Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

Resource Management Group

Environmental Science Section

Tukituki River Catchment

High Flow Allocation Modelling

Prepared by: Rob Waldron – Hydrology Resource Analyst	……………………………………………….. Signature:


Reviewed by: Rob Christie – Principal Scientist, Surface Water Quantity	……………………………………………….. Signature:

Approved: Darryl Lew, Group Manager – Resource Management	……………………………………………….. Signature:

June 2011

ISSN 1179 8513

emT 11/02

HBRC Plan No. 4258

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

executive summary

Water resources in Hawke’s Bay are facing increasing pressure from consumptive abstraction. The region is characterised by a warm dry climate with long dry periods with low rainfall and low river flow conditions. The core allocation in many Hawke’s Bay catchments is now over-allocated and the Hawke’s Bay Regional Council is currently exploring different water management options for the region.

One method used to mitigate the interruption of water supply during periods of low flow conditions (reducing the exposure to abstraction bans) is by sustainably harvesting and storing water during periods when river flows are high. High flow abstractions are an intrinsic component of water storage solutions.

This report assesses the effect of different potential methods of water allocation during high river flows within the winter and spring months of June to November, from four established hydrological sites on rivers in the Tukituki Catchment: Tukipo River at State Highway 50, Tukituki River at Tapairu Rd, Waipawa River at RDS and Tukituki River at Red Bridge.

The most suitable methods for each site have been selected based on the results of hydrological, ecological and security of water supply analyses, in terms of providing a sustainable flow (high flow allocation threshold) above which high flow allocation is made available, combined with a sustainable allocation limit (allocation cap) allowing for abstraction without adversely impacting on flow variability and instream ecological requirements, while providing an optimum security of supply.

Eight different high flow allocation scenarios were developed for each site which use high flow allocation thresholds (HFA thresholds) set as either mean or median flow combined with either of two selected high flow allocation caps (HFA caps) for each site. A flow sharing approach has been used in all scenarios where either 50% or 33% of flow above the HFA threshold up to the HFA cap is allocated for abstraction, with the rest (50% or 67% respectively) remaining in the river.

Hydrological analyses identified minor variations between the naturalised and modelled flow records for each high flow allocation scenario. Allocation scenarios with a median flow HFA threshold produced the greatest change (although still ≤17.5%) to all key hydrological statistics.

The availability of water for abstraction under each allocation scenario was assessed. The quantity of the allocation that can be utilised for abstraction depends on the duration and magnitude of high river flows.

The potential security of supply to water users provided by each high flow allocation scenario was assessed in terms of the percentage of time water is available for abstraction. All scenarios with median flow HFA thresholds provide water available for abstraction for a greater percentage of time than scenarios with mean flow HFA thresholds. Therefore all scenarios with median flow HFA thresholds provide high flow allocations with the greatest security of supply to water users.

The FRE₃ analyses (a hydrological index identified by Clausen & Biggs (1997) as the most ecologically relevant for characterizing hydrological regimes in New Zealand streams and rivers) showed that all modelled scenarios for each site alter the FRE₃ by less than 10 percent and were therefore supported as potential high flow allocation regimes.

An analysis based on the Range of Variability Approach (RVA) developed by Richter et al. (1997), investigated the degree of hydrologic alteration between the naturalised and modelled flow records.

The investigation highlighted that it is essential to first establish the key river values that require protection and identify the level of change to the natural flow regime or degree of hydrologic alteration that must not be exceeded in order to maintain and sustain the required protection of river values.

Until establishing the degree of hydrologic alteration and the level of change to the natural flow regime that will sustain and maintain the protection of key values for rivers in the Tukituki Catchment, a conservative approach to selecting the most preferable allocation methods from those modelled in this investigation was necessary. In terms of hydrologic alteration, scenarios which produced the lowest average percentage of hydrologic alteration (≤10%) were considered as potentially suitable allocation methods.

Based on the results of the hydrological and ecological analyses and the assessment of security of supply to water users, the most suitable high flow allocation scenarios for each site have been selected in terms of those which produced the least amount of change to the natural flow regime (where the disturbance to the structure and function of the riverine ecosystem is minimal) while providing a high flow allocation with the greatest security of supply to water users.

The following table presents the high flow allocation scenarios selected as the most suitable for each site:

The Selected High Flow Allocation Scenarios

The high flow allocation scenarios modelled in this investigation are all based on a 50% flow share approach. A flow sharing approach enables water to be abstracted from a river whilst maintaining a level of flow variability in the river. The practicalities of implementing high flow allocation methods based on a flow sharing approach need to be carefully considered. One possible approach could be to issue global/group water take consents to water user groups (instead of the current allocation process which issues consents to individual water users) whereby the abstraction is managed collectively by the group, employing measures such as rationing and rostering to ensure abstraction complies with any abstraction restrictions.

Before undertaking further hydrological and ecological analyses on any alternative high flow allocation methods, the regulatory tools that are currently available to implement and manage high flow allocation need to be identified and assessed to ultimately determine what type of high flow allocation methods can realistically be implemented and managed effectively.

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

table of contents

1 INTRODUCTION. 1

1.1 Background. 1

1.2 Investigation Objectives. 2

1.3 Previous Investigations. 2

1.4 Water Allocation. 3

1.5 Water Harvesting. 5

2 High Flow Allocation at Regional Councils. 7

3 RESOURCE MANAGEMENT Policy for THE Tukituki CATCHMENT. 9

3.1 Regional Resource Management Plan. 9

3.1.1 Ecological Instream Management Objectives. 9

3.1.2 Surface Water Quantity Management Objectives. 10

4 Methodology. 13

5 Existing Tukituki Catchment Hydrology. 15

5.1 Site Details. 15

5.2 Hydrological Records. 16

5.3 Hydrological Statistics. 17

5.3.1 Tukipo River at State Highway 50 - Naturalised Flow Statistics. 18

5.3.2 Tukituki River at Tapairu Rd Naturalised Flow Statistics. 21

5.3.3 Waipawa River at RDS Naturalised Flow Statistics. 24

5.3.4 Tukituki River at Red Bridge Naturalised Flow Statistics. 27

6 High Flow Allocation Scenarios. 31

7 Hydrological Consideration of High Flow Allocation Scenarios. 35

7.1 Comparison of Hydrological Statistics. 35

7.1.1 Tukipo River at State Highway 50 Scenario Flow Statistics. 35

7.1.2 Tukituki River at Tapairu Rd Scenario Flow Statistics. 37

7.1.3 Waipawa River at RDS Scenario Flow Statistics. 39

7.1.4 Tukituki River at Red Bridge Scenario Flow Statistics. 41

7.2 Availability of Water for Abstraction. 43

7.2.1 Abstraction Rates and Volumes. 43

7.2.2 Security of Supply. 46

7.2.2.1 Tukipo River at State Highway 50 Scenario Abstraction Availability. 47

7.2.2.2 Tukituki River at Tapairu Rd Selected Scenario Abstraction Availability. 49

7.2.2.3 Waipawa River at RDS Selected Scenario Abstraction Availability. 51

7.2.2.4 Tukituki River at Red Bridge Selected Scenario Abstraction Availability. 53

8 Ecological Consideration of Scenarios. 55

8.1 FRE₃ Analysis. 55

8.1.1 Biological Relevance. 55

8.1.2 Analysis of Allocation Scenario FRE₃ Values. 55

8.1.2.1 Tukipo River at State Highway 50 FRE₃ Analysis. 57

8.1.2.2 Tukituki River at Tapairu Rd FRE₃ Analysis. 58

8.1.2.3 Waipawa River at RDS FRE₃ Analysis. 59

8.1.2.4 Tukituki River at Red Bridge FRE₃ Analysis. 60

8.2 Analysis of Methods against Instream Flow Requirements. 61

8.2.1 Flow Variability. 61

8.2.2 Maintenance of Water Quality. 62

8.2.3 Habitat Requirements. 62

8.2.3.1 Instream Habitat Modelling in the Context of the Naturalised Flow Regime. 62

8.2.3.2 Fish Passage. 62

9 Range of Variability Approach (RVA) 63

9.1 Overview.. 63

9.2 IHA Summary. 66

9.2.1 Tukipo River at State Highway 50 IHA Summary. 67

9.2.2 Tukituki River at Tapairu Rd IHA Summary. 69

9.2.3 Waipawa River at RDS IHA Summary. 71

9.2.4 Tukituki River at Red Bridge IHA Summary. 73

9.3 RVA Summary. 75

9.3.1 Tukipo River at State Highway 50 RVA Summary. 77

9.3.2 Tukituki River at Tapairu Rd RVA Summary. 78

9.3.3 Waipawa River at RDS RVA Summary. 79

9.3.4 Tukituki River at Red Bridge RVA Summary. 80

10 Discussion. 81

11 Conclusion. 83

12 Recommendations. 85

13 REFERENCES. 87

APPENDIX 1

APPENDIX 2

Table InDEX

Table 1 Tukituki Catchment Minimum Flows, Core Allocatable Volumes, and Actual Core Allocation. 3

Table 2 High Flow Allocation - Regional Plan Summary. 7

Table 3 Site Details. 15

Table 4 Tukipo River at State Highway 50 Flow Records. 16

Table 5 Tukituki River at Tapairu Rd Flow Records. 16

Table 6 Waipawa River at RDS Flow Records. 16

Table 7 Tukituki River at Red Bridge Flow Records. 16

Table 8 Naturalised Flow Statistics 1969-2008. 17

Table 9 Tukipo River at State Highway 50 - Naturalised Flow Statistics (l/s) 1970-2007. 18

Table 10 Tukipo River at State Highway 50 - FRE₃ Occurrences in Naturalised Flow Record. 18

Table 11 Tukituki River at Tapairu Rd - Naturalised Flow Statistics (l/s) 1970-2007. 21

Table 12 Tukituki River at Tapairu Rd - FRE₃ Occurrences in Naturalised Flow Record. 21

Table 13 Waipawa River at RDS - Naturalised Flow Statistics (l/s) 1970-2007. 24

Table 14 Waipawa River at RDS - FRE₃ Occurrences in Naturalised Flow Record. 24

Table 15 Tukituki River at Red Bridge - Naturalised Flow Statistics (l/s) 1970-2007. 27

Table 16 Tukituki River at Red Bridge - FRE₃ Occurrences in Naturalised Flow Record. 27

Table 17 Tukipo River at State Highway 50 High Flow Allocation Scenarios. 32

Table 18 Tukituki River at Tapairu Rd High Flow Allocation Scenarios. 33

Table 19 Waipawa River at RDS High Flow Allocation Scenarios. 33

Table 20 Tukituki River at Red Bridge High Flow Allocation Scenarios. 34

Table 21 Tukipo River at State Highway 50 Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November) 35

Table 22 Tukituki River at Tapairu Rd Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November) 37

Table 23 Waipawa River at RDS Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November) 39

Table 24 Tukituki River at Red Bridge Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November) 41

Table 25 Average Monthly Abstraction Provided by SH50 Scenarios. 44

Table 26 Average Monthly Abstraction Provided by Tapairu Rd Scenarios. 44

Table 27 Average Monthly Abstraction Provided by RDS Scenarios. 45

Table 28 Average Monthly Abstraction Provided by Red Br Scenarios. 45

Table 29 Abstraction Availability for SH50 Scenarios with 100l/s HFA Cap. 47

Table 31 Abstraction Availability for SH50 Scenarios with 400l/s HFA Cap. 47

Table 32 Abstraction Availability for Tapairu Rd Scenarios with 1000l/s HFA Cap. 49

