Otiria Flood Mitigation.

We always get a massive buzz from doing work that we know improves the lives of the communities in which we ourselves live, work and play. One such project has to be that commissioned by Northland Regional Council to reduce the risk of Otiria and Moerewa flooding.

The Otiria and Waiharakeke streams are part of the Kawakawa River system. The streams have always spilled after heavy rains over wide floodplains between Otiria and Kawakawa. But for decades roads, bridges and an embankment built for a now-defunct railway line have combined to create havoc with this natural cycle – channelling floodwaters into residential properties, marae and a community rugby club in Otiria and Moerewa.

During big floods about 80 percent of the water from the Otiria Stream spilled over nearby land, as did 70 percent from the Waiharakeke Stream. The increasing frequency and severity of these events had become intolerable for these communities, so extensive stormwater management and flood mitigation work was commissioned by Northland Regional Council.

Haigh Workman was engaged by the Council to conduct a feasibility study into whether it was possible to protect these communities from floods by restoring the streams’ natural flows and, if so, how this might be achieved.

Having delivered the good news that, yes, protection was technically possible, we were then tasked with developing the preliminary design, the geotechnical study and reporting, and the detailed infrastructure engineering design resulting from these findings.

We developed a three-stage plan that diverts floodwater from the Otiria stream via a spillway beneath two of the ‘problem’ roads and into the neighbouring Waiharakeke stream – away from some of the most flood-prone housing in Otiria and Moerewa. The Waiharakeke stream is then contained in its channel by a series of overflow channels and stop-banks.

Once the feasibility, planning and design stages were completed we were briefed by our client, the Council, to provide the engineering support for the consenting process. We also provided contaminated soils investigation and remediation, project pricing for funding and contract documentation.

Then, with the project consented and given the go-ahead by our client, we undertook all elements of construction management, including roading and traffic management.

This was an extremely rewarding project that has made a huge difference to the quality of life of some of the most flood-prone communities in our district. We were very grateful to have been asked to help and delighted to have been able to do so.