2017 – Application of Risk Assessment to an Emergency Levee Design in an Urban Environment

Petros Armenis, Malcolm Barker, Peter Christensen, Graham Harrington

The Canterbury Earthquake Sequence in September 2010 and February 2011 caused large areas of land to change by differing amounts throughout Christchurch, New Zealand. Land levels fell by more than 300 mm in some areas. This increased flood risk in the tidal reaches of the Avon River. Urgent repairs were completed with the objective to restore the tidal river defences to a crest level equivalent to a 1% AEP tide level. This work needed to be completed prior to impeding spring tides.

The levees will be required for up to 20 years and then probably be rebuilt on a new alignment. To better understand the risks associated with the ongoing reliance of the levees for flood protection in the interim, a risk assessment was undertaken using conventional Australian National Committee on Large Dams (ANCOLD) practices and levee design procedures. Careful consideration was made to the performance of the existing levees under seismic, flood and tidal loading from which the societal and individual risk profiles were derived. The work included the following:

1. The identification of critical sections along both sides of the 11 km levee river alignment through consideration of the foundation and embankment construction
2. Combining seismic events with tides and flood events with tides using levee lifetimes of 1, 5, 10 and 20 years
3. Consideration of overtopping failure and piping through the levee embankment, foundation or tree roots and narrowed embankment sections owing to trees being blown over
4. Application of the International Levee handbook (CIRIA 2013) and the “Piping Toolbox” (USACEet al 2008
5. Evaluation of risk reduction with upgrade options

This paper will present the levee design and the process applied for the analysis of the levee and the upgrade options selection