A residential area in Centennial, Colorado was located in a 100-year floodplain and was susceptible to frequent flooding. The flooding potential posed risks to public safety and increased the potential for property loss.

The Mile High Flood District (MHFD), Southeast Metro Stormwater Authority (SEMSWA), and the Cherry Creek Basin Water Quality Authority (CCBWQA) chose to address this issue by designing channel improvements on Piney Creek and Antelope Creek to reduce the floodplain and eliminate a split flow on Piney Creek. Removing this split flow on Piney Creek would remove habitable structures from the floodplain designation. These groups tabbed Olsson to work on the project because of our extensive expertise in water resources engineering. We teamed with several subconsultants to bring this project to completion: Stream Landscape Architecture and Planning provided planting plans and assisted in property owner negotiations, Kumar & Associates, Inc. completed the geotechnical analysis, and ERO Resources Corporation completed 404 permitting.

We were tasked with completing a topographic and boundary survey, evaluating alternatives, conducting floodplain modeling, preparing construction documents, assisting with utility relocations, coordinating the design with several other projects on Piney Creek and Antelope Creek, and preparing Letters of Map Changes (LOMCs). We completed extensive hydraulic modeling to verify that the goal of eliminating the 100-year split flow that put structures in the floodplain designation was achieved. The project was located on private property, which required extensive easement acquisition negotiations with multiple property owners. Olsson assisted with the negotiations and prepared many design alternative iterations to find the best one to meet each individual property owner’s needs and the overall project goals.

A geomorphic assessment of Piney Creek, completed by Stantec, determined that soils in the project reach were susceptible to movement because of their sandy nature. So, we worked with Stantec to design the channel section specifically to address this challenge. Meanders in the stream alignment were spaced and located to best suit the geomorphology in the channel within the given site constraints and property owner negotiations. Pools and sandbars were graded into the channel low flow at bends to help with aggradation and degradation, and grade control structures were designed to reduce the channel slope to improve stability.

The Piney Creek channel improvements significantly enlarged the channel. The improvements also included construction of grouted sloping boulder drop structures, sheet pile cutoff walls, and bank protection, including a stacked grouted boulder wall. The Antelope Creek channel improvements included a grouted sloping boulder drop structure, a sheet pile cutoff wall, and a check structure low flow crossing. Olsson prepared a CLOMR that was approved by FEMA.

We provided construction observation and materials testing services when the time came to build the project. When construction was finished, Olsson completed an as-built survey and developed a LOMR that included two additional projects that were constructed at the same time: a second reach of Piney Creek upstream of this project location and a new culvert on Antelope Creek under Arapahoe Road.