Hurunui-Waiau Storage Options - Environment Canterburyfiles.ecan.govt.nz/public/cw-hw-zip-reports/Aqualinc 2011 Hurunui Waiau... · Hurunui-Waiau Storage Options - Modelling Assumptions

Hurunui-Waiau Storage Options - Modelling Assumptions and Results

(Version 2)

Prepared for Canterbury Water Executive

Report No C1009204/2

March 2011

w w w . a qua l i nc . c o m

CHRISTCHURCH PO Box 20-462, Bishopdale 8543, Christchurch ♦ Phone: (03) 964 6521 ♦ Fax: (03) 964 6520

HAMILTON PO Box 14-041, Enderley 3252, Hamilton ♦ Phone: (07) 858 4851 ♦ Fax: (07) 858 4847

Disclaimer:

This report has been prepared solely for the benefit of Canterbury Water Executive. No liability is accepted by

Aqualinc Research Ltd or any employee or sub-consultant of this Company with respect to its use by any other

person.

This disclaimer shall apply notwithstanding that the report may be made available to other persons for an

application for permission or approval or to fulfil a legal requirement.

Quality Control

Client: Canterbury Water Executive

Report reference: Title:

Hurunui-Waiau Storage Options (Version 2) No:

C1009204/1

Prepared by: Channa Rajanayaka, Andrew Dark, Julian Weir

Reviewed by: Channa Rajanayaka, Andrew

Dark, Julian Weir Approved for issue by:

Channa Rajanayaka, Andrew Dark,

Julian Weir

Date issued: March 2011 Project No: C1009204

This report has not been through Aqualinc's normal review process. Because of the confidentiality of some of the material in the report, internal review has been restricted to only the three authors.

Document History

Version: 1 Status: Draft for Review Author: CR, AD &

JW Reviewer:

CR, AD &

JW

Date: 17/2/11 Doc ID: Typist: JW Approver: CR, AD &

JW

Version: 2 Status: Final Author: CR, AD &

JW Reviewer:

CR, AD &

JW


JW

Version: 3 Status: Final Revised (Draft) Author: CR, AD &

JW Reviewer:

CR, AD &

JW


JW

© All rights reserved. This publication may not be reproduced or copied in any form, without the permission of

the Client. Such permission is to be given only in accordance with the terms of the Client’s contract with

Aqualinc Research Ltd. This copyright extends to all forms of copying and any storage of material in any kind

of information retrieval system.

Hurunui-Waiau Storage Options (Version 2) © Aqualinc Research Ltd Prepared for Canterbury Water Executive (Report No C1009204/2, March 2011) Page i

TABLE OF CONTENTS

Page

1 Introduction .................................................................................................................... 1

2 Overview of Results ........................................................................................................ 2 2.1 Climate Change ....................................................................................................... 4 2.2 General Comments on Storage Performance .......................................................... 4

2.3 Location of Flow Sites ............................................................................................ 4

3 General Assumptions ..................................................................................................... 6 3.1 Potentially Irrigable Areas ...................................................................................... 6 3.2 Irrigation Demand ................................................................................................... 6 3.3 Supply Reliability ................................................................................................... 7

3.4 Existing Irrigated Areas .......................................................................................... 7

3.5 Potential Expansion of Irrigated Area Through Efficiency Gains .......................... 7

3.6 Water Allocation ..................................................................................................... 8

4 Option Specific Assumptions ....................................................................................... 10 4.1 Lake Sumner Options............................................................................................ 10

4.1.1 HWP Option .............................................................................................. 10

4.1.2 NTP Option ............................................................................................... 10 4.2 Hurunui South Branch Dam .................................................................................. 10 4.3 South Branch and Lake Sumner ............................................................................ 11

4.4 Waitohi River Options .......................................................................................... 11 4.4.1 HWP Option .............................................................................................. 11

4.4.2 DPML Option ............................................................................................ 11 4.4.3 Generic Waitohi Storage with 10% Reduction in Waitohi River Flows .. 12 4.4.4 Fraser Geologic Option ............................................................................. 12

4.5 Pahau River Options ............................................................................................. 12

4.5.1 Pahau Dam Filled from In-Catchment Flows ........................................... 12 4.5.2 Pahau Dam Supplemented with Waiau River Water ................................ 13

