FAQs about flooding hazards

Selwyn District Council has with the help of Environment Canterbury identified land across the Selwyn District which may be susceptible to flooding and/or coastal hazards. On this website you can find maps showing this land. If you are interested in a particular area, we suggest you use the search function at the top right hand corner of the map. You can search by property address or town name. 

When accessing the maps on this website, we find the browser Google Chrome works best. Under the ‘Flooding map’ tab on the website, there’s a user guide about how to use the flooding map. The guide pops up as a separate window as soon as you open the map, so make sure you check it out.

The proposed changes to how natural hazards risks are managed would only apply to new projects. If your building complied with the requirements that were in place at the time it was built, and you are not proposing to change it, then you won’t need to do anything.

It’s important to note that the new proposed provisions would only apply to those areas of your property that are identified as being at risk from flooding. The rest of the property would only have to comply with the usual zone-related rules.

Any new proposed rules will come into effect once the new Proposed District Plan is fully in place, which is expected to be in the end of 2022. Until then, current rules for building and resource consents apply.

Finally, any new Land Information Memoranda (LIM) report for your property will now contain a note that indicates that your property may be susceptible to coastal inundation and/or erosion and/or flooding under certain circumstances. The note will also identify the latest technical information held by the Council. This is a legal requirement. Councils have to include in a LIM all relevant information they hold about a property in relation to natural hazards.

The key difference is that as a result of having to assess the district at risk from flooding in a 200-year flood event, a much bigger area of the district has now been identified as at being at risk from flooding. As for rules, in the new Proposed District Plan we are proposing to manage flood risks with similar restrictions to what’s in the current District Plan.

All councils in New Zealand are required to identify and manage areas that are at risk from natural hazards. Keeping our communities safe and protecting our environment is part of our job, so it’s important we do this work.

If you wanted to do work in an area that’s in the proposed Plains Flood Management Overlay, a site assessment would determine if, in a 1-in-500-year flood event, either:

• the water depth (measured in metres) times the water velocity (measured in metres per second) is greater than one; or
• the water depth is greater than one metre.

If yes, then your land is in a high hazard area.

If your land is within any of the Coastal Erosion Overlay, Coastal Inundation Overlay or Waimakariri Flood Management Overlay, it would automatically be in a high hazard area.

When investigating risk from freshwater flooding (flooding caused by heavy rain and/or Selwyn River overflowing) we did technical assessments that involved computer-based modelling. This used the most recent data from the following sources:

• LiDAR surveys taken between 2010 and 2018.
• Other existing ground-related data, such as stopbank, river and road locations and soil types, from the Selwyn District Council, Environment Canterbury (ECan), Landcare Research and Land Information New Zealand.
• Design rainfall data from NIWA which also takes into consideration different climate change scenarios.

The final product is a series of maps based on computer-generated future simulations of various size flood events. The maps show the flood extent and maximum depths that are expected to occur during various sized floods. For more detail on the technical assessments check out the following technical reports:

• ECan report R19/41 – Selwyn River/Waikirikiri floodplain investigation. The report identifies areas that may be affected by flooding from the Selwyn River/Waikirikiri.
• DHI Water and Environment Ltd report – Regional Policy Statement Modelling for SDC – District Plan. The report identifies areas that may be affected by flooding in heavy rainfall events in the Selwyn District.

The District Plan Review has also been considering tsunami, wildfire and geotechnical hazards such as land instability, liquefaction and fault lines.

For tsunami, we are not proposing any additional rules, because the types of activities that we need to manage already require a resource consent under other rules. Instead, what we are proposing is a policy that requires evacuation procedures to be considered when the resource consent application is prepared and assessed.

For the first time we have specific rules in the Proposed District Plan for how wildfire risk in rural areas of the district should be managed.

We are also identifying for the first time areas at risk of geotechnical hazard on the planning map in the Proposed District Plan. Also requirements for a geotechnical investigation are proposed to change. Check out the Proposed District Plan for more information.

Effects of climate change on risk from natural hazards have been considered as this is something that councils must do when reviewing their district plan. When doing computer-based modelling to assess areas prone to natural hazards risk, the most recent climate change predictions were built into the models.

While the current District Plan manages a risk from a 50-year flood event, in the new Proposed District Plan we are required to identify and manage areas at risk from more extreme events. The Canterbury Regional Policy Statement directs councils to assess and manage risk from a one-in-200-year flood event. Any new assessments of flood risk also need to take into account climate change effects over the next 100 years, such as increased frequency and intensity of rainstorms.

