Flood highlights need for resource consent for Levin stormwater

Stormater drains were at capacity WGP 21Aug19 - The drain at the end of Fairfield Road was at capacity. WGP 21Aug19 -

Heavy flooding in Horowhenua last weekend has highlighted the need for resource consent to be granted to Horowhenua District Council from regional council Horizons.

Stormwater infrastructure was tested on Saturday when heavy rain caused flooding throughout Levin and Ōhau, but a simple fix could provide future relief for one area of town.

Horowhenua District Council's Acting Group Manager Infrastructure Services Kevin Peel said an upgrade to the North-east Levin stormwater was carried out in 2017, which included two new pumps being installed for Kennedy Drive and Okarito Avenue.

But while the new infrastructure has provided some relief for residents, the full extent of the improvements would not be obvious until the resource consent is granted and the penstock was removed, he said.

A penstock was wooden "gate" placed over a drain at the corner of Fairfield and Roslyn Road.

"Council first started the resource consent process in 2015 and we're hopeful it will be granted in the near future," he said.

Peel said the new infrastructure was built to the recommended Annual Exceedance Probability for residential areas, which was for a one in 10-year flooding event.

Significantly larger 1350mm pipes were installed in Fairfield Road and a new outfall from the stormwater main was laid. The outlet currently has a penstock installed to restrict flow until resource consent was granted.

"The risk and frequency of flooding has significantly reduced in the area as a result of the upgrade. However, you can expect that at times of heavy rain, like what we experienced on Saturday, there will be some flooding," he said.

The upgrade was built to handle at least 103.9mm of rain in a 24-hour period, in line with NIWA's recommendations for a 10-year event.

While last weekend's rainfall was less than that, for a two-hour period it was significantly more concentrated.

"During a two-hour period on Saturday, just under 39mm of rain was recorded at a Levin catchment. This is more than the infrastructure was built to cope with," he said.

Prior to the upgrade stormwater was discharged to the Koputaroa Stream tributary through a 375mm diameter pipe and culvert at the intersection of Fairfield and Roslyn Roads.

The peak discharge from that culvert was estimated at 200 litres per second, however the 375mm pipe was undersized and additional runoff entered the tributary as a result of overland flow.

The culvert was subsequently replaced with a 1350mm diameter culvert, increasing capacity to approximately 3.800 litres per second, reducing occurrences of overland flow stormwater runoff entering the tributary.

Peel said council started the resource consent process in 2015 and was hopeful it will be granted in the near future, given the large amount of work required.

In 2015, Engineers looked at possible options for managing the stormwater and realised that the flows would need to go down the tributary that feeds into Koputaroa stream, requiring resource consent.

HDC liaised with affected landowners and a number of stakeholders, and preliminary discussions were held with Horizons.

Designs based on various storm events were considered and models created, agreement with key landowners sought, an Ecological Assessment and an Assessment of Environmental Effects were undertaken, some additional land was purchased.

The resource consent was lodged in 2017. In 2018 Horizons lodged a Section 92 seeking further information on how it proposed to mitigate a range of issues, which had since been provided.

Horowhenua District Council is continuing discussions with iwi partners and downstream landowners.