4.6 Waiau River 240 m Transfer Canal ...................................................................... 13

4.6.1 Hurunui South Branch Storage ................................................................. 13 4.6.2 Waitohi Storage ......................................................................................... 13

4.7 Mandamus River ................................................................................................... 14 4.8 Hanmer River ........................................................................................................ 14

List of Appendices:

Appendix A: Flow duration curves and hydrographs: Waitohi HWP option ........................ 15

Appendix B: Flow duration curves and hydrographs: Waitohi DPML option ...................... 17

Appendix C: Flow duration curves and hydrographs: Waitohi generic option with 10% less

Waitohi .................................................................................................................... 19

Appendix D: Flow duration curves and hydrographs: Waitohi Fraser Geologic option ....... 24

Appendix E: Flow duration curves and hydrographs: Waiau 240 m transfer canal with

Waitohi storage ........................................................................................................ 26

Hurunui-Waiau Storage Options (Version 2) © Aqualinc Research Ltd Prepared for Canterbury Water Executive (Report No C1009204/2, March 2011) Page ii

Appendix F: Flow duration curves and hydrographs: Hanmer River option ........................ 31

Appendix G: Annual Pumping Volumes: Hurunui River to Waitohi storage ....................... 33

Appendix H: Storage volumes: Hurunui River to Waitohi storage ....................................... 35

Appendix I: Flow site locations ............................................................................................ 37

List of Tables: Table 1: Summary of river flows ............................................................................................. 2

Table 2: Overview of results ................................................................................................... 3

Table 3: Existing run-of-river supply reliability ...................................................................... 7

Table 4: Hurunui River allocation regime ............................................................................... 9

Table 5: Waiau River allocation regime .................................................................................. 9

Hurunui-Waiau Storage Options (Version 2) © Aqualinc Research Ltd Prepared for Canterbury Water Executive (Report No C1009204/2, March 2011) Page 1

1 INTRODUCTION

A list of potential storage options for the Hurunui–Waiau water management zone was

compiled in consultation with ECan Water Executive staff and developers/scheme promoters

who are active in the area.

The aim of the project is to complete a high-level comparison of potential options in order to

filter out any options that are clearly not feasible. The modelling undertaken for this project

is not intended to replace or supersede more detailed modelling that has previously been

completed for specific options, nor will it remove the need for more detailed modelling and

feasibility studies to progress plausible options. Assumptions that were appropriate for

comparative purposes are not necessarily the same that would be used for more detailed

modelling of a single option.

The level of detail available for the storage options is highly variable: some are well-

developed proposals while others are concepts that have had little or no previous work done

on them.

Each storage option has been modelled in MikeBasin1 using a common set of general

assumptions. The models have been run on a daily time-step using river flow and climate

data from 1972–2009.

For each storage option the key output was the reliably irrigated area. This, combined with

cost estimates being prepared by Riley Consultants, will allow the various options to be

compared. For options that have not been considered previously, namely the Waitohi River

and Hanmer River options (described later), hydrological results have also been included to

show the impact on flow regimes.

This report contains the modelling results and the key assumptions used for each storage

option.

Once preferred options have been selected, more detailed modelling is likely to be

required. At this stage the assumptions on the following can be refined in consultation with

stakeholders:

On-farm system capacity;

Distribution losses;

Supply reliability criteria; and

Preliminary requirements and design for environmental and recreational flow

enhancements.

1 DHI (2009): MikeBasin. Mike GIS. Release 2009.0. Service Pack 5.


2 OVERVIEW OF RESULTS

In total, 14 storage options were analysed. A summary of the results are provided below, and

the various assumptions used are described later in this report. Economic assessments of the

scenarios have been supplied separately by Riley Consultants.

The Water Executive staff have specifically requested results on flow regimes for the four

Waitohi options, the ‘Waiau 240 m Transfer Canal With Waitohi Storage’ option, and the

‘Hanmer River’ option. Flow duration curves and hydrographs for these options are supplied

in Appendices A-F. In addition, Water Executive staff requested seasonal flow duration

curves for the ‘Generic Waitohi Storage with 10% Reduction in Waitohi River Flows’ option

and the ‘Waiau 240 m Transfer Canal With Waitohi Storage’ option. These curves are

presented in Appendix C and E, respectively. River flows have been separated into three

seasons to represent early irrigation season (September to December), high demand irrigation

season (January to April) and the rest of the year (May to August).

