DCR cleanfill offsets.gtu.0911012.final as issued · migration barriers, the planting of riparian vegetation and control of stock and animal and weed pests. The mitigation programme

REPORT Winstone Aggregates

Dry Creek Replacement Cleanfill:

Ecological Offset Mitigation

REPORT

Report prepared for:

WINSTONE AGGREGATES

Report prepared by:

Tonkin & Taylor Ltd

Distribution:

WINSTONE AGGREGATES 1 copy

Tonkin & Taylor Ltd (FILE) 1 copy

November 2012

T&T Ref: 85355.003

Winstone Aggregates

Dry Creek Replacement Cleanfill:

Ecological Offset Mitigation

Dry Creek Replacement Cleanfill: Ecological Offset Mitigation T&T Ref. 85355.003

Winstone Aggregates November 2012

Table of contents

1 Introduction 1

2 Existing Stream Values 1

2.1 DCR Cleanfill 1

2.2 Biodiversity offsets 2

2.3 Why not just use the SVE method? 4

2.4 Opportunities for ecological mitigation 5

3 Approach and methods used in analyses 5

3.1 Mitigation hierarchy and offsetability 5

3.2 Model and accounting framework 7

3.3 Information sources 8

3.4 Valuation of biodiversity at development and offset sites 8

3.4.1 Perennial streams 8

3.4.2 Intermittent and headwater streams 9

3.4.3 Accounting for time differences 9

3.4.4 Risk and uncertainty 9

4 Results of analyses 10

4.1 Amount and type of mitigation 10

4.2 Additionality and permanence of mitigation 13

5 Summary and conclusions 13

6 Literature cited 14

Applicability 16

Figures

Appendix A: Data inputs for offset models

Appendix B: Lengths of streams removed within the stages of the DCR footprint.



Executive summary

Winstone Aggregates (Winstone) proposes to construct a new cleanfill operation in Lower

Hutt, Wellington. The Dry Creek Replacement (DCR) cleanfill is proposed for a 13.9 ha

predominantly grazed farmland site with some patches of regenerating indigenous scrub,

early successional manuka and broadleaved forest. The cleanfill construction will require

the clearance of indigenous vegetation and the infilling of perennial and intermittent

streams and their headwaters. Effects on streams that cannot be avoided, remedied or

minimised will likely require mitigation.

This report summarises the approach and analyses used to determine the type and amount

of ecological mitigation that may be required to at least balance the ecological losses of

streams within the DCR site. In doing so, this report identifies what may be required to

provide for at least a no-net-loss outcome to address adverse effects associated with

stream removal due to the construction of the cleanfill.

Key stream values that will be impacted within the 13.9 ha DCR footprint as identified by

MWH New Zealand Ltd (MWH), are:

• The progressive piping of 952 m of perennial stream length;

• The progressive piping of 706 m of intermittent and headwater stream length;

• The loss of longfin eel and koura habitat and populations as the only At Risk

(nationally declining) species recorded within the DCR footprint. There were no

Nationally Threatened species recorded within the streams in the DCR footprint.

In addition, Boffa Miskell Ltd (Boffa Miskell) in their assessment of terrestrial ecology values

on the DCR site noted that several small areas of seepage wetlands will be impacted,

however these are colonised by exotic pasture and regarded by as induced by stock effects.

Using the Habitat Hectares biodiversity offset approach the following mitigation would be

required to achieve at least a no-net-loss outcome for the loss of stream values at the site.

• Revegetation planting of 15 m wide riparian margins and exclusion of stock from

1,510 m of perennial stream, and 440 m of intermittent and headwater streams;

and

• Monitoring of the mitigation programme to ensure that sites are planted, that

plantings establish, that fish barriers are removed and that pest control is

undertaken for a protracted period until indigenous riparian planted areas have

established.

The mitigation indicated assumes that the stream restoration will be staged according to

the phases outlined in this report and that it will comprise the removal of potential fish

migration barriers, the planting of riparian vegetation and control of stock and animal and

weed pests.

The mitigation programme should be supported by a guarantee, including availability of

financial resources where needed, that ensures the implementation of the remaining

mitigation required to fully deliver the ecological mitigation described in this report.

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1 Introduction

Winstone Aggregates (Winstone) proposes to construct a new cleanfill operation to replace its

current Dry Creek cleanfill in Wellington. The Dry Creek Replacement (DCR) cleanfill will require

the removal of streams and indigenous vegetation. Winstone consider that the removal of

streams will require mitigation to address adverse residual ecological effects not avoided or

minimised. I understand from Winstone that the removal of indigenous vegetation on the site is a

Permitted Activity, and therefore it does not constitute an impact for which mitigation is required.

This report summarises the approach and analyses used to determine the type and amount of

ecological mitigation that may be required to at least balance the ecological losses of streams

within the DCR site. In doing so, this report identifies what may be required to provide for at least

a no-net-loss outcome to address adverse effects associated with stream removal due to the

construction of the cleanfill.

This report has been prepared in accordance with our letter of engagement dated 18 July 2012.

2 Existing Stream Values

2.1 DCR Cleanfill

Stream ecological values at the DCR site are described in the report by MWH Ltd (2012). A brief

summary of the findings of that report is provided below. An associated report prepared by Boffa

Miskell Ltd (2012) describes the terrestrial vegetation values of the site and is referred to

occasionally in this report as a basis for describing the site and placing the stream values in

context.

The site is on privately owned land located in Lower Hutt, Wellington, south of State Highway 58

and approximately 4 km northwest of the current Dry Creek cleanfill facility operated by

Winstone. The site is within a predominantly grazed farmland landscape with some patches of

regenerating indigenous scrub and early successional manuka and broadleaved forest, all of which

are heavily grazed by sheep and cattle (Figures 1 and 2). Forest areas are, at most, estimated to

be 35 years old; regenerating scrub areas are much younger.

