Engineering Naturally: Lefthand Creek Restoration

In Longmont, CO, the restoration of Lefthand Creek and its floodplain to a more natural condition proved a winner for the responsible community, city, and design engineers. The restoration of 4,500 ft. of the creek south of Longmont began in October 2001 and was completed in August 2002. The successful project positively affected numerous stakeholders, including the City of Longmont, adjacent property owners, the public, Longmont’s Greenway Program, the Colorado Department of Transportation (CDOT), the Colorado Division of Wildlife (CDOW), and two ditch companies.

Background and Planning
“The restoration started as a spinoff from the Ken Pratt Boulevard extension we were designing for the city,” explains John Griffith, project manager of transportation programs for Denver, CO-based Carter & Burgess, which provided the environmental permitting, engineering, and landscape architecture design for the project.A mile of the proposed 2-mi.-long extension of Ken Pratt Blvd. would fall within the 1-mi.-wide, 100-year floodplain of Lefthand Creek. Therefore either a mile of Ken Pratt Blvd. would have to be elevated 4 ft. and the embankment would have to be protected from floodwaters at an estimated cost of $650,000 or the floodplain would need to be contained. In addition, CDOT required a 2.4-ac. site as mitigation for associated roadway projects, including the construction of bridges over Lefthand Creek and the St. Vrain River. So the project team began studying the five creeks in Longmont’s watershed as possible mitigation sites and decided this 4,500-ft. reach of Lefthand Creek was the best candidate for new wetlands.Lefthand Creek is a continually flowing stream that begins at the Continental Divide in the Rocky Mountains 27 mi. west of Longmont. The upstream watershed contains 72 mi.2 of terrain, which varies from mountain peaks to plains. Longmont, which began as a small farming community and grew into an independent city of 77,328 people, is located 30 mi. north of Denver.Once a meandering stream bordered by oxbow sloughs, streambank wetlands, and cottonwood groves, Lefthand Creek has been altered by agricultural and urban development. More than 100 years of farming and development along the creek straightened and filled in the narrow channel with silt, trash, and agricultural debris. The contractor for the project even discovered a Model T Ford buried in the silt adjacent to the unrestored channel during excavation work. Only a narrow fringe of wetland vegetation remained after most of the adjacent wetlands gradually had been graded for agricultural fields and overrun with invasive weeds. Urban and agricultural runoff contributed sediment, nutrients, and toxicants to the streamflow, degrading water quality and aquatic habitats. A hydraulic model using HEC-RAS, a floodplain hydraulic analysis software program developed by the US Army Corps of Engineers, revealed that the channel would convey only 20% of a 100-year flood event. The project evolved through several stages, with the team considering and eliminating four alternatives before arriving at the decision to do a natural restoration. The design team eliminated the first alternative of minimal channel improvements limited to the State Highway 119/Ken Pratt Blvd. bridge area due to the cost of elevating a mile of the boulevard but still leaving land planned for Longmont’s recreation center in the floodplain. Potentially unacceptable regulatory and maintenance requirements of the Federal Emergency Management Agency eliminated another alternative: constructing a dike along the top of the existing channel to contain the floodplain. A third alternative-constructing a parallel overflow channel south of the existing channel-would have required acquisition of more land outside of the existing channel corridor. Construction of a trapezoidal channel with riprap-lined sides, a 150-ft. top width, and an average channel depth of 6.5 ft. also was eliminated because the shallowest portion near the proposed State Highway 119/Ken Pratt Blvd. bridge would be required to accommodate the 100-year flood event. Opportunities to restore riparian habitats also were limited.To begin the natural restoration, the design team studied aerial photographs taken in 1949 and 1955. “Although the channel had already been straightened by 1949, we could see evidence of the meanders and terrace marks from former flood events in the aerials,” says Laura Backus, environmental scientist for Carter & Burgess.In addition to studying aerial photographs, the project team visited two stream reference reaches. As a Wildlands Restoration Volunteer, Backus had worked in the Lefthand Creek area 8 mi. upstream near Boulder. In this upstream plains area, even though years of cattle grazing had removed the native streambank willows and grasses, the morphology of the stream channel and flood terraces still was intact, according to Backus. A second riparian forest, consisting mainly of cottonwoods just downstream from the grazed property, echoed the area’s past and inspired the team with the possibility of restoring meanders, islands, fish habitats, and natural stream vegetation. All but the islands would become part of the final project.Having embarked on a greenway system that continues to grow since 1992, Longmont officials are proponents of natural restoration. They embraced the plan to restore this unused natural resource to its original state.“It made more sense to spend $1.5 million on the Lefthand Creek restoration than to put off the project and waste $650,000 raising Ken Pratt Boulevard and still leave our recreation center site and other developable land in the floodplain,” remarks David Hollingsworth, Longmont’s civil engineer in charge of the project.Challenges to the Design and Construction
When Carter & Burgess worked with the US Army Corps of Engineers for the Section 404 wetlands mitigation permit, the corps suggested following National Permit 27, “Streams and Wetland Restoration Activities.” During the application process for Colorado Senate Bill 40, “Wildlife Certification” (legislation that protects state fishing waters and stream ecosystems), CDOW voiced concerns that the nearby St. Vrain River supported populations of two endangered fish species. The common shiner (Notropis cornutus) and the stonecat (Noturus flavus) are small fish species that require moderate gradient streams with clear water, gravel bottoms, and shade from shrubs and trees. Only two known watersheds in the state shelter common shiners: the St. Vrain and Lefthand Creek watershed and the Plum Creek watershed south of Denver. “The Division of Wildlife, as a condition of the permit, stipulated that we preserve the upper 1,500 feet of Lefthand Creek in its present state for fish habitat, so we had to create the overflow channel in the floodplain for the major flood flows, which was a bit of a challenge,” Griffith explains. “We had to design the project to stay within the 365-foot right of way the city obtained from the adjacent property owners.” “The goal for flood control is to create a natural channel that will convey 5,000 cubic feet per second,” Hollingsworth adds. The first step in the new design was creating the primary channel to carry perennial flows and minor storm events. To provide flood protection, Carter & Burgess modified the design to furnish an overflow channel for floodwater conveyance. The floodplain, which was a mile wide before restoration, now is contained within the 250-ft.-wide, approximately 8- to 10-ft.-deep trapezoidal channel with 4:1 sideslopes. Then the team designed the low-flow channel within the primary channel.With the small gradient change in the area to be restored, the design team didn’t have the luxury of drop structures to slow the flow of the water and prevent erosion or to add interest to the stream. Instead the team chose to meander the low-flow channel within the channel width from side to side.

Guest author Christopher P. Ricciardiello, RLA, is a senior designer with Carter & Burgess’s Urban Design Group.