Flexible Mesh Modelling of the Upper Taieri Plain

Dunedin City Council (DCC) requires residential buildings to be built above the 100 year flood level. This approach currently creates difficulties as there are no generally established flood levels for areas vulnerable to flooding.

This places the onus on each applicant to develop a defensible flood level, increasing costs, timeframes and uncertainty. In reviewing their District Plan, council staff seek to improve this situation by developing minimum floor levels.

Otago Regional Council (ORC) have collected significant anecdotal and historic records and have done some flood risk analyses, however further work was required to develop this information into recommendations for minimum floor levels.

GHD are bridging this gap for DCC, building on the ORC work and providing estimates of flood levels and recommended minimum floor levels to support DCC planning controls. The work involves a mixture of modelling and non-modelling methods, chosen strategically to provide maximum planning benefit to Dunedin ratepayers within their project budget. It is anticipated that other areas of Dunedin may have flood hazard modelling work completed in future years as budget allows.

This paper summarises Dunedin City’s strategy for progress in their planning controls and presents the Upper Taieri Plains model, which was one element of the above work. This model is technically interesting because it uses DHI’s latest flexible mesh and GPU processing technology and contains no 1D elements.

Of particular interest will be our key learning steps as we moved from a traditional classic modelling background into flexible mesh and GPU based modelling and comparative overview of model runtimes and results using different mesh resolutions. We will also outline how modelling results plus freeboard were extrapolated to provide floor level recommendations for areas both within and beyond the predicted flood areas.