The site itself is centred on a side gully through which flows an unnamed tributary of the

Pauatahanui Stream. The tributary stream is fed by several smaller intermittent and headwater

streams as well as a section of tributary upstream from the proposed development. Stock have

access to all streams, with smaller headwater and intermittent streams within the proposed DCR

footprint impacted by historical farming-induced vegetation clearance, stock access and stock

vegetation browse. The tributary stream flows into the upper reaches of the Pauatahanui Stream

on which there are several local piped sections of stream and overhanging culverts. These may be

acting as barriers to fish passage to the unnamed tributary within the DCR project area and to

further up the Pauatahanui Stream including tributaries and intermittent reaches.

The DCR cleanfill is expected to proceed in four sequential stages. Stages are broadly

representative of the anticipated progression of the cleanfill across the site and allow a bounded

estimation of when vegetation and streams may be removed as the development progresses. For

the purpose of this report, we have assumed that stream ecological impacts associated with a

particular stage of cleanfill development will occur at the mid-point of each stage, an approach

that strikes a balance between estimating when impacts may occur and the need to group

potential impacts over time into clusters to facilitate mitigation analyses. The start time and

duration of each stage of works as anticipated by Winstone is summarised in Table 1, along with

the mid-point used in the offset modelling.

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Table 1. Stages of development of the DCR cleanfill and the mid-point included within the offset

mitigation model.

Stage Approximate start

time taken from first

year of cleanfill

development (year)

Approximate

duration of

stage (years)

Mid-point included

in offset model

(year)

1 1 1 1

2 1 7 5

3 8 20 15

4 28 30 35

Key stream values that will be impacted within the 13.9 ha DCR footprint as identified by MWH,

are:

• The progressive piping of 952 m of perennial stream length;

• The progressive piping of 706 m of intermittent and headwater stream length;

• The removal of several small areas of seepage wetlands colonised by exotic pasture and

regarded by Boffa Miskell as induced by past and present farming practices; and

• Longfin eel and koura were the only At Risk (nationally declining)1 species recorded within

the DCR footprint. There were no Nationally Threatened species recorded.

2.2 Biodiversity offsets

Environmental mitigation for developments has, until recently, relied upon negotiated

agreements or personal judgement from experts in the absence of national standards or

guidelines. Recent efforts internationally and within New Zealand have focused on developing

tools that provide robust, systematic and transparent decision-making around the type and

amount of ecological mitigation that may be required to fully balance the residual adverse effects

of developments.

The most widespread approach is that of biodiversity offsetting. That approach has been used in

New Zealand ecological assessments in recent years and is increasingly being adopted by local

government to guide biodiversity management on private land under Section 6c of the Resource

Management Act 1991 (RMA)). Most application of biodiversity offsetting in New Zealand follows

the broad principles and guidance provided by the Business and Biodiversity Offsets Programme

(BBOP), an international collaboration between business, scientists and policy makers which has

recently issued an international Standard on Biodiversity Offsetting (BBOP 2012).

Biodiversity offsetting provides an estimate of what may constitute adequate and appropriate

mitigation for the loss of indigenous biodiversity as a result of development. Offsetting addresses

residual effects after appropriate avoidance, onsite remedy and minimisation of effects has been

incorporated into the project design. While the emphasis of offsetting is on addressing

1 As classified by the Department of Conservation’s National Threat Classification Lists. Nationally At Risk (declining)

species are of a conservation status not sufficient to qualify as threatened, however they are of national conservation

concern given their current trend nationally.

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ecologically significant residual effects, there is no constraint to also addressing effects that, while

not regarded as significant, may be of sufficient size or scale such that mitigation is deemed

appropriate.

Offsetting provides a more rigorous approach to setting appropriate ecological mitigation than

the subjective, often adversarial approach undertaken to date. The overall goal is usually to

achieve at least a balance between losses and gains such that no-net-loss, or preferably a net-

gain, outcome is provided for biodiversity on the ground.

The offsetting approach is supported by a framework of principles (see below) that describe what

constitutes a robust offset and requires the use of quantitative analyses to assess the value of

biodiversity lost against the value gained through an offset. The principles driving no-net-loss

biodiversity offsetting in New Zealand include emerging best practice around what constitutes a

robust offset. These include the use of good science, the involvement of stakeholders,

demonstration of benefits additional to what would have occurred without the project

(additionality), how the offset will be secured in the long term through land protection and

financial support, and how risk and uncertainty around delivery of the offset benefits are included

in the process. BBOP summarises these as 10 principles guiding good offset design. Application of

offsetting in New Zealand has seen variants of these BBOP principles with various authors

summarising these as more concise or with greater NZ-specificity, such as the six principles

proposed by Norton (2009), the seven principles that support offsetting in the proposed draft

National Policy Statement on Indigenous Biodiversity (MfE 2011) and the principles proposed to

support the use of good practice compensation (in essence, offsetting) by the Environment Court

(for example, the Court’s decision on the J. F. Investments Limited case (C48/2006)). All of these

sets of principles have, at their core, three requirements that support robust offsetting (Gardner

et al. in press); which are:

1. Equivalence – a requirement for an explicit loss-gain calculation to demonstrate that

biodiversity gains are comparable to losses and to track delivery of the offset mitigation.

Calculation of biodiversity losses and gains requires (i) the selection of representative

biodiversity components and currencies to measure the exchange, and (ii) the definition

of an overall offset accounting system to help ensure equity in the distribution and

temporal delivery (to account for time lags between losses and gains) of biodiversity gains

compared to losses, and to safeguard offsets against failure.

2. Additionality – evidence that benefits claimed as the offset are additional to

management actions already being undertaken or committed to at proposed offset sites.

The offset activity should also not displace harmful activities elsewhere (e.g. displacement

of pests into adjoining habitat).

3. Permanence – evidence that the offset benefits are secured in the long-term, for at least

as long as the impacts of the project. Mechanisms for providing assurance around security

of offset benefits may include: ‘banking’ of offset benefits ahead of development impacts,

bonds or insurance to cover risk of non-completion, permanent protection of offset areas

through use of covenants or other legal protection mechanisms, and endowment funds to

provide long-term funding for an offset where ongoing management is needed to sustain

biodiversity benefits.