When managing the risk from future flooding, we use computer modelling that takes into consideration all the available data and then predicts the likelihood of different size flood events within a given period. But it’s important to note that natural events such as flooding don’t occur with any regularity. For example, a more extreme flood, such as one-in-200-year flood, means that in any given 200 year period, such a flood may occur once, twice or more or not at all. The probability of such a severe flood event to happen in any one year is 0.5 percent (or has 0.5% Annual Exceedance Probability (AEP)). On the other hand a less severe flood event, such a one-in-50-year period, means there’s two percent chance of it occurring in any one year.

The terms AEP (Annual Exceedance Probability) and ARI (Average Recurrence Interval) are used to describe the probability of a flow of a certain size occurring in any river or stream.

ARI is the average time period it’s predicted will pass between floods of a certain size. For example, a 200-year ARI flood flow will occur on average once every 200 years. Alternatively, AEP is the probability of a certain size of flood flow occurring in a single year. A 0.5% AEP flood flow means that a one-in-200-year flood has 0.5 percent chance of happening in any one year. In short, we also call such a flood a 200-year flood.

Therefore, the 200-year ARI flow and 0.5% AEP flow are different terms to describe a flow of the same size.

For more information on flood probabilities check out the Q&As on ECan website.

Any current or new building/resource consents aren’t affected by the proposed rules for managing flood risks until the new District Plan is fully in place. This is expected to be by the end of 2022. Until then current rules for building and resource consents continue to apply.

However, for any property that has been assessed as potentially being at risk from flooding during a 200-year flood event, a new Land Information Memoranda (LIM) report will now contain a note that indicates that the property may be susceptible to flooding under certain circumstances. The note will also identify the latest technical information held by the Council. This is a legal requirement. Councils have to include in a LIM all relevant information they hold about a property in relation to natural hazards.

The Council maintains the district’s stormwater system, to manage the effects of stormwater in urban areas, through drains, swales, pipes, soakholes and treatment devices (wetlands, basins and proprietary devices) that manage run off from roof and hard-stand areas. This is managed by the Council’s Infrastructure Strategy 2018-2048 and is reviewed under the Long-Term Plan process.

The Council is active in maintaining and improving the stormwater system and is currently carrying out $1.1million worth of upgrades to the stormwater system across the district as per the 2018/28 Long Term Plan.

The stormwater system, including overland flow (run off from properties to the street) is designed to handle up to a 1 in 50 year storm event. The cost to upgrade this system to handle a one in 200 year flood event would be prohibitively expensive.

Environment Canterbury also manages flood protection for the district’s rivers, lakes and coast line through stop banks, groynes and other measures.

The proposed rules for the District Plan also aim to ensure that any new building is protected.

The mapped areas are not a record of where land has flooded in the past. They are a result of an assessment where land may flood during a large or extreme flood event in the future. We don’t have a record of any flood as large as a 200-year flood event.

The Proposed District Plan has been notified for formal public consultation. You can make a submission on any of the proposed provisions, including how the flood areas are mapped. Make sure you have your say by 5 pm on Friday 11 December 2020.

Any submission during the formal public consultation seeking to change a mapped area will need to be supported by sufficient evidence for such a change to be accepted.

For flooding hazard the following note will be used:

The Council is undertaking a District Plan Review and through this process the Council has obtained and holds information showing that this property may be susceptible to flooding from the Selwyn River and/or in heavy rainfall events. The two reports are outlined below and can be found at www.selwyn.govt.nz/districtplanreview:

• ECan report R19/41 – Selwyn River/Waikirikiri floodplain investigation. The report identifies areas that may be affected by flooding from the Selwyn River/Waikirikiri.
• DHI Water and Environment Ltd report – Regional Policy Statement Modelling for SDC – District Plan. The report identifies areas that may be affected by flooding in heavy rainfall events in the Selwyn District.

For more information please contact the Selwyn District Council: phone: 0800 SELWYN (735 996), email contactus@selwyn.govt.nz or visit 2 Norman Kirk Drive, Rolleston.

The Council cannot advise property owners about what effect this new flood and coastal hazard information may have on your insurance. Different insurance providers will have different policies, and we suggest you contact your insurer directly to discuss your specific policy.

You can make a submission on any of the proposed provisions, including how the flood areas are mapped. Make sure you have your say by 5 pm on Friday 11 December 2020.

Any submission during the formal public consultation seeking to change a mapped area will need to be supported by sufficient evidence for such a change to be accepted.