Whanganui letters: Whanganui must prepare for unprecendented flood

Whanganui should prepare for a bigger flood than the one in 2015, writes John Archer. Photo / Giles Conley

More than a day before Cyclone Gabrielle struck Hawke’s Bay, data from Europe’s ECMWF (European Centre for Medium-Range Weather Forecasts) weather computer was used by Windy.com to correctly predict at least 55mm of rain an hour on Tuesday morning in the Bay’s 5000 square kilometres of high country.

The combined flow down Hawke’s Bay’s rivers must have totalled around 10,000 cubic metres per second (cumec) at the peak of the flooding - about 11,000 tonnes of mud-filled water every second.

The high-country catchment of the Whanganui River, north of Pipiriki, is also about 5000 square kilometres.

If a future cyclone follows a more western track than Gabrielle’s, Whanganui must also prepare for an unprecedented 10,000-cumec flood.

Whanganui’s “Great 2015 Flood” was only about 5000 cumecs and 9 metres deep at the Town Bridge. A 10,000 cumec flood squeezing through the 500-metre gap between Durie Hill and Queen’s Park hill would be several metres higher.

What buildings will be flooded by such a cyclone? Perhaps some of our clever Year 13 students could use ECMWF data to make an app demonstrating what areas of Taumarunui, Upokongaro, Aramoho, Whanganui East, Dublin Street, the CBD, Gonville, Pūtiki and Castlecliff would be inundated.

In the Whanganui headwaters, there are thousands of hectares of steep and highly erodible grasslands. With about 2000 tonnes of silt arriving at Whanganui every second, how much would be dumped on to its riverbed, streets, shop floors and house carpets? How many bridges would survive?

A big La Niña occurs every five years or so; and, as seas to the north of us become hotter, I would expect future Whanganui floods to become even higher.

I congratulate the Chronicle’s staff for your foresight in moving your offices to much higher ground. It would be prudent for any long-term municipal, housing or business investment to follow your example.

JOHN ARCHER

Ōhakune

Cyclists frustrated with pathway etiquette

I totally agree with the letter by Russell Eades (March 9). Many people do not seem to understand “keep left”. It is most frustrating for cyclists.

With the increasing amount of electric bikes now on the road, the tracks around the city are much more used than before.

More signs would help, but they really need a centre line. I find that dog walkers can be a problem as you need to be aware of the people and their dogs.

A lot of bell-ringing is needed.

KEVIN O’SULLIVAN

Whanganui

Aramoho footpath not a shared pathway

Russell Eades is right about shared pathways; however, some cyclists need to be aware that the footpath on Somme Pde is not one of these - no signage and certainly no room.

Between the railway bridge and the graveyard there is only a footpath on one side of the road and it is exactly that - a footpath. Unfortunately, many cyclists returning on the cycleway from Papaiti are not alerted to the fact that past the graveyard is not a shared pathway but a footpath, and a narrow, uneven one at that.

Spare a thought for those elderly walkers who do not hear cyclists, those young folk with prams, and dog walkers of all ages who may have a cycle-chasing pooch.

M DONNE-LEE

Aramoho