Recent opinion from legal cases provides a guide as to how biodiversity offsetting may fit within

the RMA’s avoid, remedy and mitigate framework for managing adverse effects. International

definitions of offsetting, including by BBOP, include trade of similar biodiversity types (like-for-like

or in-kind) and like-for-unlike (or out-of-kind) exchanges of different biodiversity (for instance

‘trading-up’ by exchanging a less threatened type with a more threatened type). Advice provided

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to the Transmission Gully Plan Change Board of Inquiry2 provides an opinion that like-for-like

offsets may be defined as:

‘The provision of positive effects of the same general type in relation to the same resource, to offset

site specific adverse effects caused by an activity, so as to avoid, remedy or mitigate the overall

adverse effects of a proposal in relation to that type of effect on that particular resource’

This infers that like-for-unlike offsetting or activities that do not avoid, remedy or mitigate

adverse effects of the same general manner are assumed to constitute compensation. This is also

discussed in Milne (2011).

For the DCR cleanfill we adopt the interpretation by Mr Milne and consider like-for-like exchanges

to constitute mitigation and like-for-unlike exchanges to constitute compensation, which falls

outside the realm of biodiversity offsets (and hence outside of mitigation).

2.3 Why not just use the SEV method?

The Stream Ecological Valuation (SEV) method is widely used by freshwater ecologists to assess

the loss of ecological value from streams within development sites. The method provides a good

quantitative basis for estimating loss and gain of stream ecological function. MWH has used this

method to estimate the quantum of stream values removed by this project.

The SEV method also includes a mechanism for calculating a multiplier ratio to apply to values

lost. This is used as the basis for calculating the area or length of stream required for mitigation

purposes. This Ecological Compensation Ratio (ECR) is based on key presumptions that we feel are

not relevant to the DCR project. These include:

1. It uses a universal multiplier to account for lags in time between the loss of ecological

values and their gain at an offset mitigation site. That multiplier is used to increase the

length of stream required for mitigation and presumes that mitigation will happen after

development impacts have occurred. This is not the case for DCR where most mitigation

will be started and deliver benefits prior to losses caused by the cleanfill development. As

such, the ECR is a blunt tool for addressing time lags and cannot be adjusted to account

for pre-impact mitigation or staged developments such as the DCR.

2. The ECR is calculated by using, in part, the ‘future potential‘ stream state score for an

impact site. That is, it assumes a far higher state of ecological value for a site than is

currently present and attributes all of that loss to the developer, instead of only the

portion described by the difference between the current true state and the state

following development.

Therefore, while the SEV is applicable to DCR as a method for describing and quantifying loss and

gain, the mechanism for fairly calculating a multiplier ratio is not (in this situation). Instead, we

have used the Biodiversity Offset model approach as it allows a more refined handling of time in

respect to when losses and gains occur and addresses current values of the site that are proposed

for removal. In this way we regard an offset model as more fair and robust than requiring a

landowner to be accountable for values on a site that may or may not existing in the future under

a business-as-usual management approach.

The offset model used for DCR uses the SEV data and scores collected by MWH as part of the

standard SEV method. The offset model uses them in the offset model framework in which

current state and time between losses and gains is more fairly and accurately accounted for.

2 Provided by P. Milne, counsel to the Board of Inquiry for the Transmission Gully Plan Change. P. Milne, 8 July, 2011.

5



2.4 Opportunities for ecological mitigation

The assessment of effects report by MWH identifies several opportunities for how mitigation of

residual adverse effects could be achieved.

These principally include the retirement and native revegetation of grazed pasture, and stock

exclusion and pest control within perennial, intermittent and headwater streams. There are many

potential locations for such restoration and enhancement in the vicinity of the DCR site.

One such location identified by Winstone is the area of land to the east of the DCR site (across

SH58) that forms the headwaters of the Pauatahanui Stream (the ‘mitigation site’).

The offset analysis undertaken for this report focuses on mitigation provided by the restoration of

degraded stream stretches to provide the anticipated ecological benefits to balance the residual

losses of stream values predicted within the DCR footprint.

3 Approach and methods used in analyses

This section outlines the offset approach taken, in particular i) efforts undertaken to avoid,

remedy and mitigate adverse effects prior to seeking offset solutions and whether residual

impacts are appropriate to address through offsetting, ii) the selection of offset model and

framework for addressing biodiversity losses, and information sources for spatial and site-based

ecological information and iii) the selection of attributes that describe biodiversity values and how

these are used to describe likely losses at the DCR site and anticipated gains within the proposed

offset site.

3.1 Mitigation hierarchy and offsetability

In accordance with best practice, offset mitigation should be regarded as the last option for

managing environmental effects, after avoidance, restoration on-site (remedy) and minimisation

of effects (as part of mitigation) have been applied where feasible, rather than the first or only

option.

For DCR I understand that avoidance and management of adverse effects have been addressed

through the design of the cleanfill as follows:

• Avoidance: The current design has been modified to avoid impacts on much of the

indigenous vegetation and headwater streams of this gully system, including areas

originally included in previous design considerations. I understand that the current design

is constrained by geotechnical, engineering and capacity requirements and avoidance of

further stream removal is not practically feasible, and

• Remedy: The removal of the perennial, intermittent and headwater stream sections

within the DCR footprint is total, however management of water on and through the site

offers some opportunity to produce benefits for stream function. These are principally

through the proposed piping of the tributary stream and the construction of a rock-lined

over-flow channel to the periphery of the finished cleanfill. These reduce the severity of

the effects requiring mitigation and, for the purpose of defining residual effects requiring

mitigation for this report, are considered to be actions that partially remedy adverse

effects on site. Specifically;

o Piping the tributary stream under the cleanfill will provide some continuity of

hydrological flow between upstream portions of the tributary outside of the

cleanfill footprint and down-stream confluence with the Pauatahanui Stream. We

understand that such a piped section may have limited ecological values for fish

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passage and macroinvertebrates, although it will have some values for

maintaining downstream migration and colonisation; and

o The rock-lined over-flow channel proposed for the western periphery of the

finished cleanfill will provide an above-ground link between remaining headwater

streams and streams below the cleanfill. We understand that this channel may

carry water only during high flows i.e. it may be very intermittent in nature. This

has not been included in the offset model as the design has not been finalised and

its ability to provide habitat for aquatic species may be low.