Table 33 Abstraction Availability for Tapairu Rd Scenarios with 4000l/s HFA Cap. 50

Table 34 Abstraction Availability for RDS Scenarios with 1000l/s HFA Cap. 51

Table 35 Abstraction Availability for RDS Scenarios with 4000l/s HFA Cap. 51

Table 36 Abstraction Availability for Red Br Scenarios with 2000l/s HFA Cap. 53

Table 37 Abstraction Availability for Red Br Scenarios with 8000l/s HFA Cap. 54

Table 38 SH50 Scenario FRE₃ Analysis Summary (June to November, 1970-2007) 57

Table 39 Tapairu Rd Scenario FRE₃ Analysis Summary (June to November, 1970-2007) 58

Table 40 RDS Scenario FRE₃ Analysis Summary (June to November, 1970-2007) 59

Table 41 Red Br Scenario FRE₃ Analysis Summary (June to November, 1970-2007) 60

Table 42 Summary of IHA Parameters and their Ecosystem Influences (Harkness 2010¹) 65

Table 43 Tukipo River at State Highway 50 IHA Scorecard Summary. 68

Table 44 Tukituki River at Tapairu Rd IHA Scorecard Summary. 70

Table 45 Waipawa River at RDS IHA Scorecard Summary. 72

Table 46 Tukituki River at Red Bridge IHA Scorecard Summary. 74

Table 47 SH50 RVA Analysis Summary. 77

Table 48 Tapairu Rd RVA Analysis Summary. 78

Table 49 RDS RVA Analysis Summary. 79

Table 50 Red Br RVA Analysis Summary. 80

Table 51 Selected High Flow Allocation Scenarios. 84

figure INDEX

Figure 1 Idealised River Flow Allocation on a Flow Duration Curve. 4

Figure 2 Investigation Sites. 15

Figure 3 Tukipo River at State Highway 50 - Naturalised Flow Hydrograph and FRE₃ 19

Figure 4 Tukipo River at State Highway 50 - Naturalised Flow Duration Curves. 20

Figure 5 Tukituki River at Tapairu Rd - Naturalised Flow Hydrograph and FRE₃ 22

Figure 6 Tukituki River at Tapairu Rd - Naturalised Flow Duration Curve. 23

Figure 7 Waipawa River at RDS - Naturalised Flow Hydrograph and FRE₃ 25

Figure 8 Waipawa River at RDS - Naturalised Flow Duration Curve. 26

Figure 9 Tukituki River at Red Bridge - Naturalised Flow Hydrograph and FRE₃ 28

Figure 10 Tukituki River at Red Bridge - Naturalised Flow Duration Curve. 29

Figure 11 Flow Duration Curves for SH50 Naturalised and Modelled Flow Records (June - November) 36

Figure 12 Zoomed View of Flow Duration Curves for SH50 Naturalised and Modelled Flow Records (June - November) 36

Figure 13 Flow Duration Curves for Tapairu Rd Naturalised and Modelled Flow Records (June - November) 38

Figure 14 Zoomed View of Flow Duration Curves for Tapairu Rd Naturalised and Modelled Flow Records (June - November) 38

Figure 15 Flow Duration Curves for RDS Naturalised and Modelled Flow Records (June - November) 40

Figure 16 Zoomed View of Flow Duration Curves for RDS Naturalised and Modelled Flow Records (June - November) 40

Figure 17 Flow Duration Curves for Red Br Naturalised and Modelled Flow Records (June - November) 42

Figure 18 Zoomed View of Flow Duration Curves for Red Br Naturalised and Modelled Flow Records (June - November) 42

Figure 19 Abstraction Availability for SH50 Scenarios with 100l/s HFA Cap. 47

Figure 20 Abstraction Availability for SH50 Scenarios with 400l/s HFA Cap. 48

Figure 21 Abstraction Availability for Tapairu Rd Scenarios with 1000l/s HFA Cap. 49

Figure 22 Abstraction Availability for Tapairu Rd Scenarios with 4000l/s HFA Cap. 50

Figure 23 Abstraction Availability for RDS Scenarios with 1000l/s HFA Cap. 51

Figure 24 Abstraction Availability for RDS Scenarios with 4000l/s HFA Cap. 52

Figure 25 Abstraction Availability for Red Br Scenarios with 2000l/s HFA Cap. 53

Figure 26 Abstraction Availability for Red Br Scenarios with 8000l/s HFA Cap. 54

Figure 27 Percentage Change from SH50 Naturalised FRE₃ Value for SH50 Allocation Scenarios. 57

Figure 28 Percentage Change from Tapairu Rd Naturalised FRE₃ Value for Tapairu Rd Allocation Scenarios. 58

Figure 29 Percentage Change from RDS Naturalised FRE₃ Value for RDS Allocation Scenarios. 59

Figure 30 Percentage Change from Red Br Naturalised FRE₃ Value for Red Br Allocation Scenarios. 60

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

1 INTRODUCTION

1.1 Background

The Hawke’s Bay region is located on the east coast of the North Island. The region is sheltered by the Kaweka and Ruahine mountain ranges in the west which form part of the North Island’s main chain of mountain ranges. The region is characterised by a warm dry climate with regular long dry periods with low rainfall and river flow conditions.

Water resources in Hawke’s Bay are facing increasing abstractive pressure. Many catchments in the region are now over allocated and the climate of eastern areas of New Zealand is expected to become warmer and drier (IPCC, 2007). In light of this, the Hawke’s Bay Regional Council (HBRC) is currently exploring different water management options for the region.

The HBRC manages the region’s water resources in accordance with the Regional Resource Management Plan (RRMP) 2006, and the Resource Management Act (RMA) 1991. Minimum flows^{^[1]} and core allocatable volumes^{^[2]} have been established on rivers (and streams) in accordance with Objective 41 and Policies 73 and 74 of the RRMP to ensure sufficient water is left in a river to help maintain identified river values (e.g. cultural, ecological, economical and social).

During the regions frequent dry hot summers, rivers regularly reach low flow conditions. When rivers are flowing at or below their regulatory minimum flows, abstraction bans are issued by the HBRC during which all consented abstraction is restricted.

One method used to mitigate the interruption of water supply during periods of low flow conditions, reducing the exposure to abstraction bans, is by sustainably harvesting and storing water during periods when river flows are high for use during periods of restricted abstraction or for river augmentation.

High flow allocation provides access to water for abstractive use at times when river flows are high enough to provide adequate instream habitat. High flow allocation is also referred to in other regions as supplementary allocation or B/C block allocation. High flow allocation is commonly provided to encourage water harvesting and storage (Harkness 2008).

1.2
Investigation Objectives

To assess the effect of different potential methods of water allocation during high river flows within the winter and spring months of June to November, from four established hydrological sites on rivers in the Tukituki Catchment:

▪ Tukipo River at State Highway 50

▪ Tukituki River at Tapairu Rd

▪ Waipawa River at RDS

▪ Tukituki River at Red Bridge

To determine the suitability of the modelled range of potential high flow allocation methods in terms of providing:

a) A sustainable flow (high flow allocation threshold) above which high flow allocation could be made available,

b) A sustainable allocation limit (high flow allocation cap) which will allow for abstraction without adversely impacting on flow variability and instream ecological requirements,

c) With the optimum security of water supply for the most sustainable allocation methods.

The most suitable methods for each site will be selected based on the results of hydrological, ecological and security of water supply analyses.

1.3 Previous Investigations

Two reports on high flow allocation modelling have previously been undertaken for the HBRC on the Ngaruroro River by MWH (Harkness 2008 & 2010¹).

MWH completed an investigation into potential high flow allocation from the Ngaruroro River during the May to October period (Harkness 2008). This investigation presented an overview of methods used by other Regional Councils and unitary authorities throughout New Zealand and used these as guidance to develop sixteen potential high flow allocation scenarios for the Ngaruroro River. These sixteen scenarios were modelled and a range of hydrological and ecological analyses were undertaken assessing the different scenarios in terms of effects on instream values. Four scenarios were selected as preferred high flow allocation methods.

The HBRC commissioned MWH to undertake a second investigation (Harkness 2010¹) into the potential for high flow allocation from the Ngaruroro River during the June to November months (as opposed to May to October in the 2008 investigation due to recently lengthening irrigation seasons). The four preferred high flow allocation scenarios from Harkness (2008) were reanalysed as part of the 2010 investigation along with four additional scenarios developed in consultation with HBRC.

The eight high flow allocation scenarios were modelled to determine a sustainable flow above which high flow allocation could be made available without adversely impacting on instream ecology requirements and flow variability (Harkness 2010¹).

All eight of the 2010 scenarios were identified under the methodology used, as having only minor effects on the flow regime and ecology and were recommended as possible methods for setting high flow allocation on the Ngaruroro River.

1.4
Water Allocation

Core allocation relates to the water currently allocated from rivers (and streams) in Hawke’s Bay for out of stream use.

Minimum flows and core allocatable volumes have been established on rivers (and streams) throughout Hawke’s Bay in accordance with the RRMP. These are presented in Table 9 of the RRMP (Appendix 1).

Minimum flows and core allocation volumes for the Tukituki catchment sites used in this investigation are presented in Table 1.

Table 1 Tukituki Catchment Minimum Flows, Core Allocatable Volumes, and Actual Core Allocation

Minimum flows (l/s) are set on rivers to ensure sufficient water is left in a river to maintain identified river values (e.g. cultural, ecological, economical and social) during low flow conditions. Minimum flows set in the RRMP for the Tukituki catchment are based on hydrological and instream habitat assessments. These minimum flows have recently been reviewed (Johnson 2011) and the results of which are expected to inform changes to the next RRMP.

The core allocatable volumes set in table 9 of the RRMP (Appendix 1) and presented in Table 1, are defined as the difference between the summer 7-day Q95^{^[3]} and the minimum flow. The core allocatable volume is also presented in Table 1 as an allocatable rate in litres per second. The actual core allocated volumes and rates relate to the total currently consented allocated volumes and rates for each site as of November 2010.

On the basis of allocatable volume (m3/wk), there is no over-allocation at the four sites (the Tukipo River has no allocation assigned), however in terms of allocation rate (l/s), Table 1 demonstrates over allocation at all sites.

Figure 1 Idealised River Flow Allocation on a Flow Duration Curve

A generalised picture of how high flow allocation could be managed within existing water allocation practices is presented in Figure 1.

There are a number of approaches that have been adopted to set high flow allocation in New Zealand rivers (summarised in Section 1) including the use of high flow allocation minimum flows (thresholds), allocation blocks and implementing flow sharing arrangements. Harkness (2008) explains that a high flow allocation minimum flow is required so that water harvesting only occurs when river flow is above this level. A flow sharing approach for high flow allocation (e.g. a 1:1 flow share ratio where 50% of river flow is allocated for abstraction and 50% remains in the river) would seek to maintain flow variability in the river and not unduly impact on ecological values such as flushing or disturbance flows that are essential to maintaining the instream ecosystem and channel structure (Harkness 2008).

