In 2011 RILEY provided a geotechnical feasibility study of two sites on the Nihotetea Stream, south-west of Whangarei, for a flood mitigation detention dam. The study, commissioned by Northland Regional Council, was followed by hydraulic optimisation and design reports to provide supporting information for a resource consent application for a proposed detention dam. Subsequent to obtaining resource consent, the brief was extended to providing detailed design, obtaining building consent, and managing contract documentation.
The design process encountered a number of significant engineering challenges due to the location of existing dwellings, property boundaries, and complex site geology. To enable the scheme to progress, four houses were purchased by Council and demolished in order to accommodate the left spillway. Other design challenges included the relocation of power, stormwater, sewer, and gas services.
Construction commenced in September 2014 and practical completion was achieved in December 2015. The finished earth dam has an 18m high embankment, with a fully intercepting chimney drain, a stored volume of 1.3M cubic metres of flood water, and two large emergency spillways. Due to its location in a suburban area and the consequences of failure, the dam is classified as high potential impact in terms of the New Zealand Dam safety Guidelines. Very few high potential impact dams are currently being constructed; this dam was able to gain the necessary consents due to positive community benefits.
The dam is designed to store a 1% annual exceedance probability flood event and reduce peak flows downstream, and emergency spillways at each abutment are designed for probable maximum flood.
During construction RILEY had an engineer on-site full time and provided contract administration services for Northland Regional Council. The dam was officially opened in April 2016 and has been named Hopua te Nihotetea dam, which translates as capturing the Nihotetea Stream, reflecting the detention dam's purpose.