US river restored after devastating flash flood event

In 2013 the city of Longmont in Colorado, US, experienced the most devastating flash flood event in its history. Categorized a ‘500-year event’ this flooding ravaged the St Vrain River and watershed near Lyons, Colorado, and destroyed homes, property, infrastructure, and habitat.

A heavily damaged area included Apple Valley, a 2.27-miles stretch of the river, upstream of Lyons. This low-slope reach of the river provided valuable and rare habitat to native species, but it was heavily impacted by the flood, and it remained habitat non-functional since the flood.

Ramboll was therefore hired to help restore the stream function that not only protects life and property but also returns the river to a stable ‘geomorphic equilibrium’, which means that the surface of the landscape can adjust to changes by reaching a steady state.  

“The approximate 7,400-foot reach was restored to protect life and property and improve the overall aquatic habitat and ecological function of the river by increasing the complexities of hydraulic habitats. The newly restored river will be more resilient and more capable of withstanding future flooding events, and will thrive in lush aquatic, riparian, and wildland habitat along its banks,” says Felix Kristanovich, PhD and Hydrology expert in Ramboll’s global water division.

Friendly neighbor mitigation design

The project design was modified several times to satisfy Boulder County Floodplain Development and Stream Restoration Permit requirements and Colorado Department of Transportation hydraulic criteria. Modifications were made during construction, and “friendly neighbor” mitigation design alternatives were negotiated with different land owners along the creek. 

The project was successfully constructed during an atypical dry and warm 2017/2018 winter. 

Felix, who is also a Board Member of the American Water Resources Association, will present findings and insights from this project at the 2018 AWRA conference in Baltimore November 4-8.

Learn more about the project (PPT, 35 MB).