• Minimisation: Minimisation of effects is regarded as good practice management of

ecological values on site prior to removal under the development proposed. The

minimisation of effects is regarded as a necessary step towards reducing the severity of

the adverse effects and for reducing the quantum of residual effect that needs to be

offset. Aspects of minimisation that will be undertaken as part of this project include:

o Adherence to best practice soil management and erosion prevention techniques

to minimise the risk of sediment entering downstream waterways;

o Relocation of fish and koura prior to the removal of waterways;

o Staged removal of streams as areas are needed for infilling, rather than block

culverting large sections of streams prior to each stage of cleanfill development;

o Using local indigenous plants to source material to propagate trees for riparian

mitigation planting programmes (i.e. eco-sourcing). This will minimise the loss of

local genetic diversity within plant species; and

o Where feasible, undertaking mitigation works ahead of impacts to reduce

temporal gaps in the availability of habitat and food provided by streams

currently on site.

Guidance around the use of offsetting as a mitigation tool identifies situations where offsetting is

not appropriate or may be technically challenging and may lack credible assurance of anticipated

outcomes. Such situations include sites that contain the last type of stream or remnant

population of a species (i.e. development will result in extinction or extirpation) or where a site

supports a large portion of the population or known distribution of a species or community type.

In addition, key factors that may influence the credibility of assurance offered by a developer to

provide an effective offset include the ability to fully replace lost values over a reasonable

timescale, the rarity and vulnerability of biodiversity on the site, the experience and commitment

of the developer at undertaking conservation management activities and an ability to provide

offset benefits ahead of impacts. These tests are met at the DCR site as explained below and in

the following section.

The survey and assessment report by MWH provides an evaluation of the stream ecological values

that are likely to be impacted by the DCR development and the ecological significance of those

effects. The judgements made in those reports form the foundation for identifying biodiversity

types and components that should be included in the offset model. These are explained further in

Section 3.3 and 3.4 of this report.

For DCR, the site is known to support longfin eel and koura, both of which are listed by DOC as

nationally At Risk (i.e. their status is ‘declining nationally’ which is the lowest threat classification

ranking) by the Department of Conservation’s Threat Classification ranking system. Streams

present on site are well represented elsewhere in the broader ecological area and their removal

will not substantially increase the conservation threat to eels, koura, other aquatic species or

streams in the broader landscape.

In summary, DCR avoids impacts on upper catchment streams that are of greater ecological value.

Application of good practice site management will minimise impacts on waterways and aquatic

species during the development of the project. Remedy of effects on-site will not be undertaken

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for streams (apart from benefits that may be derived from the over-flow channel) because they

will not deliver timely ecological outcomes given the time to likely cleanfill closure when

ecological restoration could be undertaken onsite. Instead, restoration of streams near the site

provides the principal means of addressing residual effects and is likely to deliver ecological

benefits in a shorter timeframe and provide greater assurance that development impacts can be

effectively offset before they occur.

3.2 Model and accounting framework

The offset accounting model chosen for DCR is the Habitat Hectares model. The model has its

origins in the research, development and guidance produced by BBOP. BBOP reviewed numerical

assessment methods currently in use worldwide and produced a modified version of one such

method, naming it the modified Habitat Hectares3 (HH) approach.

The BBOP Habitat Hectares method has been applied to several projects in New Zealand (either in

its pure form or as a slightly refined version termed the Condition-Area model), including as:

• a case study to test the BBOP methodology (Solid Energy’s Strongman Mine4);

• MainPower’s Mt Cass wind farm application ([2011] NZEnvC 384 and [2012] NZEnvC 021);

• Contact Energy’s Hauauru ma raki windfarm (BOI 2011);

• Meridian Energy’s Mokihinui Hydro Project; and

• Solid Energy’s proposed Mt William North mine (Bramley 2012).

We are also aware of several other uses of this offset model in New Zealand for windfarm, hydro-

dam and irrigation dam proposals. DOC has tested this model as part of its biodiversity offsets

research programme, including at the Winstone Hunua Quarry development project in Auckland.

The Habitat Hectares method works on the basis of trading similar biodiversity types (for example,

communities or species) between development and offset site, multiplied by their relative quality

and the area occupied. As such, the method relies upon exchanges of ‘in-kind’ biodiversity to

maintain equivalence of exchange. The method allows for the loss of some parts of biodiversity

that describe a broader community or ecosystem that is considered non-interchangeable. For

example, loss of tui at a development site may be offset by an increase in fantails at an offset site

if the overall condition of that community is maintained or enhanced relative to the existing

development site community values. This is essentially the same approach that is taken by the

Stream Ecological Valuation (SEV) method that has been used extensively around NZ (including by

MWH in its aquatic assessment for this project). That method uses up to 16 attributes that

describe aggregate stream function and allows ‘trading’ of attributes between impact and offset

site so long as the overall aggregate stream function value is at least balanced between the offset

and development site.

The HH (and the SEV) method produces a metric; units of Habitat Hectares (for perennial streams)

or Habitat Metres (for intermittent and headwater streams) which describes biodiversity value (or

for streams, ecological function value) and forms a common currency of exchange between sites.

To account for time differences between the values lost at the development site and those gained

at the offset site, a time discount is applied. The time discount effectively provides an incentive to

prioritise delivery of offset benefits close to or before development impacts occur. This is

discussed further in Section 3.4.4 of this report.

3 After the Habitat Hectares approach developed for use in the state of Victoria, Australia (Parkes et al. 2003).

4 Solid Energy New Zealand. BBOP Pilot Project Case Study – Strongman Mine, Solid Energy New Zealand Ltd,

Christchurch, New Zealand. www.forest-trends.org/biodiversiyoffsetprogram/guidelines/senz-case-study.pdf

8



3.3 Information sources

Information to populate the offset model was obtained from Winstone and the MWH freshwater

ecologist (David Cameron). The staging and extent of the DCR footprint was obtained from

Winstone and used as a GIS overlay to extract stream lengths impacted during the staged

development, and stream lengths available for restoration within the mitigation site. Streams

were transcribed from the MWH report into GIS for analysis (Figure 1).

Estimates of revegetation development at planted stream riparian sites were based on

observations of indigenous vegetation development that has occurred naturally at and around the

DCR restoration and the experience of Tonkin & Taylor with revegetation projects elsewhere.

