Work is due to begin within weeks on an enormous stormwater dam designed to help stop flooding in Whangarei's central business district.

Northland Regional Council has recently bought the final property it needed and about $2.7 million of work to build the dam itself is scheduled to begin in October.

Five houses in Kotuku St, Maunu, will be demolished at the start, and some bush cleared. Another three houses will accommodate workers and be demolished after the dam is built. The houses' construction style means they cannot be relocated.

The council's Urban Whangarei Rivers liaison committee chairman, Craig Brown, said about $5 million of the estimated $8.6 million final cost of the Kotuku detention dam project was spent buying the 20 properties needed to construct the dam in a valley between Maunu and Raumanga. Another several hundred thousand dollars went on relocating power and other utilities.


The contract to build the dam itself - to sit at the ends of Raumanga Valley Rd in Raumanga and Kotuku St, Maunu - had gone to MAP Projects, which aims to have it finished next autumn, Mr Brown said.

MAP is an experienced Rotorua-based civil contracting business that has successfully completed a number of challenging civil construction projects in recent years, including several hydro civil works and dam works.

"This is one of the largest engineering projects the regional council has undertaken and, when finished, the 18-m high dam will hold up to 1.27 million cubic metres of floodwater during heavy rain, about two-thirds the capacity of the existing Whau Valley dam," Mr Brown said.

Unlike the Whau Valley dam, which provides drinking water to much of urban Whangarei, the Kotuku dam is designed to trap vast amounts of water and then slowly release it over several days. Much of the damage caused by floods depends not only on how deep they are, but how fast they are travelling, Mr Brown says.

"In layman's terms, the Kotuku dam will help address both of these, slowing floodwaters and also cutting flood depths in the CBD by up to half a metre."