Multiple lakes and ponds on open space lands south of St. Vrain Creek were damaged during the 2013 flood when the creek jumped its banks at Breach 1 and Breach 2 and a split flow channel was formed. With funding assistance from FEMA, these three water bodies are planned to be repaired.
Lake 4 Repair Projects
Project Status: late stage of design and beginning of project permitting (May 2017)
Ownership, Location, and Use
Rock’n WP Ranch Lake No. 4 (Lake 4) is a lined, off-channel water storage reservoir with a decreed capacity of 600 acre-feet. The reservoir is owned jointly by the St. Vrain and Left Hand Water Conservancy District (District) and Boulder County with the Parks and Open Space Department (BCPOS) managing the county’s interests. The reservoir, also known as Western Mobile Lake 4, is located just south of St. Vrain Creek and west of 61st Street between Lyons and Longmont, and within the Western Mobile Open Space. The District uses Lake 4 to store water for later release to St. Vrain Creek to augment out-of-priority depletions for members who pay a fee under its Water Activity Enterprise Fund. For Parks & Open Space, the reservoir is a significant feature in the Western Mobile Open Space Complex that provides local habitat benefits and the potential for enhancing in-stream flows during low flow periods. It may also have a role in future public use envisioned for this area.
BCPOS and the District propose to restore the reservoir’s water storage capacity and delivery capability while also increasing the reservoir’s resiliency in future flooding. The repair project includes the repair of two breaches in the reservoir’s embankment, the addition of flood and erosion protection features, and the repair and partial realignment of the outlet pipeline that delivers water stored in the reservoir to St. Vrain Creek. The project also includes the removal and disposal of approximately 200,000 cubic yards of flood debris deposited within the reservoir during the flood event. The debris consists largely of sediment that will be used as fill material on this and other county flood recovery projects on adjacent county lands.
Design Documents, Design Reports, & Other Studies
- Western Mobile Lake 4 Dam Rehabilitation, Final Design (March 22, 2017)
- Western Mobile Lake 4 Sediment Removal, Final Design (March 3, 2017)
- Note: includes repairs to northwest embankment of the reservoir
Description of Project Components
The eastern embankment of Lake 4 functions as a jurisdictional dam. This dam is regulated by the State Engineer’s Office (SEO), Dam Safety Branch. BCPOS and the District have proposed to the SEO that the appropriate classification for the dam is – Small, Low Hazard. As part of this breach repair, a clay-core cut-off wall and a new outlet structure are being constructed to seal the reservoir and facilitate water releases under the reservoir’s adjudicated water rights and consistent with the SEO’s design criteria. The repair project will also include a new emergency spillway at the north end of the eastern embankment designed to convey potential future flood flows. A large breach in the northwestern embankment will also be repaired. This repair will include a clay-core cut-off wall. It will also incorporate a rundown channel that will facilitate the entry of up-gradient flood flows into the reservoir in a controlled manner. The spillway and rundown channel, along with other erosion control measures, are intended to protect the integrity of the reservoir and work in conjunction with other nearby flood recovery projects to improve the resiliency of facilities in the area during future flooding.
Concurrent with the flood repairs, BCPOS and the District propose to repair the Lake 4 Outlet Pipeline between the dam’s new outlet structure and St. Vrain Creek. The existing outlet pipeline did not function dependably before the flood. The repair includes lining about 2,100 feet of existing pipeline and constructing about 300 feet of new pipeline between 61st Street and St. Vrain Creek across the county’s Ramey property. A measurement structure and outfall will be co-located with an outfall from A-Frame Pond at St. Vrain Creek. For accounting purposes, the measurement components are being designed such that the Lake 4 releases will be measured separately from any flows from A-Frame Pond prior to entering the creek.
In addition, BCPOS and the District are seeking a cost effective means for disposing of the very large volume of flood sediment currently in Lake 4. Left in place, this sediment would reduce the water storage capacity of the reservoir. So, as part of the repair project, it is proposed to remove and dispose of the material from Lake 4. A portion of the sediment will be used as fill material in this project and other flood repair projects on adjacent BCPOS lands. It is proposed that the remaining portion of the sediment be placed in the former location of Lake 3, an unlined lake located of Lake 4 that was severely impacted by the 2013 flood and does not presently hold water.
Much of the project area lies in areas where gravel mining has occurred. While the Lake 3 area and other lands adjacent to Lake 4 are owned by Boulder County, Martin Marietta Materials (Martin Marietta) currently holds the mineral rights and permits for a mining operation with Boulder County Land Use (#SU-96-18) and the State Division of Reclamation, Mining, and Safety (M-1974-015). However, the Lake 4 Repair project is not a construction activity associated with any current mining permit. Boulder County is finalizing an agreement with Martin Marietta in order to place fill material within Martin Marietta’s existing mining permit area and to use staging areas and access roads that have been used during past mining operations.