Flood hazard areas

Risk from flooding is not the same in all areas across the district, and ranges from areas where no risk is identified to areas where there are potentially significant hazardous conditions created by flood water depths or the speed at which the water is moving.

  • Low and medium hazard areas generally have a flood water depth ranging from 0 mm to 1 metre
  • High hazard areas generally have a depth of more than 1 metre.

Note: Water velocity can mean, for instance, in a high hazard area the depth could be less than 1 metre.

Flood water depths shown on the interactive map are the height above ground level.

Depths and water speed (velocity) are calculated using ground contours and computer models for a 0.5% Annual Exceedance Probability (1 in 200 year) or 0.2% Annual Exceedance Probability (1 in 500 year) storm event. The Annual Exceedance Probabilities mean that there is a 0.5% or 0.2% chance that a storm event of that magnitude will occur within any year.

Please note: The interactive map indicates both flood hazard and flood depths for properties.  The hazard (or risk) for a site is calculated from the depth and velocity (speed of movement) of any floodwater.

The Annual Exceedance Probability levels come from the Canterbury Regional Policy Statement. The Council is required to give effect to the Canterbury Regional Policy Statement and therefore has to use these flood events in its assessment of risk.

Areas excluded from flood hazard areas

Some identified areas have already gone through assessments for flood risk – normally through a plan change or subdivision process, and have either had resource consents approved that set out floor heights or works that must be done to mitigate flood risk. For areas that have been through a plan change process, there are rules in the District Plan that set floor heights or other methods for these particular areas.

As a full assessment has already been carried out that allows for development to have occurred, these areas are proposed to be excluded from the new high hazard flood area provisions and are identified on the planning maps.

Diversion and displacement of floodwater

Development of a site, for example through earthworks or placement of structures, can in some cases affect the flow of floodwater, or how deep the water is. Moving the path of floodwater is referred to as diversion.

For example, a new building may be placed so that it redirects the flow of flood water onto roads or neighbouring properties where it can adversely affect traffic and drainage, and cause damage or create risk from floodwater for other people and property.

Displacement relates to the level of floodwater. For example, if earthworks raise the level of one site, it may mean that adjacent sites take more of the floodwater as a consequence. Displacement can also occur if the intensity of building development increases, leaving less area for floodwater to pond, which can result in higher water levels.

Floor heights and freeboard

Freeboard is a distance between the height of floodwaters and the floor level of a building. A minimum floor level height may be required to provide a safety factor to avoid buildings becoming inundated with water during a flood event.

If the floor level requirement in the District Plan is less than that of the Building Act, the Building Act requirements will still need to be met. If the District Plan has a higher requirement, then that will need to be provided for even if it is higher than that required by the Building Act. This is because the District Plan can impose a greater level of protection in areas where higher flood levels could be expected.

Climate change

It is recognised that climate change will lead to a rise in sea level over time and that this will affect coastal processes, and flooding in coastal areas and further inland. The information has been used within flood modelling and includes an allowance for sea level rise and changing rainfall as a result of ongoing climate change. Sea level rise, as it may affect part or the full extent of the Waimakariri District coast, is not part of the draft proposed plan change.

Questions and answers

Why have some areas been excluded from the high hazard flood area on the maps (being the red hatched areas shown on the District Plan map series A1-E4)?

Some areas have gone through assessments for flood risk – normally through a plan change or subdivision process, and have either had resource consents approved that set out floor heights or identified works that must be done to mitigate flood risk.  For areas that have been through a plan change process, there are rules in the District Plan that set floor heights or other methods for these particular areas.  As a full assessment has already been carried out that allows for development to have occurred, these areas are proposed to be excluded from the new provisions.

What does 0.2% and 0.5% Annual Exceedance Probabilities (AEP) mean and how/why have these levels of risk been used?

The figures are used to describe the probability of a flood of a certain size occurring in any one year.

A 0.5% AEP is another way of saying a one in 200 chance of a flood occurring in any one year. 0.2% similarly means a one in 500 chance that a flood of this type could occur in any one year.

The AEP levels come from the Canterbury Regional Policy Statement. The Council is required to give effect to the Regional Policy Statement and therefore has to use these events in its assessment of risk.

Aren’t the Council’s flood protection works supposed to protect us from flooding? Why do we need this plan change as well?

The Council provides some flood protection works that are designed to provide protection to property and infrastructure from moderate to heavy rainfall events, but are not designed to cope with extreme flood events. The rainfall that flood protection and drainage works are designed to cope with is at a level that could be expected once every 10 years to once every 50 years.

The District Plan change is required to provide protection to human life and to houses in extreme rainfall events that result in flooding. In events of this nature it is expected that existing drainage systems including streams and rivers would be overwhelmed and some damage would be caused to infrastructure and property.

What is displacement and diversion of floodwater and why does it matter?

Development of a site, for example through earthworks or placement of structures, can in some cases affect the flow of floodwater, or how deep the water is.

As an example, a new building may be placed so that it redirects the flow of floodwater onto roads or neighbouring properties where they can adversely affect traffic, drainage and the damage or risk from floodwater for other people and property. Moving the path of floodwater is diversion.

Displacement relates to the level of floodwater, if earthworks raise the level of one site, it may mean that other adjacent sites take more of the floodwater as a consequence. In an area, displacement can occur if the intensity of building development increases, leaving less area for floodwater to pond, which can result in higher water levels.

What is ‘freeboard’ and how has it been assessed?

Freeboard is a distance between the height of floodwaters and the floor level of a building.  For new buildings a minimum floor level height may be required to provide a safety factor to avoid buildings becoming inundated with water during a flood event.

How has climate change been factored into this plan change?

It is recognised that climate change will lead to a rise in sea level over time and that this will affect coastal processes, and flooding in coastal areas and further inland.  The information has been used within flood modelling and includes an allowance for sea level rise and changing rainfall as a result of ongoing climate change.

Why has the Council previously allowed development to happen in new residential areas that are at risk to flooding or liquefaction?

Since the previous rules were developed the Council has received hazard and flood modelling information which increases the understanding of risks posed by natural hazards.

The information that is available about natural hazards wasn’t available to the extent that it is now. Now that this information is available, the Council has a responsibility to manage and minimise risks to protect people and property. The District Plan needs to be updated to respond to this new information and plan for management of risk situations.