High flow allocation should ideally occur during winter and spring when river flows are higher, so that harvesting of flow has only a small proportionate effect on reducing river flows (Harkness 2008).

1.5
Water Harvesting

Harkness (2008) describes water harvesting as:

….the practice of diverting a portion of river flows into a suitable storage for subsequent use during periods of reduced water availability. This enables utilisation of the available water resources for maximum productive benefit while reducing pressure on water resources during period of limited availability.

Water harvesting can be carried out in various ways:

▪ Setting higher minimum flows (above the core allocation) for category B or C water permits, where users accept a lower priority and security of supply. This ensures category A permits have access to the core allocation.

▪ Taking water during times of higher river flow during the irrigation months.

▪ Diverting water to storage over winter months, when demand is low and river flows are high, and to utilise the stored water to meet irrigation demand during summer that could otherwise not be met from the available flow allocation.

▪ Setting higher minimum flows to allow water abstraction for frost protection purposes.

1

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

3 RESOURCE MANAGEMENT Policy for THE Tukituki CATCHMENT

3.1 Regional Resource Management Plan

The Hawke’s Bay Regional Resource Management Plan (RRMP) which was made operative in 2006 sets out a policy framework for managing resource use activities across the region.

3.1.1 Ecological Instream Management Objectives

The following objectives and policies in the RRMP are relevant to the Tukipo, Tukituki and Waipawa Rivers in terms of seeking to sustain, maintain or improve the existing aquatic ecosystems.

Objective 25: Surface Water Quantity

The maintenance of water quantity of the rivers and lakes in order that it is suitable for sustaining aquatic ecosystems in catchments as a whole….while recognising the impact caused by climatic fluctuations in Hawke’s Bay.

Objective 27: Surface Water Quality

The maintenance or enhancement of the water quality of rivers, lakes and wetlands in order that it is suitable for sustaining or improving aquatic ecosystems in catchments as a whole…

Objective 40: Surface Water Quantity

The maintenance of the water quality of specific rivers in order that the existing species and natural character are sustained…

Policy 71: Environmental Guidelines – Surface Water Quality

To manage the effects of activities affecting the quality of water in rivers…in accordance with the environmental guidelines set out below in Table 7 of the RRMP.

Table 7 of the RRMP – Surface Water Quality Guidelines

Note: * Upstream of Fernhill Bridge;

** Between Fernhill Bridge and Expressway Bridge;

*** Downstream of Expressway Bridge.

Objective 41: Surface Water Quantity

The maintenance of water quantity of specific rivers in order that the existing aquatic species and the natural character are sustained…

3.1.2
Surface Water Quantity Management Objectives

The following objectives and policies in the RRMP are relevant to the Tukipo, Tukituki and Waipawa Rivers in terms of the management and use of surface water.

Objective 25: Surface Water Quantity

The maintenance of the water quantity of the rivers and lakes in order that it is suitable for sustaining aquatic ecosystems in catchments as a whole and ensuring resource availability for a variety of purposes across the region, while recognising the impact caused by climatic fluctuations in Hawke's Bay.

Objective 26: Surface Water Quantity

The avoidance of any significant adverse effects of water takes, uses, damming or diversion on lawfully established activities in surface water bodies.

“The demand for water in the Hawke’s Bay region is rising, particularly as a result of increasing crop and pasture irrigation. Inefficient use of water exacerbates problems during summer droughts. The demand for surface water needs to be managed in a manner which ensures that water availability is maintained and water is allocated fairly, the impact of droughts is minimised, and economic development is not unnecessarily curtailed.” (Harkness 2010¹)

Policy 34: Role of Non-Regulatory Methods

Education and co-ordination for encouraging efficient use of water, for example water harvesting, use of storage and consideration of alternative water supply, and avoiding wastage of water.

Policy 35: Regulation – Water Allocation

a) To manage the taking of water where the effects of that take may be more than minor.

b) To manage the cumulative adverse effects of small takes, particularly in catchments:

i) that are located in an area of low annual rainfall

ii) where the geology has a low storage capacity

iii) for which the location is such that there is a high potential for increased use

Policy 35 manages and controls water takes through the resource consent process.

Policy 37: Resource Allocation – Minimum Flows and Allocatable Volumes

a) To manage takes from those rivers listed in Table 9 of this Plan in accordance with the minimum flows and associated allocatable volumes set out in that table.

b) To establish minimum flows and allocatable volumes for additional rivers in accordance with the approach set out in Table 9 or as a result of research demonstrating that lower minimum flows or higher allocatable volumes are sustainable. Council will use the Plan Change procedure of the First Schedule of the RMA to introduce or change these.

c) To ensure the protection of aquifer recharge from the effects of minimum flows.

Policy 37 establishes that takes from rivers will be managed in accordance with prescribed minimum flows and upper minimum flows and allocable volumes (Harkness 2010¹).

Policy 39: Decision Making Criteria – Water Allocation

To allocate water from rivers in accordance with the following approach:

a) The water requirement for each resource consent applicant will be determined on the basis of reasonable needs and the efficiency of end use, requiring an applicant to determine how much water is required for their activity (for irrigation takes, see also Policy 42).

b) Where the demand for water within a stream management zone11 is greater than the allocatable volume as a result of a consent application for a new activity, a consent will not be issued except where it can be considered under d).

c) Where the demand for water within a stream management zone is greater than the allocatable volume as a result of a change to the minimum flow for that stream management zone the HBRC will adopt any or all of the following approaches:

i) Review all consented takes from that water body at the same time.

ii) Give preference to the renewal of existing resource consents, over the granting of new consents where it can be demonstrated that the allocation is still required.

iii) To encourage the establishment of user groups or the seasonal or long-term transfer of water permits in accordance with Policy 34.

iv) Where over-allocation still exists, to reduce the allocation on a pro-rata basis except that where the consent holder has been advised (e.g. in the consent document) that the water allocated may no longer be available for allocation at the time of consent renewal, in which case the consent may not be renewed.

v) To encourage the use of alternative water sources.

d) Water may be allocated over and above the allocatable volume, subject to a substantially higher cut-off level than that specified in Table 9 provided that any such additional allocations will not have any adverse effect on other lawfully established activities, nor any other significant adverse environmental effect and assuming allocation is subject to the implementation and/or consideration of (a), (b) and (c).

Harkness (2010¹) identified that Policy 39 establishes the overall approach for the allocation of surface water. This policy recognises that the type of water management required for the region’s surface water bodies is variable. As such, Policy 39 sets out that HBRC can allow for periods when water can be allocated over the allocatable volumes (e.g. for water harvesting purposes). The ecological protection of the river, including the maintenance of a natural “flushing” effect is the baseline consideration for any allocations which are made under this scenario.

Objective 41: Surface Water Quantity

The maintenance of the water quantity of specific rivers in order that the existing aquatic species and the natural character are sustained, while providing for resource availability for a variety of purposes, including groundwater recharge

Policy 73: Environmental Guidelines – Surface Water Quantity

a) To sustain aquatic ecosystems by establishing a minimum flow in a river as that level which will maintain the existing ecosystem.

Policy 74: Implementation of Environmental Guidelines – Surface Water Quantity

a) Resource Allocation: To define the allocatable volume as being the difference between the summer 7- day Q95 and the minimum flow.

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

5 Existing Tukituki Catchment Hydrology

5.1 Site Details

The Hawke’s Bay Regional Council (HBRC) operates a number of hydrological sites in the Tukituki catchment. The locations of the four sites within the Tukituki catchment that are used in this investigation are shown in Figure 2 with Table 3 providing some additional site details.

Figure 2 Investigation Sites

Tukituki High Flow Allocation Sites Cropped

Table 3 Site Details

5.2
Hydrological Records

Each site has a long-term rated flow record and a naturalised flow record which are detailed in the following tables.

Naturalised flow records were produced by Harkness (2010²) for a number of sites in the Hawke’s Bay region. Naturalised flow records represent the natural river flow before any abstractions or discharges occur.

Table 4 Tukipo River at State Highway 50 Flow Records

Table 5 Tukituki River at Tapairu Rd Flow Records

Table 6 Waipawa River at RDS Flow Records

Table 7 Tukituki River at Red Bridge Flow Records

The naturalised flow records for the four sites are used for all modelling and analyses in this investigation

5.3
Hydrological Statistics

A range of hydrological statistics have been calculated from the naturalised flow record for each site. Table 8 presents key statistics calculated from the full available naturalised records.

Table 8 Naturalised Flow Statistics 1969-2008

Note: MALF calculated as Jul-Jun 7 Day Moving Average, FRE₃ = 3 x median flow

Hydrological statistics have been calculated to describe the variability of each flow record. Flow variability (i.e. the frequency, intensity and duration of flows) is important in the assessment of instream flow regimes (MfE 1998). The modelling undertaken as part of this investigation (detailed in later sections of this report) uses the naturalised record from 01-Jan-1970 to 31-Dec-2007, a total of 30 years of record.

Statistics have been calculated for the June to November months (which represent the winter and spring months) and for the full record between 1970 and 2007. The statistics are presented for each site in Sections 5.3.1 to 5.3.4. After modelling various high flow allocation scenarios (detailed in Section 1), these statistics are recalculated and compared to the naturalised statistics (Section 7.1).

A method of describing the frequency that river biota are disturbed by flood flows is to calculate the FRE₃, which is the number of times per year the flow exceeds three times the median flow (Harkness 2010¹).

The occurrence and frequency of FRE₃ flows in the naturalised records are presented in Sections 5.3.1 to 5.3.4. FRE₃ values are presented on an annual basis for the June to November period and the entire year (January to December). Further analyses of FRE₃ flows and comparisons between naturalised and modelled flow records are detailed in Section 8.1.

The Q95 flow value is that which is exceeded 95% of the time. Similarly the Q80 and Q90 are the flows that are exceeded 80% and 90% of the time, respectively.

Naturalised flow hydrographs and flow duration curves (which represent the percentage of time any given flow is equalled or exceeded in a flow record) for each site are also presented in Sections 5.3.1 to 5.3.4.

5.3.1
Tukipo River at State Highway 50 - Naturalised Flow Statistics

Table 9 Tukipo River at State Highway 50 - Naturalised Flow Statistics (l/s) 1970-2007

The river flow statistics calculated for the June to November months are as expected, mostly higher than those for the full record with the exception of the Q95.

Table 10 Tukipo River at State Highway 50 - FRE₃ Occurrences in Naturalised Flow Record

The naturalised record shows a large proportion of FRE₃ events (approximately 65%) occur during the June to November months. The June to November months show an average of 8 events per year compared to 12 for the full year. The average frequency of FRE₃ flows during June to November is one occurrence every 24 days. In comparison the average frequency of FRE₃ flows for the full year reduces to one occurrence every 30 days.

The hydrograph in Figure 3 plots the naturalised flow record (1970-2007) for Tukipo River at State Highway 50 (with June to November months are highlighted in red) and the FRE₃ flow calculated for the site. It is clear from the hydrograph and Table 10 that FRE₃ events are most frequent during the June to November months.

Figure 3 Tukipo River at State Highway 50 - Naturalised Flow Hydrograph and FRE₃

The naturalised flow duration for the Tukipo River at State Highway 50 over all months from January to December (black curve) is compared with the periods, June to November (blue curve) and December to May (red curve) in Figure 4.

