Causes of flooding
Flooding is the most common natural hazard in Canterbury. Three main types of flooding affect our low-lying region - river flooding, local runoff flooding and coastal overtopping.
Vast areas of the region are vulnerable to a degree of flood risk. Disastrous floods have struck many areas in Canterbury at some time over the past 100 years. But what causes this flooding and how do they originate?
Read our brochure about living on a floodplain.
Three types of flooding explained
The rivers of Canterbury fit into two distinct types – large alpine river catchments, and shorter foothill catchments.
Alpine rivers are rivers with catchments that extend back to the Main Divide.
These include (from south to north) the Waitaki, Rangitata, South Ashburton, Rakaia, Waimakariri, Hurunui and Waiau Rivers.
Heavy rain on the West Coast of the South Island spills to the eastern side of the Main Divide of the Southern Alps and falls into the upper catchments of Canterbury’s alpine rivers.
If enough rain spills over then floods occur. Rain that affects the upper catchments of the alpine rivers occurs when there are northerly conditions in Canterbury. This means that floods often coincide with hot, dry conditions on the Canterbury Plains
Alpine rivers tend to have large catchments and as a result, flood flows may take 24 hours to reach the populated parts of the plains. While this may be adequate time to prepare for potential floods, the braided nature of these rivers means the riverbeds and flow patterns are continually changing. This creates a high degree of uncertainty around river flow and the location where breakouts may occur.
Rivers with their origins in the foothills on the western edge of the Canterbury Plains have smaller catchments.
These rivers flood in rain events originating from the southwest, but more commonly during rain events with an easterly component. These rivers flood under completely different conditions and at different times to the alpine rivers.
Foothill rivers include (from south to north) the Waihao, Pareora, Opihi and its tributaries, Orari, Hinds, North Ashburton, Selwyn, Ashley, Waipara, Conway, Kowhai and Clarence.
Because of their relatively small catchments, these rivers respond rapidly to heavy or prolonged rainfall. In many cases it may only take a few hours for heavy rain to generate a flood that reaches townships on the Canterbury Plains.
These floods rise and recede at a more rapid rate than alpine-fed rivers but there is much less time to issue flood warnings making them a potentially greater risk to the people of Canterbury.
Local runoff flooding
This is flooding that originates from small streams, drains and overland surface flow following high intensity, short duration rainstorms or more prolonged, lower intensity rainfall events. It doesn't originate from any river or major stream but in major floods will exacerbate the effects of river flooding.
Stormwater flooding is the term used in an urban environment and generally results from blockages in sumps, drains, culverts or stormwater systems.
Local runoff can occur almost anywhere in Canterbury and often very quickly. In general, runoff collects in low areas and swales (historic flood flow channels) and any flooding that overtops lower areas tends to be shallow and widespread. Local runoff problems are difficult to predict in terms of frequency of occurrence and their potential effects are impossible to map.
To combat the effects of local runoff when building a home:
- Site the dwelling clear of any obvious low areas or swales
- Local runoff will tend to back up on the up-plain side of obstacles such as roads or irrigation headraces. If possible, elevate the floor of the house above downstream obstacles. Solid fences or hedges on the downplain side of a house site may also lift flood levels
- Consider elevating the floor above the minimum required. Increasing the floor height of a new dwelling by even a small amount, (say 200 mm) could reduce the risk of it being affected by floodwaters
Runoff will usually be contained in swales, drains and low areas so keep them clear of blockages such as overgrown vegetation, loose piles of garden debris left adjacent to the drain, fences, footbridges or undersized culverts.
Coastal inundation can threaten some coastal communities, endanger stock, and cause long-term damage to farmland.
Many low-lying parts of the Canterbury coastline are prone to coastal inundation. Coastal inundation occurs when a combination of high tides, low pressure systems, easterly winds and heavy swells drive the sea up and over the beach crest.
As well as coastal inundation, heavy sea conditions can also cause Canterbury river mouths to block, which may result in river flooding adjacent to the coast.