Section 2.3 provides a discussion on the representative physical location of the sites for

which the hydrographs and duration curves have been derived. Hydrographs are presented

for the period 2005-2007. As demonstrated in the flow statistics presented in Table 1, these

were the driest hydrological years over the period simulated.

Table 1: Summary of river flows

Hurunui River

at Mandamus

Waitohi

River

Waiau River at

Marble Point

Mean annual1 flow (m

3/s) 54.1 1.6 94.9

Maximum annual1 flow (m

3/s) 71.0 2.4 138.1

Minimum annual1 flow (m

3/s) 38.2 1.0 68.1

Year of the minimum annual1 flow 2005 2007 2005

1 Hydrological year, from 1 July to 30 June for the period of 1972 to 2009

Table 2 presents a summary of the irrigated areas and considered storage volumes for each

scenario assessed.


Table 2: Overview of results

Scenario

Lake Sumner

Hurunui

South

Branch

dam

South

Branch

& Lake

Sumner

Waitohi Pahau Waiau 240 m

transfer canal

Man-

damus

River

Hanmer

River HWP

1 NTP

2 HWP DPML

3

Generic

with 10%

Waitohi

reduction

Fraser

Geologics4

In-

catchment

Waiau

pumping

Hurunui

south

branch

storage

Waitohi

storage

Irrig

ate

d a

rea

s (h

a) Hurunui A-

Block5 13,760 13,760 13,760 13,760 13,760 13,760 13,760 13,760 13,760 13,760 13,760 13,760 13,760 -

Waiau A-

Block5 - - - - - - - - - 33,688 - - - 33,688

New

B-Block 9,500 20,000 34,000 45,000 31,000 32,500 32,500 36,500 2,100 5,500 34,000 32,500 27,000 5,500

Total 23,260 33,760 47,760 58,760 44,760 46,260 46,260 50,260 15,860 52,948 47,760 46,260 46,250 39,188

Reservoir volume

(Mm3)

27.5 31 111 27.5 (LS)

111 (SB) 104 104 104 104 20 29 93 86 80 40

On-farm storage

total volume

(Mm3)

- 20 - - - - - - - - - - - -

Total storage

volume (Mm3)

27.5 51 111 139 104 104 104 104 20 29 93 6

86 7

80 40

1 HWP = Hurunui Water Project 2 NTP = Ngai Tahu Properties Ltd 3 DPML = Direct Project Management Ltd 4 This is option ii) by Fraser Geologics Ltd that maximises hydro power generation. Option i) is hydrologically equivalent to the option by DPML. 5 ‘Hurunui A-Block’ and ‘Waiau A-Block’ are the potentially irrigated areas assuming existing allocation is used efficiently. Refer to Section 3.4 for further discussion. 6 For comparison with the Hurunui South Branch option

7 For comparison with the Waitohi DPML storage option


2.1 Climate Change

The scenario ‘Generic Waitohi Storage with 10% Reduction in Waitohi River Flows’ is

presented as a coarse assessment of the effects of climate change, and also to assess the

relative contributions to storage of water supplied by the Waitohi River versus that supplied

from the Hurunui River (via pumping).

Overall, the 10% reduction resulted in no change to the area that could be reliably irrigated

and resulted in an average increase in pumping from the Hurunui River of 1.18 Mm3/year,

which equates to approximately 0.07% of the average Hurunui flow. Relative flow

contributions to Waitohi storage from the Waitohi River and pumping from the Hurunui

River for the DPML option are 40% and 60%, respectively. These percentages change to

37% and 63% for the ‘Generic Waitohi Storage With 10% Reduction in Waitohi River

Flows’ option.

The annual flow contributions from the two rivers for the two options are presented in

Appendix G. Charts showing the relative contributions of Hurunui and Waitohi water to

Waitohi storage are also provide in this appendix. Appendix H presents hydrographs of

storage volumes in the Waitohi storage reservoir for both the ‘DPML’ option and the

‘Generic Waitohi Storage with 10% Reduction in Waitohi River Flows’ option. These graphs

were specifically requested by the Water Executive staff.

