Client: Boulder County, Parks & Open Space
Contract Value: $2,333,917
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Stream restoration at Hall Ranch located just west of the town of Lyons was required to restore approximately 1.2 miles of South St. Vrain creek severely damaged by flooding of September 2013. Boulder County through its Parks and Open Space Department (BCPOS) owns and manages the majority of the project areas (also referred to as Hall Meadows). Colorado State Highway 7 borders the construction limits to the north, and Old St. Vrain Road (CR 84S) borders the construction limits on the south. The construction areas also contain a few private properties, as well as land owned by the City of Longmont. South St. Vrain Creek is an alluvial system that was highly altered during the historic 2013 flood event. Natural alluvial channels in lower gradient reaches generally meander through the valley, occasionally shifting lateral and/or vertical position on the landscape during large flood events. In the case of the recent flood, South St. Vrain Creek experienced an episodic shift in the channel planform and cross-section geometry and substantial sediment aggradation and deposition occurred throughout the project area. Thus, the channel widened as the banks receded and new flow paths formed through the floodplain. Without intervention, the channel could continue to shift position, so to minimize the threat to existing infrastructure, engineered improvements were required to stabilize the channel and restore ecologic function.
SCOPE OF WORK:
- The Project consists of main and overflow channel grading and reshaping, restoring floodplain connectivity, installation of instream rock structures, bioengineered bank stabilization, incorporation of engineered large wood structures at stream banks and floodplain, riparian and upland revegetation collectively using mitigation measures to reduce the impact of future flood (that will be sustainable), resilient and benefit ecological values.
- Secure permits Mobe, project controls, erosion control, demobilization, and creek restoration at various reaches per construction drawings and specifications.
- Site prep, including survey, sediment and erosion control Best Management Practices (BMP), soil analysis and testing, removing, and stockpiling topsoil.
- Clearing & grubbing within limits of disturbance especially woody material having DBH < 6″ at 4.5′ above ground surface.</>
- Remove abandoned utilities and flood generated trash and debris along the project corridor.
- Remove flagged trees i.e. trees over 6″ for re-use and salvage trees > than 12″ with root wad intact for reuse in large wood structures (LWD).
- Dewater temporary excavations in and off channel.
- Selective native streambed stripping, stockpiling and replacement of existing native river bottom material (sand, gravel, cobbles).
- Channel and floodplain excavation in conformity with construction drawings.
- Sort and stockpile of large, alluvial rounded rock and boulder material within the grading limits.
- Haul excess unwanted material to predetermined disposal site.
- Final excavation and shaping of reconstructed creek bottom after other mass grading to create to create pools, bank contouring and low flow channel.
- Install large wood structures of different types for bank stabilization, flood plain roughness and ecological values.
- Installation instream rock structures such as riffle crest, riffle floodplain sill, floodplain sill overflow rock ramp and knick point stabilization.
- Install instream boulders for channel roughness and complexity.
- Install bank stabilization structures such willow cuttings in cobble toe, willow cuttings in existing rip rap, and boulder toe construction.
- Prepare final grades, incorporate soil amendments, install erosion control fabric, seeding and mulching for riparian and upland revegetation.
- Plant trees, shrubs, dormant log poles, perennial (tubelings), willow cuttings and willow fascines shown on construction drawings.
- Cage specified plantings against beaver and small mammal herbivory.
- One year plant establishment warranty (corrective measures if needed) and/or temporary watering as determined by the contractor for plant survival.
- Restore all disturbed areas within project limits and project closeout.