Overland Flow - Flood Prediction and Validation from a 100 Year Event

Auckland Council has identified buildings likely to be affected by overland flow. This has been done region wide using LiDAR and GIS analysis and mapping tools. In the north area the assessment of overland flow issues has also included property inspections to ground-truth the desk studies. Several hundred flood mitigation projects have been scoped from this work and several projects have been completed.

A short duration, high-intensity storm hit northern Auckland on 3 July 2012. The 10, 20 and 30 minute duration intensities reached 100 year ARI. This storm caused the worst flooding for northern Auckland since 2000 and resulted in 70 buildings being flooded. Of these 70 buildings, 80% of the flooding was caused by overland flow and were located outside the 100 year ARI floodplains.

This paper shows the close relationship between predicted flooding from overland flow and actual flooding from the storm affected area and the benefit of mitigation works where they have been already undertaken.

The focus of this paper is:

• The process of ground-truthing potential overland flow flooding issues
• Simple predictive and preventative measures.
• Validating predicted overland flow flooding with the 3 July event
• Overland flow flooding from small contributing catchments
• Assessing the effectiveness of flood mitigation works.