2.2 General Comments on Storage Performance

In order to provide a high level of supply reliability, sufficient storage is needed to meet

irrigation demand in dry years when the availability of run-of-river water is limited. To

mitigate the risk of consecutive dry years, the storage typically needs to re-fill between

irrigation seasons.

In the options modelled for this project, storage volumes are typically drawn down to (or near

to) the minimum level approximately three times in the period simulated. In the majority of

other years a relatively small amount of storage is utilised. Water from storage is typically

required in the second half of the irrigation season. After being drawn down, storage levels

often recover by mid-winter. The storage would therefore be full for a significant amount of

time. This is illustrated by the storage volume hydrographs in Appendix H.

The variability in the annual storage requirements and the ability of the storage to refill

rapidly following drawdown indicate that, in the majority of years, the storage options being

considered could provide additional water for other purposes such as environmental flow

enhancements, recreational releases, additional hydro generation and/or temporary reductions

of the run-of-river takes. With careful planning, it is believed that this can be done without

compromising on irrigation supply reliability. These flow enhancements should be explicitly

included in future, more detailed modelling of preferred storage options.

2.3 Location of Flow Sites

Appendices A-F present hydrographs and flow duration curves for the four Waitohi options,

the ‘Waiau 240 m Transfer Canal With Waitohi Storage’ option, and the ‘Hanmer River’

option. Maps showing the locations of these sites are presented in Appendix I.


The reported Waitohi flows represent the residual flows once all irrigation releases have been

removed. It is assumed that all irrigation releases are removed from the lower Waitohi River

(i.e. the Hurunui River is not used for conveyance of releases). This is the worst case

scenario in terms of flows in the lower Waitohi.

The Hurunui flows are represented nominally at State Highway 7 (SH7), as they include the

return flow from the Waitohi, but do not include the Pahau River contribution. In presenting

these results, it is assumed that all irrigation takes occur upstream of SH7. There are

potentially irrigable areas adjacent to the lower Dommett Plains reach of the Hurunui River

(below the Pahau River confluence). In practice, water can be taken from the Hurunui River

below the Pahau River confluence, but it has been modelled as being taken above the

confluence. This is because there is insufficient information on tributary inflows and

groundwater interactions (etc.) to enable a reasonable account of the additional available

flow. These assumptions present a worst case scenario for flows in the lower Hurunui River.

The Hanmer River flow regime results are immediately downstream of the dam, and include

all water released from storage. As there are no major tributary inflows, the results are

representative for flows between the dam and the confluence with the Waiau River.

The reported Waiau River flows are downstream of the existing Amuri Scheme intake. It is

assumed that all water for irrigation is taken at this point. As is the case for the Hurunui

River, there are downstream areas adjacent to the Waiau River that are potentially irrigable,

but the downstream reach of the river is not explicitly included in the model. It is therefore

assumed that all water is taken within the Culverden Basin.


3 GENERAL ASSUMPTIONS

Each model incorporates key assumptions, some of which are common to all models. Model

outputs are sensitive to the assumptions used, and therefore it is important to consider these

assumptions when comparing the results in this report to those from other studies.

The following paragraphs overview the common assumptions. Assumptions that are specific

to each model scenario are described later.

3.1 Potentially Irrigable Areas

The potentially irrigable areas in the Hurunui, Waipara and Waiau catchments have been

identified previously in the Canterbury Strategic Water Study, and in the North Canterbury

Storage Options report (Riley Consultants, et al, 2010)2. With the exception of the Hanmer

Basin, it was assumed that it is practical to deliver water from storage to anywhere in the

potentially irrigable areas. Some pumping may be required to deliver water.

3.2 Irrigation Demand

Irrigation demand was calculated on a daily basis using a soil-moisture balance and irrigation

scheduling model over the period 1972-2009. It was assumed that the entire irrigated area

was pasture (which has the greatest seasonal water use of all standard crops grown in New

Zealand), with well-managed spray irrigation. Soil water holding capacities were

proportioned from Landcare’s Fundamental Soils GIS layer, and appropriate irrigation

regimes were set up in the model for light, medium and heavy soils.

