Can Protracted Drought Undermine the Structural Integrity of California’s Earthen Levees?

There is a crucial need to improve our understanding of the potential threats of extreme events, such as the ongoing California drought, on critical geotechnical infrastructure and take subsequent actions in a timely manner to adapt our infrastructure. The need is more pronounced for levees because their functionality to protect limited water resources and dry land is more critical during drought (Hanak and Lund 2012; Vahedifard et al. 2015a). The frequency and severity of California’s drought are anticipated to worsen in a warming climate (Port and Hoover 2011). TheMillennium Drought (1997–2009), that severely impacted the economy and environment in southeastern Australia (AghaKouchak et al. 2014a), is often considered as an archetype of an extreme event that California should prepare for. The Millennium Drought significantly affected critical infrastructure like river ecosystems, dry land and irrigated agriculture, and surface water storage in natural and engineered reservoirs (Van Dijk et al. 2013). The event led to significant reduction in production of water-intensive crops (e.g., rice and cotton) by up to 99%, resulting in over 4.5 billion Australian dollars in Federal Government drought assistance and investments in multibillion dollar water infrastructure for improving preparedness and response (e.g., Van Dijk et al. 2013; Howden et al. 2014). The 2012–2015 California drought has been one of the most extreme on record, characterized by low precipitation and high temperatures (Shukla et al. 2015; AghaKouchak et al. 2015). Fig. 1(a) displays snapshots of drought conditions from 2012 to 2015 based on the standardized soil moisture index (SSI) (Hao et al. 2014). As Fig. 1(a) shows, the most severe drought on record occurred in May 2014, and it continues to remain in a critical condition. The severity of drought is in part a result of the lack of snow pack, which affects the overall exploitable water supply throughout the state. As Fig. 1(a) indicates, there is also a shortage of soil moisture, which comes in addition to the water shortages caused by lack of snow packs. Figs. 1(b and c) show the 4-year average precipitation and temperature, respectively. The concurrent extreme low precipitation [Fig. 1(b)] and high temperature [Fig. 1(c)] was estimated to be a 200-year extreme event (AghaKouchak et al. 2014b). This paper intends to stimulate discussions among the geotechnical engineering community regarding the effects of California’s prolonged drought on levees in the Central Valley and Sacramento– San Joaquin Delta (Delta). There is a clear gap in the state of our knowledge in terms of structural-scale assessment of levees under extreme drought (CACC 2015). We call for further investigations to quantitatively assess the impact of California’s protracted drought on the shortand long-term behavior of levees and also for a timely action to strengthen and improve the resilience of California’s levees to cope with drought. This paper discusses, from a geotechnical engineering perspective, how the current California drought might threaten the integrity of levee systems. Catastrophic levee failures and major damages that occurred in similar drought situations are shown and discussed to illustrate the devastating impacts that the California drought might impose on existing levees. Several drought-induced, thermo-hydro-mechanical weakening processes are also discussed. Although the focus of this paper is more on levees throughout the Delta, the drought-induced weakening processes discussed herein also threaten levees in similar conditions in Northern California.