DCR cleanfill offsets.gtu.0911012.final as issued · migration barriers, the planting of riparian...
Transcript of DCR cleanfill offsets.gtu.0911012.final as issued · migration barriers, the planting of riparian...
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.
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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.
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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
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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
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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.
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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).
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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
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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
• stock-proof fencing and riparian planting of 90 m of
perennial stream
• stock-proof fencing and riparian planting of 65 m of
intermittent and headwater stream P1b as shown in
Figure 3.
25 P1
(Pauatahanui
mainstem)
• stock-proof fencing and riparian planting of 830 m of
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.
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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
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Winstone Aggregates November 2012
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.
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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