Irrigation demand model was dynamically linked to the irrigation supply, so that soil

moisture deficits accumulate when no water is available. When water subsequently becomes

available, the model will attempt to ‘catch-up’ the deficit, if able.

An on-farm system capacity of 5 mm/day (0.58 l/s/ha) was used for all irrigated areas. This

is a typical assumption for irrigation of pasture in Canterbury. It was also assumed that there

were no seepage or operational losses (by-wash) in distribution systems. This assumption

was made for two reasons:

1. Some of the options considered in the project are at a very early conceptual level, and

distribution arrangements have not been considered. Ignoring distribution losses for

all options allowed a fairer comparison of options; and

2. One of the targets of the Canterbury Water Management Strategy (CWMS) is to

increase irrigation efficiency. Although some losses are unavoidable in practice, one

of the aims of scheme design should be to minimise losses.

2 Riley Consultants, Aqualinc Research Ltd and Pattle Delamore Partners Ltd. (2010): Canterbury Water

management Strategy – North Canterbury Storage Options. Final Report Version 0.3. 9 August 2010.


3.3 Supply Reliability

The area that could be reliably irrigated from each storage option was determined using a

two-stage reliability criteria:

Mean irrigation-season average supply-demand ratio to be greater than 94%; and

Periods of restrictions exceeding 10 consecutive days will occur in no more than 10%

of the irrigation seasons modelled.

For storage-based systems the second stage of the criteria will tend to determine the overall

reliability, as restrictions occur when the storage volume is depleted. The reliability criteria

can therefore be loosely interpreted as the storage emptying once every 10 years on average.

For comparison, the reliability of the existing run-of-river irrigation supply from ‘A’

allocation blocks has been calculated for both the Waiau and Hurunui rivers. These are

presented in Table 3. Based solely on the supply-demand ratio, the Waiau River is at the

lower end of what is normally considered ‘good’ reliability, while the Hurunui River is in the

‘marginal’ category, just below ‘good’ reliability. Both the Waiau and Hurunui rivers are

highly unreliable (in terms of the criteria set for this project) with respect to the number of

occurrences of 10 consecutive days or more restrictions.

Table 3: Existing run-of-river supply reliability

River supply

Avg.

supply/demand

ratio

No. of periods of 10

days or more

consecutive

restrictions

(1972-2010)

Waiau River 94.7% 70

Hurunui River 93.2% 67

3.4 Existing Irrigated Areas

Based on consented takes and local knowledge, the area currently irrigated from Hurunui

River A-Block water is approximately 5,240 ha under the existing Balmoral Irrigation

Scheme, plus an additional 1,000 ha (approximately) from private individual takes.

Similarly, the area currently irrigated from the Waiau River A-Block water is 14,500 ha

under the Waiau Plains scheme and 419 ha under the Waiareka Downs Scheme, plus an

additional 2,000 ha (approximately) of private individual takes. The actual area irrigated

from private individual takes is uncertain due to inconsistencies in the consents database.

The reliability of supply for these existing takes is presented in Table 3.

3.5 Potential Expansion of Irrigated Area Through Efficiency Gains

By improving the efficiency of on-farm irrigation and scheme distribution systems, some

expansion of irrigated area could occur either prior to, or in parallel with, the development of


storage-based schemes, with no reduction of supply reliability from the status quo. This has

been discussed in detail in a previous report (Riley, Aqualinc & PDP, 2010).

The existing irrigation schemes (Balmoral and Waiau Plains) were originally designed to

provide water for border-dyke irrigation, with an overall system capacity of 0.7-1.0 l/s/ha.

Some of the existing private irrigation takes are also border-dyke, with higher system

capacities than modern spray irrigation systems. Spray irrigation systems for crops and

pasture require 0.5-0.6 l/s/ha.

The exact amount of water lost to leakage in the existing schemes’ distribution systems is

unknown, but it is understood that there are some relatively leaky races. There is no

information available on the amount of water lost in the distribution races associated with

private irrigation takes. As irrigators within the schemes have converted from border-dyke to

spray, water has theoretically become available for re-allocation to new users. However, in

practice it has not yet been possible to re-allocate this water while there is a combination of

border-dyke and spray irrigators in the schemes due to distribution constraints. When a

higher proportion of irrigators in the schemes convert to spray, it will be possible to re-

allocate water that that has been ‘freed up’.

