Opinion: Flood protection key focus for Greater Wellington Regional Council

Penny Gaylor.

Weekly column by Kāpiti's Greater Wellington Regional Council representative Penny Gaylor.

As the year draws to a close the flooding around the Wellington region has highlighted the importance of Greater Wellington's flood protection focus.

And the value of mātauranga Māori, or knowledge held by mana whenua to manage our rivers, has come to the forefront with Te Ātiawa ki Whakarongotai placing a rāhui on the entire Waikanae awa system after last week's flooding.

Swimming and fishing in the river system and at the seashore are prohibited until the rāhui is lifted.

Iwi Pou Tiaki from Waikanae ki Uta ki Tai have implemented a rāhui – a traditional iwi practice that places a restorative restriction for access upon an area – be placed upon the entire Waikanae awa system and foreshore area, for at least the next five days, while the Waikanae water system is monitored for potential pollution, sediment and silt damage from the recent flash floods.

The Te Horo beach community and surrounding farming area had a massive impact.

I'm sure you've seen photos showing the spread of the waters.

GWRC had done maintenance work on the Mangaone Stream as recently as August, and got a contractor in there on Thursday to do some emergency clearance.

A public meeting is being organised by locals for early February so we can all consider what more we can do to prepare for the increasing rainfall that GWRC's climate change research has predicted for our west coast water catchment.

This week the Waitohu Stream mouth is being cut by GWRC.

Our flood protection teams have not only been busy on the Kāpiti Coast, but throughout our region.

Here's some interesting detail about what happened on our side of the Tararuas.

Waikanae catchment:

Waikanae River peaked at 255m3/s at around 1pm on Monday, December 6 - between a 20-10 per cent AEP flow (5-10 year ARI event) rainfall continued through to 4am Wednesday, December 8.

Waikanae Water Treatment plant rain gauge
72 hour rainfall (9am Sunday, December 5 to 9am Wednesday, December 8) – 183mm – 5-2 per cent AEP rainfall (20-50 year ARI event).

Ōtaki catchment

Ōtaki River peaked at 580m3/s at around 7pm on Tuesday, December 7 - less than a mean annual event.

Taungata rain gauge - 72 hour rainfall (9am Sunday, December 5 to 9am Wednesday, December 8) – 243mm – <20 per cent AEP rainfall (<5 year ARI event).

Waitohu catchment

Waitohu Stream peaked at 46m3/s at around 5.30pm on Tuesday, December 7 - approximately a mean annual event.

Here's a layperson's definition of what those numbers mean for us.

m3/s is cubic metres (1000litres equals one cubic metre) passing by a (gauging) point each second. Eg. 100m3/s is 100,000litres passing by a point per second.

AEP is another way of stating return period percentage for each watercourse, eg. for the Waikanae River this was a 5-10 year return period event.

By comparison, a 100 year return period event has a 1 per cent chance of occurring each year.

A mean annual event has a 100 per cent chance of occurring annually.

Overall, Waikanae and Porirua catchments had very high rainfall events (both 20-50 year return period events) which led to high flows in the discharging watercourses.

A key point with this event was the intensity of the rainfall and the resulting sharp peak flows in the streams and rivers.

NIWA advice is that this type of rainfall event is likely to become more common, leading to more flood events in the western rivers and streams.

Merry Christmas, and take care of one another this summer, especially at our beaches and rivers.