NGONGOTAHĀ FLOOD REVIEW PROJECT - DECEMBER 2019 UPDATE

17 Dec 2019

The Ngongotahā Flood Review project aims to reduce the risk and impact of future storm events in Ngongotahā. This flyer provides an update on some of the recent work we've completed, particularly in relation to upcoming stream maintenance works and the assessment of potential flood mitigation options. It's been a busy few months since our last update in September and we'll provide another update on progress in February next year.

STREAM BANK REPAIRS AND MAINTENANCE
In the past two months, preparations and planning for summer maintenance works have been finalised and development has continued on the longer-term maintenance plan. To minimise risk and avoid future damage, a number of large trees were removed in the upper catchment area.

A lot of work was completed last summer and, as we move into Summer 2019 / 2020 that will continue including activity near the A&P Showgrounds and further large tree removal, vegetation management, fencing, tree planting and fish habitat restoration.

Over summer, existing spillways (pathways that carry away surplus water) will be enhanced by contouring, weed control, and placing rockwork to strengthen inlets and outlets.

CIVIL DEFENCE AND READINESS PLANNING AND CONSENTS
We've completed the Emergency Evacuation Plan for the Ngongotahā catchment. A working group has been established for the Paradise Valley Community Response Plan and potential members have been identified for the working group to develop the Ngongotahā Village Community Response Plan.

A workshop will be held regarding the Ngongotahā Village Community Response Plan with the goal to complete the Community Response Plans for the Ngongotahā and Paradise Valley communities in 2020. We'll let people know the dates for these workshops once confirmed.

PLANNING AND CONSENTS
Scoping is still underway regarding the District Plan review with additional risk assessments required by BOPRC to satisfy Regional Policy Statement requirements. Over the coming months RLC and BOPRC will work together to develop and review current urban flood hazard catchment models with a view to using this to develop flood risk assessments for these catchments to, in turn, inform a review of the District Plan.

FLOOD MITIGATION OPTIONS
An initial ‘high-level list’ of options was assessed using a 12-point criteria, which considered features including hydrology, water quality, ecology, land ownership, cultural aspects and likely costs. Six options were selected in conjunction with the Community Reference Group in August.

We are seeking flood mitigation options that will:

  • Accommodate a possible 'one in one hundred' year event (1% AEP = a 1 percent Annual Excedence Probability, i.e. a 1 in 100 chance that a river will experience a flood of that size in any given year).
  • Maintain the Ngongotahā Stream's environmental and recreational values.
  • Allow for climate change.
  • Take an adaptive approach i.e. options that can be enhanced in the future

The three upper-catchment (Above State Highway 5) options are being worked through by Bay of Plenty Regional Council’s Rotorua-based Land Management team. These options involve conversations with landowners about creating detention bunds which hold and slow stormwater; protecting areas vulnerable to erosion; and exploring land-use changes.

Meanwhile, the lower-catchment (Below State Highway 5) options are being modelled and the three options are as follows:

  • Raising Western Road - This option looks at raising a section of Western Road either side of the intersection with Brake Road. The heightened road level would act as a flood defence.
  • Overland flow path - This option looks at creating a flow path to the south of the stream to convey water in times of flood, diverting it away from residential areas.
  • High flow bypasses - This option would enhance existing bypasses and look to create others at sections of the stream where water naturally ponds in times of high flow.
Modelling the stream & rainfall
Modelling uses data to simulate flood events, enabling engineers to see how a river or stream, and its flood defence systems, will be affected by various factors, including volumes of water, durations of rainfall and design elements. The model is repeated multiple times with different scenarios and each ‘run’ can take two or three days to complete.

Next Steps:
Further modelling is currently underway to find out which of the options (by themselves or combined) will be most effective in safeguarding the Ngongotahā community in the future.
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