Project overview

In 2019, Tonkin + Taylor delivered a stormwater masterplan for the Rotorua urban area to protect property and infrastructure from flooding and facilitate new development.

This in turn led to design and construction of an upgrade to an existing stormwater detention dam (Linton Park Dam), new stormwater detention dams (Morey Street East Dam and Morey Street West Dam), as well as wetlands, culvert and channel upgrades. Construction of the works for the three High Potential Impact Classification flood detention dams was completed in 2024.

Tonkin + Taylor is continuing to support Rotorua Lakes Council with ongoing dam safety management services.

Rotorua urban stormwater catchments

Rotorua urban stormwater catchments

Services

  • Flood modelling and catchment planning.
  • Options development and concept design.
  • Geotechnical and site investigations.
  • Environmental and ecological assessment.
  • Potential Impact Classification (PIC) and dam break modelling.
  • Resource Consent and Building Consent, including under emergency provisions and fast-tracking for Linton Park Dam.
  • Detailed design of dams and other flood infrastructure.
  • Construction monitoring and supervision of dams and other flood infrastructure.
  • Dam safety management services, including preparation of a Dam Safety Management System (DSMS), Dam Safety Assurance Programme (DSAP), training, provision of Damtec, interpretation and evaluation of surveillance, and other support for implementation and compliance of their DSMS and DSAP.
  • Certification by Recognised Engineers under the Building (Dam Safety) Regulations 2022.

Client

  • Rotorua Lakes Council

Year

  • 2019 – 2025

Linton Park Dam Upgrade

This High PIC, earth dam was originally constructed by others for flood detention in 1998. The 2021-2024 remedial works designed by Tonkin + Taylor addressed several dam safety deficiencies, including an urgent lack of spillway capacity. These works included dam raising, addition of a concrete crest wall, replacement of the concrete primary spillway, and addition of a new auxiliary spillway. The design addressed complex ground conditions, including liquefiable foundations in some locations, and soft compressible foundations in other locations. The works were progressed under a fast-tracked staged Building Consent process and via emergency provisions in the Resource Management Act.

Drone view of Linton Park Dam

Drone view of Linton Park Dam


Linton Park Dam-Ground improvements-driven timber piles

Linton Park Dam: Ground improvements – driven timber piles

Morey Street East Dam

The Morey Street East Dam is a High PIC, 17m high flood detention dam with a crest-full storage volume of 151,000 m3. The dam is a zoned earthfill embankment with a chimney drain and downstream horizontal blanket drain. The primary spillway has a low-level 0.3 m diameter inlet, and high-level 1.05 m diameter manhole riser inlet, discharging a 4 m diameter manhole drop structure, then to a 1.8 m diameter culvert. The auxiliary spillway comprises a concrete weir block discharging to a grouted riprap lined open channel cut into natural ground around the right abutment. Construction was completed between 2022 and 2024. Tonkin + Taylor’s detailed design addressed challenging volcanic soil foundations, including pumice, ash and liquefiable sands at both Morey Street Dams.

Drone view of Morey Street West Dam

Drone view of Morey Street West Dam


Morey St East

Morey St East Dam primary spillway construction

Morey Street West Dam

The Morey Street West Dam is a High PIC, 10m high flood detention dam with a crest-full storage volume of 106,000 m3. The dam is a zoned earthfill embankment with a chimney drain and downstream horizontal blanket drain. The primary spillway has a low-level 0.4 m diameter inlet, and high-level 1.05 m diameter manhole riser inlet, discharging to a 4 m diameter manhole drop structure, then to a 2 m diameter culvert. The auxiliary spillway comprises a concrete ogee crest discharging to a concrete lined open channel cut into natural ground around the right abutment. Construction was completed between 2022 and 2024. Tonkin + Taylor’s detailed design addressed challenging volcanic soil foundations, including pumice, ash and liquefiable sands at both Morey Street Dams. Other challenges for Morey Street West Dam included relocation of a wastewater line from under the dam footprint, which required specific seepage control details for future dam safety resilience.

Drone view of Morey Street West Dam

Drone view of Morey Street West Dam


Morey Street West Dam: Embankment and chimney drain construction

Morey Street West Dam: Embankment and chimney drain construction

Engineering Challenges + Innovation

Complex Geotechnical Conditions: Both Linton Park and Morey Street projects required extensive geotechnical investigations due to complex foundations and seismic risks.

  • Morey Street dams: As noted above, the design had to accommodate challenging volcanic soil foundations, including pumice, ash and liquefiable sands. Local borrow materials for fill were also volcanic and required careful specification. Over 100,000 m³ of earthworks were involved, including cut-to-waste and engineered dam fill sourced from on-site borrow areas, which had to meet strict specifications for stability and permeability.

 

  • Linton Park dam: Preload, delayed construction of the concrete crest wall following settlement, flexible joints between sections of the crest wall, and defensive drainage and seepage control measures were used to manage risks associated with soft foundations. Geogrid basal reinforcement of embankments and driven timber piles were used to manage deformation under seismic conditions, including potential for liquefaction. The foundation challenges also required detailed analysis of seismic hazard, seismic stability and deformation considering liquefaction and cyclic softening, using FLAC, settle3D and Plaxis.

The Linton Park dam upgrade also faced challenges regarding cost and availability of borrow materials and limited space for embankment raising due to proximity of private property, utilities including a power pylon, and natural wetlands. Solutions involved offsite borrow, adoption of the crest wall and a complex 3D geometric design to minimise material and space requirements.

Fast-tracked Consenting and Emergency Works: The original Linton Park dam did not meet modern dam safety standards, necessitating urgent remedial works under emergency provisions of the Resource Management Act and a fast-tracked building consent working closely with the regulatory authority and their regulatory reviewers.

Hydraulic Complexity: The remedial design had to accommodate extreme rainfall scenarios and climate change projections, complex 3D hydraulic behavior, and areas already constrained and prone to flooding. The PIC assessment had to utilize innovative GIS-based approaches to identify the incremental consequences of a hypothetical dam failure on the 2000-3000 buildings affected by flooding in both the natural and dam break flood scenarios.

Concurrent Construction Strategy: For Morey Street, two dams and a wetland were built simultaneously, requiring advanced resource scheduling and logistics to optimize efficiency and minimize disruption.

Integrated Wetland System: The inclusion of a stormwater treatment wetland alongside the Morey Street detention dams is an innovative approach to improve water quality while managing flood risk. This dual-purpose design supports ecological restoration and enhances natural character.

Community + Environmental Impact

Cultural and Ecological Considerations: Mana whenua was involved throughout the process, including site blessings and input into resource consent documentation. Ecologists monitored construction to relocate native species and mitigate ecological impacts, with restoration planting planned post-construction.

Climate Resilience and Housing Enablement: These projects are part of Rotorua’s Stormwater Master Plan, designed to future-proof infrastructure against climate change and enable housing development in growth areas. The Crown contributed $15 million to support upgrades, highlighting the strategic importance of these works.

Regulatory and Safety Framework: Ongoing compliance includes five-yearly dam safety reviews under national regulations, ensuring continued protection for downstream communities.

For more details on these projects, see Rotorua Lakes Council for Linton Park East and Morey Street

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