If irrigation was applied with a reasonable and efficient peak water use of between 0.5-0.6

l/s/ha and if scheme distribution efficiency was improved, then the area able to be irrigated

within the existing A-Block allocations would be greater than the existing irrigated areas.

In Table 2, the ‘Hurunui A-Block’ and ‘Waiau A-Block’ irrigated area values assume these

on-farm and in-scheme efficiency gains have already occurred to the maximum possible

extent. These areas can be irrigated under the existing allocation policy and with the same

supply reliability as existing takes (Table 3). Because the expansion of irrigated area by

means of efficiency gains can occur independent of storage development, these areas have

been included with the existing irrigated areas in Table 2. The storage volumes in Table 2

enable these areas (both the existing areas, and the additional areas achieved through

efficiency gains) to be irrigated with improved reliability as per the criteria discussed in

Section 3.3.

In practice, it may not be possible to achieve 100% of the efficiency gains that are

theoretically possible. The maximum area irrigable from the existing A blocks may therefore

be smaller than reported in Table 2. In this case it would only be the A block areas that

would change; the new areas irrigated from the B block would be independent of the

efficiency gains achieved. For example, if only 50% of the potential efficiency gains in the

Hurunui catchment were achieved, the Hurunui A block area for all storage options would

reduce to approximately 10,000 ha.

3.6 Water Allocation

Allocation regimes for the Hurunui and Waiau rivers were specified by ECan for this project.

These are summarised in Table 4 and Table 5. The Hurunui allocation regime is based on

Variation 8 (now withdrawn) of the NRRP. The A-Block allocation limit for May–Sep has

been modified to ensure that the A and B blocks do not overlap. The volumetric cap on the

B-Block that was proposed in Variation 8 has been ignored.


Table 4: Hurunui River allocation regime

Month of the year

Flow (m3/s)

Jan Feb Mar Apr May –

Jul Aug Sep

Oct –

Dec

A-Block minimum flow 15 12 12 15 12 13 15 15

A-Block allocation limit 6.7 6.7 6.7 6.7 11.7 11.7 11.7 6.7

B-Block minimum flow 26.7 23.7 23.7 23.7 23.7 24.7 26.7 26.7

B-Block allocation limit

(Mandamus flow < 40) 10 7.5 7.5 10 10 10 10 15

B-Block allocation limit

(Mandamus flow > 40) 15 15 15 15 10 10 10 15

Table 5: Waiau River allocation regime

Minimum flow

(m3/s)

A-Block

(m3/s)

Gap size

(m3/s)

B-Block (m3/s)

reserved for

in-catchment use

B-Block (m3/s)

for storage, out-of-

catchment, hydro use

20 18 18 6 Not specified

The NRRP divides the Hurunui River into two reaches: Amuri Plains and Dommett Plains.

An allocation block is set for each. It has been assumed that irrigable areas from both reaches

can be supplied by the storage options considered in this report.

In order to meet the targets of the CWMS, it is likely that additional allocation rules would be

required, for example flushing flow or recreational releases from storage. It is beyond the

scope of this project to design and model these rules. However, by modelling the storage

options with a relatively high reliability criteria, it can be assumed that in the majority of

years there would be sufficient water to provide for these additional releases while still

maintaining a reliable supply for irrigation.


4 OPTION SPECIFIC ASSUMPTIONS

Further to the general assumptions presented above, assumptions that are specified to

each option are described below.

4.1 Lake Sumner Options

Two Options have been considered for Lake Sumner, one by Hurunui Water Project

(HWP) and another by Ngai Tahu Properties Ltd (NTP). Each of these is described

below.

4.1.1 HWP Option

The HWP’s Lake Sumner proposal has been based on their Resource Consent

application3. The option incorporates the construction of a weir to raise the Lake

Level and create active storage for ten months of the year. Additional details are:

Reservoir active storage: 27.5 Mm3 (for ten months per year)

Reservoirs operating range: 541.8-543.8 m amsl approx.