The percentage of time spent at or above any given flow is highest during the June to November period representing the winter/spring months. During the summer/autumn months of December to May, the greatest percentage of time is spent at lower river flows.

Figure 4 Tukipo River at State Highway 50 - Naturalised Flow Duration Curves

5.3.2
Tukituki River at Tapairu Rd Naturalised Flow Statistics

Table 11 Tukituki River at Tapairu Rd - Naturalised Flow Statistics (l/s) 1970-2007

The river flow statistics calculated for the June to November months are as expected, mostly higher than those for the full record with the exception of the maximum and Q95.

Table 12 Tukituki River at Tapairu Rd - FRE₃ Occurrences in Naturalised Flow Record

The naturalised record shows a large proportion of FRE₃ events (approximately 65%) occur during the June to November months. The June to November months show an average of 7 events per year compared to 10 for the full year. The average frequency of FRE₃ flows during June to November is one occurrence every 27 days. In comparison the average frequency of FRE₃ flows for the full year reduces to one occurrence every 35 days.

The hydrograph in Figure 5 plots the naturalised flow record (1970-2007) for Tukituki River at Tapairu Rd (with June to November months are highlighted in red) and the FRE₃ flow calculated for the site. It is clear from the hydrograph and Table 12 that FRE₃ events are most frequent during the June to November months.

Figure 5 Tukituki River at Tapairu Rd - Naturalised Flow Hydrograph and FRE₃

The naturalised flow duration for the Tukituki River at Tapairu Rd over all months from January to December (black curve) is compared with the periods, June to November (blue curve) and December to May (red curve) in Figure 6.

Figure 6 Tukituki River at Tapairu Rd - Naturalised Flow Duration Curve

5.3.3
Waipawa River at RDS Naturalised Flow Statistics

Table 13 Waipawa River at RDS - Naturalised Flow Statistics (l/s) 1970-2007

The river flow statistics calculated for the June to November months are as expected, mostly higher than those for the full record with the exception of the maximum and Q95.

Table 14 Waipawa River at RDS - FRE₃ Occurrences in Naturalised Flow Record

The naturalised record shows the majority of FRE₃ events (approximately 60%) occur during the June to November months. The June to November months show an average of 5 events per year compared to 9 for the full year. The average frequency of FRE₃ flows during June to November is one occurrence every 34 days. In comparison the average frequency of FRE₃ flows for the full year reduces to one occurrence every 42 days.

The hydrograph in Figure 7 plots the naturalised flow record (1970-2007) for Waipawa River at RDS (with June to November months are highlighted in red) and the FRE₃ flow calculated for the site. It is clear from the hydrograph and Table 14 that FRE₃ events are most frequent during the June to November months.

Figure 7 Waipawa River at RDS - Naturalised Flow Hydrograph and FRE₃

The naturalised flow duration for the Waipawa River at RDS over all months from January to December (black curve) is compared with the periods, June to November (blue curve) and December to May (red curve) in Figure 8.

Figure 8 Waipawa River at RDS - Naturalised Flow Duration Curve

5.3.4
Tukituki River at Red Bridge Naturalised Flow Statistics

Table 15 Tukituki River at Red Bridge - Naturalised Flow Statistics (l/s) 1970-2007

The river flow statistics calculated for the June to November months are as expected, mostly higher than those for the full record.

Table 16 Tukituki River at Red Bridge - FRE₃ Occurrences in Naturalised Flow Record

The naturalised record shows a large proportion of FRE₃ events (approximately 70%) occur during the June to November months. The June to November months show an average of 7 events per year compared to 10 for the full year. The average frequency of FRE₃ flows during June to November is one occurrence every 26 days. In comparison the average frequency of FRE₃ flows for the full year reduces to one occurrence every 35 days.

The hydrograph in Figure 9 plots the naturalised flow record (1970-2007) for Tukituki River at Red Bridge (with June to November months are highlighted in red) and the FRE₃ flow calculated for the site. It is clear from the hydrograph and Table 16 that FRE₃ events are most frequent during the June to November months.

Figure 9 Tukituki River at Red Bridge - Naturalised Flow Hydrograph and FRE₃

The naturalised flow duration for the Tukituki River at Red Bridge over all months from January to December (black curve) is compared with the periods, June to November (blue curve) and December to May (red curve) in Figure 10.

Figure 10 Tukituki River at Red Bridge - Naturalised Flow Duration Curve

1

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

6 High Flow Allocation Scenarios

A number of high flow allocation scenarios were developed for modelling work on the Ngaruroro River by Harkness (2008). After completing a review of the methods used by other regional councils and unitary authorities throughout New Zealand, only three councils (Horizons Regional Council, Marlborough District Council and Otago Regional Council) were identified as having adopted specific high flow allocation methods within their regional plans.

The three methods already in use by councils made up the first three allocation scenarios out of sixteen scenarios developed by Harkness (2008). The other scenarios developed were based on variations of these with different high flow allocation thresholds and allocatable volumes. All sixteen scenarios only allowed water to be available for abstraction during the May to October period; the winter and spring months when river flows are typically higher.

The high flow allocation thresholds were based on long-term mean or median flows which act as a threshold flow above which water can be allocated for abstraction. The scenarios combined high flow allocation thresholds with a range of maximum abstraction caps (high flow allocation caps). The reason for incorporating maximum abstraction caps is explained in the excerpt from Harkness (2010¹):

….modelling the scenarios without a maximum abstraction cap allowed for very large abstractions during flood events. The modelling process would assume that all allocation was abstracted which resulted in a residual river flow with flood peaks and freshes dramatically diminished. This is unrealistic, not only because of the sheer volume modelled as being abstracted, but also because in many high river flow events the water becomes very turbid and silt laden and this becomes a physical limitation on water harvesting. Sediment laden water has adverse effects on pumping equipment and can lead to sediment deposition behind dams if diverted there.

The sixteen scenarios were modelled using the Ngaruroro River at Fernhill naturalised flow record to assess the impacts that different potential allocation methods would have on the river. Four scenarios with a mean high flow allocation threshold (HFA threshold) were selected as preferred high flow allocation methods.

Harkness (2010¹) undertook further high flow allocation modelling work using the June to November period instead of May to October used in the 2008 investigation due to recently lengthening irrigation seasons. The four preferred methods from Harkness (2008) which used a HFA threshold set as the long-term mean were adopted for re-modelling in this second investigation and an additional four scenarios incorporating HFA thresholds set at the long-term median flow were also included.

All eight of the 2010 scenarios were identified as having only minor effects on the flow regime and ecology and were recommended as possible methods for setting high flow allocation on the Ngaruroro River. Harkness (2010¹) suggested it came down to a preference of what quantity of water should be made available for allocation.

These eight high flow allocation scenarios have been adopted and adapted for modelling high flow allocation in the Tukituki River catchment.

For each site, the range of scenarios use HFA thresholds set as either mean or median flow (derived from the full naturalised flow record for each site - presented in Section 5.3), with either of two selected high flow allocation caps (HFA caps) for each site. A flow sharing approach has been used in all scenarios where either 50% or 33% of flow above the HFA threshold up to the HFA cap is allocated for abstraction, with the rest (50% or 67% respectively) remaining in the river.

The HFA caps selected for the Ngaruroro scenarios were 2000l/s and 5000l/s. Although these were essentially arbitrary figures the lower cap was close to the existing Ngaruroro River at Fernhill RRMP core allocatable rate of 1581l/s (956189m³/wk).

HFA caps were developed for the Tukituki catchment sites by comparing the naturalised flow regimes for the Ngaruroro and each Tukituki site in terms of key flow statistics including for example; the mean and FRE3 flows. The ratio between the Ngaruroro HFA caps and the Ngaruroro key flow statistics were analysed and allocation limits for the Tukituki sites were derived based on equivalent proportions.

Given that the Harkness (2010¹) investigation found that even scenarios with the higher HFA cap of 5000l/s had only minor effects on the flow regime and ecology, it was decided that the highest HFA caps for the Tukituki sites be set at higher levels equating to twice the 5000l/s cap for the Ngaruroro.

The work resulted in two HFA caps being set for each site. Further analysis was undertaken comparing the proposed HFA caps to current core allocated rates (Table 1 in Section 1.4). The lower HFA caps for all sites except the Tukipo River at State Highway 50 (which has a very low allocated rate of 13l/s) were close to the core allocation (an average between the three Tukituki and Waipawa River sites of 110% of core allocation).

The scenarios developed for each site are presented in the following tables.

Table 17 Tukipo River at State Highway 50 High Flow Allocation Scenarios

Note: Median and mean are from entire period of flow record

Table 18 Tukituki River at Tapairu Rd High Flow Allocation Scenarios

Note: Median and mean are from entire period of flow record

Table 19 Waipawa River at RDS High Flow Allocation Scenarios

Note: Median and mean are from entire period of flow record

Table 20 Tukituki River at Red Bridge High Flow Allocation Scenarios

Note: Median and mean are from entire period of flow record

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

7 Hydrological Consideration of High Flow Allocation Scenarios

7.1 Comparison of Hydrological Statistics

For each scenario, flow statistics and flow duration curves for the naturalised and modified flow records are compared in Sections 7.1.1 to 7.1.4. All results presented are for the June to November months between 1970 and 2007 (inclusive).

The magnitude of change as a percentage (% ∆) of the naturalised flow record is presented for the median, mean and maximum values for each scenario.

7.1.1 Tukipo River at State Highway 50 Scenario Flow Statistics

Table 21 Tukipo River at State Highway 50 Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November)

There are only minor changes to the low flow statistics (Minimum, Q80, Q90, Q95) and the maximum flow between the naturalised and modified flow records shown in Table 21.

SH50 scenarios 1 to 4 lower the median flow by less than 1% with scenario 6 showing the greatest change, lowering the median by 14.12%. Scenario SH50 6 also shows the greatest change to the mean flow (-9.72%).

All scenarios with the higher HFA cap of 400l/s (SH50 2, 4, 6 and 8) show slightly greater change to the median, mean and maximum flows.

The flow duration curves for the naturalised and modified SH50 flow records with the FRE₃ value over-plotted are compared in Figure 11.

A closer view of the 5% to 65% range where most of the variation away from the original naturalised flow data occurs is provided in Figure 12.

Figure 11 Flow Duration Curves for SH50 Naturalised and Modelled Flow Records (June - November)

Figure 12 Zoomed View of Flow Duration Curves for SH50 Naturalised and Modelled Flow Records (June - November)

7.1.2
Tukituki River at Tapairu Rd Scenario Flow Statistics

Table 22 Tukituki River at Tapairu Rd Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November)

There are only minor changes to the low flow statistics (Minimum, Q80, Q90, Q95) and the maximum flow between the naturalised and modified flow records shown in Table 22.

Tapairu Rd scenarios 1 to 4 lower the median flow by less than 1% with scenario 6 showing the greatest change, lowering the median by 12.74%. Scenario Tapairu Rd 6 also shows the greatest change to the mean flow (-9.64%).

All scenarios with the higher HFA cap of 4000l/s (Tapairu Rd 2, 4, 6 and 8) show slightly greater change to the median, mean and maximum flows.

The flow duration curves for the naturalised and modified Tapairu Rd flow records with the FRE₃ value over-plotted are compared in Figure 13.

