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Executive Summary

Purpose

The main aim of this guidance manual is to help local authority staff – including river managers, engineering staff and asset managers – to manage and minimise the risks posed by increased flood risk due to climate change. More specifically, the manual provides good practice guidance for incorporating climate change impacts into flow estimation. It does this by providing:

  • information on the key effects of climate change on flood risk
  • methods for estimating changes in the frequency and/or magnitude of rainfall
  • methods for converting changes in rainfall to changes in flow rate
  • methods for converting changes in flow rate to changes in inundation
  • some case studies to illustrate these methods.

The manual offers a list of options but is neither exhaustive nor prescriptive. In other words, it is not a handbook for flood estimation or flood risk management.

This guidance 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.

Climate change and flooding

Climate change is expected to lead to increases in extreme rainfall, especially in places where mean rainfall is expected to increase. Extreme rainfall is the most common trigger for floods in New Zealand. Wetter weather in some areas may also change the antecedent conditions, which means that floods might occur more often. Changes in storminess are harder to predict, but it is likely that tropical cyclones will be more intense, and such weather systems can transform into intense sub-tropical lows that bring heavy rainfall to New Zealand. Places that currently receive snow are likely to see a shift towards increasing rainfall instead of snowfall, along with changes in the snowline and snow depth. Sea-level rise will increase base levels for coastal river reaches. All of these factors need to be considered when looking at estimations of flooding.

Climate change is also likely to have a number of indirect effects. For example, changes in precipitation will lead to changes in sediment transport, in turn affecting the riverbed levels. This could be complicated in the coastal reaches of rivers because sea-level rise slows the flow of water out to the sea. If this is likely to be a significant issue for your catchment, then detailed modelling of these effects may be required. Antecedent conditions are also likely to be affected; for example, catchments in the east and north are likely to be drier on average. Warmer temperatures and windier conditions are also likely to affect evapotranspiration. These factors may be important for estimating non-flooding flows, such as water resource availability.

The climate is naturally variable, and on a decadal timescale New Zealand rainfall and flooding are affected by the Interdecadal Pacific Oscillation, or IPO. The IPO is a cycle of 15 to 30 years between warm and cool waters in the north and south Pacific, and has a significant role in modulating the climate of New Zealand. Over the next 50 years or so the changes in climate resulting from increases in greenhouse gases are in the same order of magnitude as IPO variability, so both IPO and greenhouse gas effects may need to be considered.

Screening and advanced methods

The methods discussed in this manual are standard techniques for incorporating climate change impacts into flow estimation. They range from simple and straightforward engineering approaches to complex scientific models. However, the fundamental choice is between screening methods and advanced methods. This decision will depend on the value of assets and the size of the community under consideration. Screening methods are appropriate for detecting issues which require further investigation, but insufficient for very large assets such as floodable urban areas. Some physical settings (eg, coastal rivers) require particular advanced modelling approaches.

Estimating changes in rainfall due to climate change

This manual provides projected changes in annual mean temperature for the six greenhouse gas emission scenarios developed for the Intergovernmental Panel on Climate Change (IPCC), for each region of New Zealand. These temperature changes can then be used with a range of factors for estimating increases in extreme rainfall to provide a basic screening method for estimating changes in rainfall. A number of more advanced methods for estimating rainfall are also discussed. These include weather generators, empirical adjustments, analogue selection from observed data, downscaling of global models, regional climate models and mesoscale weather models. When selecting the most appropriate method you should weigh up data availability, the desired accuracy and the expertise available.

Estimating changes in flood flows

This manual discusses estimating flood flows from the new estimates of rainfall that incorporate climate change impacts. The manual describes several screening and advanced methods, highlighting how climate change can be incorporated into flow estimation by discussing in more detail three models that are presently used in New Zealand: TP108, RORB and TopNet. Again, when selecting the most appropriate method you should weigh up data availability, the desired accuracy and the expertise available.

Estimating changes in inundation

Coastal and low-lying riverine communities are particularly vulnerable to increased inundation due to climate change impacts on rainfall, river flow and sea level. Screening methods are available to identify areas potentially susceptible to increased inundation. Advanced hydrodynamic modelling methods are also described for use in situations that require greater precision, particularly when considering coastal inundation.

Using case studies

Case studies are presented to provide illustrations of how the various methods have been applied in the real world. We present four case studies, with each one showing the three steps of estimating rainfall, converting rainfall to flows, and converting flows to inundation. The case studies demonstrate how a mixture of methods – including screening or more advanced methods – can be used together to estimate the impact of climate change on flood flows.

Discussion of issues for engineering

Incorporating climate change estimates into flow estimation can reveal various issues pertinent to engineering design. The manual discusses some of these issues, such as the appropriate use of historical records, clear reporting flow estimates, dealing with uncertainties in estimates, using professional judgement and appropriate scenario choice.