Waterways are often the most visible feature of a stormwater system for the communities they pass through. Communities can be highly engaged with their valued waterways and often seek ways to restore, interact with and improve their environmental sustainability. Therefore, waterway design must deliver hydraulic engineering performance, ecological values as well as other community concerns like aesthetics. This requires a balance of often conflicting perspectives.
Hydraulically, waterways need to convey baseflow, small storm freshes, and floods. However, the other drivers result in the form of a waterway being complex, needing to incorporate varying morphology, slope, ground conditions, vegetation, habitat features, engineering structures and spatial constraints, to list but a few. Accommodating all this is complicated by competing requirements; for example achieving flood conveyance while providing a healthy, sustainable habitat. The first would see wide, open, smooth, stable waterway cross-sections while the second seeks nearly the opposite with narrower, dynamic normal-flow channels incorporating shading, plant cover, bank undercutting and variations in velocity and depth.
This paper presents two different project examples where hydraulic, ecological, landscape and other multidisciplinary design elements have been successfully balanced, and identifies the key project features that can be applied in other waterway designs.
Firstly, Christchurch City Council’s Dudley Creek Flood Remediation project aims to restore the pre-earthquake flood risk to the Flockton Street Area. It involves 2 km of flood channel widening through private property and road reserve, plus a 790 m flood bypass conduit.
In the second example, NZ Transport Agency’s MacKays to Peka Peka Expressway on the Kapiti Coast includes over 5 km of new and enhanced waterway, to mitigate that lost to the Expressway. This too must balance the significant flood risk that is a feature of urban and rural areas around the project, while supplying suitable ecological mitigation.