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Hydrology data from June floods

30 Jul 2018

Hydrology data from June floods

Wigan Bridge NR2

The flood event in Tolaga Bay over Queen’s Birthday weekend was recorded as a one in 5.7 year event, hydrology data has revealed.

Although the event resulted in significant damage, the rainfall was centered in a narrow band that mainly affected the Uawa and Pakarae catchments.

At Willowbank and Panikau, the highest intensity falls were 55mm and 60mm in one hour but these do not represent the overall storm size, with other gauges in the area recording much lower rainfall.

A one in 5.7 year event equates to an 18 percent chance that a flood of that size could happen every year.

In comparison, the water levels in the Waipaoa River at Te Karaka over 11-12 June were the second highest on record.

River levels in the Waihora, Mangatu and upper Waipaoa rivers were all exceedingly high and combined to form a very significant, one in 70-year event.

The preceding storm event contributed significantly to the size of the flood as catchments were already wet and rivers running high.

The river peaked above the 9 metre mark at Kanakanaia for 7 hours, compared to Cyclone Bola where it held above 9 metres for 40 hours.

Data shows the flood control scheme worked as designed with the water held within the scheme to prevent widespread flooding across the Poverty Bay flats. Rainfall intensity was moderate to high at a range of sites in the foothills of the Waioeka / Raukumara Range, the Waipaoa headwaters and in the Wharerata ranges.

A more detailed report on the hydrology data for both storms will be presented at a Council meeting on 16 August.


PHOTO: Flooding at Wigans Bridge in Tauwhareparae following June rainfall events