PRIME PUMP: When you need to over-pump an entire river

Gravel deposits swept into Nelson City’s largest river after flooding in August last year exposed the city to what was described by experts as an unreasonable flood risk.

As such, local environment infrastructure firm Nelmac was tasked to remove gravel build-up and instal a flood wall to reduce erosion at a natural turn in the river.

It was found that the safest, and least environmentally disruptive way to achieve this was to work on a dry riverbed. The Maitai River has a large catchment area and a reservoir prone to overflow. With a winter 2023 deadline, the work was time-critical.

Nelmac contracted Prime Pump, described as New Zealand’s largest dedicated civil pump supply business, to undertake the bypass of a 300-meter section of the river, leaving the exposed riverbed for the construction team to work in.

“With an estimated 750l/s flowing down the Maitai at any given time, that was no small task,” says George Everest, hire manager, Prime Pump.

Over-pumping helped to minimise the impact of heavy machinery on the river floor, and provided a greater opportunity to find and move river species while works were under way.

“We used BBA pumps, for their reliability and efficiency, deploying three BA180s and one BA300. To limit risk, the team used five discharge lines, each running the full 300m length of the bypass,” says Everest.

Prime Pump also had a crew on site 24/7 to monitor for water level fluctuations.

Based on the success of the initial bypass, Nelmac and Prime Pump over-pumped a second, 200m section of the river, using two additional BA150 pumps to handle the deeper water.