Pipiri - June 2022 update No images? Click here Tēnā koutou katoa, We wanted to give you another short update on the progress for the Ngongotahā flood mitigation work that the Councils are working on together, and some information about the storm event Rotorua experienced overnight on 30 May 2022, including high flows in the Ngongotahā Stream. The 30 May event resulted in a stream flow classified as just above a 20% Annual Exceedance Probability (AEP) flood event, this means that event has a 20 percent chance of occurring in any one year (previously described as a 1 in “5 year” event). During this event, the stream level at the State Highway 5 bridge reached a stage of 4.2 metres, which is 0.6 metres below the estimated spill level for streambank overflow at Western Road and Brookdale Drive. By comparison, the 2018 event was assessed as more than a 1% AEP, which means that it has a 1 percent chance of occurring in any one year (previously described as a 1 in “100 year” event). The flood mitigation work undertaken to date and proposed is designed to provide protection to the township in the 1% AEP event. Bay of Plenty Regional Council has assessed the 30 May storm in the model being used to design the flood mitigation work, i.e. floodway, high flow bypasses, raising of Western Road and various stopbank enhancements, to compare predictions to survey information collected after the flood event. We’ve found that the actual flood levels during the 30 May event closely matched the model’s predictions, and the discrepancy was well within the design tolerance of the proposed flood mitigation work. This gives us greater confidence in the model and the proposed designs. While the results are not significant enough to quantify the net benefit of catchment improvements to date, the operations team did report that the stream maintenance that has been undertaken and reported in our previous updates/e-pānui was effective in preventing major debris blockages during the storm. During the event on 30 May, Rotorua Lakes Council staff were visiting properties in the Ngongotahā area and advising of a potential need to evacuate, thankfully this never eventuated. There were early and regular updates on various Facebook and media channels also. The 30 May event was a good reminder that during periods of heavy rain the community needs to remain vigilant, check in with their neighbours and whānau and keep up to date with any advisories to evacuate, e.g. through the Rotorua Lakes Council Facebook page or local radio stations. As an update on progress with the flood mitigation works, Regional Council has engaged a specialist to work with one owner of land that the proposed floodway crosses, to look at options to mitigate the effect of the proposed floodway on their operations. This work is in progress and anticipated to be complete shortly. Regional Council is also working on resource consent for the high flow bypasses, ground investigation and design for proposed stopbanks, and the processes to legalise permanent land access for all of the proposed mitigation infrastructure for construction and maintenance purposes. The flood mitigation work is split into two phases: Phase 1 is the high flow bypasses and may commence as early as December 2022. Phase 2 is the proposed stopbanks, floodway, and raising of Western Road, this Phase has more complex challenges in terms of land access and approvals and we are hoping work on this can commence in the 2023 earthworks season. If you have any questions please contact helen.creagh@boprc.govt.nz |