Performance of Levees (Stopbanks) during the 4 september 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, Earthquakes

Abstract

The objective of this paper is to summarize the performance of the levees (or stopbanks) along the Waimakariri and Kaiapoi rivers during the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquakes. Shortly after their arrival in the Canterbury area in the mid-nineteenth century European settlers started constructing drainage systems and levees along rivers (Larned et al. 2008). In particular, flooding of the Waimakariri River and its tributaries posed a constant threat to the Christchurch and Kaiapoi areas. The current levee system is a culmination of several coordinated efforts that started in earnest in the 1930s and is composed of both primary and secondary levee systems. The primary levee system is designed for a 450-year flood. Damage estimates for scenarios where the flood protection system is breached have been assessed at approximately NZ$5 billion (van Kalken et al. 2007). As a result, the performance of the levee system during seismic events is of critical importance to the flood hazard in Christchurch and surrounding areas. During the 2010 Darfield and 2011 Christchurch earthquakes, stretches of levees were subjected to motions with peak horizontal ground accelerations (PGAs) of approximately 0.32 g and 0.20 g, respectively. Consequently, in areas where the levees were founded on loose, saturated fluvial sandy deposits, liquefaction-related damage occurred ( i.e. , lateral spreading, slumping, and settlement). The performance summary presented herein is the result of field observations and analysis of aerial images (New Zealand Aerial Mapping 2010, 2011), with particular focus on the performance of the levees along the eastern reach of the Waimakariri River and along the Kaiapoi River. In the sections that follow, we first present background information about the levee system. This is followed by an overview of the performance of the levees during the Darfield …