The Stormwater and Tide Interface in Christchurch

Christchurch city is mostly built on a floodplain of the Waimakariri River. This floodplain tapers gently toward the coast so the flooding risks at coastal interface is a particularly important area to understand in order to manage this risk into the future. Tidal flooding risk exists not only directly at the coast but also several kilometres inland where the tidal backwater effects extend.

This paper outlines the methods that have been used in Christchurch to estimate and plan for combined tidal and stormwater flooding risk using analyses of historical data and hydraulic modelling approaches. The results of these analyses have been used to develop planning rules and infrastructure programmes.

Recent tidal extreme events have focused attention on the trends in annual high tides. The Christchurch tidal statistics have now been updated following the previous review as recently as 2011. The trends in annual high tides since 1926 have been analysed and a comparison has been made with the higher trends in the last 20 years.

These new statistics are being used to set new finished floor levels for residential and commercial buildings and other infrastructure. They will also be used to re-map the flooding zones in the next revision of the Christchurch District Plan.

The data will also be used to inform Christchurch City Council’s future approaches to floodplain management. Council has invested heavily in floodplain management infrastructure since the earthquakes. Major strategic tidal flood protection infrastructure was damaged during the earthquakes and have now been stabilized. This is to allow time for development of long term plans and financing of interventions which will be resilient or adaptable to ongoing sea level rise.