Waikawau/Hannahs Bay wetland rehabilitation

Waikawau/Hannahs Bay reserve sits on the eastern shore of Lake Rotorua, adjacent to the airport – a quietly significant place where upstream catchments drain through to one of the region’s few remaining urban wetlands.

When the extension of the Rotorua Airport runway resulted in the loss of 4.33 hectares of wetland, Rotorua Lakes Council (RLC) needed a considered, consent-compliant response. Proposed remediation works were planned between RLC, local iwi and the Eastside Community Collective as part of the Waikawau/Hannahs Bay Reserve Management Plan.

In mid-2020, RLC engaged Pattle Delamore Partners (PDP) to undertake investigation and design services for the restoration. As part of this project, we conducted geotechnical investigations to characterise the soils and groundwater across the project area. Findings of the investigations, along with topographical survey results provided by RLC, were then used to inform the concept, preliminary and detailed design.

The catchments feeding the wetland run through a 2.10 m diameter culvert under the runway, flowing into a central channel that runs to an overflow weir roughly 100 m from the lake shore – the structure that primarily controls water levels before flows discharge to Lake Rotorua.

The rehabilitation centred on lifting the central channel weir and adding notches to allow specified residual flows while diverting as much water as practical to the areas identified for rehabilitation. Diverted flows were directed to predefined rehabilitation zones through a combination of spill weirs, culverts, ground reshaping, earth bunds, and wetland overflows.

Each outlet structure was designed to maintain function through the existing channel while maximising the water quality benefits. The result was the restoration of 5.90 ha of existing wetland and the creation of an additional 1.51 ha – meeting the requirements of Resource Consent 64785.Beyond design, we supported RLC through preparing cost estimates, technical specifications, and tender documentation in line with NZS 3910, and assisting with contractor procurement and construction supervision.

What we delivered:

• Geotechnical investigations to characterise site conditions and informdesign
• Analysis of flow diversion options to support wetland establishment andrehabilitation
• Concept, preliminary and detailed design of diversion structures – weirs,culverts, inlets and outlets – and associated earthworks, to create andenhance wetland areas
• Cost estimates, engineer’s estimate and schedule of quantities
• Technical specifications and construction drawings
• Tender documentation compliant with NZS 3910:2013
• Tendering support and construction supervision of physical works toestablish the wetland areas.