A closer view of the 5% to 75% range where most of the variation away from the original naturalised flow data occurs is provided in Figure 14.

Figure 13 Flow Duration Curves for Tapairu Rd Naturalised and Modelled Flow Records (June - November)

Figure 14 Zoomed View of Flow Duration Curves for Tapairu Rd Naturalised and Modelled Flow Records (June - November)

7.1.3
Waipawa River at RDS Scenario Flow Statistics

Table 23 Waipawa River at RDS Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November)

There are only minor changes to the low flow statistics (Minimum, Q80, Q90, Q95) and the maximum flow between the naturalised and modified flow records shown in Table 23.

RDS scenarios 1 to 4 lower the median flow by less than 1% with scenario 6 showing the greatest change, lowering the median by 10.71%. Scenario RDS 6 also shows the greatest change to the mean flow (-9.32%).

All scenarios with the higher HFA cap of 4000l/s (RDS 2, 4, 6 and 8) show slightly greater change to the median, mean and maximum flows.

The flow duration curves for the naturalised and modified RDS flow records with the FRE₃ value over-plotted are compared in Figure 15.

A closer view of the 5% to 65% range where most of the variation away from the original naturalised flow data occurs is provided in Figure 16.

Figure 15 Flow Duration Curves for RDS Naturalised and Modelled Flow Records (June - November)

Figure 16 Zoomed View of Flow Duration Curves for RDS Naturalised and Modelled Flow Records (June - November)

7.1.4
Tukituki River at Red Bridge Scenario Flow Statistics

Table 24 Tukituki River at Red Bridge Residual Flow Statistics (l/s) for High Flow Allocation Scenarios (June - November)

There are only minor changes to the low flow statistics (Minimum, Q80, Q90, Q95) and the maximum flow between the naturalised and modified flow records shown in Table 24.

Red Br scenarios 1 to 4 lower the median flow by less than 1% with scenario 6 showing the greatest change, lowering the median by 17.50%. Scenario Red Br 6 also shows the greatest change to the mean flow (-7.54%).

All scenarios with the higher HFA cap of 8000l/s (Red Br 2, 4, 6 and 8) so slightly greater change to the median, mean and maximum flows.

The flow duration curves for the naturalised and modified SH50 flow records with the FRE₃ value over-plotted are compared in Figure 17.

A closer view of the 5% to 75% range where most of the variation away from the original naturalised flow data occurs is provided in Figure 18.

Figure 17 Flow Duration Curves for Red Br Naturalised and Modelled Flow Records (June - November)

Figure 18 Zoomed View of Flow Duration Curves for Red Br Naturalised and Modelled Flow Records (June - November)

7.2
Availability of Water for Abstraction

All high flow allocation scenarios incorporate high flow allocation caps (HFA caps) which are designed to limit the potential high flow abstraction from a river so as not to adversely impact on flow variability and instream ecological requirements. HFA caps represent the maximum potential abstraction provided under each scenario. The availability of this potential abstraction depends on the duration and magnitude of high river flows.

7.2.1 Abstraction Rates and Volumes

Average monthly abstraction rates and volumes provided by the range of high flow allocation scenarios for each site are presented in Table 25 to Table 28.

Average monthly abstraction data can be used as an estimate of future available abstraction rates provided under each high flow allocation scenario and enables future estimates of dam/storage fill times to be calculated.

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

7.2.2 Security of Supply

In addition to understanding the volume of water made available under each allocation scenario, it is also critical to assess the proportion of time the allocated water provided by each scenario is likely to be available for abstraction. Only when river flows are above the high flow allocation thresholds (HFA thresholds) set in each scenario is water allowed to be abstracted. Conversely when river flows are below the HFA threshold, high flow abstraction is restricted.

The range of scenarios modelled on each site use HFA thresholds set as either mean or median flow, above which water is made available for abstraction. Scenarios incorporate one of two selected HFA caps for each site (to limit the total abstraction from the river) and either 50% or 33% of flow above the HFA threshold (up to the HFA cap) is allocated for abstraction. The total water allocated for abstraction by each scenario is limited by the HFA cap.

Using naturalised flow duration curves produced for the June to November period for each site (presented in Section 5.3), the potential security of supply provided by each high flow allocation scenario has been assessed in terms of the percentage of time water is available for abstraction. The greater proportion of time water is available for abstraction the greater the security of supply to water users.

The following sections identify the percentage of time river flows are above the HFA thresholds (above which abstraction can begin), the percentage of time 50% of the total allocation is available for abstraction and the percentage of time 100% of the total allocation is available for abstraction.

7.2.2.1
Tukipo River at State Highway 50 Scenario Abstraction Availability

The SH50 statistics presented in Table 29 and Table 30 show that river flows are above the median flow HFA threshold for a greater percentage of time than the mean flow HFA threshold.

Comparing the abstraction availability for scenarios with the lower HFA cap of 100l/s (Table 29 and Figure 19) shows that SH50 scenarios 5 and 7 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 29 Abstraction Availability for SH50 Scenarios with 100l/s HFA Cap

Figure 19 Abstraction Availability for SH50 Scenarios with 100l/s HFA Cap

Comparing the abstraction availability for scenarios with the higher HFA cap of 400l/s (Table 30 and Figure 20) shows that SH50 scenarios 6 and 8 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 30 Abstraction Availability for SH50 Scenarios with 400l/s HFA Cap

Figure 20 Abstraction Availability for SH50 Scenarios with 400l/s HFA Cap

The scenario that provides the highest security of supply in terms of greatest percentage of time above its HFA threshold and greatest abstraction availability is scenario SH50 5 which with a median flow HFA threshold, provides 50% and 100% of allocation for abstraction more than 61% and 56% of the time respectively.

7.2.2.2
Tukituki River at Tapairu Rd Selected Scenario Abstraction Availability

The Tapairu Rd statistics presented in Table 31 and Table 32 show that river flows are above the median flow HFA threshold for a greater percentage of time than the mean flow HFA threshold.

Comparing the abstraction availability for scenarios with the lower HFA cap of 1000l/s (Table 31 and Figure 21) shows that Tapairu Rd scenarios 5 and 7 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 31 Abstraction Availability for Tapairu Rd Scenarios with 1000l/s HFA Cap

Figure 21 Abstraction Availability for Tapairu Rd Scenarios with 1000l/s HFA Cap

Comparing the abstraction availability for scenarios with the higher HFA cap of 4000l/s (Table 32 and Figure 22) shows that Tapairu Rd scenarios 6 and 8 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 32 Abstraction Availability for Tapairu Rd Scenarios with 4000l/s HFA Cap

Figure 22 Abstraction Availability for Tapairu Rd Scenarios with 4000l/s HFA Cap

The scenario that provides the highest security of supply in terms of greatest percentage of time above its HFA threshold and greatest abstraction availability is scenario Tapairu Rd 5 which with a median flow HFA threshold, provides 50% and 100% of allocation for abstraction more than 62% and 57% of the time respectively.

7.2.2.3
Waipawa River at RDS Selected Scenario Abstraction Availability

The RDS statistics presented in Table 33 and Table 34 show that river flows are above the median flow HFA threshold for a greater percentage of time than the mean flow HFA threshold.

Comparing the abstraction availability for scenarios with the lower HFA cap of 1000l/s (Table 33 and Figure 23) shows that RDS scenarios 5 and 7 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 33 Abstraction Availability for RDS Scenarios with 1000l/s HFA Cap

Figure 23 Abstraction Availability for RDS Scenarios with 1000l/s HFA Cap

Comparing the abstraction availability for scenarios with the higher HFA cap of 4000l/s (Table 34 and Figure 24) shows that RDS scenarios 6 and 8 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 34 Abstraction Availability for RDS Scenarios with 4000l/s HFA Cap

Figure 24 Abstraction Availability for RDS Scenarios with 4000l/s HFA Cap

The scenario that provides the highest security of supply in terms of greatest percentage of time above its HFA threshold and greatest abstraction availability is scenario RDS 5 which with a median flow HFA threshold, provides 50% and 100% of allocation for abstraction more than 59% and 53% of the time respectively.

7.2.2.4
Tukituki River at Red Bridge Selected Scenario Abstraction Availability

The Red Br statistics presented in Table 35 and Table 36 show that river flows are above the median flow HFA threshold for a greater percentage of time than the mean flow HFA threshold.

Comparing the abstraction availability for scenarios with the lower HFA cap of 2000l/s (Table 35 and Figure 25) shows that Red Br scenarios 5 and 7 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 35 Abstraction Availability for Red Br Scenarios with 2000l/s HFA Cap

Figure 25 Abstraction Availability for Red Br Scenarios with 2000l/s HFA Cap

Comparing the abstraction availability for scenarios with the higher HFA cap of 8000l/s (Table 36 and Figure 26) shows that Red Br scenarios 6 and 8 with median flow HFA thresholds, provide 50% and 100% of allocation for abstraction for the greatest percentage of time.

Table 36 Abstraction Availability for Red Br Scenarios with 8000l/s HFA Cap

Figure 26 Abstraction Availability for Red Br Scenarios with 8000l/s HFA Cap

The scenario that provides the highest security of supply in terms of greatest percentage of time above its HFA threshold and greatest abstraction availability is scenario Red Br 5 which with a median flow HFA threshold, provides 50% and 100% of allocation for abstraction more than 67% and 63% of the time respectively.

Tukituki River Catchment High Flow Allocation Modelling Report

Attachment 1

8 Ecological Consideration of Scenarios

8.1 FRE₃ Analysis

The FRE₃ statistic is a measure of flow variability (being the number of times per year the flow exceeds three times the median flow) incorporating both a frequency and intensity component (Harkness 2010¹).

Clausen & Biggs (1997) undertook a study to identify the most ecologically relevant hydrological indices for characterizing hydrological regimes in New Zealand streams (paper included in Appendix 2). The FRE₃ variable was selected to be the most ecological useful flow variable in New Zealand streams and it was suggested that FRE₃ is the best overall flow index for classifying habitats of benthic biota (periphyton and invertebrates) in New Zealand streams and rivers (Clausen & Biggs 1997).

The FRE₃ method was used by Harkness (2008 & 2010¹) as the ecological basis for the broad assessment of biological consequences of high flow allocation scenarios on the Ngaruroro River. The same method has been adopted for analysis on the Tukituki catchment sites.

8.1.1 Biological Relevance

The following extract from Harkness (2010¹) describes the biological relevance of flood frequency:

Flood frequency is of biological relevance primarily by provision of an important flushing function, thereby regulating water quality parameters and limiting periphyton accrual by sloughing of excessive periphyton growth.

The percentage of change from the naturalised FRE₃ score to the altered flow regime FRE₃ score provides a guide as to the likely impact to instream biological communities from the flow regime change.

Two key theories underlie this assumption:

i) Benthic communities are assembled according to the existing flow regime, and ‘significant’ changes in the flow regime are likely to result in significant changes to the biological composition of those communities.

ii) Benthic communities provide primary (periphyton) and secondary (macroinvertebrates) energy production functions, and changes in the abundance or diversity of these organisms can have wider reaching responses to production and energy flux in the aquatic environment.

8.1.2 Analysis of Allocation Scenario FRE₃ Values

The modelled flow records (based on the different allocation scenarios) for each site, have been analysed in terms of the percentage change to FRE₃ from the naturalised records.

