This part covers:
- methods you can use to assess the likely change in flooding from climate change
- how to estimate the effects of climate change on rainfall, flood flow and inundation
- some of the implications climate change has for engineering design
- case studies of these methods being used.
Choosing a method to estimate the impact of climate change on flooding
A range of methods are available to estimate the impact of climate change on flooding. The best method will provide a level of detail that is appropriate to the scale or importance of the decisions which will be based on your assessment of climate change impacts on flooding. The method you choose will depend on a number of factors, such as the size of your community or the value of the assets at risk. The consequences of a potential flood event will dictate the level of detail and resources required.
The methods described in this section fall into two main categories: basic screening tools and advanced methods. Screening methods are simpler and can be used to show if there is a potential risk posed by climate change impacts. Advanced methods provide a more detailed assessment of potential risks and are used where the screening suggests there could be an impact. Some examples of screening and advanced methods are presented in the boxes below. However, readers may also want to consult sections 3, 4 and 5 of the source report for a more detailed discussion of the different methods available.
Methods to estimate the impact of climate change on flooding differ in their complexity, data requirements and reliability of results. As the method becomes more complex, so too does the expertise needed to carry out the method. While some methods are intended for all practitioners to apply, the use of the more advanced methods may require expert practitioners.
There may be some situations that require more complex or detailed modelling approaches. For example, if you need to identify which river basins in a region might see a significant change in flood hazard, you could reasonably use a screening method. However, if you are re-evaluating the design flood for a major flood mitigation scheme, using a full risk assessment approach as outlined in Preparing for Climate Change, then you would use the advanced methods.
We recommend you consider three questions when deciding how to assess the impact of climate change on flooding and what the most appropriate method is to use:
- what data does your council have access to?
- what accuracy and precision do you need?
- do you have access to the expertise and technology to undertake the analysis and modelling?
Methods for estimating changes in rainfall
The first step in estimating the effects of climate change on river flood flows is to calculate the change in rainfall. The methods that can provide estimates of how climate change may affect extreme rainfall generally convert projections of climate change into a time-series of rainfall (eg, a design storm).
| Screening methods | Advanced methods |
|---|---|
|
|
Methods for estimating changes in flow
After estimating the change in rainfall, the next step is to convert that rainfall into the amount of water flowing in a river. Historical data and ongoing data collection are vital components of any estimates of future flood flows. While climate change will mean that future flow statistics will be different from past statistics, they are necessary to calibrate or test any model of river flow. Past extreme events may be used as indicators of future trends, and can be invaluable in assessing how climate change has affected river flows.
| Screening methods | Advanced methods |
|---|---|
|
|
Methods for estimating changes in inundation
Climate change affects inundation through the combination of changes on rainfall, river flow and sea level. Coastal and low-lying riverine communities are particularly vulnerable to increased inundation. There is a range of methods to estimate how changes in flood flows may affect inundation levels. Each method converts flood flow data into an estimate of flood height, speed of flow and spatial extent over land.
| Screening methods | Advanced methods |
|---|---|
|
|
Implications for engineering design
Incorporating climate change estimates into flow estimation can reveal various issues pertinent to engineering design. Some of these issues are discussed here.
Using historical records
Gradual shifts in climate and flood risk have important implications for engineering design. An essential element of a ‘design flood’Footnote 5 study is the prediction of the future risk of extreme floods. As the climate changes, historical observations will be less indicative of future events. In other words, future flood statistics may diverge from historical statistics. Statistical flood data analysis methods, and their applications, will need to change to reflect this.
Historical data is still useful to calibrate hydrological models, to serve as a benchmark to see how flooding is indeed changing, as well as being useful in certain screening and advanced methods discussed in this guide. Also, since flood risk will change as climate changes, it will be necessary to consider the future time horizon that you are planning for and determine flood risk for that specific period. The relevant time horizon will be based on considerations such as the lifetime of the asset you are designing or the legacy or permanency of the decision you are making.
Reporting and providing information
When preparing a report or presenting information on rainfall, flow and inundation estimates, it is important to comment clearly on the parameters used in the assessment, what has been considered and what was beyond the scope of the project. This includes the climate change scenarios chosen, the assumptions made, and the basis for the choices of parameters used in the modelling.
Uncertainties
There may be significant uncertainties in the estimates made of rainfall, flow and inundation. These arise through uncertainties in things like rainfall inputs, parameter choices in modelling, errors in modelling and assumptions about antecedent conditions. The combined effects of these uncertainties could be as large as the expected climate change impacts. However, because climate change is likely to have a significant impact on flow, and much of that impact can be calculated, these broader uncertainties should not prevent efforts to include climate change in flow estimation. Where possible you should try to estimate the error bounds of the calculations.
