Executive Summary
Global climate change is predicted to increase the temperature of the atmosphere between 1.4°C and 5.8°C by the end of the century if no action is taken to reduce greenhouse gas emissions. This will have an impact on flooding, as the warmer air holds more moisture, and this moisture can produce not only more rain but more intense weather systems.
The New Zealand Climate Change Office of the Ministry for the Environment has produced a series of guidance materials to help local government assess the impacts of climate change. These materials provide information on climate change impacts as well as a generic risk assessment framework that can apply to flood risk as well as to other weather-related natural hazards.
This report provides more specific guidance for councils on how to handle the possible impact of climate change when assessing flood risk. Firstly, a simple screening test is recommended to assess whether climate change is likely to significantly affect flooding in a region. If so, it is recommended that a further, more detailed analysis be carried out for each catchment of interest.
More accurate projections of expected future flood risk will provide local government with a better basis for community consultation and informed decision-making on necessary levels of flood protection works.
This report first outlines the application of the screening test, which will help councils identify whether changes in flood risk are likely to be significant. It then outlines a more detailed methodology to assess the impacts of climate change on flooding. The methodology involves using weather models to estimate the impact of expected temperature changes on future rainfall. The weather models generate estimates of the future rainfall. Hydrological modelling is then used to convert future rainfall to future river flows, including peak flow levels. Finally, inundation models can be used to convert that peak flow to the area, depth, and flow speed of flood waters. Many councils are already using hydrological and inundation modelling when assessing current flood risk for their communities. The methodology described here to assess the impacts of climate change fits well into this more general framework.
The detailed methodology outlined in this report, as well as the application of the simple screening test, make a number of assumptions and carry a range of uncertainties. The report discusses these assumptions and uncertainties, and the extent to which the results obtained by this methodology can be a robust tool for decision-making.
