In 2011 Riley provided a geotechnical feasibility study of two sites on the Nihotetea Stream, south-west of Whangārei, for a proposed detention dam to mitigate flooding.
The study was followed by hydraulic optimisation and design reports to provide supporting information for a resource consent application. After the consent was approved, the brief was extended to providing detailed design, obtaining building consent and managing contract documentation.
In the design process, we encountered some significant engineering challenges due to the location of existing homes, property boundaries and complex site geology.
The earth dam features:
an 18m-high embankment, with a fully intercepting chimney drain
a stored volume of 1.3 million cubic metres of floodwater
two large emergency spillways.
Due to its suburban location and the consequences of failure, the dam is classified as high potential impact under the New Zealand Dam Safety Guidelines.
During construction Riley had an engineer on site full-time and provided contract administration services for the council. Building started in September 2014 and finished in December 2015.
The dam was officially opened in April 2016 and named Hopua te Nihotetea. This translates as capturing the Nihotetea Stream, reflecting the detention dam's purpose.