Ken Mitchell Lakes

Challenges

Ken Mitchell Lakes consists of a series of gravel pits created from sand and aggregate mining activities along the South Platte River in Brighton, Colorado.  The City of Brighton (Brighton) acquired the gravel pit complex with the intention of converting it into lined water storage reservoirs.

RJH has been providing engineering consulting services to Brighton since 2006 to assist Brighton with the planning, design, monitoring, construction, development, and repair of the Ken Mitchell Lakes gravel pit reservoir system.  Key challenges have included:

• A soil-bentonite barrier wall was designed and installed by others. Following construction, the cutoff wall did not meet the groundwater inflow criteria established by the Colorado Office of the State Engineer for lined gravel pit reservoirs.
• The reservoir slopes were experiencing significant wave erosion.
• The reservoir lacked a riverside spillway and was vulnerable to a breach from flooding in the South Platte River.

Solution

RJH was initially retained to evaluate the barrier wall and provide recommendations for remediation.  Borings were advanced through the cutoff wall at locations of excessive seepage to evaluate if the excessive infiltration was from seepage through the cutoff wall or around the cutoff wall through the bedrock.  RJH identified that “sand windows” were the cause of excessive seepage and likely resulted from collapse of the cutoff trench excavation during extended construction shutdowns.  RJH prepare final design documents for rehabilitation and provided on-site field engineering.  The reservoir was tested after remediation and met seepage requirements.

RJH was subsequently retained for additional design and construction services that included 4,900 linear feet of slope protection, four soil-bentonite cutoff wall crossings that were installed as part of a pipeline project, and three grouted riprap spillways to mitigate flooding in the South Platte River.