The Ministry for the Environment works closely with scientists and local government staff to understand the potential impacts of climate change on the natural resources managed by territorial local authorities. This manual focuses on the effects of flooding from freshwater systems such as rural and urban rivers (ie, not coastal flooding). Through both inundation and erosion, flooding can have an impact on local authority infrastructure such as roading, water supply and irrigation, wastewater systems and drainage, and river flood protection works. Flooding also affects other assets, both private and public, including houses, businesses and schools.
The manual provides best practice information and guidance for integrating climate change into flow estimations. It has been written primarily for river managers and flood engineering staff in local government, but is also likely to be useful to designers of public and private infrastructure and consulting engineers. More specifically, it provides:
- information on the key effects of climate change on flooding
- methods for estimating changes in rainfall
- methods for converting changes in rainfall to changes in flow rates
- methods for converting changes in flow rate to changes in inundation
- case studies to illustrate these methods.
This manual has been used as a primary source of information for a summary document, Preparing for future flooding: A guide for local government in New Zealand. Preparing for future flooding also shows how you can consider the consequences of future flood risk in a risk management framework and highlights options and principles for managing future flood risk. These principles include:
- adopting a precautionary approach
- ensuring adaptive management
- taking a low-regrets or even no-regrets approach to risk treatment
- avoiding locking in options that limit further adaptation in the future
- targeting progressive risk reduction
- planning an integrated, sustainable approach.
Preparing for future flooding is targeted at those who are involved in local government decision-making, in particular strategic and policy planners, asset managers, natural hazards analysts, river managers, emergency management and ‘lifeline’ utilities and infrastructure managers.
1.2 Structure of the manual
After this introduction, chapter 2 summarises what we know about the impacts on flooding of climate change in New Zealand, using the most recent projections from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report from 2007.
The next three chapters describe a series of methods that can be used in consecutive steps to:
- estimate the impacts of climate change on future rainfall (chapter 3)
- convert changes in rainfall to changes in run-off flows (chapter 4)
- convert changes in flows into changes in inundation (chapter 5).
Each of these chapters discusses a number of tools which are available for each of the three steps. The tools we describe fall into two main categories: screening tools and advanced methods. This categorisation is in line with the approach taken in the Climate Change Effects and Impacts Assessment Manual (Ministry for the Environment, 2008a, referred to here as the Climate Change Effects manual). Screening tools are usually simple yardsticks that aim to show whether there is a potential risk posed by climate change impacts. It is appropriate to use mid-range greenhouse gas emission scenarios for an initial screening using this type of tool, although a range of scenarios can then be used if the initial screening highlights a potential issue. (See the Climate Change Effects manual, p 64, for more on developing scenarios.)
Advanced methods are used when screening suggests a significant impact is possible (Climate Change Effects manual, p xi), and provide a more detailed assessment of the potential risks. Chapters 3, 4, and 5 each conclude with a table which summarises the advanced methods and guidance to help select the most appropriate method. A risk management approach suggests the need to use a range of climate change scenarios to provide options for decision-makers when carrying out an advanced study.
To help ground this screening and modelling in real-world situations, chapter 6 has a series of case studies illustrating different approaches to working through the processes described in this manual. The case studies provide examples of where particular options were selected and linked together to estimate the change in flood hazard as a consequence of climate change.
Finally, chapter 7 discusses some of the issues raised by consideration of climate change impacts that are of particular relevance to engineering design.
1.3 Using the manual
In most cases, users of this manual will need to progress through the consecutive steps of estimating changes in rainfall, then river flow, then inundation (chapters 3, 4 and 5). Projects to assess the impacts of climate change on flood hazards will need to select tools that provide a level of detail appropriate to the decisions which will be based on the project results. This risk management approach means that the method chosen will depend on a number of factors, such as the size of the community or the value of the asset at risk. More advanced methods have more certain predictions, but this increase in certainty requires increased resources (in terms of expertise, person time and data input requirements).
Where the consequences of the potential flood event are high, this increase in resources is justified. For example, a project to identify which river basins in a region are likely to experience a significant change in flood hazard could reasonably use a screening method. However, a project to re-evaluate the design floodFootnote 1 for a major flood protection system, using a full risk assessment approach as advocated in the Climate Change Effects manual, would use one of the advanced tools from each of the three chapters. Some of the advanced methods described in this manual will require technical expertise and experience to use, and also for interpreting the results.
1.4 Supporting guidance
In addition to this manual, a range of complementary guidance is available on climate change and hazard management from the Ministry for the Environment, including:
- Climate Change Effects and Impacts Assessment: A Guidance Manual for Local Government in New Zealand (2nd edition, May 2008)
- Coastal Hazards and Climate Change: A Guidance Manual for Local Government in New Zealand (2nd edition, July 2008).
This manual aims to be self-contained to the extent that it includes all key information on climate change impacts in New Zealand and the estimation of the effects of these on flood magnitude. However, there are areas of overlap, and the two guidance manuals listed above should be used alongside as key resources. It is also important to note this manual is not a handbook for design flood estimation: it provides a list of possible options, but is neither exhaustive nor prescriptive.
The Ministry for the Environment also provides guidance for local government planners on a range of topics on the Quality Planning website (http://www.qp.org.nz/). Current guidance that may be of use for managing flood hazards and climate change includes:
- Climate Change Guidance Note
- Natural Hazards Guidance Note
- a number of articles relating to flood hazards in the Quality Planning library
- a report on natural hazard management in the Quality Planning research category.Footnote 2
Standards New Zealand released the standard NZS9401, Managing Flood Risk: A Process Standard, in November 2008. It outlines a risk management approach to managing flooding. In addition, a River Manager’s Guide is being compiled for local government at the time of publishing this manual.Footnote 3 The guide will collate existing local body knowledge on statutes and principles, agencies and stakeholders, natural environment management, flood management, operations and maintenance, sediment management, water quality and human interaction (such as recreation and other human use of the river resource).
Back to footnote reference 1 Design flood is the flood expected to occur from a hypothetical storm of specific storm duration and recurrence interval. For example, a 25-year/24-hour design storm means that the storm duration is 24 hours and the recurrence interval is 25 years. See the Glossary.