Preventing Flood Disasters - A Better Option than Creating Them

Christchurch desperately needs new affordable housing for families displaced by the 2010 and 2011 earthquakes. Gravel deposits on the alluvial fans of the large north Canterbury rivers provide attractive building sites in terms of foundation conditions with a low risk of liquefaction. However, the fact they are alluvial fans means that a river has flowed there at some stage in the past. We will present a case study of the flood hazard assessment that was undertaken for an 80 hectare residential development in close proximity to the Cust Main Drain, which in name might sound rather innocuous but in fact has flood flows in excess of 200 m3 /s.

The Cust Main Drain has stopbanks on both sides that are built to a height in excess of a 2% annual exceedance probability flood event. So sounds like it’s already protected? Go ahead and start building? NO!! It was exactly this type of thinking that Neil Ericksen highlighted as flawed in his 1986 report “Creating Flood Disasters?” The “protection” provided by stopbanks encourages intensification of development on floodplains due to the perceived protection which they provide and the risk actually increases. Our assessment of flood risk at the proposed development site was based on considering two key questions - What would happen if the stopbanks failed? What would happen if a flood larger than expected occurred?