Author(s): Terry Van Kalken; Tony Oliver; Ian Heslop; Tony Boyle
Linked Author(s):
Keywords: Floodplains; Modelling; Embankment breach
Abstract: The City of Christchurch is located on the Canterbury Plains at the foot of the Southern Alps on New Zealand’s South Island. The city is at risk from potential flooding from the Waimakariri River, a substantial braided gravel bed river typical of those which flow out of the mountains and onto the Canterbury Plains. 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 piping failures for much smaller flood events. The potential flood damages to Christchurch in a Probable Maximum Flood are approximately NZ$5 billion. During extreme flood events in the Waimakariri River the most likely risk scenario involves breakouts to the north (affecting Kaiapoi) and to the south (affecting Christchurch). It is possible that the main river channel will continue to carry large flows, although complete avulsion (course or channel change) could occur to the north or to the south. A secondary flow channel, contained by a secondary line of embankments tying into natural high river terraces could substantially reduce potential flood damages to Christchurch City. This paper discusses the river and floodplain investigations undertaken for a range of design scenarios for both the current flood protection system and the secondary embankment proposal on the southern floodplain. The investigations were undertaken using a 1D/2D mathematical modelling approach, including a dynamic embankment breaching feature to represent the progressive failure of the river banks in the considered scenarios. The models were used to assess both the existing flood inundation extents for up to a 10, 000 year return period event (6, 000m3/s) as well as the impacts of the proposed scheme in terms of changes in water levels and flow patterns. The model results were published to large scale inundation maps and were used in the community consultation process prior to final design.
Year: 2007