Is Motueka town really below sea level?

July 27th, 2012
[by David Armstrong]

Motueka is actually below sea level, right? That's often said to be a fact by people who have lived here for a long time and told to newcomers. But it's an urban myth - a story thast becomes true because so many people pass it on.

If the central part of town was below sea level, then the only way large amounts of stormwater could escape to sea would be either by pumping or by waiting until low tide and letting it out. But we have no pumping stations, only floodgates, and stormwater eventually clears no matter what the tides are like. And water only flows downhill.

And we must remember that the sea level out in Tasman Bay is much the same (though with slight delays) as it is in the Moutere Inlet, and therefore in the Woodlands Canal and up into Thorps Bush. Actually, due to some funnelling effect, the high tide level in the Woodland Canal is probably a little higher than in the Inlet, though the overgrown vegetation in the canal slows any rapid buildup up the canal.

So if the town was lower than high tide in the bay, then the water should rush up the Woodland Canal and swamp Thorps Bush and New World twice a day!

Seeking to establish the truth, Motueka Online examined the topographical maps of the town, held by council and available at TDC's Motueka Service Centre within the "Explore Tasman" computer mapping system.

The figures given there are for mean (average) sea level, half-way between high and low tide. Normally high tide is up to about 1.5 metres higher than these figures, and the largest king tide would be no more than 2.2m higher.

To give some examples then, Taylor Avenue (behind the council service centre) is 3.0m above mean sea level, which is also higher (though by not a large margin) than the highest high tide we could get. And Bowers Lane, behind Hotel Motueka, is 4.5m above mean sea level.

Things get a bit more interesting at the southern end of High Street. The entrance roundabout is officially at 2.5 metres, so it could become wet at a very large high tide. And places between High Street and the Moutere Inlet north of Wharf Road are, of course, lower still and need the protection of the stopbank there.

And of course the groundwater flooding problems during wet events over lower lying land east of Thorp Street at the Staples Street end are well known, especially to those living in that area.

So it's official - most of the Motueka township is definitely nowhere near below high-tide sea level. But we shouldn't be complacent. A global sea level rise due to climate change of around 0.5m will begin to make king tides a real concern.

Comment by Peter Garlick:
[Posted 8 July 2012]

I have never heard comments about Motueka being below sea level, but I've often been involved in discussions about the low lying nature of the town. I would have thought that the most immediate natural hazards to the town are flooding from the Motueka River or flooding from storm surge.

As you point out, heavy rain on the town shouldn't cause flooding if the drains are clear and the floodgates are operating correctly. I looked at the TDC website to see if a natural hazards report for Motueka was available. I found nothing. I did find the following general information.

"Coastal erosion and inundation hazard risks increase during periods of extreme high tide, strong onshore wind and storm surge. These hazards may be locally generated within Tasman and Golden Bay, or may result from more distant events such as cyclonic or tsunami events propagating larger waves into the bays, as occurred for example during Cyclone Drena in 1997. The present day extent and probability of various coastal hazard risks are both expected to increase as a result of climate change projections of increased wind frequency and intensity and sea level rise."

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