Failing Culvert and Scour Hole – North Douglas Creek
Walter C. Pennington, PE, CFM
Merrick & Company
Denver, Colorado
Reconstructed Culvert Outlet with Energy Dissipation – North Douglas Creek
Unsafe Degraded Channel – North Douglas Creek
Improved Urban Restoration Channel – North Douglas Creek
North Douglas Creek is a tributary to Monument Creek located on the west side of the City of Colorado Springs, Colorado. North Douglas Creek conveys stormwater runoff from a fully developed six-square-mile watershed. The majority of the drainageway, apart from the last 2,500 feet, is a steep, concrete-lined trapezoidal channel. As a result of steep grades and an urbanized watershed, North Douglas Creek experienced up to 25 feet of vertical channel degradation at the downstream side of the Interstate 25 (I-25) corridor, which caused property losses, damage to utility crossings, and failure of a 12-foot by 14-foot culvert. North Douglas Creek was also a public safety hazard and provided an unwanted source of sediment load into Monument Creek.
The City of Colorado Springs sought to address the issues with North Douglas Creek by designing sustainable and durable channel improvements for a 1,100-foot channel section of the creek. The resulting North Douglas Creek stabilization project was a high-profile project for the Colorado Springs Stormwater Enterprise (SWENT) and was a major piece of their Intergovernmental Agreement with Pueblo County. The $4.6M construction project was partially funded by a Federal Emergency Management Agency (FEMA) Hazard Mitigation Grant Program (HMGP).
Unique challenges for this project included managing channel velocities in excess of 35 feet per second exiting a 12-foot x 14-foot culvert under I-25; split flows returning to the channel from 25 feet above the main channel thalweg; design of grade control structures for 25 feet of vertical fall along the 1,100-foot reach; balancing energy dissipation options; private property acquisition; public safety; incorporation of three storm drainage outfalls; constructability; permitting; political expectations; maintenance access; and the grant program schedule.
Six energy dissipation alternatives were analyzed and evaluated for effectiveness and safety. An innovative impact basin design was selected to dissipate the extreme culvert exit velocities. Channel erosion protection measures were minimized through the use of several strategically placed rock vane structures. Two-dimensional hydraulic modeling was utilized to guide the channel design decisions in addition to understanding the complex split flows between two transportation crossings. Grouted grade control structures utilized an innovative grout mix and construction technique which allowed the use of smaller boulders for the core of each structure.
Project construction was completed in March 2023.
About Walter C Pennington, PE, CFM
Coming SWalt Pennington is the Surface Water Practice Leader at Merrick & Company, where he brings a wealth of technical expertise in the area of rivers and stream ecosystems. With a focus on stream restoration and river processes, river recreation, storm drainage, floodplain management, and hydrology and hydraulics, Walt plays a pivotal role in shaping the landscape of surface water-related projects.
A proud graduate of Texas A&M University, Walt holds a bachelor's degree in Civil Engineering and a master's degree in Water Resources. His commitment to excellence is reflected in his status as a Certified Floodplain Manager (CFM) and a professional engineer licensed in six states.
Beyond his professional pursuits, Walt is a devoted father of three and a resident of Broomfield, Colorado. In his leisure time, he finds joy in diverse outdoor activities, including golfing, camping, hunting, fishing, and participating in competitive sports. Walt's passion for both his work and personal interests underscores his dedication to making a positive impact in the field of river engineering.