Professional judgement
Your professional or expert judgement will form an important and valuable part of the process of flow and flood estimation. This judgement could be applied to scenario choice, the choice of modelling parameters, the interpretation of past data, and in estimating confidence in the final results. Indeed, this judgement may be most important when considering issues that have yet to be quantified.
Scenario choice
The estimates of rainfall, flow and inundation developed by the procedures outlined here, and further explained in the source report, are likely to be used as the main input in your risk assessment of future flooding. To help in your risk assessment you will need to choose a number of climate change scenarios to span a range of future possibilities. For example, you might examine the consequences of a base level of temperature rise of 2ºC by 2100, but also consider the consequences of a greater rise in temperature (for example, at least 3ºC rise).
Setting freeboard levels
‘Freeboard’ is a term used to describe a factor of safety above a design flood level for flood mitigation works. Freeboard allows for the uncertainties in hydrological predictions, wave action, modelling accuracy, topographical accuracy, final flood defence levels and the quality of the digital elevation models. The increase in flood levels associated with climate change is in addition to freeboard, because the uncertainty freeboard incorporates remains in future climate scenarios. Therefore, freeboard should not contain the ‘core’ component of climate change impacts, but it may be increased to account for climate change uncertainties.
Research
There is significant climate change research in progress at the time of writing that may aid your flow estimation and engineering design. This includes more detailed information on extremes in temperature, wind and rainfall, changes in offshore waves and storm surge, changes in storm paths and intensity, and changes in snowfall and accumulation. Much of this research is due to provide results within the next few years (2010−2013). Planners, hazard analysts and engineers will need to be alert to the arrival of this information and the implications it has for their work. Decisions need to be made now on the best information available, but you will also need to be flexible enough to take into account further improvements in understanding of climate change. Most importantly, you should not lock in options that minimise your ability to adapt at a later date.
Case Study 1: The Hutt River
In the early 1990s, there was concern that climate change could increase the risk of Lower Hutt being inundated if the stopbanks downstream of the Taita Gorge were overtopped. A study was undertaken to assess the likelihood of this happening and the process followed is discussed below.
Step 1:Obtain catchment rainfall data. Data had to be at hourly time steps to provide enough detail to enable adequate simulation of river flows. The data also needed to realistically reproduce the known spatial variation of rainfall over the catchment.
Step 2:Convert rainfall to flood flows. A rainfall-to-flow model was built for the catchment to convert rainfall into river flow. The model was designed to allow the spatial variation of rainfall to be taken into account. The model was calibrated using data from a 1986 storm to test how well the model simulated the real river flows.
The rainfall amount for each annual flood event was then successively increased by 5 per cent, 10 per cent and 15 per cent. The percentage increases in the rainfalls were chosen in order to bracket the increases likely to occur as a result of climate change and provide a range of potential risk scenarios. The modified rainfalls were then run through the model to form corresponding climate change-affected flows. The peak flows were then extracted from the data to derive the changes in the design flows used for sizing the stopbanks along the Hutt River.
Step 3:Calculate changes in flood inundation. The final step in the investigation was to turn the increased flows into water levels and compare the new levels with the heights of the existing stopbanks to see if the banks would be overtopped.
Conclusions: The Hutt River study concluded that:
- the rainfall-flow model was able to accurately and reliably estimate flows in the Hutt River
- spatial rainfall patterns for each storm give more representative results than a standard pattern based on storage gauges
- rainfall increases of 5, 10 and 15 per cent lead to flow increases of 6.7, 13.4 and 20.3 per cent, respectively
- the present stopbanks are designed to protect against a 1 in 450-year event. It would be unlikely that present stopbanks at Taita would be overtopped for this design event, even if the rainfall increased with climate change to the maximum extent examined. Climate change would reduce that level of service, but not to an unacceptable level in this case.
Case Study 2: Leith Lindsay flood protection scheme, North Dunedin
The Leith Stream, and its tributary Lindsay Creek, poses a flood hazard in the reaches flowing through the urban area of North Dunedin. A study was undertaken to look at the possible changes in flood risk due to increases in rainfall intensity associated with climate change and to assess the performance of the proposed flood mitigation scheme. The approach was to gather records of storm rainfalls and flood flows for recent events recorded in the Leith catchment. A rainfall-losses/run-off routing model, calibrated for recent storms, was used to assess the expected increase in peak flood flow resulting from expected increases in design storm rainfall intensities. Finally, hydraulic models were used to assess the expected increase in water levels and the increase in flood hazard.
