Life in the Floodplain - Flood Mitigation Concept to Construction

Edgecumbe township is located between major river, canal and stop bank flood protection systems. The town is subject to severe stormwater flooding in relatively minor events. The 28 January 2011 rainfall event was considered to be significantly smaller than the 10-year stormwater design standard, however a number of properties were flooded. 

Harrison Grierson was commissioned by Whakatane District Council (WDC) to investigate flooding issues in the north-western quadrant of Edgecumbe. The objective of the project was to alleviate stormwater flooding in order to reduce risk to people and property, while also reducing stormwater ingress into the wastewater system. This project was carried out in parallel with improvements to the town’s wastewater system to address issues relating to stormwater infiltration into the wastewater system, which has resulted in wastewater overflows in the town.

The principal outcomes of this investigation were to: 

• Identify houses at risk of flooding for a 10-year ‘2090’ design rainfall event. 
• Identify, conceptualise, and evaluate potential remedial options within WDC’s available capital works budget for Edgecumbe. 

The option available within Council budget was a 1.05m diameter gravity stormwater relief pipe, which would reduce water levels by up to 300mm for a 10-year ‘2090’ rainfall event. 

From the perspective of a fresh Environmental Science Graduate, this paper will discuss the conceptual design process and challenges experienced during this high profile flood mitigation project, which was successfully constructed in early 2013.