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Irrigation District Underdrain Replacement

The Casper-Alcova Irrigation District’s Master Plan identified the CU-046 underdrain as being structurally deficient and in need of immediate repair or replacement. The underdrain conveys runoff under the main canal and consists of a dual 5’x5’ cast-in-place concrete culvert, concrete approach aprons, and concrete wing walls on each end. This structure was built as part of the Bureau of Reclamation Kendrick Project in the late 1930’s. Collapse of this structure would allow a severe breach of the main canal which would result in hundreds of producers not having irrigation water as well as the potential of severe flooding downstream of the culvert crossing. WLC’s initial task was to prepare a feasibility study for either rehabilitating or replacing the structure. The study also included a drainage study of the contributing basin. The preferred method of rehabilitation was to slip line the rectangular culverts with circular HDPE pipe and grout the annular space. This alternative would reduce the capacity of the culverts to convey less than the 100-year storm. The replacement alternative consisted of removing the existing culverts and replace it with a 5’x10’ RC box culvert. Although this alternative was more expensive than slip lining, the CAID chose to replace the culvert with the RC box culverts to maintain the historic runoff conveyance quantity. The construction of the project had to be completed before the irrigation water was turned on for the season. Any delays to the completion of the project would result in severe financial impacts to downstream water users. Our solution to this challenge was retain the existing concrete approach aprons and wing walls at each end of the culvert. This obviously reduced the cost to the project but also eliminated the need for time consuming forming and curing time for new concrete.