Adjustable Weir Providing Glood Passage and Landscaping in New Plymouth CBD

In 2008 New Plymouth District Council created a pond on the Huatoki Stream as part of a landscape revitalisation project in New Plymouth’s CBD. To avoid increasing flood risk upstream, a 1.2m high by 5m wide weir that adjusts to stream flows was specified. The installation was completed in August 2009.

The pond created by the weir, while attracting people to the stream’s edge, increases backwater levels during floods. To achieve high amenity in both low and high flows, the weir would need to “collapse” at a critical stream flow but be easily reinstated afterwards.

To meet these objectives, an Obermeyer weir was specified. This is a curved steel blade, hinged at the stream bed and counterbalanced by a rubber bladder of compressed air which continuously adjusts in response to a water depth sensor upstream. During a flood the bladder deflates, lowering the blade and maximising the channel capacity.

This paper discusses the logistics of specifying and installing the weir, and offers further applications for the technology locally. It demonstrates the benefits of this solution including allaying flood risk, supporting a fish pass structure, a smooth resource consent process, limiting maintenance and providing a visually dynamic signal of stream flow.