Leavitt Reservoir Expansion

Challenges

Leavitt Reservoir is used primarily for agricultural irrigation throughout Shell Valley in northern Wyoming.  The original reservoir was too small to reliably meet irrigation demands during dry summers and late in the growing season.  The Leavitt Reservoir Company desired to increase the reservoir storage from about 643 acre-feet (ac-ft) to 6,600 ac-ft to enhance late-season irrigation supply and provide increased downstream streamflows during low-water periods to benefit riparian vegetation and aquatic habitat.

Solution

RJH was involved in earlier stages of this project, providing geologic and geotechnical engineering services during initial assessments, environmental permitting, and planning studies.  RJH provided key geologic input for selecting the preferred alternative of constructing a new, larger dam upstream of the existing dam.  The new dam includes an approximately 85-foot-high, 4,200-foot-long zoned earthen embankment and a multi-level intake tower.  The project also includes about 5.5 miles of pipeline, creation of about 22 acres of wetlands, recreation facilities, and realignment of a county road.

The geologic conditions at the site are complex, with highly plastic mudstone in the foundation and poorly cemented sandstone in the foundation and left abutment.  RJH performed specialized laboratory testing and consulted with geologic and geotechnical experts to confirm the mudstone was not a fatal flaw.

To initiate the design stage, RJH performed a week-long concept review and risk workshop.  RJH staff led and facilitated a qualitative risk assessment to identify key risk drivers and formulate potential mitigation measures to incorporate during investigations, design, and construction.  RJH led the work from initial evaluation and assessment of the existing dam through identification of dam safety issues, alternatives development, permitting, final design, and construction engineering.