Stream Restoration and Innovative Grade Control in an Arid Developing Urban Area
Jeanne Boyle, PE, CFM
Jeffrey Butson, PE, CFM
Merrick & Company
Denver, Colorado
Authors:
-- Jeanne Boyle, PE, CFM, Merrick & Co.
-- Jeffrey Butson, PE, CFM, Merrick & Co.
-- Jim Watt, PE, CFM, Mile High Flood District
The two-mile Happy Canyon Creek design reach is a sinuous and steep drainageway located in the developing southern suburbs of Denver, CO. The watershed is seeing rapid population growth, leading to increased runoff and degradation in the stream bed and banks. The project is being completed as a partnership between the land developer, the Mile High Flood District (MHFD), and the City of Lone Tree (CoLT). The goals of the project are:
Restoration of degraded stream bed and bank areas
Reconnection of the creek to its floodplain to reduce erosive shear stresses
Protect against future erosion from an anticipated increased urban runoff
Provide reduction of flow rates to below pre-development conditions for downstream flood protection.
The flood peak reduction method used was peak-shaving ponds, where structures were used to moderately reduce lower frequency flood flows but pass higher frequency flows without attenuation. In addition, for lower frequency storm events, the timing of peak flows along the main channel was modified compared to tributaries that allowed for peak flow reductions to below pre-development levels.
Techniques used for stabilization included the use of vegetated soil lifts, rock riprap with interstitial fill material, sculpted concrete drop structures, and rock riffle grade control. A technique to wash in smaller materials into rock riprap voids to provide a well compacted hard protection that is visually appealing was implemented.
To reduce initial construction costs and to preserve natural appearance, selected stabilization improvements were not constructed, such as laying slopes flatter and installing rock riprap erosion protection in well vegetated areas. An adaptive management and monitoring plan have been developed to assist the MHFD and CoLT to identify and address potential future erosion issues. The anticipated maximum limits of disturbance to flatten these slopes were identified and permanent improvements, such as trails, were located beyond those limits. Adaptive management drawings were developed.
This project included permitting efforts, including a USACE Regional General Permit. To compensate for a significant channel length reduction at a proposed roadway crossing, several secondary low-flow channels were provided to create aquatic and riparian habitat areas.
About Jeanne Boyle, PE, CFM
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About Jeffrey Butson, PE, CFM
Jeff is a registered professional engineer with 24 years of experience in water resources engineering, civil site design, dam engineering, lake dredging, and mine reclamation. He is currently managing the design and construction of multiple stream restoration projects in the Denver and Colorado Springs metropolitan areas. Other water resources experience includes detailed hydrologic analysis, open channel and closed conduit hydraulic studies, scour analysis, and NFIP studies. Dam engineering experience includes planning and construction of new dams, dam safety, and rehabilitation. Site design experience includes grading and drainage design for greenfield sites and retrofitting or existing sites.