For permanent streams, information on present (baseline), potential (benchmark), post-impact

(for the piped stream) and restoration stream areas was obtained from the SEV report and SEV

model prepared by MWH. For intermittent and headwater streams, information on stream quality

was obtained from the MHW report.

Biodiversity types that were included in the offset models are listed in Table 2. These comprise

types that the MWH report regards as ecologically significant as well as types that are well

represented within the site. These types form the non-interchangeable components of

biodiversity for which no-net-loss is separately the goal.

Table 2. Biodiversity types included within the offset models.

Biodiversity type Description Residual effects regarded

as more than minor?

Included in offset models?

Perennial streams Main tributary stream Yes (MWH) Yes

Intermittent and

headwater streams

Side gully streams and

headwater flow paths

Yes (MWH) Yes

Seepages Stock-induced

seepages dominated

by exotic plant species

No (MWH) No, however seepages are

present within the

proposed mitigation site

and will benefit from the

proposed management.

3.4 Valuation of biodiversity at development and offset sites

The HH (or for intermittent and headwater streams a similar Habitat Metres (Hm)) model was run

separately for perennial streams, and intermittent and headwater streams. They were both

modelled such that the offset site and management chosen generated a net-gain in ecological

outcome for each.

The attributes used to describe the ecological values for both of the ecological types differed and

are explained below.

3.4.1 Perennial streams

Estimates of permanent stream ecological values prior to development, after development and

before and after restoration were taken from the SEV assessment model applied by MWH

(Appendix A: Table 1). Timescales for the achievement of restoration of functional attributes for

9



the offset stream was based on discussions with MWH and took into consideration likely

progression of planted riparian margins, the benefits of removal of stock access to streams and

the benefits of the removal of barriers to fish passage on downstream sections of the Pauatahanui

Stream (Appendix A: Table 2).

3.4.2 Intermittent and headwater streams

Estimates of intermittent and headwater stream values applied the semi-quantitative indexes of

stock damage and riparian vegetation cover used by MHW to describe general stream condition

(Appendix A: Table 3). Timescales for the improvement of these indices at restored stream sites is

based on discussions with MHW, and the experience of the author.

3.4.3 Accounting for time differences

Accounting for the difference between when biodiversity values are removed and when they are

replaced is a key aspect of mitigation estimates, whether they are solely based on judgement or if

they include quantitative modelling. There is extensive literature around the use of time-lag

discounting as well as some case history of use in New Zealand. A recent review commissioned by

DOC (Denne 2012) calculated an appropriate time discount rate of 0.8%5, which has been applied

in the DCR offset models to the habitat hectares (perennial streams) and habitat metres

(intermittent and headwater streams) generated for each biodiversity component, the results of

which is compounded annually.

3.4.4 Risk and uncertainty

Risk and uncertainty has been incorporated into the offset mitigation design by taking a

precautionary approach to the assessment of existing values and predictions of biodiversity value

generated through mitigation activities. In addition, BBOP advocates for monitoring of mitigation

sites to confirm the effectiveness of mitigation at generating predicted benefits and adaptive

management to respond to under-delivery of mitigation gains.

Precautionary aspects in the offset design for this project which are intended to address risk and

uncertainty include:

1. Incorporating timescales for restoration of biodiversity at mitigation sites that reflect

longer periods for ecosystem delivery based on experience from projects elsewhere;

2. Limiting the ecological benefits achievable for restored streams to no more than 80% of

their potential value in recognition that natural variability can result in outcomes that

differ from those predicted. This is in spite of certainty that underlying improvements will

occur at restoration sites for species recolonisation, improvement in ecological functions,

indigenous vegetation development and provision of habitat and food resources;

3. Potential benefits of the rock-lined over-flow channel are not included in the mitigation

modelling even though it is likely that with even intermittent flows the channel will

5 BBOP gives guidance as to selection of a discount rate to address time delays, but does not prescribe a rate. Previous uses have

variously applied rates of 3% (Norton 2010), 3.5% (Stevens 2010), 3.5% (Bramley 2012), 4.4% (Ussher 2011) and 8.74% (Lee et al. 2007)

per annum. A review as part of Habitat Equivalency Analysis (Julius 1999) (a marine-orientated offsetting tool originating from the

USA) notes that when discounting interim service losses, such as time lags in delivery of biodiversity benefits for an offset, the

consumer rate of time preference should be used as the discount rate. That review recommended a discount rate of 3.0%.

A recent review of discounting as related to biodiversity offsetting in New Zealand (Denne 2012) arrived at a similar conclusion; that

the consumer rate of time preference should be used as the discount rate. Denne recommends that a risk-free rate of time preference

would be 0.8% per annum, but that such a rate does not take into account of risk and uncertainty associated with proposing

biodiversity gains in the future in return for removing certain values now.

10



provide some habitat for instream fauna, particularly if the channel includes pools that

are designed to retain water; and

4. As explained in Section 4.1, planting associated with stream mitigation has not been

counted in terms of its likely benefits for terrestrial biodiversity. In reality, planted

riparian margins will deliver ecological benefits that are not important to stream

restoration, such as provision of food for birds and habitat for invertebrates and lizards.

The offset models do not ‘count’ these benefits as the modelling addresses only aquatic

impacts and mitigation. Therefore, this represents a conservative approach to estimating

the overall benefit of the proposed mitigation programme.

Risk of default of mitigation requirements is addressed in Section 4 and focuses on the use of

existing regulatory mechanisms to provide assurance that mitigation is implemented, maintained

and that it delivers the anticipated benefits.

4 Results of analyses

4.1 Amount and type of mitigation

The type, amount and timing of removal of ecological values from the proposed DCR site is

described in the report by MWH and is summarised in Appendix B of this report. Examination of

the staged development shows that loss of ecological values varies in magnitude over the life of

the cleanfill. For perennial streams, loss is broadly consistent throughout the life of the

development. For intermittent and headwater streams, loss is concentrated in the first few years

with less in later years.

The timing of loss of ecological values is important as Winstone proposes to set up ecological

mitigation works such that the anticipated benefits at least balance the impacts of successive

stages before those impacts occur. This means that many of the ecological benefits from

mitigation will be realised on the ground prior to the reduction of values within the DCR footprint.

