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material having been eroded out during the 2017 incident. In general, the geologic maps created during original construction were found to be very accurate. The thicknesses of the concrete chute slabs in the FCO Spillway chute, however, were found to be generally much thicker than the nominal 38 cm (15 inch) minimum thickness specified during original design, with thicknesses ranging up to 2.1 m (7 ft) and an average concrete slab thickness on the order of about 1 m (3.5 ft). This larger thickness resulted in additional demolition and excavation in the removal of these concrete slabs, larger thicknesses of new leveling concrete, and deeper underdrains, extending to rock through the leveling concrete beneath the new structural concrete. 3.2.3. Hydraulic Analyses and Modeling Hydrologic and hydraulic analyses, using 1D, 2D, and 3D analytical models and a physical model of the FCO Spillway chute constructed at Utah State University, were used to verify and improve the designs for both spillways. The details of these analyses are presented in a separate companion paper [3]. 3.2.4. FCO Spillway Chute Structural Concrete Designs The structural reinforced-concrete designs for new chute wall and slab sections were developed to meet modern spillway design standards and to address any potential areas for improvement identified in the original design. Presented in Table 2 is a list of potential design and/or construction areas for improvement identified by the OER and the Forensic Investigation Team appointed to provide an independent assessment of the potential causative factors that led to the incident. Also shown are the elements included in the new structural-concrete design intended to address these areas for improvement. Fig. 8 illustrates several of these elements. 3.2.5. Secant Pile Wall/RCC Designs for Emergency Spillway During the final design of the interim measures for the Emergency Spillway, designers concluded that a capacity higher than the 850 m3/s (30,000 ft3/s) design objective for the upcoming 2017-2018 Winter Flood Season was needed. This was because it was recognized that a major modification or replacement would eventually be needed in order for the Emergency Spillway to safely pass its original rated design capacity of 10500 m3/s (371,000 ft3/s) for a PMF flood event. Further, it would likely require 5 to 10 years to implement such a major project. Consequently, the interim repairs/upgrades were needed to be able to convey flows that might reasonably be discharged through the Emergency Spillway during the next 5 to 10 years. While not established during the initial alternatives analyses, a target flow of 2800 m3/s (100,000 ft3/s) was later adopted during the final design phase as a design objective for the Emergency Spillway for the next 5 to 10 years. 77