DESIGNING FOR A FLOOD THAT SHOULDN’T HAPPEN

The MacKays to Peka Peka Expressway Project passes through Paraparaumu and Waikanae, north of Wellington. Resource consenting involved rigorous stormwater assessment and detailed modelling of flood risk in critical areas, including a crossing of the Waikanae River.

One unusual aspect was that the Expressway crosses an historical river overflow path (identified in the District Plan), running north from the Waikanae River, through parts of the Waikanae township, to the adjacent Waimeha Stream. The overflow is now protected by the Waikanae River stopbanks and therefore should not flow in the 1% AEP design event. However, a stopbank breach would flood residential areas, and the Expressway embankment could potentially block the passage of these overflows. The challenge was defining what constituted a “design” (or “super-design”) event, how it could be modelled appropriately and the implications for the Expressway design. Further complications included identifying a suitable Waikanae River flow to use, the failure rate of the stop bank and the relative timing of the associated flood in both the river and the receiving Waimeha catchment.

As a consequence of this work, the Expressway includes a designed floodway to capture any breach flows, conveying them under the Waimeha Stream Expressway bridge via an additional 50 m land span, and returning them to the existing historical flow path downstream.

This paper outlines the methods applied in investigating and understanding the location of the breach, its failure mode, how its progressive development was modelled and the consequences of that modelling on the design and on the flood resilience of the community. The design was tested in MIKE FLOOD software in order to find an appropriate solution that maintained flood resilience.