The weather events of this summer have been hugely challenging for some communities throughout New Zealand and will continue to be so for quite some time. Central and local government are facing some critical decision making as we grapple with the challenge of where to rebuild homes and critical infrastructure in light of clear evidence that previous decisions on where to site homes and critical infrastructure have a few questions marks hanging over them. I noted that questions had been raised regarding the performance of the Ōtorohanga stopbanks during recent rainfall events and if the town was vulnerable to flooding in an extreme rainfall event. It’s a fair question, so I thought it might be useful to provide a response. Firstly, it’s important to appreciate how the system is designed. There are effectively three parts to the system. The river channel itself, the floodway or floodplains (the large flat area between the river channel and the stopbanks) and the stopbanks. The floodplains are designed to be inundated during larger rainfall events, and this is what happened during the most recent event, as shown in a photo in the King Country News when the river breached its main channel. While the floodplains were inundated, there is still significant capacity within the stopbanks themselves to cope with extra floodwater, should this be required. The stopbanks were designed to cope with a flood event equivalent to that of the February 1958 event, which at that time was considered to be a 100-year event. In addition, the stopbanks are constructed with additional freeboard of 2 feet (0.61m) over and above the design flood level. This is to accommodate uncertainties such as debris blockages and wind generated wave action etc. The design flow capacity of the floodway (flow area between the stopbanks) is approximately 50% larger than the 1958 flood flow. So, the Ōtorohanga flood protection scheme has been designed to withstand large events. However, the scheme’s ability to provide this protection is reliant on good maintenance. The height and structural integrity of the stopbanks is clearly important, along with ensuring that any infrastructure adjoining the stopbanks is not having an adverse impact on performance. This is a role that is undertaken by Ōtorohanga District Council, supported by technical expertise and advice from Waikato Regional Council when required. The regional council is responsible for river management, with regular inspections to ensure channel capacity is maintained. ŌDC and the regional council have a good working relationship, and the regional council is not aware of any issues that could compromise performance of the scheme. The most recent review of the Ōtorohanga scheme’s hydraulic model was undertaken in 2015, and showed the stopbank design levels were appropriate to accommodate the 100-year flood. These reviews are done approximately every 10 years. Comparing rainfall events is not necessarily straight forward from a river flow perspective, and a 100-year rainfall event does not necessarily generate a 100-year river flow, as catchments can be quite different and complex. An analysis of rainfall and river levels for the period from January 18 to February 3 this year was undertaken by the regional council’s hydrology team. Rainfall of 253mm in Te Kuiti was assessed to be in excess of a 250-year event, yet the Waipa River level was assessed as a 5-to-10-year level. However, the Waitomo stream (measured at the Aranui Caves Bridge) was assessed as a 50-to-100-year level. We have over 600km of stopbanks in the Waikato, and these are all checked regularly by regional council engineers.
