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Golden Bay New Zealand- Takaka

Paynes Ford Station

 
  
       
 
 
 
 

 

Flooding Risk in Paynes Ford and Golden Bay

There have been several flooding events in the area since the weather station started operation in 2007. None of them as bad as the historical big flood of July 1983 with knee deep water running through the house. Still, in November 2008 and March 2016 water was flowing under the house through the ventilation grills.

Flooding in Paynes Ford and the Bay in general is a regular occurrence, but even then the rain can be very localised and affect flooding and slip patterns. For example the all time record rainfall of December 2011, resulting in devastation to roads and houses all over Golden Bay did not cause a major flooding event in Paynes Ford due to the fact that the heavy rain for most part didn't fall in the catchment areas of the Takaka or the Waingaro Rivers.

The confluence of the Takaka River and the Waingaro at Paynes Ford can lead to exciting stuff when enough rain falls in the Kahurangi National Park, more so when it is high tide. At that point the Waingaro tends to breach the somewhat elevated State Highway 60 at about 300 meters after the Paynes Ford Bridge, coming from Takaka (see arrow). Water will then further flood the paddocks at the Takaka River side (these will be taking in water from the Takaka River as well at that stage) and back up against the Paynes Ford cliffs and push back towards the downstream side of the house.

SH60

The Waingaro is shorter than the Takaka River and therefore tends to peak earlier at Paynes Ford. Another factor in water flow of the Takaka River is the Cobb Hydro Dam, operated by Trustpower. The Cobb Dam can provide an initial buffer until it fills up to its maximum retention level, but after that an automated process will open the spill gates to prevent the Dam from overflowing which could potentially damage the very structure of the dam.

The Cobb Dam issue:

It has been questioned in the community how much the Cobb Dam affects flooding events. One thing to remember is that the water flow from the Cobb Lake only partially makes up the volume that eventually ends up at Paynes Ford which at first instance is measured at Harwoods. During the big flood of 1983 the Dam "spilled" 188 m3/sec, the flow at Harwoods at that time was 689 m3/sec. Furthermore estimates of how long it takes the water to reach Paynes Ford from when it comes out of the spill gates are varying and also dependent on the flow volumes at the time. This could take 3-5 hours or even more. Apart from the fact that the dam spilling is an automated process, all these factors make it difficult to use the Cobb Dam as a flood prevention structure, not to mention the fact that this has nothing to do with the actual purpose of the dam in the first place. Having said all this, I would still like to think that some guidelines could be developed for the Cobb Dam to spill earlier, based on extreme heavy rain forecasts or early in heavy rain events which are forecasted for extended periods (say more than 36 hours continuously). Lower lake levels early on may just make the difference between "just" paddocks being completely flooded or a true emergency at the end of a 50 year rain event.

Most seasoned people of the Bay are well aware of the flooding risks in their area and have their own reference points to monitor the situation. As flooding is a real re-occurring risk in Paynes Ford, I'd like to share mine here. I normally start monitoring these more closely after 24 hours of heavy rain and in combination with further rain forecasts of course.

Tide Time Table:

see Golden Bay weekly latest edition, tide watch timetables, somewhere near the back of the paper. Alternatively search online for Port Tarakohe tide timetables.

The Takaka River flow:

see also the Takaka River at Harwoods flow and ultimately, after the Takaka River and the Waingaro come together, the Takaka River at Kotinga flow. Generally when the Kotinga flow reaches 650 m3/sec, water will start flowing into the paddocks at Paynes Ford and when it reaches more than 1100 m3/secs we can expect the Waingaro to start overflowing SH 60.

The Waingaro flow: see Waingaro at Hanging Rock

As a rule of thumb things are starting to get tricky at about the halfway point of the 1983 extreme levels and with never all the circumstances the same, our household is normally on full alert by then.

The Cobb Lake level: see Cobb Lake Level

The Cobb Dam starts spilling at a progressive rate after the level reaches 807.70 meters above sea level. At 808.25 meters water will overflow the spillway gates into the culvert chute and at 812.29 meters we would experience a dam breach and a truly serious problem. At 808 meters the spill will be about 30 m3/secs and at the 812 meters level the dam will spill >1250 m3/secs. As a reference point, in July 1983 the maximum Cobb Lake level was about 808.95 meters with a spill of 188 m3/sec. It takes the spill water about several hours at least to reach Paynes Ford.

The 1983 flood was a one in 50 year event. Most likely period for flooding starts in July and ends in December, but that's all just statistics. The upside of the Bay in terms of flooding is that the flood water generally quickly recedes. It can go from a truly alarming situation (the incredible noise of two rivers in flood swooshing past either side of the house is quite something!) to "did that actually happen just now?" in a timespan of only a few hours.

Eventually there will be a big flood again. Best not to forget what happened in 1983 and be somewhat prepared. A good read with many pictures is "The Wrath of the Waters" by Arnold Clark about the 1983 flood. The back cover shows a picture with the current location of the Paynes Ford weather station in the background. The foreground shows the Paynes Ford Bridge with a section of the bridge washed away and a power pole leaning precariously against what is left of the bridge....

"Mental note to self-let's try and get over the bridge well before that happens next time."

Example of flood 24 March 2016: GoToVideo

 

©2017 -  René van Sint Annaland

 

 

 
         
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