Stormwater treatment devices are a best management practice that are used to reduce the risk of adverse environmental effects and decrease the contaminant concentrations to below acceptable trigger limits in sensitive receiving environments. The majority of devices in New Zealand tend to be specified and designed according to either Auckland Council’s Technical Publication 10, Christchurch City Council’s Wetland, Waterways and Drainage Guide or The New Zealand Transport Agency’s Stormwater Treatment Standard for State Highway Infrastructure. These documents provide a good general overview of various devices, performance and treatment applications however, the operating effect of the device with regards to the hydraulic grade line is often misinterpreted or unconsidered by designers. Consequently this can lead to under designing the treatment device for the required water quality flows or, at worst, make the device inoperative all together.
Device driving head, tailwater from a downstream receiving waterbody and location of upstream diversion structures are all examples of design considerations that can affect the hydraulic operation of the device. Climate change may also provide future tailwater problems with rising sea levels at coastal outfalls.
This paper will present hydraulic design considerations, beyond the standard guidance information, for stormwater treatment devices and discuss implications on the stormwater network.