Potential flooding risk from the Waimakariri River

The existing flood protection scheme, comprising a line of flood embankments along the rivers edge, provides protection up to a maximum flow of 4730 m3/s, approximately a 450 year return period flood event. However due to the nature of the braided river, there is a risk of embankment failure due to undercutting and pipingfailures for much smaller flood events.

The City of Christchurch, the second largest in NZ,  is at risk from major flood events in the Waimakariri River as the primary embankments that protect the city may breach through overtopping or scour due to an avulsion (course change) of the river. It is possible that the main river channel will continue to carry large flows, although complete avulsion (course or channel change) could occur. A secondary flow channel, contained by a secondary line of embankments tying into natural high river terraces could substantially reduce potential flood risk.

DHI undertook river and floodplain investigations for a range of design scenarios for both the current flood protection system and the secondary embankment proposal on the southern floodplain. The model includes a dynamic embankment breaching feature, to represent the progressive failure of the river banks in the considered scenarios.

The model was used to assess both the existing flood inundation extents for up to a 10,000 year return period event (6,000 m3/s) as well as the impacts of the proposed scheme in terms of changes in water levels and flow patterns.