(2 m operating range)

Minimum outlet flow: 9 m3/s (though when reservoir is empty, outlet

flow is less than this minimum)

4.1.2 NTP Option

NTP’s Lake Sumner proposal has been based on information supplied by NTP in a

letter to Pattle Delamore Partners Ltd on 18 December 2009. Primary proposal details

are:

Reservoir active storage: 31 Mm3 (all year round)


(2.25 m operating range)

Minimum outlet flow: 9 m3/s (though when reservoir is empty, outlet

flow is less than this minimum)

On farm storage: Sufficient to supply 12 days of irrigation

4.2 Hurunui South Branch Dam

This option, supplied by HWP, involves the construction of a dam on the south branch

of the Hurunui River. Details have been derived from HWP’s Resource Consent

application and include:

Reservoir active storage: 111 Mm3

Reservoirs operating range: 605-630 m amsl approx.


3 SKM (2009): Hurunui Water Project. Resource Consent Application & Assessment of Environmental Effects.

Final. June 2009.


Minimum outlet flow: Set at 4.5 m3/s downstream of North Esk

confluence4

4.3 South Branch and Lake Sumner

This option by HWP includes storage in both Lake Sumner and in the south branch.

Details have been derived from HWP’s Resource Consent application and combined

the operating features described in Sections 4.1.1 and 4.2. It has been assumed that

irrigation shortfall is supplied first from Lake Sumner and then from the south branch

reservoir (if required).

4.4 Waitohi River Options

Three options have been proposed for storage within the Waitohi catchment with

water filling from the Waitohi River (in-catchment) and from the Hurunui River. One

of the three options was supplied by HWP, one by Direct Project Management Ltd

(DPML) and one by Fraser Geologics Ltd. A similar reservoir site is proposed for the

DPML and Fraser Geologic options, which differs from the HWP option. However,

the active storage volume and operating range for both sites is similar. Details are:


Reservoirs operating range: 25 m operating range

Minimum outlet flow: 130 l/s

4.4.1 HWP Option

For HWP’s Waitohi option, water is pumped from the Hurunui River into storage at a

maximum capacity of 8 m3/s.

When available, demand is met from run-of-river supply from the Hurunui River.

Excess water is pumped into storage. Shortfalls in the run-of-river supply are made

up with releases from storage.

4.4.2 DPML Option

DPML’s Waitohi option incorporates a level head race along the 320 or 315 m amsl

contour from the Hurunui River to the Waithoi River. At the end of this race, water is

pumped into storage. The head race doubles for both supply to storage and supply to

farm distribution; hence the race capacity is set to the maximum irrigation demand.

When available, demand is met from run-of-river supply from the Hurunui River.

Excess water is pumped into storage. A maximum diversion for the Hurunui has been

set at 21.7 m3/s, which is the combined available flow currently proposed under the

NRRP.

4 Aqualinc’s interpretation of the proposal was that the 4.5 m

3/s minimum flow would be maintained at the

catchment outlet (i.e. downstream of the North Esk River confluence). It is now understood that HWP’s model

maintains 4.5 m3/s immediately downstream of the dam, which would result in more water being released from

the dam. This may have some effect on the performance of the storage option.


Shortfalls in the run-of-river supply are made up with releases from storage. Water

released from storage is used for hydro power generation.

4.4.3 Generic Waitohi Storage with 10% Reduction in Waitohi River Flows

This option considers generic storage in the Waitohi catchment but with Waitohi

River inflows to the reservoir reduced uniformly by 10%. It is closely comparable to

the ‘DPML Option’. This scenario is presented as a coarse assessment of the effects

of climate change, and to assess the relative contributions to storage of water supplied

by the Waitohi River versus that supplied from the Hurunui River (via pumping). A

simple 10% reduction was chosen as an upper limit to the predictions of river flows

made by NIWA to account for future climate change.

4.4.4 Fraser Geologic Option

Fraser Geologic’s Waitohi option involves a gravity-fed tunnel from the Hurunui

River main stem (i.e. below the south branch confluence) through to the Waitohi

catchment. Releases from storage are used for hydro power generation.

There is a range of potential operating scenarios for this option. Two options (outer

bounds) for operating rules have been considered:

i) When available, demand is met from run-of-river supply from the Hurunui

River. Anything above this, within the allocation blocks, is then available

for diversion to storage (via the tunnel). Water is released from storage to

make up the shortfall in run-of-river supply; and

ii) Hydro power generation is maximised by diverting all available water

from A and B blocks into storage, and releasing to meet all irrigation

demand.

