Abstract
Increased demand for power generation coupled with changing seasonal water uncertainty has caused a worldwide increase in the construction of large hydrologic engineering structures. That said, the soon-to-be-completed Grand Ethiopian Renaissance Dam (GERD) will impound the Blue Nile River in Western Ethiopia and its reservoir will encompass ~ 1763 km2 and store ~ 67 Gt (km3) of surface water. The impoundment will undergo maximum seasonal load changes of ~ 28 to ~ 36 Gt during projected seasonal hydroelectric operations. The GERD impoundment will cause significant subsurficial stresses, and could possibly trigger seismicity in the region. This study examines Coulomb stress and hydrologic load centroid movements for several GERD impoundment and operational scenarios. The maximum subsurficial Coulomb stress applied on optimally oriented fault planes from the full impoundment is ~ 186 kPa and over 30% of our model domain incurs Coulomb stresses ≥ 10 kPa, regardless of the impoundment period length. The main driver behind Coulomb stress and load centroid motion during impoundment is the annual, accumulated daily reservoir storage change. The maximum Coulomb stresses from the highest amplitude season of five long-term operational scenarios are around 36, 33, 29, 41, and 24% of the total maximum stresses from the entire GERD impoundment. Variations in annual Coulomb stresses during modeled GERD operations are attributed to the seasonal load per unit area, and partially to the initial seasonal water level. The spatial patterns and amplitudes of these stress tensors are closely linked to both the size and timing of GERD inflow/outflow rates, and an improved understanding of the magnitude and extent of these stresses provides useful information to water managers to better understand potential reservoir triggered seismic events from several different operational and impoundment strategies.
Highlights
Large scale hydrologic loading and reservoir operations have the ability to affect the surrounding lithosphere and crust as well as the interconnected hydrogeological systems
The specific goals of this work are to provide meaningful answers to the following science questions: (1) What are the Coulomb stresses at depth on optimally oriented fault planes as caused by hydrologic load changes from different filling schedules as well as from several seasonal operation plans at the Grand Ethiopian Renaissance Dam (GERD)? (2) What are the main hydrologic factors that affect these subsurficial stresses? We utilize daily hydrologic load arrays from several filling and operational scenarios to derive Coulomb stresses on optimal planes in a 3D elastic half-space to answer (1), and we investigate the relationships between both load area density and starting reservoir water levels with Coulomb stress results to answer (2)
We note that the impoundment scenarios each started on January 1 and nearly the first half of the first year in each filling plan displayed very little to no impoundment
Summary
Large scale hydrologic loading and reservoir operations have the ability to affect the surrounding lithosphere and crust as well as the interconnected hydrogeological systems To this end, large impoundments and reservoir operational cycles would alter local potentiometric groundwater surface elevations. Past research has shown that the initial impoundment and subsequent reservoir operations of large dams have altered groundwater levels for hydraulically connected systems (Zhang et al 2014; Zhao et al 2016). These connections have the capability to destabilize slopes and to eventually trigger failure events (Teimouri and Khalkhali 2018). This instability is mostly caused by variations in hydrostatic pressure from the changing groundwater levels and the fluctuations in the hydraulically connected water levels of the reservoir (Fredlund and Rahardjo 1993; Paronuzzi et al 2013; Xia et al 2015; Zhang et al 2012)
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