Overpumping the Maitai River

Gravel deposits swept into Nelson City’s largest river after flooding in August 2022, exposed the city to unreasonable flood risk. The deposits in the Maitai (Mahitahi) river raised water levels and inundated nearby land, leaving the same areas prone to flooding in future severe weather events.

Local environment infrastructure firm Nelmac developed a plan to address the issue while being mindful of the sensitive location.

Their plan included:
- Removing gravel build-up
- Slight adjustment to the course of the river
- Installing a flood wall reduce erosion at a natural turn in the river.

The safest, and least environmentally disruptive way to achieve this was to work on a dry riverbed.

That's when Nelmac contacted Prime Pump.

Stage 1 of the Maitai River bypass involved over-pumping the river 300m. Five pump lines were used to manage flow and to ensure continuity.

Working against the clock with high flows.

The Maitai River has a large catchment area and a reservoir prone to overflow. With a winter 2023 deadline, the work was time-critical.

First, Nelmac created a temporary dam to guide the river to the side which the pumps were set up. Prime Pump then began over-pumping 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.

Over-pumping in this way, 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. Nelmac’s environmental team and local iwi were on site to transfer acquatic life and keep an eye out for taonga.

Prime Pump used BBA pumps, choosing them for their reliability and efficiency. The three BA180s and one BA300 moved 750 litres per second at peak flow. To limit risk, the team used five discharge lines, each running the full 300m length of the bypass.

The Prime Pump team remained on site 24/7 to maintain optimal flow and to monitor for water level fluctuations.

In four days, Nelmac had adjusted the river course, installed the protective flood wall and removed the gravel deposited during the earlier flooding.

Following the success of this work, Nelmac opted to over-pump a second, 200m section of the river, adding two BA150 pumps to the fleet to manage this deeper section of the Maitai.

 NELMAC contractors removing gravel deposits and installing rock retaining to reduce riverbank erosion.

*Formerly known as Prime Pump, now Prime Fluid Management