2.2.4 Feasibility Study Alternatives and Selected Alternative

After the concept study and submittal of the PAD, Black Canyon commenced a feasibility study for the Project in 2021 to further refine the selected configuration from the concept study. Three Project configuration alternatives were explored in the feasibility study: Alternatives 1, 2, and 3. These alternatives were further subdivided into Alternatives 1A, 1B, 1C, 1D, 2A, 2B, and 3. Alternative 5 from the concept study was explored as Alternative 1 in the feasibility study. Alternatives 1A, 1B, 1C, and 1D were created by developing alternative locations for the intake structure and the necessary changes to the access tunnels and tailrace conveyance associated with a change in intake structure. Alternatives 2A and 2B differ in the location and type of the lower reservoir intake.

Refinements were made to the upper reservoir proposed in Alternative 5 from the concept study. Dam type options were evaluated with consideration for a range of factors including technical feasibility with the site and subsurface conditions, material availability and suitability, construction cost and schedule, and long-term operation and maintenance. Concrete-faced, rockfill dam (CFRD) and roller-compacted concrete (RCC) dam types were considered the most appropriate for the upper reservoir.

A variety of other dam types were considered, including zoned earthfill embankment, asphalt core rockfill, geomembrane-faced rockfill, faced symmetrical hardfill, and buttress dams. These options were ruled out for a number of reasons, including but not limited to insufficient quantity of impervious borrow material, higher construction labor costs, susceptibility to damage, degree of maintenance, or otherwise inferior to the CFRD and RCC options. The RCC type was selected as the preferred dam type option due to a faster dam construction schedule; the ability to construct a portion of the dam and start filling the upper reservoir before completion of the reservoir; needing less overall material handling (cut and fill) for the RCC dam, and because substantially less fill material is needed (1.1 million cubic yards of excavation for RCC, 5.4 million cubic yards of excavation for CFRD); and the reservoir operating range can be at a higher elevation, providing greater energy storage than the CFRD option. One risk with the RCC dam selection is alkaliaggregate reaction (AAR), which is present at the existing Seminoe Reservoir dam and is likely to be an issue with utilizing on-site aggregate materials obtained from excavating the access tunnels and materials obtained at the upper reservoir location. However, in modern RCC applications, large proportions of fly ash or other pozzolan is used to mitigate both AAR and freezing and thawing deterioration.

The upper reservoir was resized to accommodate a higher generation capacity (840–1,000 MW, 924 MW nominally) with the same 10-hour storage period that the Alternative 5 from the concept study achieved. With the modified dam type, the storage of the upper reservoir was increased from 8,842 ac-ft to 13,427 ac-ft.

The location of the underground powerhouse complex in Alternative 1 (Alternative 5 from the concept study) was determined to be too close to the postulated contact between the Seminoe Dam Granite and the Flathead Formation, a location that carries risk in the construction of the required large caverns. Challenges were present with the access tunnels and conveyance configuration of Alternative 1, both of which cross multiple

horizons of the Madison Limestone and Amsden Formation of shales. To address these geologic concerns, Alternative 2 was developed to maintain most underground excavation within the Seminoe Dam Granite and avoid potential structural lineaments. Alternative 2 had the secondary benefit that the intake was located in deeper water and a modified access tunnel route provided easier access to the tunnel portal, but in turn had a longer tailrace than Alternative 1. Alternative 3 was created as a combination of features of Alternatives 1 and 2. The powerhouse location for Alternative 3 was selected to take advantage of the Seminoe Dam Granite for the caverns and maintain a shorter tailrace.

The upper reservoir, power facilities, penstock, and draft tube tunnels were shared among all considered alternatives when comparing alternatives to one another to assess comparative costs. All alternatives and their subdivisions were mostly within estimating error of each other. Instead, a pairwise comparison based on the Water Resources Assessment Methodology (WRAM) used by the U.S. Army Corps of Engineers was performed to determine the favored option based on comparative cost, geotechnical risk, ease of access, ease of approvals, and schedule predictability. Alternative 2, specifically 2A, was supported by the results of the weighted pairwise comparison not necessarily due to being the lowest cost choice, but because it is seen as the option with the least geotechnical risk and greatest predictability of schedule.

Due to the refinements made in changing the dam type from earthfill to RCC, the construction schedule of the RCC dam would be arranged to complete the northeast corner of the dam as early as possible, allowing for the collection of natural precipitation during the construction period. Early placement of the geomembrane lining would be required and a plug may have to be left in the intake approach channel. It is assumed that precipitation during the construction period will be insufficient to supply the necessary water for commissioning of the Project pump-turbines. See Sections 4.1 and 4.2 for discussion on initial fill and make-up water sourcing.

Of note, refinements for the lower intake structure and conveyance profile were substantially different from the concept study alternatives. The lower intake structure had been identified as having many challenges, from the potential of ice, a fluctuating lower reservoir that is operated by an entity other than Black Canyon, underwater excavation, and the potential need for fish screening. The lower intake would be a precast structure floated into place and sunk at the outlet of the tailrace tunnel onto a pre-excavated ledge. The lower intake would include a horizontal intake and fish screens that are cleanable and removable. The elevation of the lower intake and size of the fish screen have been set based on Reclamation’s historical water levels of Seminoe Reservoir and assuming a fish screen velocity of 2 feet per second (fps) is applicable.

The conveyance profile for the alternatives from the concept study as shown in Figure 2.2-6 includes a vertical shaft immediately downstream from the upper reservoir inlet/outlet structure, then a horizontal headrace conduit to the powerhouse and tailrace. The conveyance profile presented in the feasibility study’s selected alternative reduces the difficult tunneling,as determined by the geologic data review, by modifying the conveyance profile to include a small vertical shaft from the upper reservoir intake. A roughly 3,300foot-long headrace tunnel that daylights for 600 feet in a gulley in the existing topography

before running into rock for the last 1,800 feet of the headrace tunnel where the vertical shaft is located.

Figure 2.2-4 presents the alternatives explored in the feasibility study. A detailed discussion of the selected configuration is provided in Exhibit A. Figure 2.2-5 presents the plan view of the selected alternative. Figure 2.2-6 presents the profile view of the selected alternative; of note is the updated conveyance profile compared to the concept study’s proposed conveyance profile. Exhibit F presents a 40-sheet drawing set of the selected Project alternative.