Optimisation of Earthworks, Runoff Attenuation and Breakout Flood Conveyance using Infoworks and MX Modelling

Ravenswood is a proposed 135 ha residential and commercial development north of Christchurch. This development is located in the flood path of a breakout from the Ashley River, requiring careful management of flood risk.

Space restrictions within the developed area, plus the breakout floods mean that it was not possible to fully mitigate local runoff peaks within the development. Instead, a system was designed whereby a realigned stream/flood path through the development was used to provide the additional local-flood attenuation that the conventional treatment wetlands/attenuation areas could not achieve. This proposed system was balanced against managing stream ecology, passage of breakout floods, and maintaining the existing flow balance of diverging flood paths downstream. This also needed to be achieved in the context of achieving a cut-to-fill earthworks balance across the site, while creating an attractive environment, fitting of its location as the focal area of the development.

This approach required both earthworks and terrain design modelling with MX and hydraulic modelling with InfoWorks CS, to optimise the future land form and stream design. The resulting system includes a series of weirs and culverts within the stream system which will cause water to pond behind them to provide attenuation, yet allows larger flood flows to pass safely.