Using a Dynamically Coupled Model for Hazard Mapping and Option Assessment

The Oakley Catchment is a long narrow stormwater catchment on the Auckland Isthmus, which experiences significant stormwater flooding issues. The stormwater response to extreme events creates flooding across a broad floodplain adjacent to the creek. State Highway 20, Stage 1 was completed in 2007/08 and is now programmed for extension to Waterview starting in 2011. NZTA engaged AECOM to undertake modelling of the proposed extension, the results of which were used in their assessment of effects report to the Environmental Protection Agency (EPA). Concurrently, Auckland Council commissioned AECOM to undertake Flood Hazard Mapping and Options assessment to mitigate catchment flooding in the Oakley catchment. The programming of these complex (and interdependent) projects was time constrained and thoughtful sequencing was required to ensure timely delivery.

Flooding in Oakley results in complex flow regimes including a wide range of velocity profiles and flow directions in the creek. Recent advancements in modelling software allowed for complete dynamic modelling of flows by coupling the creek, pipe network (1D model) and floodplains (2D model) into one complete, higher confidence, dynamic model. This model has been used to determine flooding and subsequently options for the catchment. Options investigated included beheading the upper catchment, releasing water forward or by attenuating the water.

This paper outlines the initial options investigated and the model runs completed. It discusses the different applications the model was used for during the last 12 to 18 months. The paper also discusses the advantages and disadvantages of using a coupled 1D/2D model for option assessment and discusses the lessons learned.