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St. Vrain Creek Reach 3 Flood Damage Restoration

Boulder, Colorado

Significant flooding and damage occurred along a 4-mile stretch of St. Vrain Creek between the Town of Lyons and the Town of Hygiene in Boulder County in September 2013. Very heavy rains in the mountains west of Lyons created a 500-year flood event with flows of greater than 12,000 cfs in the creek.

Engineering Analytics was retained by Boulder County Parks and Open Space (BCPOS) to provide design for repair of one embankment dam, two reservoirs, and seven embankment breaches along the creek. The overall project goal was to provide a design that met the requirements of the NRCS Emergency Watershed Protection Program and FEMA funding constraints. The design was prepared to mitigate the impacts of the 2013 flooding along the creek and to reduce the impact of future flooding on life and property, while restoring reservoir capacity and the riparian and wildlife habitat throughout the reach.

The initial site studies included geotechnical field and laboratory investigations, hydraulic analyses, geomorphology study, sediment transport analyses, and characterization of aquatic and terrestrial species of the corridor.

Preliminary and final designs were prepared for the 30 foot tall embankment, slurry wall cutoff, and replacement low level outlet works. The embankment breach repairs were designed to resist flood flows and reduce underseepage and internal erosion during flood flows. Embankments were founded on the undisturbed native soils below sediment and disturbed materials.

The restoration design included in-channel structures and bioengineering work to improve geomorphology and sediment transport; ecological restoration within a restoration corridor adjacent to the channel; and preliminary grading and design plans for the proposed channel and breach repairs. The design provided detailed site plans including overall grading, stream restoration, and planting and seeding work, including details and cross sections.