A significant change to the value of FRE₃ as a result of a proposed activity (e.g. high flow allocation) may indicate the risk of a change in the biological community (MfE 1998).

When Harkness (2008 & 2010¹) assessed the change to FRE₃ flows between the natural and modelled flow records on the Ngaruroro River, only scenarios which produced a change of ≤10 percent to FRE₃ value were supported as potential allocation regimes, so as to limit the potential impact to benthic communities, and the impact to the wider aquatic environment. The same criteria was used for this investigation with only scenarios producing a change of ≤10 percent to FRE₃ being supported for further analysis.

Results of the assessments for each site are presented in Tables and Figures in Sections 8.1.2.1 to 8.1.2.4.

8.1.2.1
Tukipo River at State Highway 50 FRE₃ Analysis

Table 37 SH50 Scenario FRE₃ Analysis Summary (June to November, 1970-2007)

Figure 27 Percentage Change from SH50 Naturalised FRE₃ Value for SH50 Allocation Scenarios

All modelled SH50 allocation scenarios show less than 10 percent change in the mean and total occurrences of flows exceeding three times the median flow and to the FRE₃ value.

8.1.2.2
Tukituki River at Tapairu Rd FRE₃ Analysis

Table 38 Tapairu Rd Scenario FRE₃ Analysis Summary (June to November, 1970-2007)

Figure 28 Percentage Change from Tapairu Rd Naturalised FRE₃ Value for Tapairu Rd Allocation Scenarios

All modelled Tapairu Rd allocation scenarios show less than 10 percent change in the mean and total occurrences of flows exceeding three times the median flow and to the FRE₃ value.

8.1.2.3
Waipawa River at RDS FRE₃ Analysis

Table 39 RDS Scenario FRE₃ Analysis Summary (June to November, 1970-2007)

Figure 29 Percentage Change from RDS Naturalised FRE₃ Value for RDS Allocation Scenarios

All modelled RDS allocation scenarios show less than 10 percent change in the mean and total occurrences of flows exceeding three times the median flow and to the FRE₃ value.

8.1.2.4
Tukituki River at Red Bridge FRE₃ Analysis

Table 40 Red Br Scenario FRE₃ Analysis Summary (June to November, 1970-2007)

Figure 30 Percentage Change from Red Br Naturalised FRE₃ Value for Red Br Allocation Scenarios

All modelled Red Br allocation scenarios show less than 10 percent change in the mean and total occurrences of flows exceeding three times the median flow and to the FRE₃ value.

8.2
Analysis of Methods against Instream Flow Requirements

The HBRC objectives and policies set out in the RRMP seek to maintain or improve existing aquatic ecosystems. Any high flow allocation regime needs to be set in the context of these instream management objectives and policies (Harkness 2010¹).

Harkness (2010¹) lists three main components that define a flow regime’s ability to support an ecological community as identified by MfE (1998):

▪ Flow Variability - Variations in flow are important ecologically and provide processes such as flushing of accumulated sediment, algae and detritus.

▪ Water Quality - The flow is a significant determinant of water quality parameters such as dissolved oxygen, nutrients, pH and toxic contaminants.

▪ Habitat (space and food producing area) - Many river biota have preferences for certain ranges of water depth and velocity, i.e. hydraulic habitat. Flow, interacting with the river’s morphology (cross-section and slope), determines this available hydraulic habitat.

All high flow allocation scenarios for each site have been assessed in relation to these three ecological requirements.

8.2.1 Flow Variability

The frequency of biologically significant events (freshes or flushing flows) has particular significance to the duration of periphyton accrual periods and the maintenance of acceptable water quality (Harkness 2010¹).

Freshes can also be a functional requirement for selected fish species, which rely on pulse events such as migration and spawning cues. Key determinants of the rate and extent of periphyton accrual are closely related to nutrient supply and flow velocities (Harkness 2010¹).

The likely effect of reduced flow variability in gravel and cobble bed rivers, is an increase in periphyton biomass which can produce flow-on effects to the river ecosystem (MfE 1998). Problematic periphyton biomass can accumulate over periods of continuous low flows which typically occur in Hawke’s Bay during the summer/autumn months. During the winter/spring months, river levels and flow velocities are consistently higher than those of the summer low flow period and combined with lower water temperatures and shorter daylight hours during this period, the potential for abundant periphyton growth is limited (Harkness 2010¹).

To limit the potential impact to ecological values, only scenarios which produced a change of ≤10 percent to the naturalised FRE₃ value, were to be supported as potential allocation regimes which maintain an acceptable level of flow variability.

Sections 8.1.2.1 to 8.1.2.4 present the mean, total and frequency of flows exceeding three times the median flow for all allocation scenarios applied to each site. All modelled high flow allocation scenarios for each site produce minimal change of less than 10% to the FRE₃ value and are therefore supported as potential regimes under which acceptable flow variability can be maintained in each river.

8.2.2
Maintenance of Water Quality

The high flow allocation modelling undertaken on the Ngaruroro River by Harkness (2008 & 2010¹) looked at the effect of the 8 allocation scenarios (which have been adopted and adapted for use in this investigation) on water quality of the Ngaruroro River. The Ngaruroro reports found that the modelled scenarios resulted in a limited effect on flood frequency and that due to allocation only occurring when river flows were greater than mean or median flows (depending on the scenario) adverse impacts to water quality were not anticipated.

An analysis of water quality data for the Tukituki catchment has not been included in this investigation. The 8 scenarios modelled on each Tukituki catchment site are based on the same scenarios used in the Ngaruroro investigations (detailed scenario information can be found in Section 1). It should be noted there are different water quality issues associated with the Tukituki catchment than with the Ngaruroro catchment (e.g. effluent discharges from municipal waste water treatment plants) but given that all of the modelled Tukituki scenarios result in only minimal changes to the low flow statistics (Section 7.1) and minimal effects to flood frequency (Section 8.1), as with the Ngaruroro investigation, adverse impacts to water quality in the Tukituki catchment resulting from the potential high flow allocation scenarios are also not anticipated.

8.2.3 Habitat Requirements

8.2.3.1 Instream Habitat Modelling in the Context of the Naturalised Flow Regime

Instream habitat modelling has been undertaken on rivers in the Tukituki catchment and documented in a report by Johnson (2011). The analysis undertaken in this report focuses on how predicted flow conditions change the amount of available instream habitat for a range of species in relation to low flow conditions.

The modelling undertaken in this investigation relates to high flow allocation above mean and median flows so it is expected that the available instream habitat will be uncompromised by any of the modelled high flow allocation scenarios for each site.

8.2.3.2 Fish Passage

The high flow allocation modelling undertaken on the Ngaruroro River by Harkness (2008 & 2010¹) looked at the fish passage requirements for diadromous in the Ngaruroro River. These investigations found that given the limited variation in flood frequency and magnitude imposed by the 8 allocation scenarios (which have been adopted and adapted for use in this investigation), none of the scenarios would be expected to affect fish passage within the river itself or significantly affect the river mouth opening regime.

The 8 scenarios modelled on each Tukituki catchment site are based on the same scenarios used in the Ngaruroro investigations and given that all of the modelled Tukituki scenarios also result in minimal effects to flood frequency and magnitude, adverse impacts to fish passage in the Tukituki River catchment are also not expected.

The Ngaruroro reports recommend that river mouth openings should be monitored by the Regional Council and that as far as practicable, the river mouth should be manually kept open during spring and early summer. It is recommended the same approach to river mouth openings is adopted for the Tukituki River.

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9 Range of Variability Approach (RVA)

9.1 Overview

The ecological integrity of riverine ecosystems is dependent upon the natural flow regime of the river system. Maintaining natural variability in the flow regime is critical for conserving the structure and function of riverine ecosystems (Sanford et al. 2007).

A method referred to as the Range of Variability Approach (RVA) described by Richter et al. (1997) has been developed to comprehensively characterise ecologically relevant attributes of a flow regime in order to develop flow-based river management targets that incorporate the concepts of natural hydrological variability and river ecosystem integrity.

The RVA uses the pre-development natural variation of 33 hydrological parameters called indicators of hydrologic alteration (IHAs), as a reference for defining the extent to which natural flow regimes have been altered (TNC 2009).

The IHA parameters are used to assess hydrological alteration resulting from anthropogenic activities (e.g. water abstraction) in terms of magnitude, timing, frequency, duration, and rate of change (Richter et al. 1996). The IHA parameters provide information on some of the most ecologically significant features of the flow regime that influence aquatic ecosystems.

The 33 IHA parameters are divided into the five major groupings. These are detailed in Table 41 along with their ecological influences.

The ‘Indicators of Hydrologic Alteration v7.1’ is a software tool developed by The Nature Conservancy (TNC 2009) which enables users to implement the RVA to assess the degree of hydrologic alteration attributable to human impacts within a river ecosystem.

The IHA software has been used in this investigation to assess how the natural flow regime could be affected as a result of the potential high flow allocation method scenarios (outlined in Section 1) for each Tukituki catchment site. The IHA software calculates statistics for each IHA parameter from both the naturalised and eight modelled flow records for each site. The IHA software produces a scorecard table presenting the resulting statistics which are compared and summarised in Section 9.2.

In developing the RVA, Richter et al. (1997) defines six basic steps for setting, implementing and refining management targets and rules for a specific river or river reach. The first three steps relate to characterising the natural hydrological regime of a river and setting river management targets (environmental goals) to use as design rules for developing a set of management rules or a management system that will enable the targeted flow conditions to be met during most, if not all years. The last three steps are concerned with the implementation of the management system, monitoring, assessment and revision of the established targets.

In this investigation the RVA analysis characterises the natural (pre-impact) inter-annual range of stream-flow variation for each site using the 33 IHA parameters. The range of natural variation for each IHA parameter is used as a basis for defining the initial flow management targets. The RVA analysis generates a series of hydrologic alteration factors which quantify the degree of alteration of the 33 IHA flow parameters in relation to the established targets. The RVA analysis and results for each site are detailed in Section 9.3.

Richter et al. (1998) states that using the RVA, river managers strive to keep annual values of each hydrologic parameter within a targeted range of values that defines some portion or all of the natural range of variability in the parameters. These RVA management targets should be based, to the extent possible, on available ecological information. At the same time, the RVA is meant to enable river managers to define and adopt readily interim management targets before conclusive, long-term ecosystem research results are available. Richter et al. (1998) states that an adaptive management approach, whereby interim management targets and management actions are prescribed and implemented, system response is monitored, and management targets and the prescribed flow regime are adjusted based on monitoring results and ecological research is fundamental to successful application of the RVA.

The RVA targets used in this investigation are intended to be used as interim management targets that will be adjusted and refined in the future.

To undertake the analyses, datasets were constructed for each site, which combined the naturalised flow record and modelled flow data for each scenario. The IHA software uses daily flow data for a sufficiently long hydrological record in order to obtain reliable pre- vs. post-impact comparisons (TNC 2009). This means one dataset of continuous flow record is required with a defined break where naturalised flow ends and altered flow begins, e.g. abstraction starts or a dam is built etc. (Harkness 2010¹).

Hydrological datasets have been constructed for each modelled high flow allocation scenario to analyse with the IHA software, combining the naturalised flow data with the modelled flow data.

