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The most common weather-related cause of river flooding in New Zealand is heavy rainfall, which can greatly increase water levels in rivers and lakes and cause water to overflow into surrounding areas.
The magnitude of a flood depends on many factors in addition to the intensity and duration of rainfall. Other contributing factors include the land forms and surface features of the land, the vegetation and soil characteristics of the catchment, the wetness of the catchment before the storm (known as the antecedent or initial conditions) and evaporation.
Rainfall-driven floods range in both duration and extent, and may result from:
In some parts of New Zealand such as Otago and Southland, flooding can be exacerbated by the melting of snow. Warm temperatures and rainfall on a deep snowpack can lead to rapid snowmelt, which can sometimes cause or exacerbate a flood. There are also other types of flooding, such as inundation by groundwater or high sea levels, or even dam-break floods, however these are not discussed in this guide.
The impact of floodwaters on communities will also depend on non-weather-related factors, including how many people and what assets are at risk, and the effectiveness of flood mitigation and flood warning systems.
To examine the likely impact of climate change on flooding in New Zealand, this guide uses projections of future changes in annual mean temperature and rainfall set out by region. The projections for climate change in New Zealand have been made using six greenhouse gas emission scenarios developed for the Intergovernmental Panel on Climate Change (IPCC). The scenarios consider different combinations of socio-economic profiles, energy use and transport choices into the future. Preparing for Climate Change provides more information on how the New Zealand projections were developed.
Figure 1 shows projected patterns for temperature change. Because natural effects cause the New Zealand climate to vary from year to year, the changes are specified in terms of the average change for the period 2030–2049 (referred to below as 2040), and for 2080–2099 (referred to as 2090), relative to the climate of 1980–1999 (1990). Tables 2 and 3 in Preparing for Climate Change provide the numerical values for the projections of annual mean temperature and rainfall change for each region of New Zealand.
These temperature changes can be used to estimate increases in rainfall to provide a basic screening method for estimating changes in rainfall. A number of more advanced methods for estimating changes in rainfall are also highlighted in Part Two of this guide.
Note the different temperature scales for 2040 and 2090. These maps are intended to illustrate broad geographical patterns of climate change within New Zealand. They should not be used as definitive predictions of climate change for specific geographical locations. Projections for specific regions are provided in Tables 2 and 3 of Preparing for Climate Change.
Figure 1: Projected mid-range changes in annual mean temperature (in °C) relative to 1990
This figure shows two maps of New Zealand, giving projected changes over New Zealand in annual mean temperatures in degrees Celsius relative to 1990 for 2040 and 2090 from the twelve-model averages.
For 2040 the projected changes in mean annual temperature for New Zealand range from 0.8 degrees (South-Westland and Stewart Island) to 1.0 degrees (Canterbury-Kaikoura and most of the North Island). For 2090 the projected changes in temperature range from 1.8 degrees (Stewart Island) to 2.2 degrees (most of the north of the South Island, and most of the North Island).
Climate change is expected to lead to increases in the frequency and intensity of extreme rainfall, especially in places where mean annual rainfall is also expected to increase. Therefore, changes in seasonal and annual rainfall patterns, as well as changes in extreme rainfall, will be important factors for understanding future flooding. Generally, wetter conditions in some areas may also change the antecedent or initial conditions, so that floods could occur more often.
Places that currently receive snow are likely to see a shift towards precipitation falling as rainfall instead of snowfall as average temperatures rise and freezing levels climb to higher elevations.
Changes in climate can also affect the magnitude of a flood by indirect means. For example, any change to the balance of sediment transported within a river, storminess, sea levels or even the cycles of natural variability in the climate can all have an effect on river processes and flooding.
When assessing future flood risk you will need to consider all of these factors to see how they interact to give you a picture of future flood risk in your area. How climate change is likely to affect each of these factors is discussed in more detail below.