Such ‘mitigation banking’ provides a greater level of assurance that effective mitigation will

deliver promised benefits and allows monitoring and adaptive management to be applied if

necessary to ensure that the mitigation continues to deliver biodiversity benefits prior to most

impacts.

Table 3 presents a summary of outputs from the offset models. These represent guidance on the

appropriate type and amount of mitigation required to fully offset the residual adverse effects of

the DCR development. Spreadsheets containing the models and calculations are attached to the

electronic version of this report.

The accumulation of losses and gains over time is shown in Table 4 for both types of stream. In

each case the mitigation instigated by Winstone at the proposed time of cleanfill development is

sufficient to balance the residual loses of the cleanfill development.

For perennial streams, a substantially greater length of stream than will be impacted is required

to be managed to generate offset benefits that at least equal the value of the losses. The ratio of

stream length removed to stream length restored is 1: 1.6, while the ratio of stream area

removed to stream area restored is 1: 2.5. The timing of the mitigation works (Table 5) is

sufficient to generate offset gains mostly in advance of the impact caused (Table 4a).

11



Table 3. Summary of mitigation required to achieve at least a no-net-loss of ecological values for

the proposed DCR development. Grey shaded cells represent offset model outputs for losses and

gains. To achieve no-net-loss of values, shaded cells across a row should add to be equal to or

greater than zero.

Ecological type Loss within DCR footprint Gain from mitigation over a 35-year

period

Area/ Length

removed

(ha/ m)

Habitat Hectares or

Habitat Metres

removed (HH or Hm)

Area/ Length

replaced (ha/

m)

Habitat Hectares or

Habitat Metres

replaced (HH or Hm)

Perennial streams 952 m

(0.09 ha)

-0.036 HH 1,510 m

(0.22 ha)

0.036 HH

Intermittent and

headwater streams

706 m -205.5 Hm 440 m 206.1 Hm

Table 4. Cumulative losses (DRC footprint) and gains (mitigation site) over time from the start of

the cleanfill development for perennial streams (a) and intermittent and headwater streams (b).

a) Perennial streams

Year 1 5 10 15 20 25 30 35

loss -0.001 -0.017 -0.017 -0.020 -0.020 -0.020 -0.020 -0.036

gain 0.008 0.012 0.018 0.021 0.024 0.024 0.031 0.036

b) Intermittent and headwater streams

Year 1 5 10 15 20 25 30 35

loss -102.1 -168.0 -168.0 -205.5 -205.5 -205.5 -205.5 -205.5

gain 8.9 55.6 122.6 146.5 161.2 175.4 189.0 206.1

For intermittent and headwater streams, a considerably shorter length of stream compared to

that removed is required to generate the mitigation benefits needed. The reasons for this are the

low quality of the streams within the DCR site and the great restoration potential for similar

streams in the mitigation site. The timing of the mitigation works (Table 5) is not sufficient to

balance the losses at the time periods shown in Table 4b, however over 35 years the losses do

outweigh the gains (as riparian planting benefits accrue over time).

Winstone has indicated that the mitigation site located to the east of the DCR site (encompassing

streams P1 and P1a – P1e with a planted riparian margin) meets the area or stream length

requirements of the offset mitigation modelling. That site contains sufficient stream length to

address mitigation needs for all impacts on intermittent and headwater streams in the upper

12



reaches (streams P1b, P1c, P1d and P1e) and perennial streams in the reaches of the Pauatahanui

Stream above and below the confluence with the DCR stream. MWH notes in its report that the

site supports seepages and small wetland areas and that these would benefit from the removal of

stock grazing and riparian planting proposed for streams.

Table 5 provides an indication of the stream lengths within the proposed offset mitigation site

and when these would require fencing and planting after the start of cleanfill development.

Table 5. Schedule for restoration activities and timing

Timing (years from

start of DRC cleanfill

development)

Stream

identifier

Management action required to generate mitigation benefits

1 P1

(Pauatahanui

mainstem)

• Remove fish barriers on the Pauatahanui Stream in the

vicinity of the confluence of that stream and the DCR

tributary stream

1 P1

(Pauatahanui

mainstem

• stock-proof fencing and riparian planting of 590 m of

perennial stream

1 P1e, P1d, P1c • stock-proof fencing and riparian planting of 375 m of

intermittent and headwater streams P1e, P1d and P1c

as shown in Figure 3.

5 P1

(Pauatahanui

mainstem)

and P1b


perennial stream


intermittent and headwater stream P1b as shown in

Figure 3.

25 P1

(Pauatahanui

mainstem)


perennial stream

Management requirements that accompany the above actions include:

1. Riparian margin planting comprises a 15 m wide strip of native trees and shrubs along

each side of all stream and headwater sections that contribute towards the mitigation

lengths; and

2. Possum control (and incidental rat control) and control of invasive ecological weeds is

undertaken for at least 20 years over all riparian planted areas until trees are fully

established.

It is assumed that the piped sections of the DCR tributary will be unable to support fish passage

and that mitigation plantings will not receive intensive rodent control.

13



4.2 Additionality and permanence of mitigation

The proposed mitigation site for DCR meets the requirement of additionality in that:

• The landowner has no plans to fence or plant the streams and grazed pasture involved;

• Farming is the current land use of the site and Winstone is not aware of any plans to

cease farming at that location; and

• There are no current plans that Winstone is aware of to improve fish passage on the

stream sections on the Pauatahanui Stream in the vicinity of the offset mitigation site.

The potential for displacement effects from pests that may inhabit riparian vegetation areas that

are proposed for clearance will be addressed by Winstone by undertaking control of possums

(and incidentally, rats) immediately prior to vegetation clearance.

The guarantee of permanence of offset gains is a key part of providing a credible and robust

replacement of biodiversity lost through development. Long-term security of the offset mitigation

site (or any other site should this proposed site change to an alternative location) would be best

served by either:

1. Ensuring that the characteristics of stream and riparian areas forming the basis of no-net-

loss mitigation reach a state whereby local planning regulations (for example RMA

provisions under the District Plan) afford protection from subsequent vegetation

clearance and land development; and/or

2. Improving legal protection of the land over which the mitigation is conducted such that

the land is protected from development that may reduce the ecological value of the site,

for example by placement of a conservation covenant or other legal instrument over the

mitigation area.