Table 41 Summary of IHA Parameters and their Ecosystem Influences (Harkness 2010¹)

9.2
IHA Summary

The IHA software produces a scorecard table presenting statistics calculated for each IHA parameter, which compares the naturalised records (pre-impact) to the scenario modelled records (post-impact) for each Tukituki catchment site.

The IHA scorecard tables produced for each site are summarised in Sections 9.2.1 to 9.2.4. The IHA analysis was undertaken using non-parametric (median/percentile) statistics.

The scorecard tables display statistics for each hydrological parameter:

▪ The median flow value for both the naturalised and scenario modelled records for each IHA parameter

▪ The deviation factor which represents the deviation of the modelled record from the naturalised record

▪ The significance count for the deviation values. To calculate the significance count, the software program randomly shuffles all years of input data and recalculates (fictitious) pre- and post-impact medians 1000 times. The significance count is the fraction of trials for which the deviation values for the medians were greater than for the real case. A low significance count (minimum value is 0) means that the difference between the pre and post-impact periods is highly significant. A high significance count (maximum value is 1) means that there is little difference between the pre and post-impact periods (TNC 2009).

▫ NOTE: The IHA v7.1 user manual (TNC 2009) notes that it is important to understand that in some infrequent situations the algorithm may generate very low significance counts when there is very little apparent difference between the pre and post-impact periods. This can occur when the deviation factor between the pre and post-impact periods is zero or very small, and the overall distribution contains a large number of values right at or very near the centre of the distribution. In this situation a low significance count actually means that the lack of difference between the two periods is highly significant, in a statistical sense, because randomly rearranging the data rarely yield a larger deviation factor than the original data.

The scorecard tables in Sections 9.2.1 to 9.2.4 present the statistics calculated for the following parameter groups:

▪ Parameter Group 1 - the median monthly flows for the June to November period

▪ Parameter Group 2 - the median of the minimum and maximum flows

▪ Parameter Group 4 - the high and low pulse count (The high pulse threshold for each site is set as close to the naturalised FRE₃ (three times the median) threshold as the software will allow

▪ Parameter Group 5 - the rate change of rising and falling flows

In addition to the scorecard table, the IHA software also produces a message report, providing details on any warnings regarding the results produced by the IHA software. Warnings regarding dates of extreme flows (Parameter Group 3) were given for each site and as such have been omitted from the IHA analysis in this report.

9.2.1
Tukipo River at State Highway 50 IHA Summary

In terms of median monthly flows, Table 42 shows that scenarios SH50 5 to 8 (scenarios with a median flow high flow allocation threshold) show the greatest change from the naturalised record. Scenarios SH50 6 and SH50 8 produce the highest deviation in median flow values from the naturalised record and produce deviation factors of up to 0.23 (equivalent of a 23% deviation). SH50 6 and SH50 8 also produce the lowest significance counts meaning the differences from the naturalised (pre-impact) flow are the most significant.

In terms of minimum flow parameters, all scenarios show minimal change. Scenarios SH50 6 and SH50 8 show slightly higher deviation factors combined with low significance counts for the 30-day maximum, 90-day maximum and base flow index parameters. Scenarios 1, 3, 5 and 7 show the least significant impact on all group 2 parameters.

The scenarios with the highest HFA cap of 400l/s (SH50 2, 4, 6 and 8) show the greatest impact on the high pulse count in terms of deviation factor and significance count. The other four scenarios show no change in the high flow pulse count.

Overall the least deviation from the naturalised flow record (<5%) is associated with scenarios SH50 1, 2, 3, 4, 5, and 7.

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9.3 RVA Summary

As mentioned in Section 9.1, the RVA analysis characterises the natural (pre-impact) inter-annual range of stream-flow variation for each site using the 33 IHA parameters. The range of natural variation for each IHA parameter has been used as a basis for defining the initial flow management targets (RVA targets).

Richter et al. (1997) refers to the management target for any given parameter being expressed as a range of acceptable values with lower and upper boundaries.

In the RVA analysis, the full range of naturalised flow records (pre-impact data) for each parameter is divided into three different categories, low high and middle. The boundaries between these categories are based on percentile values which are specified by the user.

As with the work undertaken on the Ngaruroro River by Harkness (2008 & 2010¹), non-parametric RVA analyses were undertaken using the IHA software default category boundaries set at 17 percentiles above and below the median. This produces three categories of equal size: the lowest category contains all values less than or equal to the 33rd percentile and the highest category contains all values greater than the 67th percentile. The middle category represents the RVA target range for each parameter, containing all values falling in the range of the 34th to 67th percentiles (lower and upper boundaries respectively). These RVA targets are intended to be used as interim management targets that will be adjusted and refined in the future based on further monitoring and research.

The IHA software computes the expected frequency with which the post impact (modelled data) values of the IHA parameters should fall into each category (i.e. 33% for each of the three categories in this case). The frequency with which the post-impact values actually fall within the categories (referred to as the observed frequency) is computed. A Hydrologic Alteration factor based on the observed and expected frequencies is calculated for each category (TNC 2009) as:

(Observed Frequency - Expected Frequency) / Expected Frequency

A positive Hydrologic Alteration factor means that the frequency of values in the category has increased from the pre-impact to post-impact data. A negative value means it has decreased (Harkness 2010¹).

The degree of hydrologic alteration, D, can be expressed as a percentage, (Shiau & Wu 2004):

Where:

N_o = observed number of post-impact years for which the value of the hydrologic parameter falls within the RVA target range

N_e = expected number of post- impact years for which the parameter value falls within the RVA target range.

Richter et al. (1998) developed a system to classify the degree of hydrologic alteration (also adopted by Shiau & Wu 2004). The range of possible (absolute) average values for the degree of hydrologic alteration (0–100%) was divided into three classes of equal range:

1) Low degree of alteration = 0-33% representing little or no alteration

2) Moderate degree of alteration = 34-67%

3) High degree of alteration = 68-100%

A second stage of classification has been used by Shiau & Wu (2004) which defines the degree of overall alteration based on the D values of all the 33 IHA parameters:

1) Overall low alteration = the degree of hydrologic alteration of each IHA belongs to the low alteration category; i.e. the D values of all IHAs are less than 33%.

2) Overall medium alteration = at least one of the 33 IHAs belongs to the moderate degree of hydrologic alteration category but none belongs to the high alteration category.

3) Overall high alteration = at least one IHA belongs to the high degree of hydrologic alteration category.

In this investigation it is proposed that both stages of classification are trialled to assess the suitability of all high flow allocation scenarios for each site.

An ideal high flow allocation scenario would result in the modelled (post-impact) flow regime attaining the target ranges at the same frequency as that which occurred in the natural (pre-impact) flow regime.

Using the two-stage classification system, a suitable high flow allocation scenario would be one where the 33 IHA parameters are not significantly altered, producing a low degree of overall hydrologic alteration where the disturbance to the structure and function of the riverine ecosystem is minimal.

The RVA analyses of the eight potential high flow allocation scenarios for each site are summarised (including tables detailing the hydrologic alteration percentage and class) in Sections 9.3.1 to 9.3.4.

The RVA analysis results tables present the statistics calculated for the following parameter groups:

▪ Parameter Group 1 - the median monthly flows for the June to November period

▪ Parameter Group 2 - the median of the minimum and maximum flows

▪ Parameter Group 4 - the high and low pulse count (The high pulse threshold for each site is set as close to the naturalised FRE₃ (three times the median) threshold as the software will allow

▪ Parameter Group 5 - the rate change of rising and falling flows

As with the IHA analysis detailed in Section 9.2 the IHA software produced warning messages relating parameter group 3 for each site and as such have been omitted from the RVA analysis in this report.

9.3.1
Tukipo River at State Highway 50 RVA Summary

The results of the RVA analysis in Table 46 show that scenarios SH50 1, 3, 4, 5 and 7 produce the least impact on the natural flow regime with a low degree of hydrologic alteration for all IHA parameters. The percentages of hydrologic alteration for these scenarios for all IHA parameters were within +/- 33%. These five scenarios are therefore classed as having overall low alteration.

Scenarios SH50 2, 6 and 8 fell within the overall medium alteration class. Scenario SH50 2 results in a moderate degree of alteration to the July median flow parameter. Scenario SH50 6 had three IHA parameters classified as moderate degrees of alteration (October flow, 90-day maximum flow and fall rate parameters). Scenario 8 results in moderate hydrologic alteration to the October flow, 90-day minimum flow and 90-day maximum flow parameters.

In terms of the average percentage of hydrologic alteration for all parameters, scenarios SH50 1, 3, 4, 5 and 7 show lowest percentage of alteration (≤10%).

Table 46 SH50 RVA Analysis Summary

9.3.2
Tukituki River at Tapairu Rd RVA Summary

The results of the RVA analysis in Table 47 show that scenarios Tapairu Rd 1-4 (all scenarios with mean flow high flow allocation thresholds) produce the least impact on the natural flow regime with a low degree of hydrologic alteration for all IHA parameters. The percentages of hydrologic alteration for these scenarios for all IHA parameters were within +/- 33%. These four scenarios are therefore classed as having overall low alteration.

Scenarios Tapairu Rd 5-8 fell within in the overall medium alteration class. Scenario Tapairu Rd 6 produces the greatest degree of alteration to the IHA parameters. Three IHA parameters (June flow, October flow and fall rate) result in percentage of alteration between +/- 50%.

Scenarios Tapairu Rd 5 and 7 result in moderate alteration to the June and September flow parameters and Tapairu Rd 8 shows a moderate alteration to only the August flow parameter.

In terms of the average percentage of hydrologic alteration for all parameters, scenarios Tapairu Rd 1, 2, 3, 4, 5 and 7 show lowest percentage of alteration (<10%).

Table 47 Tapairu Rd RVA Analysis Summary

9.3.3
Waipawa River at RDS RVA Summary

The results of the RVA analysis in Table 48 show that scenarios RDS 1-4 and RDS 7 produce the least impact on the natural flow regime with a low degree of hydrologic alteration for all IHA parameters. The percentages of hydrologic alteration for these scenarios for all IHA parameters were within +/- 33%. These five scenarios are therefore classed as having overall low alteration.

Scenarios RDS 5, 6 and 8 are placed in the overall medium alteration class. RDS 5 shows a moderate alteration to only the September flow parameter. Scenarios RDS 6 and 8 result in moderate hydrologic alteration to the July flow, October flow and 30-day maximum flow parameters.

In terms of the average percentage of hydrologic alteration for all parameters, scenarios RDS 1, 2, 3, 4, 5 and 7 show the lowest percentage of alteration (≤10%).

Table 48 RDS RVA Analysis Summary

9.3.4
Tukituki River at Red Bridge RVA Summary

The results of the RVA analysis in Table 49 show that scenarios Red Br 1-4 (all scenarios with mean flow high flow allocation thresholds) and Red Br 5 and 7 (scenarios with a median flow high flow allocation threshold and lowest allocation limit of 2000l/s) produce the least impact on the natural flow regime with a low degree of hydrologic alteration for all IHA parameters.

Scenario Red Br 8 which allocates 33% of flow above the median with the highest HFA cap of 8000l/s fell within the overall medium alteration class and results in the moderate alteration of five IHA parameters; July, August and October flows; the 90-day maximum flow and fall rate parameter. Scenario Red Br 6 is placed in the overall high alteration class due to producing a high degree of alteration to the fall rate parameter. Three other parameters for this scenario also result in moderate alteration; the August flow, 90-day maximum flow and rise rate.