Figure 2 shows that the projected change in the average annual rainfall has a pattern of increases in the west (up to 5 per cent by 2040 and 10 per cent by 2090) and decreases in the east and north (exceeding 5 per cent in places by 2090). This annual pattern results from the changes in rainfall in the dominant seasons of winter and spring.
Figure 2: Projected mid-range changes in annual mean rainfall (in %) relative to 1990.
Text description: This figure shows two maps of New Zealand, giving projected changes over New Zealand in precipitation in percent relative to 1990 for 2040 and 2090 from the twelve-model averages.
For 2040 the projected changes in mean annual precipitation for New Zealand range from an increase of 7.5 percent (West Coast of the South Island) to a decrease of 5 percent (along a thin coastal strip from Kaikoura north to East Cape, and in Northland). For 2090 the projected changes in precipitation range from an increase of over 10 percent (West Coast of the South Island) to a decrease of 7.5 percent (in patches along the coastal strip from Kaikoura north to East Cape, and in Northland).
Projected changes in seasonal rainfall, as shown in figure 3, suggest increased westerlies in winter and spring will bring more rainfall in the west of both islands and drier conditions in the east and north. During autumn and summer, drier conditions are expected in the west of the North Island, and rainfall increases are possible in summer for Gisborne and the Hawke’s Bay.
Figure 3: Projected mid-range changes in seasonal mean rainfall (in %) for 2090 relative to 1990.
This figure shows four seasonal maps of projected changes in seasonal mean rainfall (in percentage) over New Zealand for 2090 relative to 1990 (twelve model average) for the A1B scenario.
For summer, the map shows increases in seasonal mean rainfall of up 10 percent in Hawkes Bay and parts of the East Cape. Smaller increases of up to 7.5 percent are projected in most parts of Marlborough and Canterbury, the Wairarapa and southern Bay of Plenty. Decreases of up to -5 percent are shown in the east of the North Island and Kapiti region, the northern West Coast and parts of Southland. Smaller decreases over Stewart Island, the Waikato Region and south Westland are projected.
For autumn, increases of up to 7.5 percent in seasonal mean rainfall are projected for Marlborough, Canterbury, Otago, Westland, Western Bay of Plenty, and from East Cape down through Hawkes Bay and the Wairarapa. Decreases of up to -5 percent are shown for Northland, Waikato, Wanganui, most of the central North Island and the region around the northern end of the Southern Alps.
For winter, marked increases in seasonal mean rainfall are projected, particularly in the west, with the West Coast of the South Island, Southland, parts of Taranaki, Manawatu and the Waikato increasing by over 10 percent. Decreases in the east of up to -7.5 percent in Northland and from East Cape down through Hawkes Bay and the Wairarapa are expected, as well as down the east coast of the South Island as far as Canterbury. Smaller decreases in the Bay of Plenty and parts of the Waikato are shown.
For spring, marked increases in seasonal mean rainfall of over 10 percent are projected for the West Coast of the South Island and Southland. Smaller increases of up to 5 percent are expected for Otago, the north-west of the South Island and most of Taranaki and the Manawatu. Marked decreases of more than -7.5 percent are shown in Northland, Auckland, Coromandel, East Cape, Hawkes Bay and parts of the Wairarapa. Smaller decreases in the central North Island and Marlborough/Canterbury regions are expected.
In regions where seasonal rainfall is expected to increase more than the seasonal evaporation rate, it is likely there will be wetter initial conditions (eg, wetter soils, higher lake levels). Conversely, if seasonal rainfall is projected to decrease, then initial conditions would be expected to be drier. Increases in temperature and wind are also likely to increase evapotranspiration (the amount of water lost to the atmosphere from soil and plants), resulting in drier initial conditions. These factors may also be important for estimating base flows for purposes such as water resource management.
Climate change will have the biggest impact on New Zealand river floods through changes in the frequency and intensity of extreme rainfall. This is because extreme rainfall is the most common trigger for flooding in New Zealand.