Step 1:Calculate the increase in storm rainfall. The study used expected annual mean temperature changes by 2080 (from 0.4 to 3.1°C), as recommended for the Otago region in the 2004 edition of Preparing for Climate Change, to increase design storm rainfall intensities. These changes suggested, for example, that the rainfall intensities for a 12-hour duration, 1 in 100-year event, could increase by between 3 and 21 per cent. The 2008 edition of Preparing for Climate Change revised the expected temperature change for Otago by 2090 to 2.0°C average and a range of 0.8 to 4.6°C. Based on these latest projections, the rainfall intensities for a 12-hour duration, 1 in 100-year event, would be expected to increase by between 6 and 37 per cent, with a mid-range value of 16 per cent. This shows the importance of using the latest climate change information available and re-evaluating the impact of climate change from time to time as new information comes to light.
Step 2:Convert rainfall to flow rate. The study used projected percentage increases in storm rainfall with a calibrated rainfall losses/run-off routing model to determine flood flows for the Leith Lindsay catchment. The study found that the 1 in 100-year flood peak for the Leith Stream (above the tidal limits) could increase, on average, from the present day value of 171 m3/s to 200 m3/s, a 17 per cent increase in flow. The design flood estimates determined in Step 2 were then used with models to assess the performance of the proposed flood mitigation scheme.
Conclusions: The Otago Regional Council found the flood magnitude for a given standard of protection is expected to increase, but also that there was some uncertainty about the magnitude of the increase. The results showed the proposed scheme would also perform safely under the extreme and long-range climate change scenarios developed using the 2004 edition of Preparing for Climate Change.
Case Study 3: The Buller River
In a report commissioned by the Ministry for the Environment in 2005 a combination of weather, hydrological and inundation modelling was used to look at the impact of three different climate change scenarios on flood inundation for Westport.
Step 1: Choose the climate change scenarios. The three scenarios chosen assumed changes in temperature of 0.5, 1.0 and 2.7 ºC. It was also assumed that the initial relative humidity remained the same across each scenario.
Step 2: Calculate the changes in rainfall and flow. To determine the impact of temperature on rainfall, weather models were used to replicate the rain from three historical rainfall events. These events were then remodelled, but with the initial conditions based on the three climate change scenarios. The rainfall increased by 3, 5 and 33 per cent on average for the three events, through both an increase in the water-holding capacity of the air as well as through changes in the intensity of the storms. The flow for each of the rainfall events was estimated using the Topnet model. The resulting percentage increases in river flow, averaged over the three events, were 4, 10 and 37 per cent.
The average changes in flow were then used to change the rainfall value in a 1-in-50-year design storm. The design storm and its climate-changed versions were used to estimate the flow that would result in the Buller River. No changes were made to the antecedent conditions to allow for other factors, such as a wetter catchment from increases in annual rainfall resulting from climate change. No changes were made to the river flow characteristics either, although changes could be expected as rainfall events are likely to be more intense.
Step 3: Calculate the changes in inundation. The changes in flow were then used to estimate the inundation that may result for the town of Westport. The three scenarios increased the area of inundation from 13, to 30, to 80 per cent.
Conclusions: The results of this study suggest that even with a 1ºC change in temperature (which might be regarded as a mid-high projection for 2050 or a mid-low projection for 2100), there could be significant changes in the level of risk of flooding for Westport over the next 50 to 100 years (Figure 4). This would have important implications for the level of protection provided by current infrastructure.
Figure 4: 1-in-50-year inundation areas in downtown Westport, with the present climate (left) and a mid-high scenario for 2080 (right).
Read a description of this image
Figure 4: 1-in-50-year inundation areas in downtown Westport, with the present climate (left) and a mid-high scenario for 2080 (right).
The figure shows two aerial maps of Westport. The left hand map is for the present climate and shows that only 4% of Westport would get flooded in a 1 in 50 year event. Levels of inundation in that event are shown to be generally below 0.5 m. The right hand map shows the flooding that might be expected from a 1 in 50 year event in 2080 for a mid-high scenario. The map shows that 80% of Westport is flooded, with levels exceeding 1 metre in many places.
Back to footnote reference 3 A hyetograph is a graph showing the distribution of rainfall over a period of time.
Back to footnote reference 4 A hydrograph is a graph showing changes in river flow over a period of time.
Back to footnote reference 5 Design floods are hypothetical floods of specific storm duration and recurrence interval. They are used for planning and flood plain management investigations.
See more on...
Part Two: Estimating the impact of climate change on flooding
May 2010
© Ministry for the Environment