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 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 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. 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 i MAY 2014 REPORT NO. 2505 | CAWTHRON INSTITUTE 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. ii CAWTHRON INSTITUTE | REPORT NO. 2505 MAY 2014 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”