Torrential totals

DECEMBER DROP-OUT: The Waikura Valley bridge paid the price for a whopping 263mm rainfall on December 15 last year. Picture supplied

Parts of the Gisborne-East Cape district have had more rain in the past seven months than they normally get in a year.

The excess rain, and the erodible, sedimentary geology of the region explain why so many roads have been damaged, particularly in the top half of the district.

On Wednesday morning of this week, eight district roads were still closed, 16 were open to four-wheel-drive only, and there were eight Waka Kotahi NZTA notifications for highways in the region.

Since November last year there have been six to eight heavy rain events, depending on location.

The East Cape part of the region copped the most and heaviest falls by far, with a one-day downpour of 230 millimetres at Te Puia on March 23 — the heaviest fall in 24 hours since the beginning of the year.

The biggest one-month rainfall total so far this year was 622mm at Te Puia in March — the biggest March rainfall there in five years — while the Waikura Valley was just behind with 604mm over July.

Te Puia's rain total to the end of July was 1589mm — well over its whole year average of 1447mm (averaged over a 10-year period).

The March and July events, added to those of earlier months, brought millions of dollars worth of new and additional roading damage to that already experienced earlier.

Highway closures in the Waioeka Gorge, around Ruatoria, and at the Motu River mouth once again showed the area's vulnerability to being cut off.

The rain saw the Motu River rise to over 5.5m — half a metre short of the all-time record set in 1964 — and undercut the road.

This happened at almost the same spot as the tragedy of August 5, 1900, when 16 children aged 5 to 13, and two adults, drowned when their canoe overturned in the flood-swollen river.

On that day the children, from Maraenui on the north side of the river, were being taken to the school at Omaio on the other side.

Since November last year there have been bridge washouts, and the coast has been cut off, but fortunately no human casualties.

Ruatoria to the north has had six days since November last year when the 24-hour total has been 100mm or more, and again its year-to-date total is almost equivalent to its whole year average.

The Waikura Valley is well-known as a rain catcher, its biggest one-day fall since last November being a whopping 263mm on December 15.

Since then, the valley has had eight days with 24-hour falls of 100mm or more.

Its total rain for the year to the end of July is 2108mm — double its average total for a whole year.

The Waikura Valley takes the title for overall wettest in the district since November, with three days in a row — March 21 to March 23 — totalling a torrential 394mm.

Similar rainfall has been recorded all over East Cape and the top end of the Raukumara Ranges, with totals equivalent to a full year's worth having been recorded down to Tolaga Bay.

Lesser totals have been recorded further inland and in the southern part of the region.

Even so, Gisborne residents have had some 340mm more rain than at the same time last year.

The airport rain gauge recorded 866mm to the end of July - closing in on the 30-year full year's average of around 980mm.

With five months to go, one La Nina ended and another expected to begin, Gisborne could reach the record of 1432.3mm set in 1938.