In addition, a guarantee should be provided that financial resourcing will be available to support

the implementation of the management actions needed to generate the offset mitigation benefits

(fencing, fish passage works, planting, pest control, maintenance etc). The financial mechanism

should be directly related to the amount of outstanding residual ecological impact that needs to

balance predicted impacts at a given point in time or over a set time period (for example, 5 years).

Given that most impacts from DRC happen later in the cleanfill development, and that mitigation

is proposed from an early stage, the amount of residual impact not yet mitigated at any one time

is in most cases quite small. This should be reflected in the size of the financial guarantee.

It is expected that the ecological mitigation agreed upon for DCR would be described in an

Ecological Mitigation Management Plan (or similar) which would also include the monitoring

required for Winstone to demonstrate that mitigation actions are in place on the ground and are

delivering the necessary biodiversity benefits upon which this offset calculation is predicated.

5 Summary and conclusions

The proposed DCR cleanfill will result in the loss of streams and indigenous vegetation values.

Surveys of the ecological values and their extent on site confirm that the cleanfill will have

ecologically significant effects and that mitigation should be provided for the loss of perennial,

and intermittent and headwater streams.

A Habitat Hectares offset model was applied to the residual effects of the proposed DCR cleanfill

to provide an estimate of the type and amount of mitigation required to generate benefits that at

least balance the unavoidable adverse effects of the development on aquatic ecology values.

Winstone has proposed that mitigation will be undertaken in a staged manner such that

mitigation will be in place prior to impacts occurring. The mitigation proposed will comprise the

14



removal of potential fish migration barriers, the planting of riparian native forest and the control

of stock and animal pests. Assuming that timing and range of management actions, the following

mitigation would be required to achieve at least a no-net-loss outcome for the loss of indigenous

values at the site;

• Revegetation planting of 15 m wide riparian margins and exclusion of stock from 1,510 m

of perennial stream, and 440 m of intermittent and headwater streams; and

• Monitoring of the mitigation programme to ensure that sites are planted, that plantings

establish, that fish barriers are removed and that pest control is undertaken for a

protracted period until indigenous riparian planted areas have established.

The mitigation should be supported by a guarantee, including availability of financial resources

where needed, that ensures the implementation of the remaining mitigation required to fully

deliver the ecological mitigation described in this report.

6 Literature cited

BBOP. 2012. Standard on Biodiversity Offsets. Business and Biodiversity Offset Programme; BBOP,

Washington, D.C.

Board of Inquiry. May 2011. Final report and decision of the Board of Inquiry into the Hauauru ma

Raki wind farm and infrastructure connection to the grid. Volume 1 of 3.

Boffa Miskell Ltd, 2012. Dry Creek Replacement Cleanfill, SH58, Porirua: Assessment of terrestrial

ecological effects. Unpublished report prepared for Winstone Aggregates, June 2012.

Bramley, G. 2012. The extent of Brunner coal measures on Denniston and Stockton Plateaux’;

Appendix 2 to the statement of evidence prepared for the resource consent hearing for Solid

Energy NZ’s proposed Mt William North coal mine

Denne, T. 2012. Discounting for biodiversity offsets. Unpublished final report prepared for the

Department of Conservation, NZ. Covec Ltd.

Gardner T., von Hase A., Brownlie S., Ekstrom J., Pilgrim J., Savy C., Stephens T., Treweek J., Ussher

G., Ward G., and ten Kate K. Biodiversity offsets and the challenge of achieving no net loss. In

press.

Julius, B. 1999. Discounting and the treatment of uncertainty in natural resource damage

assessment. Technical paper 99-1. National Oceanic and Atmospheric Association, Maryland

Lee, W. G., Barker, G. Innes, J and Overton, J. 2007. Calculating biodiversity offsets for the

Mokihinui Hydro Proposal. Unpublished draft report prepared for Anderson Lloyd lawyers on

behalf of BCL Energy Limited. Landcare Research Contract Report LC708/056, Landcare

Research Limited, Hamilton.

Ministry for the Environment. 2011. Proposed National Policy Statement on Indigenous

Biodiversity. Ministry for the Environment.

MWH, 2012. DCR Cleanfill, Pauatahanui: Assessment of effects on stream ecology. Unpublished

report prepared for Winstone Aggregates, June 2012.

Norton, D. A. 2009. Biodiversity offsets: two New Zealand case studies and an assessment

framework. Environmental Management 43(4) 698-706.

Norton, D. A. 2011. Evidence in chief of David Andrew Norton. Environment Court hearing into

the Mt Cass wind farm, Christchurch.

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Parkes D, Newell G, Cheal D. 2003. Assessing the quality of native vegetation: the ‘habitat

hectares’ approach. Ecological Management and Restoration. 4: S29- S38.

Stevens, R. T. 2010. Evidence in chief of Richard Theo Stevens. Board of Inquiry into the Hauauru

ma Raki wind farm and infrastructure connection to the grid.

Ussher G.T. 2011. Evidence in chief of Graham Thomas Ussher. Environment Court hearing into

the Mokihinui Hydro Project, Christchurch.

16



Applicability

This report has been prepared for the benefit of Winstone Aggregates with respect to the

particular brief given to us and it may not be relied upon in other contexts or for any other

purpose without our prior review and agreement.

Tonkin & Taylor LTD

Environmental and Engineering Consultants

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

pp.

.......................................................... ...........................….......…...............

Dr Graham Ussher Dr Brett Ogilvie

Restoration Ecologist Project Director

Technical Review Dr Matt Baber

gtu

P:\85355\85355.0030\WorkingMaterial\DCR cleanfill offsets.gtu.0911012.final as issued.docx

Figures

Figure 1. DCR development footprint (shaded white) and location of perennial and

intermittent and headwater streams within the footprint (red) and in the vicinity of DCR

(green). The proposed mitigation area encompasses P1, P1a (perennial stream

stretches) and P1b – P1e (intermittent and headwater streams). Image courtesy of MWH

(as presented in its report).

