Proposed Work | Project Timeline | Project Contacts
In July 2004 when the Rangitaiki River stopbanks were under considerable stress from floodwaters, there was a breach at Sullivan’s Bend upstream from Edgecumbe on the true right (east) bank. The resulting huge volume of uncontained floodwaters entered parts of eastern Edgecumbe town (in particular homes in Konini Place and Hydro Road); overwhelmed the Reids Floodway and eventually swamped approx 17,000ha of farmland and infrastructure on the Rangitaiki Plain. The breach waters almost inundated the Transpower Substation facility to the south east of the town and loss of power supply to the whole Eastern Bay was only narrowly averted. The western (especially the south western) urban margins of Edgecumbe were also flooded by stormwater overflows from the rural Omeheu drainage system (a problem the town has faced since the 1987 earthquake when ground levels dropped by over two metres).
While floodwaters entered the Fonterra complex and caused some damage, it was fortuitous that this facility was shut for off season maintenance.
Environment Bay of Plenty (EBOP) spent considerable time and resources repairing river scheme assets in the Eastern Bay of Plenty between July 2004 and mid 2007. Government assisted financially with these response and recovery (repair) costs. EBOP has been and still is putting substantial resource into addressing stopbank weaknesses along the Rangitaiki River – particularly adjacent to the Edgecumbe urban area and upstream and downstream of the town.
However, the serious July 2004 flooding event highlighted a need to develop a higher standard of flood and hazard mitigation around Edgecumbe and the adjacent Plains. EBOP and Whakatane District Council (WDC) discussed this issue with Government and in 2005 the Minister of Civil Defence Emergency Management invited the two Councils to jointly present a Business Case for solutions to reduce community risk from flooding. Engineering and risk management reports were commissioned to identify the options and to assess the costs and benefits of various mitigation works and measures.
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As part of the Business Case, WDC and EBOP commissioned OPUS to produce a suitable flood protection design for Edgecumbe and its environment. The resulting engineering report identified that:
- The hydraulic conveyance of the design flood event (100 year) in the river and Reids Floodway is marginal.
- Reids Floodway is very narrow in the lower reaches and this greatly restricts its overall capacity. In addition, the constriction of the Floodway by the SH 2 bridge is also noted. New Zealand Transport Authority (NZTA) has agreed to extend the bridge length to address this issue.
- The foundation conditions make the stopbanks prone to piping failures under flood conditions.
- Dredging is unlikely to significantly increase long-term river conveyance capacity and is a high cost option.
- Local stormwater is a problem in the west of Edgecumbe where low lying parts of the urban area are reliant upon a rural standard drainage scheme.
A number of other recommended flood mitigation measures arose from the engineering report and Councils have implemented or are exploring these. For example maintaining the use of building controls such as minimum floor levels; monitoring and maintenance of the river mouth cross section; weather radar to improve risk prediction and optimisation of Matahina and Aniwhenua hydro lakes for limited but useful flood water storage.
The recommended works to reduce the risk of major flooding in floods up to 1% AEP (annual exceedance probability), i.e. in a 1 in 100 year event equivalent to the July 2004 flood, are as follows:
Environment Bay of Plenty responsibility:
- Improvements to Reids Floodway capacity by widening the lower narrow 4km section. Estimated cost is $5.5m.
- Add a control gate to the inlet spillway of Reids floodway to allow possible management of floodwaters between the Floodway and main river channel in emergency situations. That said, there is no intention to increase the frequency of flow down the spillway. Estimated cost $3.1m.
- Stopbank geotechnical investigations and repair/strengthening along the length of the protection works including Edgecumbe urban area. In progress and estimated cost $1.5m. The above wider Rangitaiki River Scheme works estimated to cost $10.1m are the responsibility of EBOP and will provide improved flood protection to both the Edgecumbe urban area and the Plains between Edgecumbe and Thornton. The following recommended urban area works estimated to cost $3.2m are largely a WDC responsibility.
New Zealand Transport Authority responsibility:
- Widen and upgrade the SH 2 bridge across the floodway to remove flow restrictions (NZTA responsibility).
Whakatane District Council responsibility:
- Edgecumbe South East – raised left floodway bank, deflector bank and emergency flood pump provision. This will benefit Hydro Road and Konini Place properties and the Transpower Substation. Estimated Cost $690k.
- Edgecumbe South West – low bund/bank to isolate urban area from rural floodwaters; floodgates to prevent Omeheu backflow floodwaters impacting town and a permanent flood pump. This will benefit the residential areas regularly flooded such as Totara and Kanuka Streets and will help prevent stormwater inflows to the sewage system. Estimated cost $818k.
- Edgecumbe North West – low bund/bank and ground contouring to isolate urban area from rural floodwaters; floodgate and flood pump provision. This will benefit urban area and sewage system. Estimated cost $891k.
- Edgecumbe North East – raised left bank floodway bank, new section of return stopbank and emergency pump provision. This will primarily benefit Fonterra and Eastpack. Estimated cost $809k.
A cost benefit analysis determined that all of the proposed flood mitigation works outlined above are economically beneficial.
The proposed works are required to ensure the current river scheme design standard (100 year flood event) can be met and that Edgecumbe and the Plains are protected from a July 2004 equivalent flood.
The location of infrastructure and the local topography seriously limits the practicality (in terms of costs and benefits) of any higher level of flood defences in the Scheme area at this point. However it needs to be reinforced that while flood defences can be used to manage and mitigate risk, there will always be a possibility of the defences being overwhelmed.
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Resource consent for the works has been lodged and is currently being processed.
Construction works that Whakatane District Council are responsible for, have commenced in 2010.
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