Thursday Update #26

Christmas in the Vines

Vine Aid Christmas in the Vines 18th December 2011
2pm – 8pm

Vine Aid Poster (940kb pdf) (Please download and print)
Following on from our inaugural Vine Aid earlier on in the year we are going to hold a regular event in December each year, Vine Aid Christmas in the Vines!
Live Music & Entertainment on the Riverway Homes Stage, BBQ, Spit Roast, Chicken Racing, Christmas Bring and Buy Sale, Father Christmas, Bouncy Castle, Candy  Floss, Pop Corn & everything Christmas! A none profit charity event organised by the Vine Aid Charitable Trust with all proceeds being split between three local community  groups.
1.      Tongariro  walking/cycle track.
2.      Turangi Playground.
3.      Coastguard Turangi.
Entry is a gold coin donation.

We are grateful to Bob and Julie Neville for their support for the Tongariro River Trail project.

A Festive Hello from River Vineyard and Restaurant Turangi!

Visit to the Rangipo Dam

Committee members, Virginia Church, John Wheeler, Eric Wilson and Neville Young visited the Rangipo Dam with Genesis staff members, Bonny Lawrence, Environmental Manager, Neil Jelley, Hydrologist and Charlotte Fletcher, Environmentalist on Monday 5 December 2011.

We had a frank, informative discussion.

We visited because we have raised the issue of severe sedimentation in the river downstream of the State Highway 1 bridge (SH1). We asked if the cause was the discharge of sediment from the Rangipo Dam and Genesis was keen to show us the workings of the Rangipo Dam and make the point that the operation of the Dam is not responsible for the sedimentation we have seen.

We accept the following factors:

  • the total reservoir is small in size and not able to hold the levels of sediment that we have seen in the lower river after flushing
  • the area where sediment is trapped is limited to an area smaller than a rugby field
  • that the reservoir fills quickly and in flood overflows taking sediment with it
  • that the sediment in the settling area is flushed only in times of a flood of more than 100 cumecs (but only on selected floods.)

So where has the sediment come from?

The explanation given is

  • Unconsolidated Volcanic Ash on the slopes of Mt Ruapehu can clearly be seen along the Rangipo Desert
  • In January 2011 cyclone Norma dumped torrential rainfall in this usually low rainfall area moving a huge amount of sediment in the Whangaehu and Tongariro catchments.
  • That the sediment is carried in pulses in times of flood
  • That it will take some time to move this sediment through the river
  • That the river slows just below the SH1 bridge and dumps the sediment in the area we call the Braids and that only sand and silt is carried beyond that point

So what are the implications. We argue that

  • the river previously had a gravel bottom to the point of entry to the Lake and was deep enough to allow boats with a reasonable draft to go upstream as far as the Downs pool.
  • the river is unabe to carry the sediment into the lake as the energy of the river has been reduced (this has not been reduced in flood flows over 160 m3/s at Poutu Intake)
  • the problem is made worse by a lake level that is higher for longer than pre control gates (Not a Genesis issue).

The counter argument presented is

  • that from SH1 bridge downstream the area  is a delta which is a developing landform and as such grows from deposition of sediment which is what we are seeing and that if left to its own devices given the existing flow level and lake level the river will open a new mouth to the lake. Currently this looks like it will flow through Deep Creek and enter the lake alongside the Tokaanu tailrace.
  • that if there was no Hydro scheme on the river then this would still happen.
  • As such it is a community issue as to what should happen in this section of the river. Let the river behave as it wants to or intervene.

In conclusion

I can’t blame Genesis for the Mt Ruapehu eruption nor for cyclone Norma. I do accept that most sediment is carried in times of flood. I also accept that the Rangipo Dam has an insignificant impact on the deposition of sediment that I have seen in the river.

Genesis have offered to bring Graeme Smart to Turangi for a public meeting to explain the processes that are happening on the river and to discuss the implications and consider a desired community outcome.

Report Protecting NZ’s Rivers released.

Read it online:

The report sets out the issues facing our rivers. It is well worth a read.

Protecting New Zealand's Rivers publication cover.

Tongariro at Turangi – last 3  months River-flow

Tongariro at Turangi – River-flow



Eric Wilson