For both options, a maximum diversion for the Hurunui has been set at 21.7 m3/s,

which is the combined available flow currently proposed under the NRRP.

Hydrologically, proposal i) is essentially the same as the proposal by DPML, and so

the reader is referred to the results under DPML’s option.

4.5 Pahau River Options

Two options for storage in the Pahau River catchment have been proposed, one using

only in-catchment flows from the Pahau River, and the other supplementing storage

with Waiau River water, pumped into the dam. For both Pahau River options a

minimum outlet flow of 400 l/s was assumed, as per Variation 8 of the NRRP.

4.5.1 Pahau Dam Filled from In-Catchment Flows

For this option, the Pahau River reservoir is filled only from in-catchment flows.

Details are:





4.5.2 Pahau Dam Supplemented with Waiau River Water

This option supplements storage via pumping from the Waiau River. Waiau A-Block

irrigation takes are supplied from the Waiau River first. Any flow above this is

available for pumping into the Pahau storage site. Based on information provided by

Riley Consultants, the storage volume for this option is the maximum achievable for a

25 m operating range at the proposed dam site, without building additional saddle

dams.

The model was initially run with a higher maximum pump capacity. This was

incrementally reduced until the pump capacity started to constrain the reliably

irrigable area.

Further details are:




Maximum pump capacity: 5 m3/s (partially optimised)

4.6 Waiau River 240 m Transfer Canal

These options consider how in-catchment storage could be reduced with additional

run of river infrastructure, in this case supplying water from the Waiau River along

the 240 m contour. A canal capacity of 20 m3/s was assumed.

Two options have been developed, one with storage in the south branch of the

Hurunui River and one with storage in the Waitohi River.

4.6.1 Hurunui South Branch Storage

For this scenario, storage volumes have been partially optimised based on the

reservoir site in the Hurunui River south branch. The resulting storage volume should

therefore be compared to the ‘Hurunui South Branch’ option (Section 4.2). Caution

should be used when comparing these results with any other scenario.

4.6.2 Waitohi Storage

For this scenario, storage volumes have been partially optimised based on the

reservoir site in the Waitohi River. The resulting storage volume can be compared to

the ‘DPML’ storage option (Section 4.4.2). Again, caution should be used when

comparing these results with any other scenario.


4.7 Mandamus River

Under this option, a reservoir situated in the Mandamus River catchment is filled from

in-catchment flows and from additional water pumped from the Hurunui River main

stem. A maximum diversion from the Hurunui has been set at 21.7 m3/s, which is the

combined available flow currently proposed under the NRRP.

Details include:




Maximum pump capacity: 21.7 m3/s

Minimum outlet flow: 760 l/s (as specified in Variation 8 of the NRRP)

4.8 Hanmer River

The Hanmer option proposes to fill a dam on the Hanmer River from in-catchment

flows only. Details are:




Minimum outlet flow: 420 l/s (calculated as 2/3rd

of MALF)


Appendix A: Flow duration curves and hydrographs: Waitohi HWP option



Appendix B: Flow duration curves and hydrographs: Waitohi DPML option



Appendix C: Flow duration curves and hydrographs: Waitohi generic option with 10% less Waitohi






Appendix D: Flow duration curves and hydrographs: Waitohi Fraser Geologic option



Appendix E: Flow duration curves and hydrographs: Waiau 240 m transfer canal with Waitohi storage






Appendix F: Flow duration curves and hydrographs: Hanmer River option



Appendix G: Annual Pumping Volumes: Hurunui River to Waitohi storage


Relative contributions of Hurunui and Waitohi water to Waitohi storage

DPML Option

Generic storage with 10% less Waitohi flow


Appendix H: Storage volumes: Hurunui River to Waitohi storage

DPML Option


Generic Waitohi Storage with 10% less Waitohi flow


Appendix I: Flow site locations



Documents

Hurunui-Waiau Storage Options - Environment Canterburyfiles.ecan.govt.nz/public/cw-hw-zip-reports/Aqualinc 2011 Hurunui Waiau... · Hurunui-Waiau Storage Options - Modelling Assumptions