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.

The Client: Nelmac

The Project: Keeping the river bed dry to enable flood mitigation work.

Problem:
Elevated water levels and increased flood risk during severe weather events.

Challenge:
Nelmac, the local environmental infrastructure firm, needed to address the gravel build-up and reduce flood risk while minimizing environmental impact. Their plan involved removing the gravel, adjusting the river’s course slightly, and installing a flood wall. This required working in a dry riverbed to avoid disrupting the river ecosystem.

Solution:
The Prime Pump team set up BBA vacuum-assisted self-priming pumps to manage a 300-meter section of the Maitai River. This operation involved pumping 750 liters per second to expose the riverbed for construction. Prime Pump deployed three BA180s and one BA300 pump, using five discharge lines to handle peak flows. The team worked around the clock to maintain optimal flow and monitor water levels.

Result:
Within four days, Nelmac successfully adjusted the river course, installed the flood wall, and removed the gravel. Following this, a second 200-meter section was over-pumped with additional BA150 pumps. The project minimised environmental disruption and enabled effective flood mitigation work, improving the city’s resilience to future flooding.

*Formerly known as Prime Pump, now Prime Fluid Management.