocation allowed in current WARMP.

9. Alternative options to the current flow sharing rule.  Keep the allocation of 15 m3/s of B class water as in the existing plan, but with a block by block flow sharing arrangement above the minimum flow up to full allocation, using a more equitable 1:1 flow sharing ratio. This would see full B class abstraction implemented at about 40 m3/s11 , which would result in ~90% macroinvertebrate habitat retention relative to the median flow, providing for invertebrate food production.  Reduce the B class allocation limit to ~50% of MALF (i.e. ~6.7 m3/s), which would still provide for slightly more than existing allocation from the Wairau, but would substantially reduce the possible future impact of abstraction on mid-to-low range flows that potentially support benthic production, thus alleviating the requirement for flow sharing.

10. A higher minimum flow for supplementary allocation. Consider creating a gap between B and C class allocation. The median flow is widely used as a minimum flow for supplementary ‘flow harvesting’ allocation. A pragmatic alternative would be to prescribe seasonal C class minimum flows based on seasonal median flows. This would allow more efficient use of water while mitigating for effects on habitat and trout and salmon passage and to a small extent also on potentially productive benthic habitat. It would also be worthwhile considering either flow sharing above the C class minimum flow(s), or a limit (cap) on C class allocation, to avoid significant reduction of flushing flows, and possibly encouraging off-season abstraction to storage.

11. ACKNOWLEDGEMENTS

Thanks to Val Wadsworth (MDC) for providing information and discussing concepts. Thanks also to Neil Deans (Nelson Marlborough Fish & Game) for discussing his perception of key issues.

11 ~10 m3/s minimum flow, then 15 m3/s allocation plus a 15 m3/s flow share for the river = 40 m3/s. Could step the flow sharing in 1 m3/s increments (i.e. 1 m3/s for abstraction then 1 m3/s for the river).

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