Climate Exchange

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Decision tools are provided to inform the decision-making process.

They present a set of methods and examples for filtering and

summarizing complex information based on repeatable standard-

ized criteria so that decisions can be rationalized and traced. In most

cases, it is suggested that these tools be implemented in a workshop

environment, where there is scope for multiple iterations.

A decision framework for climate change adaptation

A decision framework has been developed that provides for a

balanced and justifiable prioritisation of sustainable adaptations to

climate change and which is flexible to change. The current level

of uncertainty surrounding the timing and geographic location of

future climate effects requires this risk-based framework. It incor-

porates formal decision-making processes and employs a toolbox

approach because the diversity of decisions to be made is not condu-

cive to a one-size-fits-all approach.

While there are many ways that priorities for action could be

established, it is likely that a staged and risk-centred approach,

involving a successive narrowing down and refinement of the issues

of concern, will be an important part of setting priorities.

With

climate change singled out for attention, an example three-stage

approach is as follows:

• Stage 1 involves an assessment of priorities across all the

climate change effects of relevance for the geographical region

of interest, to identify priority climate change effects and areas

most vulnerable to them

• Stage 2 involves risk mapping of priority (high risk) areas for the

selected climate change effects, as identified in Stage 1

• Stage 3 involves identifying preferences among alternative

adaptation schemes to address the priority climate change effect

(identified in Stage 1) and the risk identified in the priority

locations (identified in Stage 2).

Funding for the toolbox was provided by New Zealand’s Ministry of

Business, Innovation and Employment. It was produced by research-

ers in the National Institute of Water and Atmospheric Research,

MWH New Zealand, Allan Planning and Research, GNS Science and

the Building Research Association of New Zealand.

Case study: Modelling future flooding in Westport, New Zealand

Flooding is the most frequent natural hazard in New Zealand.

Under climate change, rainfall events in New Zealand are fore-

cast to become more intense, causing greater storm run-off and a

decrease in the protection afforded by measures such as levees.

Urban environments in New Zealand and around the world are

particularly vulnerable to extreme weather and flooding events.

A key finding of the New Zealand Government’s review of flood

risk management was that good information on the nature of the

flood hazard was crucial to the management of flood risk.

This

is backed up by Regional Council Strategy documents which

state that more research is needed for “development and imple-

mentation of updated techniques for modelling and mapping

to determine the economic risk of river flood hazards that are

applied consistently regionally and nationally”

and “to provide

a more robust and defensible position to address hazard risk

more effectively, and to give decision makers confidence”.

The Impacts of Climate Change on Urban Infrastructure and

the Built Environment toolbox was used in Westport, which is

particularly vulnerable to flooding because it is on the flood plain

between the Buller River and the Orowaiti Estuary,

an old channel of the Buller River that carries a

substantial flow during large floods. Westport is

vulnerable to inundation from a combination of river

floods and high sea levels. It is therefore important to

understand whether climate change could lead to any

further increase in flood risk for Westport.

Experience in Westport shows how physically-

based climate, hydrological and hydrodynamic

models can be used together to simulate changes

in meteorological and hydrological processes under

future climates, and how the effect of those changes

on projections of flood inundation and risks to

people and assets can be evaluated. Using a historic

1-in-50-year event as a baseline, we predicted how

the severity of that event would change under

several future climate and sea level rise scenarios.

Statistically downscaled projections from global

climate models were used to define appropriate

adjustments to the historical rainfall and tempera-

ture measurements. Using a hydrological model,

these data were used to simulate flood hydrographs

at the Te Kuha gauging station upstream of Westport.

The resulting hydrographs predicted for the future

time period 2080-2099 correspond to events in the

current climate with recurrence intervals of 78, 98

and 113 years for the B1, A1B and A2 IPCC SRES

scenarios respectively. The flood hydrographs

provided upstream boundary conditions for a 2D

hydrodynamic model simulating inundation of the

Buller flood plain. Predictions for the inundated area

increase from 50 per cent of Westport town in the

current climate to 67 per cent, 70 per cent, and 72

per cent for the B1, A1B and A2 scenarios for the

2080-2099 time period. Resulting maps of inunda-

tion depths and velocities allow detailed planning for

the mitigation of flood events. We used the hazard

assessment tool RiskScape

to calculate the impact

of the flood on people and assets (buildings, contents

and vehicles) within the inundated area. The predic-

tions showed that under the A1B 2080-2099 scenario,

present day Westport could expect risk to life clas-

sified as ‘medium’ or greater to 560 people, building

damage of NZ$72 million and contents damage of

NZ$68 million.

This and other flood assessment information is

currently being considered by the decision-makers

and public of Westport and surrounding communi-

ties. Several flood adaptation options for Westport are

being considered and compared, and an early indica-

tion is that improvements to stopbanks are likely to

be the best option for flood protection and are worthy

of further detailed investigation and consultation.

Additionally, raising houses above the flood level is

feasible in many parts of Westport’s residential areas,

but is less feasible for large buildings and commercial

areas. A long-term plan to renew/rebuild structures at

higher foundation levels to progressively reduce flood

risks is also being discussed.