Flood hazards
Flooding is the most common natural hazard in Canterbury/Waitaha. Three main types of flooding affect our region – river flooding (fluvial), surface flooding (pluvial), and coastal flooding.
Here are the current issues faced around flood hazards in Waitaha.
Issue: Flooding and related risks to communities and assets in Waitaha
Impact of flooding
River flooding (fluvial flooding) occurs when a river or stream overflows its banks and inundates normally dry land.
Surface flooding and ponding (pluvial flooding) occurs when the rate of rainfall exceeds the rate at which water can infiltrate the soil, or when the soil becomes saturated, and excess water runs off.
Coastal flooding (or coastal inundation) occurs when extreme weather causes low-lying coastal land to be inundated with sea water.
This can happen when a combination of factors such as high tides, low-pressure systems, strong onshore winds (winds blowing from sea to land) and large ocean swells drive the sea up and over the top of the beach.
Surface flooding in Otahuna Valley
A flood hazard occurs when floodwater resulting from any of these processes interacts with people and assets including culturally significant sites.
Flooding can result in houses and other buildings being damaged. Productive farmland can be impacted by inundation, riverbank erosion, scour, deposition of river alluvium, salt water damage, and crops, pasture and livestock can be lost. Infrastructure can be damaged, causing disruption to communities, transport and business activity, and can pose a threat to human life.
Many parts of Waitaha are vulnerable to one or more types of flooding. Settlements located near river mouths can be impacted by flooding from the complex interactions and rising water levels within coastal lagoon and river mouth environments.
Coastal flooding near the Orari River
Climate change
Flooding in Waitaha will be exacerbated by climate change. With ongoing sea level rise more areas will become susceptible to coastal flooding.
Waitaha has experienced several recent flooding events such as from the Rangitata River in 2019, regionwide flooding (especially the Ashburton district) in May 2021, and coastal flooding in parts of the Timaru and Selwyn districts in May 2022.
Increases in winter rainfall, along with greater rainfall totals and intensities during storm events will produce more surface flooding and increase river flood flows. Together with sea level rise, this means that large coastal flood events are expected to occur more frequently.
Rangitata River flooding at SH1
In addition to the built environment, many areas along our coast and rivers have high natural and cultural values, especially natural features such as dunes, lagoons, estuaries, and hāpua.
Natural features such as barrier beaches, dune environments, and coastal wetland areas can help manage or reduce the effects of natural hazards. Natural features such as dune systems may also need space to retreat into as the climate changes.
Current actions or approach
The Canterbury Regional Policy Statement (CRPS) sets out a hierarchy for dealing with natural hazards. The order of priority is avoidance, then mitigation, and then response and recovery.
The importance of natural features to help avoid or mitigate natural hazards is recognised, and these features are to be maintained, protected and restored as appropriate.
The role of physical mitigation works such as stop banks, sea walls, resilient buildings and infrastructure are also recognised as being acceptable where the natural hazard risk cannot be avoided.
Avoiding development in areas prone to flooding and other natural hazards cannot always be achieved. For example, infrastructure that is located near the coast or rivers in our region includes ports, large tracts of state highways, and the national rail network. New infrastructure may need to be provided in similar locations to support existing communities.
Alternatives to physical works such as the relocation, removal or abandonment of existing structures is to be considered. These directions are implemented through the provisions of district and regional plans as well as working with stakeholders to identify agreed solutions.
- The CRPS also sets out the specific direction for managing the risk from flood hazards in Waitaha
- Development in areas that are subject to severe flood hazard (flooding that meets the High Hazard Definition) needs to be avoided for new residential areas and avoided or mitigated in existing urban area
- Where development is proposed in areas that are subject to flood hazard (in a 200-year Average Recurrence Interval (ARI) event) mitigation of flood risk is required.
District plans implement this direction by restricting subdivision and building in areas that are subject to flooding. Environment Canterbury also implements this direction in some coastal areas through the provisions of the Regional Coastal Environment Plan.
Flood modelling to support planning
We undertake flood modelling across the region and provide information and site-specific advice to help implement the provisions in district plans and for other purposes such as Civil Defence Emergency Management.
Managing risk from river flooding
The CRPS also helps manages the risk of river flooding by providing for the removal of vegetation and bed material from riverbeds to maintain the flood-carrying capacity of rivers as well as to protect essential structures and prevent or control erosion. We implement this direction through the provisions of the Canterbury Land and Water Regional Plan.
Flood protection schemes
We have flood, drainage and erosion management responsibilities to minimise the impacts of flooding on communities through the delivery and maintenance of flood protection schemes. We currently manage 59 river control and drainage schemes from Kaikōura to Waitaki.
Within these established schemes we maintain $691m of flood protection assets including stop banks, groynes spurs and planting to manage erosion and flooding or provide drainage. These schemes and assets are protected through the Flood Protection and Drainage Bylaw 2013 (amended January 2019) from inappropriate modification, damage, or destruction.