The May 2021 flood event caused significant damage to the Springfield Township including inundation and water entering several dwellings.

Following this event the Council committed to the community that it would carry out a review of the flood event, assess the outstanding risk to the township, and identify potential mitigation options.

May 2021 Flood Event.

  • The Council engaged consultant engineers to review and analyse the May 2021 flood event with a second phase scope to identify any available mitigation options.
  • Statistics for 13 Mile Bush Rainfall Gauge (Used for this study) Rainfall data | Environment Canterbury (
  • Rainfall for the event exceeded 300mm.
  • Event duration was approximately 80 hours.
  • Annual Exceedance Probability (AEP) for this event was less than 0.5% (Meaning the event was a 1 in 200 year event)
  • The AEP for shorter durations contained within the total 80-hour event varied and was as high as 40% (1 in 2.5).
  • Intensity of the rainfall was not extreme; the issue with this rain event was the volume of water that fell.

Catchment Learnings and assessment

  • Detailed assessment was completed of the catchment and review of available rainfall data.
  • A hydraulic model was created for the township and this was calibrated and verified based on the May 21 flood event.
  • This model was then used to analyse flood risk to the township in future likely events.
  • Bishops Creek has a large catchment above the township, critical time event (time it takes water to flow through) is approximately 6 hours.
  • Current capacity of Bishops Creek through township is about 7m³/s.
  • Learning from May 21 - this was a 0.5%AEP (Annual Exceedance Probability or chance of occuring in any given year) - 1:200yr Flood event for the 72 hour rain event.
  • Approximately 380mm of water fell during this rain event.
  • This event had a peak flow of approximately 15m³/s in Bishops Creek (twice the capacity of the channel).
  • When taking into account the effects of climate change and a 2% AEP (1:50yr) event for the six hour critical event for the catchment, the assessment showed this will produce approximately 23m³/s of water from Bishops Creek.

Option assessment

  • A number of different flood mitigation solutions were investigated given a very high level assessment. These included - channel works, stop banks, detention dams, lifting houses, flood flow diversion.
  • After a high-level assessment some options were narrowed down and run through hydraulic model. From this a preferred option could be selected.
  • The preferred option selected is the Bishops Creek diversion based on the 1950's catchment board design, including channel works.
  • While this option is expensive, it will require a feasibility study to be completed, and will not guarantee protection for all dwellings in the township.

Next Steps

Springfield Next Steps.pdf [PDF, 91 KB]

Immediate Actions

The Council will:

  • Design and Implement ‘minor’ channel upgrades and stormwater improvements within Springfield to mitigate ‘minor’ flooding (not to 50 year event level and able to be implemented independently of diversion scheme).

Long Terms Actions

  • Confirm the agency who will lead the Bishop's Creek diversion project and the role of other partner agencies (Selwyn District Council, ECan, central government, Waka Kotahi New Zealand Transport Agency).
  • Lead agency to commission Feasibility Report to assess and confirm following the points:
  • Planning assessment, understand resource consent issues (consents required, activity status and supporting technical studies).
  • Consultation with affected landowners to assess potential alignment options.
  • Consultation and confirmation to determine scheme performance e.g. 1 in 50 year event without freeboard (or alternative), applicable climate change scenario.
  • Consultation with external stakeholder e.g. Waka Kotahi, KiwiRail.
  • Refine options for final design noting factors such as levels and ecology
  • Optimise and confirm final design.
  • Update scheme concept design including assessment of properties protected, refine cost estimate, and assess scheme technical and financial feasibility.
  • Secure funding