In terms of the average percentage of hydrologic alteration for all parameters, scenarios Red Br 1, 2, 3, 4, 5 and 7 show the lowest percentage of alteration (<10%).

Table 49 Red Br RVA Analysis Summary

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10 Discussion

This investigation assesses the effect of different potential methods of water allocation during high river flows within the winter and spring months of June to November, from four established hydrological sites on rivers in the Tukituki Catchment (Figure 2 Section 5.1):

▪ Tukipo River at State Highway 50

▪ Tukituki River at Tapairu Rd

▪ Waipawa River at RDS

▪ Tukituki River at Red Bridge

Eight different high flow allocation scenarios developed for each site have been assessed in terms of providing a sustainable flow (high flow allocation threshold) above which high flow allocation is made available, combined with a sustainable allocation limit (allocation cap) allowing for abstraction without adversely impacting on flow variability and instream ecological requirements, while providing an optimum security of supply.

The high flow allocation scenarios use high flow allocation thresholds (HFA thresholds) set as either mean or median flow combined with either of two selected high flow allocation caps (HFA caps) for each site. A flow sharing approach has been used in all scenarios where either 50% or 33% of flow above the HFA threshold up to the HFA cap is allocated for abstraction, with the rest (50% or 67% respectively) remaining in the river.

The FRE₃ flood frequency has been identified as the most ecologically relevant hydrological index for characterizing hydrological regimes in New Zealand streams and rivers (Clausen & Biggs 1997). An ecologically based analysis was undertaken which assessed the change to FRE₃ between the natural and modelled flow records.

To limit the potential impact to benthic communities and to the wider aquatic environment, criteria was employed from previous studies where only scenarios producing a change to the FRE₃ of less than or equal to 10 percent, were supported as potential allocation regimes and suitable for further analysis.

The FRE₃ analyses showed that all of the modelled potential high flow allocation scenarios for each site alter the FRE₃ by less than 10 percent and were therefore supported as potential allocation regimes.

An analysis based on the Range of Variability Approach (RVA) developed by Richter et al. (1997) was undertaken to assess the degree of hydrologic alteration between the naturalised (pre-impact) and scenario modelled (post-impact) flow records in relation to 33 hydrological parameters (IHA parameters).

Two previously developed systems to classify the degree of hydrologic alteration for each IHA parameter and the degree of overall alteration by each scenario have been trialled and applied to the RVA results.

Both systems of classification are relatively simple to apply to the RVA results. Both systems use a three class scheme by which to classify the degree of hydrologic alteration, but a three class scheme could be considered too simplified (Shiau & Wu 2004). A more comprehensive and effective scheme could be based on the continuous variation of hydrologic alteration. Consideration also needs to be given to the importance of each individual IHA parameter to identify if more weight should be given to certain parameters when classifying a potential allocation method.

Irrespective of what classification scheme is used or developed in the future, for any river, it is essential to establish the key river values that require protection and identify the level of change to the natural flow regime or degree of hydrologic alteration that must not be exceeded in order to maintain and sustain the required protection of river values.

Until establishing the degree of hydrologic alteration and the level of change to the natural flow regime that will sustain and maintain the protection of key values for rivers in the Tukituki Catchment, a conservative approach to selecting the most preferable allocation methods from those modelled in this investigation is necessary. In terms of hydrologic alteration, scenarios which produced the lowest average percentage of hydrologic alteration (≤10%) were considered as potentially suitable allocation methods.

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11 Conclusion

▪ Tukipo River at State Highway 50

Scenarios SH50 1, 3, 5 and 7 with the lower HFA cap of 100l/s and scenario SH50 4 with the higher HFA cap of 400l/s, result in the least overall deviation (IHA analysis) and hydrologic alteration from the naturalised flow record (RVA analysis). These five scenarios also result in minimal change to all key hydrological statistics and minimal change to the FRE₃ flood frequency from the naturalised flow record. SH50 scenarios with median flow HFA thresholds have been identified as providing the greatest security of supply to water users.

Taking into account all analyses undertaken as part of this investigation, the high flow allocation scenario which provides the greatest security of supply with the least change to the natural flow regime for the Tukipo River at State Highway 50 is scenario SH50 5 (If river flow is greater than median, then 50% of flow above the median is allocated up to a maximum of 100l/s).

▪ Tukituki River at Tapairu Rd

All scenarios with allocation flow thresholds set at mean flow (Tapairu Rd 1, 2, 3 and 4) plus scenarios Tapairu Rd 5 and 7 (median flow HFA threshold and lower HFA cap of 1000l/s) result in the least overall deviation and hydrologic alteration from the naturalised flow record. Although two of the monthly IHA parameters produce slightly higher degrees of hydrologic alteration, the average degree of alteration for all parameters is relatively low and of similar value to the mean HFA threshold scenarios. These six scenarios also result in minimal change to all key hydrological statistics and minimal change to the FRE₃ flood frequency from the naturalised flow record. Tapairu Rd scenarios with median flow HFA thresholds have been identified as providing the greatest security of supply to water users.

Taking into account all analyses undertaken as part of this investigation, the high flow allocation scenario which provides the greatest security of supply with the least change to the natural flow regime for the Tukituki River at Tapairu Rd is scenario Tapairu Rd 5 (If river flow is greater than median, then 50% of flow above the median is allocated up to a maximum of 1000l/s).

▪ Waipawa River at RDS

All scenarios with allocation flow thresholds set at mean flow (RDS 1, 2, 3 and 4) plus scenarios RDS 5 and 7 (median flow HFA threshold and lower HFA cap of 1000l/s) result in the least overall deviation and hydrologic alteration from the naturalised flow record. These scenarios also result in minimal change to all key hydrological statistics and minimal change to the FRE₃ flood frequency from the naturalised flow record. RDS scenarios with median flow HFA thresholds have been identified as providing the greatest security of supply to water users.

Taking into account all analyses undertaken as part of this investigation, the high flow

allocation scenario which provides the greatest security of supply with the least change to the natural flow regime for the Waipawa River at RDS is scenario RDS 5 (If river flow is greater than median, then 50% of flow above the median is allocated up to a maximum of 1000l/s).

▪ Tukituki River at Red Bridge

All scenarios with allocation flow thresholds set at mean flow (Red Br 1, 2, 3 and 4) plus scenarios Red Br 5 and 7 (median flow HFA thresholds and lower HFA cap of 2000l/s result in the least overall deviation and hydrologic alteration from the naturalised flow record. These scenarios also result in minimal change to all key hydrological statistics and minimal change to the FRE₃ flood frequency from the naturalised flow record. Red Br scenarios with median flow HFA thresholds have been identified as providing the greatest security of supply to water users.

Taking into account all analyses undertaken as part of this investigation, the high flow allocation scenario which provides the greatest security of supply with the least change to the natural flow regime for the Tukituki River at Red Bridge is scenario Red Br 5 (If river flow is greater than median, then 50% of flow above the median is allocated up to a maximum of 2000l/s).

The details of the four scenarios selected as the most suitable for each site are presented in Table 50.

Table 50 Selected High Flow Allocation Scenarios

The high flow allocation scenarios modelled in this investigation are all based on a 50% flow share approach. A flow sharing approach enables water to be abstracted from a river whilst maintaining a level of flow variability in the river. The practicalities of implementing high flow allocation methods based on a flow sharing approach need to be carefully considered. One possible approach could be to issue global water take consents to water user groups (instead of the current allocation process which issues consents to individual water users) whereby the abstraction is managed collectively by the group, employing measures such as rationing and rostering to ensure abstraction complies with any abstraction restrictions.

Alternative approaches to high flow allocation which do not include any flow sharing arrangements may pose fewer difficulties to management practices. Further work would be required to assess the impact on natural river flow regimes of any alternative approaches.

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13 REFERENCES

Clausen, B & Biggs, JB 1997, Relationships between Benthic Biota and Hydrological Indices in New Zealand Streams, Freshwater Biology, vol. 38, no. 2, pp 327-342.

Intergovernmental Panel on Climate Change (IPCC) 2007, Climate Change 2007: Impacts, Adaptation and Vulnerability - Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK.

Harkness, M 2008, Ngaruroro River High Flow Allocation, Report prepared for Hawke’s Bay Regional Council by MWH, Wellington.

Harkness, M 2010¹, Ngaruroro River High Flow Allocation: June to November Period. Prepared for Hawkes Bay Regional Council, MWH, Wellington.

Harkness, M 2010², Flow Naturalisation for Six Hawke’s Bay Catchments: Tutaekruri, Waipawa, Tukipo, Tukituki, Maraetotara and Porangahau. Prepared for Hawkes Bay Regional Council, MWH, Wellington.

Hawke’s Bay Regional Council 2006, Hawke’s Bay Regional Resource Management Plan (RRMP), Environmental Management Group Technical Report, Operative 28 August 2006.

Johnson, K 2011, Tukituki Catchment Instream Flow Assessments, Hawke’s Bay Regional Council - Environmental Management Group Technical Report.

MfE 1998, Flow Guidelines for Instream Values Part A & B, Ministry for the Environment, Wellington.

Richter, BD, Baumgartner, JV, Powell, J & Braun, DP 1996, A Method for Assessing Hydrologic Alteration within Ecosystems, Conservation Biology, vol. 10, no. 4, pp 1163-1174.

Richter, BD, Baumgartner, JV, Wigington, R & Braun, DP 1997, How Much Water Does a River Need?, Freshwater Biology, vol. 37, no. 1, pp 231-249.

Richter, BD, Baumgartner, JV, Braun, DP & Powell, J 1998, A Spatial Assessment of Hydrologic Alteration within a River Network, Regulated Rivers: Research and Management vol. 14, no. 4, pp 329-340.

Sanford, SE, Creed, IF, Tague, CL, Beall, FD & Buttle, JM 2007, Scale-dependence of natural variability of flow regimes in a forested landscape, Water Resources Research, vol. 43, W08414, doi:10.1029/2006WR005299.

Shiau, JT & Wu, FC 2004, Feasible Diversion and Instream Flow Release Using Range of Variability Approach, Journal of Water Resources Planning And Management, vol. 130, no. 5, pp 395-404.

The Nature Conservancy (TNC) 2009, Indicators of Hydrologic Alteration Version 7.1 User’s Manual

[1] Minimum Flow (RRMP definition): A critical flow set to ensure sufficient water is left in a river to maintain the life-supporting capacity of aquatic ecosystems and/or other identified values, during low flow conditions.

[2] Allocatable volume (RRMP definition): The volume of water flow available for out-of-stream use e.g. irrigation. It is the volume of the total river flow available over a set period (e.g. the average daily flow or average seven day flow) that may be abstracted from a river or stream without causing the minimum flow to occur so often as to cause a continuing change in the nature of the aquatic ecosystem.

[3] Q₉₅: This is the flow that is exceeded 95 percent of the time.

Table 25 Average Monthly Abstraction Provided by SH50 Scenarios		Table 26 Average Monthly Abstraction Provided by Tapairu Rd Scenarios
Table 27 Average Monthly Abstraction Provided by RDS Scenarios		Table 28 Average Monthly Abstraction Provided by Red Br Scenarios