Ngongotahā Flood Review Project

Pepuere | February 2023 update

Tēnā koutou katoa,

 As you will all be aware, extensive rain occurred in Rotorua and the Ngongotahā catchment over Auckland Anniversary Weekend. There was some minor overtopping of the Ngongotahā Stream at Brake Road and Brookdale Place over the long weekend, thankfully there was no major flooding as a result of this.  We were expecting another 150mm of rain within the already saturated catchment on Wednesday 1 February,  fortunately that did not transpire and the river remained well within its banks.

You may recall that the Ngongotahā Review Action Plan was developed in response to the recommendations of the Independent Panel Review Report following the Ngongotahā 2018 flood event. Bay of Plenty Regional Council and Rotorua Lakes Council continue to work closely together on implementing the actions of this plan. We believe that the work undertaken over the last few years, and outlined to you in previous e-panui helped manage flood risk over the weekend, helping to prevent more significant overtopping. This work has included significant tree clearance adjacent to the stream throughout the catchment, construction of 10 detainment bunds within the upper catchment to hold back stormflows, 800 tonne of rock armouring, and improvements across various spillways within the stream, e.g. vegetation removal and benching. Climate Change is increasing the severity and frequency of these storms, please remain vigilant in times of heavy rain – especially on major floodplains like Ngongotahā.

Please remember to check in with neighbours and whānau and keep up to date with any advisories to evacuate, e.g. through the Rotorua Lakes Council or Bay of Plenty Civil Defence Facebook pages or local radio stations.

Bay of Plenty Regional Council and Rotorua Lakes Council are continuing to work collaboratively on the project to finalise design and implement further significant flood mitigation works along the stream, including the construction of an improved floodway, high flow bypasses, stopbank and road raising for floodwater containment. The parties continue to work together to test different designs which will enable this further flood mitigation work to proceed while mitigating the impacts on these private landowners as much as possible. We are moving closer to a solution and hope to be able to update the community on this in the near future.

If you have any questions please contact helen.creagh@boprc.govt.nz

Overview of Anniversary Weekend Event

 The storm over the weekend landed a total rainfall of 270mm at the Relph Rd recorder and 320mm at Upper Oturoa Rd between Thursday night and Monday morning.  Clustered rainfall over Friday night and again Saturday night generated two separate high flow events in Ngongotahā Stream. Flow rates peaked at 43.8 m³/s at 8am Saturday morning with flood flow contained within the streambanks before receding then rapidly rising again to a peak flow of 48.5 m³/s at 7:35am Sunday morning. The peak on Sunday morning resulted in minor overtopping around Brake Road and Brookdale Place. Council staff and contractors were able to contain overflow with no reports of inundated homes due to stream overflow. We are aware that local residents were concerned about flooding risk and we endeavoured to keep everyone informed of the emerging situation.

 The graph below plots hourly rainfall recorded at Relph Road and water level recorded at the SH5 stream gauge Ngongotahā Stream levels peaked at 4.9m on Saturday morning and 5.1m on Sunday morning. It is important to note that the peak on Sunday morning was as a result of less than half of the preceding days rainfall. This was a reflection of catchment saturation at that time.

Regional and district council staff sand-bagged the Western Road / Brake Road spill point; crew remain on site removing large debris in both the upper and lower catchment and are responding to reports of erosion in the lower reaches in preparation for continued rainfall this week.