Opunake has a problem – we are going to have more floods. They will be greater than any we have had before. The rain is going to get worse, not better. This is not my opinion, but that of climate scientists. Climate change has come to Opunake.

The picture shows the catchment area which produced the Opunake floods on 6 August 2015. Those who attended the mayor’s meeting about the floods will recognise the picture.

How much water?
It is impossible to predict how much water we could face, Scientists have models of rainfall, and they will make estimates using different assumptions. However, they may be wrong. It is amazing how often exceptional events occur.
Meteorologists say there was nothing special about the weather pattern on 6 August. It was rain as usual, but a bit heavier for a short time.  South Taranaki District Council data indicates that the total rainfall and duration (245 mm in 72 hours) equated to a return period of about 1 in 80 years, with the highest intensities estimated at up to 40 mm/hr.
What would have happened if the very heavy rain had lasted one or two hours longer? That might double the amount of water! If you live in Opunake expect your carpet and furniture to be ruined within about four years.

History
We can compare our recent flood to events in the past. From 1800 to the present, Opunake has had 10 significant rainfall flood events, according to NIWA. The earliest recorded “heavy rain” was in February 1883. It was throughout, Taranaki, Waikato, Auckland and Northland. Two people drowned.

1893
In February 1893 the Waiaua Bridge at Opunake was wrecked and the river impassable – all Opunake houses were flooded except for “one or two”.
In March, the Heimama Bridge was washed away, likewise for the Mongohotu stream. This left a gap in the South Road about 9.14 meters wide. The road between Rahotu and Pungarehu was 61 centimetres under water and the gravel washed away.

1924 & 1925
In March 1924, many Opunake river banks were altered in rain events. The next year the big one hit: flooding all over our district, Ihaia Road was washed out, Te Namu Stream recorded its highest ever level.
The Waiaua River brought a broad torrent of dirty water into Opunake – culverts were overcome and the whole town flooded with significant damage to houses, furniture, gardens and roads.

1936
In February 1936 pigs drowned in South Taranaki! Opunake was flooded by the Hihiwera Stream. Opunake was completely isolated, when both road and rail approaches washed out.
The Waiaua River wrecked the rail bridge. The hydroelectric scheme was devastated – the flood filled the head race and lake entry with thousands of tonnes of silt and rocks. Te Namu Stream at the Opunake factory rushed across the flat. Again, there was significant property damage.
 
1956
Flooding in July 1956 at Opunake – this was a bit different from many of the earlier events, because it involved just the lower North Island. A depression to the north of us caused the flooding and cold temperatures.

1976
 In February 1976 a Civil Defence Emergency was called in Opunake where we recorded 9.5 cm of rain in 24 hours.  Opunake recorded 12.7 cm of rain over 48 hours. Patiki Road recorded 16.6 cm of rain in 24 hours.

1998
You may well remember this event which was over three-days in October and impacted on much of the North Island, but Opunake was the worst hit. Many homes damaged.

2008
There were minor floods in Opunake in July 2007, before a major event in April 2008. Massive rainfall blocked the intake to the Opunake water treatment plant. The storm caused the highest river flows ever recorded in the area between Opunake and Oakura. Flood waters from the Stony River severely damaged houses near Okato. Destroyed were the Dover Road bridge, two Katikara Stream bridges, and a Mangatete Stream footbridge. In an old dairy factory pigs swam.

Future
Scientists predict more frequent and heavier rain for Taranaki. I would suggest we consider 6 August as a warning.
Council cannot rebuild Dieffenbach Street because they do not know the capacity required in the storm water system. Already we have seen a new water detention pond built and overcome.
We have no hydrological and engineering reports at all regarding new works to protect the lower end of Gisborne Terrace, Ponderosa Place and Ihaia Road. I walked round there in the rain (before the fire-fighters arrived) and saw the depth of the water (just below the floorboards in many cases).

Council plans
I bet you have not read the STDCs “2015-2025 Long Term Plan”. The name itself is awful. It should be called “What we intend to do for you”. Or perhaps they should call it “How we will spend your money”.
The Councillors voted for the Plan on 29 June this year. They did so after issuing a draft and hearing submissions. The plan is without a provision for flood mitigation in Opunake, because it had not become an issue.

Urgent action
STDC officers have decided on urgent action. This is fantastic news! They have commissioned research to assess our problem and develop options. The money for this is coming from the storm-water budget. After this first report, you can expect more research as the selected option is developed with costs and a timeframe.

Short-term
Council’s consultants will to do preliminary flood and hydraulic modelling of the township. They will report in October. This modelling will enable Council to establish appropriate designs for pipe sizes and detention and (most important) give them the ability to investigate long-term options. 
Council has begun a review of the Detention Pond looking specifically at improving the design of the screen and secondary flow path.

Long-term
Council’s current work must be seen as but one step towards an integrated, comprehensive scheme to protect the whole of Opunake and manage the catchment. This is not a job to do piecemeal. Do it once, do it right.
            The major options involve either the diversion of catchment run-off from the town or major improvements to drainage systems and waterways within the town.
I believe we must divert the water in the catchment away from the town. To advance the diversion option, Council is already talking to the Taranaki Regional Council about new channels. But there is a rub: history suggests the rivers may not cope!  Once the research is complete the capital expenditure will have to be included in next year’s Annual Plan.

Private land
We can expect necessary work to be on private land. Most landowners will cooperate. Nevertheless, Council should ensure it has the power to do what needs to be done. Council may have to buy land.

Who pays?
The question of who pays relates somewhat to how much money will be needed. The district as a whole should help Opunake. Opunake should also help itself.
That means (say) half the money for capital works should come from general rates. The other half should come from a special rate targeted at Opunake and the catchment area. Fortunately, Council’s debt level is not too bad.
Our regional council should also contribute. Thus, the financial strength of the region can be used to help Opunake. I was told by a council officer that the regional council’s chief executive is open to suggestions. When it comes to local government funding, it is a good idea to spread the pain.

Robert Shaw
robert@porirua.net