Flooding in Waitaha Canterbury
Flooding is the leading cause of damage from natural events in Aotearoa New Zealand and Waitaha Canterbury has experienced many floods. Many of our homes and businesses, as well as infrastructure such as roads and rail, have been built in areas that are vulnerable to flooding.
We experience three main types of flooding in Waitaha: river flooding, surface flooding, and coastal flooding.
Types of flooding
River flooding
River flooding, or fluvial flooding, occurs when a river or stream overflows its banks and floods normally dry land. Waitaha’s rivers fall into three main categories.
Alpine rivers
Rangitata River alpine river flooding, 2019
Alpine rivers have catchments that extend back to the Main Divide, or Kā Tiritiri o te Moana the Southern Alps. During north-westerly weather conditions, heavy rain falls on Te Tai Poutini West Coast, with the winds pushing some of this rain to spill eastward over the mountains. This heavy rain in the upper catchments of our alpine river systems generates runoff that flows down to Kā Pākihi Whakatekateka A Waitaha Canterbury Plains.
Alpine river floods usually happen during hot and dry weather on the Plains, so those impacted by flooding may not see any rain.
Flood flows can take 24 hours to reach the populated parts of the Plains, but the changing braided nature of our alpine rivers makes it hard to predict where river breakouts may occur.
Our alpine rivers include: Waitaki, Rangitata, Rakaia, Waimakariri, Hurunui, and Waiau Uwha.
Foothill rivers
Kōwhai River Kaikōura foothill flooding, 1993
Foothill rivers have catchments in the foothills which sit between Kā Tiritiri o te Moana Southern Alps and the Canterbury Plains or coast. They are generally much smaller than alpine river catchments.
Flood flows usually happen during rainfall with an easterly or south-west component.
Because of their smaller catchments, these rivers respond rapidly to rainfall. It may only take a few hours for this kind of flooding to reach populated parts of the Waitaha Plains.
Our foothill rivers include: Waihao, Pareora, Ōpihi and its tributaries, Ōrāri, Hinds, Ashburton, Waikirikiri Selwyn, Rakahuri Ashley, Waipara, Conway, and Kōwhai.
Lowland rivers
Lowland rivers in Waitaha are mostly spring-fed, single channel rivers located near the coast. Flooding in and around these rivers usually occurs during long rainfall events.
Flooding often results from high water levels in these rivers preventing the adjacent low-lying land from draining, rather than from direct breakouts or overflows.
Our lowland rivers include: Huritini Halswell, Ōtākaro Avon, Ōpāwaho Heathcote, and Ruataniwha Cam.
Surface flooding
Surface flooding near Lincoln, 2022
Surface flooding, or pluvial flooding, occurs when the rate of rainfall exceeds the rate at which water can soak into the soil, or when the soil becomes saturated and excess water runs off.
Surface flooding doesn’t originate from rivers, but it will worsen the effects of river flooding in major rainfall events. High river levels can also make surface flooding worse by preventing water from draining away.
Surface flooding can occur almost anywhere in Waitaha and often happens quickly. Water collects in low areas, historic flood flow channels, or where obstacles like raised roads, hedges, or irrigation features prevent water from moving down-plain.
Surface flooding in urban areas is often referred to as stormwater flooding. Stormwater flooding occurs when the increased runoff from sealed surfaces like roads and driveways is greater than the capacity of stormwater systems, or if they are not working properly because of blockages or pump failures.
Coastal flooding
Coastal flooding, coast north of Timaru, 2022
Coastal, or seawater flooding, happens when a combination of high tides, low-pressure systems, strong onshore winds, and elevated swells drive the sea up and over the natural beach crest or coastal stopbank.
Many parts of Waitaha’s coastline are vulnerable to coastal flooding. It can threaten coastal communities, endanger stock, and cause long-term damage to productive land. Vulnerable locations in Waitaha include low-lying areas between Waitarakao/Washdyke Lagoon and the Ōrari River, between the Rakaia River and Taumutu, Leithfield and Amberley Beach, and estuary margins.
Coastal flooding combined with heavy sea conditions can also cause river mouths to block, which may worsen river flooding next to the coast.
Find out more about coastal hazards in Waitaha and how we monitor them.
Check river flow and rainfall
Flood protection in Waitaha
Find out how we work to reduce the impacts of flooding in Waitaha.