A warmer atmosphere increases the water-holding capacity of the air. This means that, assuming other factors remain the same, rainfall is likely to be more intense. The expected percentage increase in extreme rainfall is around 8 per cent per degree Celsius of temperature increase. So if we expect a 1 to 2°C temperature rise by the end of the century, we could estimate that the intensity of extreme rainfall in the future might increase by 8 to 16 per cent.
However, the relationship between rainfall intensity and flood magnitude depends on several factors and is not linear. For example, an 8 per cent increase in rainfall intensity does not lead to an 8 per cent increase in flood peak discharge (when there is the greatest amount of water in the river), and does not lead to an 8 per cent increase in inundation (the area flooded). In many cases, you will need to combine your understanding of the current rainfall/run-off/inundation processes with the expected increases in rainfall to determine the resulting increases in flow and inundation. Methods for determining changes in the rainfall/run-off/inundation processes are discussed further in Part Two.
Table 7 in Preparing for Climate Change provides more detail on the recommended percentage adjustments per degree of warming to apply to extreme rainfalls for various average recurrence intervals and for different rainfall durations.
Places at lower altitudes that currently receive snow are likely to see a shift towards more precipitation falling as rainfall instead of snowfall, as freezing levels climb to higher elevations due to rising temperatures. For rivers where the winter precipitation currently falls mainly as snow and is stored until the snowmelt season, there is the possibility of larger winter river flows. These impacts have not yet been quantified, but are in addition to the temperature-driven increases in extreme rainfall that result from a warmer atmosphere.
Changes in precipitation will lead to changes in the amount and size of sediment a river can transport, which will then affect riverbed levels and channel width. Increases in rainfall intensity may lead to changes in river channel morphology, leading in turn to changes in the location and likelihood of inundation. For example, extra sediment may be deposited in the bed of a river, raising the level of the bed and thus reducing the flood-carrying capacity of the channel. As a result, for a given river flow rate, less water can be carried in the channel and more water will overflow, causing flooding. The opposite situation may also occur, where an increase in floodwaters in a channel gives greater water velocities, allowing the river to transport more sediment than is being deposited. This can lead to increased erosion and degradation in the channel and subsequent effects further downstream.
Projections of future sea-level rise due to climate change (see Preparing for Coastal Change) will cause lower freeboardFootnote 2 on coastal flood-mitigation structures, increased inland influence of tides and a flattening of river slopes in coastal reaches in some locations. The reduction of a river’s slope reduces the energy of the flood flow, increases the depth of flow and reduces the sediment-transporting capacity potentially leading to aggradation in the channel.
A risk-based approach can be used to assess the sensitivity to different amounts of future sea-level rise. Preparing for Coastal Change provides guidance on planning for future sea-level rise and recommends assessing the potential consequences of a range of future possible sea-level rise values.
Based on our understanding of physical processes in the atmosphere it is likely that climate change will bring increased storminess. ‘Storminess’ can refer to the number of storms, or to their intensity, which in turn could be judged on the basis of strong winds or heavy rainfall. It is also likely that tropical cyclones will be more intense, and such weather systems can transform into intense sub-tropical lows that bring heavy rainfall, damaging winds, waves and storm surge to New Zealand.
There is also the potential for flooding to be exacerbated in coastal areas by increased frequency and magnitude of wind set-up and storm surge, which result when high winds and decreased barometric pressure during storms raise the local sea level (see Preparing for Coastal Change). This may be important for river mouth areas and coastal stormwater systems.
The climate is naturally variable, and New Zealand’s climate is affected by the Interdecadal Pacific Oscillation (IPO). The IPO brings decadal fluctuations in winds and rainfall over New Zealand and this leads to variations in river flow and flooding. Changes in ‘climate’ over the next 50 years or so are in the same order of magnitude as IPO variability. Therefore, both IPO and climate change effects may need to be considered when calculating flood risk. Section 2.3 in the source report has more discussion on the implications of the IPO for flooding.
Back to footnote reference 2 Freeboard is a term used to describe a factor of safety above a design flood level for flood protection or control works. See Part Two for more detail.