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Performance of Levees (Stopbanks) duringthe 4 September 2010 M w 7.1 Darfield and22 February 2011 M w 6.2 Christchurch,New Zealand, EarthquakesRussell A. Green, John Allen, Liam Wotherspoon, Misko Cubrinovski, Brendon Bradley, Aaron Bradshaw, Brady Cox, and Thomas AlgieRussell A. Green, 1 John Allen, 2 Liam Wotherspoon, 3Misko Cubrinovski, 4 Brendon Bradley, 4 Aaron Bradshaw, 5 Brady Cox, 6and Thomas Algie 7INTRODUCTIONThe objective of this paper is to summarize the performance ofthe levees (or stopbanks) along the Waimakariri and Kaiapoirivers during the 4 September 2010 M w 7.1 Darfield and 22February 2011 M w 6.2 Christchurch, New Zealand, earthquakes.Shortly after their arrival in the Canterbury area in themid-nineteenth century European settlers started constructingdrainage systems and levees along rivers (Larned et al. 2008).In particular, flooding of the Waimakariri River and its tributariesposed a constant threat to the Christchurch and Kaiapoiareas. The current levee system is a culmination of severalcoordinated efforts that started in earnest in the 1930s and iscomposed of both primary and secondary levee systems. Theprimary levee system is designed for a 450-year flood. Damageestimates for scenarios where the flood protection system isbreached have been assessed at approximately NZ$5 billion(van Kalken et al. 2007). As a result, the performance of thelevee system during seismic events is of critical importance tothe flood hazard in Christchurch and surrounding areas.During the 2010 Darfield and 2011 Christchurch earthquakes,stretches of levees were subjected to motions with peakhorizontal ground accelerations (PGAs) of approximately 0.32g and 0.20 g, respectively. Consequently, in areas where thelevees were founded on loose, saturated fluvial sandy deposits,liquefaction-related damage occurred (i.e., lateral spreading,slumping, and settlement). The performance summary presentedherein is the result of field observations and analysis of1. Department of Civil and Environmental Engineering, VirginiaTech, Blacksburg, Virginia U.S.A.2. TRI Environmental, Duluth, Minnesota U.S.A.3. University of Auckland, Auckland, New Zealand4. University of Canterbury, Christchurch, New Zealand5. University of Rhode Island, Kingston, Rhode Island, U.S.A.6. University of Arkansas, Fayetteville, Arkansas, U.S.A.7. Partners in Performance, Sydney, Australiaaerial images (New Zealand Aerial Mapping 2010, 2011), withparticular focus on the performance of the levees along theeastern reach of the Waimakariri River and along the KaiapoiRiver.In the sections that follow, we first present backgroundinformation about the levee system. This is followed by anoverview of the performance of the levees during the Darfieldand Christchurch earthquakes. Next, we discuss the relationshipbetween the severity of damage to the levees along thedowntown stretch of the Kaiapoi River and the response of thefoundation soils. Finally, we present a summary of the findingsand draw conclusions.BACKGROUND OF THE LEVEE SYSTEMThe Waimakariri River flows from the Southern Alps acrossthe Canterbury Plains between Christchurch, to the south,and Kaiapoi, to the north, and empties into Pegasus Bay inthe east (Figure 1). The river drains a mountainous catchmentarea of 3,566 km 2 and poses the most significant floodhazard in New Zealand (van Kalken et al. 2007). Early effortsby European settlers to realign and contain the river withinits banks were piecemeal and only partially successful (e.g.,Wotherspoon et al. 2011). To better coordinate the efforts andto ensure equal flood protection to both Christchurch andKaiapoi, the Waimakariri River Trust was established in 1923(Griffiths 1979). In response to the 1926 floods (Figure 2),the Trust implemented a major river improvement scheme in1930, known as the Hays No. 2 Scheme. Among other things,the scheme entailed an overall improvement of the levee systemalong the Waimakiriri River. However, these improvementswere unable to prevent the major floods in 1940, 1950,and 1957. These floods prompted a further river improvementscheme in 1960, which entailed benching existing levees andconstructing new levees.doi: 10.1785/gssrl.82.6.939Seismological Research Letters Volume 82, Number 6 November/December 2011 939