Figure 2. Vegetation types within the DCR development footprint. Indigenous broadleaved

forest (dark blue), manuka forest (brown), manuka scrub (green) and early regeneration

(light blue) contribute to the riparian vegetation cover associated with streams within the

DCR site. Image courtesy of Boffa Miskell (as presented in its report).

Appendix A: Data inputs for offset models

• Perennial streams

Table 1. Ecological function attributes from the Stream Ecological Valuation (SEV) model used

to describe stream condition for the DCR main tributary (T1 in Figure 1) and the time

predicted to reach the new (post-piping) state. Values given for benchmark (SEV

Predicted), baseline (SEV Current) and post-piping (SEV Impact) condition are from the

SEV model developed by MWH.

Ecological Function type Ecological Function

DCR main tributary (T1 in MWH report)

benchmark (P)

baseline (C)

post-piping (I)

Time after piping that post-piping state is achieved (years)

Hydraulic Natural flow regime 1 0.85 0.55 1

Hydraulic Connectivity to floodplain 0.55 0.55 0.05 1

Hydraulic Connectivity for migrations 1 0.65 0.3 1

Hydraulic Connectivity to groundwater 0.73 0.48 0.1 1

Biogeochemical Water temperature control 0.83 0.58 0.83 1

Biogeochemical Dissolved oxygen maintained 1 1 1 1

Biogeochemical Organic matter input 0.8 0.29 0 1

Biogeochemical Instream particle retention 0.7 0.79 0.03 1

Biogeochemical Decontamination of pollutants 1 0.82 0.25 1

Biogeochemical Floodplain particle retention 0.61 0.45 0.03 1

Habitat provision Fish spawning habitat 0.88 0.71 0.05 1

Habitat provision Habitat for aquatic fauna 0.98 0.72 0.55 1

Biotic Fish fauna intact 1 0.67 0 1

Biotic Invertebrate fauna intact 0.85 0.73 0 1

Biotic Aquatic biodiversity intact 0.92 0.35 0 1

Biotic Riparian vegetation intact 1 0.51 0 1

Table 2. Ecological attributes from the Stream Ecological Valuation (SEV) model used to

describe stream condition for the mitigation site main tributary (P1 and P1a in Figure 1)

and the time predicted to reach the new (restored) state. Values given for benchmark

(SEV Predicted), baseline (SEV Current) and post-restoration (SEV Impact) condition are

from the SEV model developed by MWH.

Ecological Function type Ecological Function

Mitigation stream (Pauatahanui Stream; P1 and P1a in MWH report)

benchmark (P)

baseline (C)

post-restoration (I)

Time after start of management that post-restoration state is achieved (years)

Hydraulic Natural flow regime 0.78 0.64 0.78 20

Hydraulic Connectivity to floodplain 0.55 0.55 0.55 1

Hydraulic Connectivity for migrations 1 0.3 1 1

Hydraulic Connectivity to groundwater 0.73 0.48 0.73 10

Biogeochemical Water temperature control 0.8 0.53 0.8 10

Biogeochemical Dissolved oxygen maintained 1 1 1 1

Biogeochemical Organic matter input 0.8 0.17 0.8 20

Biogeochemical Instream particle retention 0.7 0.7 0.7 1

Biogeochemical Decontamination of pollutants 1 0.85 1 2

Biogeochemical Floodplain particle retention 0.61 0.44 0.61 10

Habitat provision Fish spawning habitat 0.88 0.63 0.88 20

Habitat provision Habitat for aquatic fauna 0.87 0.59 0.87 20

Biotic Fish fauna intact 1 0.67 0.7 1

Biotic Invertebrate fauna intact 0.85 0.85 0.85 1

Biotic Aquatic biodiversity intact 0.92 0.33 0.85 20

Biotic Riparian vegetation intact 0.98 0.38 0.98 35

• Intermittent and headwater streams

Table 3. Ecological attributes used to describe intermittent and headwater stream condition

within the DCR footprint and proposed mitigation site and the time predicted for

restored steams to reach the new (restored) state. Scores provided for condition are on

a scale of 1 (degraded/ most damaged) to 5 (very good quality/ least damaged).

Stream name

Stream length (m) Attribute benchmark baseline

post-restoration

Time after start of management that post-restoration state is achieved (years)

DCR footprint streams

T1a 190 stock damage 5 1 5 n/a

T1a 190 riparian shade 5 3 5 n/a

T1b 180 stock damage 5 1 5 n/a

T1b 180 riparian shade 5 1 5 n/a

T1c 110 stock damage 5 1 5 n/a

T1c 110 riparian shade 5 2 5 n/a

T1d 40 stock damage 5 3 5 n/a

T1d 40 riparian shade 5 5 5 n/a

T2 120 stock damage 5 1 5 n/a

T2 120 riparian shade 5 2 5 n/a

Mitigation streams

P1b 180 stock damage 5 1 5 10

P1b 180 riparian shade 5 2 5 35

P1c 140 stock damage 5 2 5 10

P1c 140 riparian shade 5 2 5 35

P1d 100 stock damage 5 2 5 10

P1d 100 riparian shade 5 2 5 35

P1e 210 stock damage 5 2 5 10

P1e 210 riparian shade 5 1 5 35

Appendix B: Lengths of streams removed within the stages

of the DCR footprint.

• Perennial streams

Stream length (m)

Cleanfill development stage 1 2 3 4

Year at which removal is recorded in the offset model 1 5 15 35

Total (m)

Stream T1 320 66 437 823

Stream T1b 25 25

Stream T1a 41 41

Stream T1c 23 23

Stream T1d 40 40

Length removed per stage 25 384 66 477 952 m

• Intermittent and headwater streams

Stream length (m)

Cleanfill development stage 1 2 3 4

Year at which removal is recorded in the offset model 1 5 15 35

Total (m)

Stream T2 201 201

Stream T1a 180 180

Stream T1b 134 66 200

Stream T1c 50 75 125

Grand total 706 m

Documents

DCR cleanfill offsets.gtu.0911012.final as issued · migration barriers, the planting of riparian vegetation and control of stock and animal and weed pests. The mitigation programme