Engineering Naturally: Lefthand Creek Restoration
A transportation project encompasses wetland mitigation and restores habitat.
By Christopher P. Ricciardiello
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.
The project benefited the city and the adjacent property owners by making 230 ac. of land originally in the floodplain available for future development. This was achieved by confining the 100-year floodplain within a larger trapezoidal flood channel. The restoration provided the public with a natural area for enjoyment and added a link to Longmont's greenway recreational path system. By confining the floodplain, the project reduced the cost of building the State Highway 119/Ken Pratt Blvd. extension for CDOT while fulfilling the wetland mitigation requirements of the United States Army Corps of Engineers Section 404 permit. Additionally CDOW was able to maintain stream reaches of valuable vegetation and fish habitats and guide the construction of the new stream channel to create a sustainable fishery and a new fish and wildlife habitat.
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.
Construction proceeded from downstream to upstream and began in October 2001. Work consisted of 190,000 yd.3 of earthwork, a temporary irrigation sprinkler system, a concrete bikeway, 95 trees, and wetland plantings and landscaping. Construction of the low-flow channel began the first week of December 2001. Aquatic and Wetland Company of Fort Lupton, CO, a subcontractor of the state's Duran Excavating, cut the primary alignment of the low-flow channel with a bulldozer. In the 10-ft.-wide channel, the bulldozer cut 29 minor meanders every seven to nine channel widths, approximately 70–90 ft. apart. The next stage of grading the channel bottom was accomplished with a trackhoe with an articulated thumb. Within the meanders, the contractor tilted the bed to the inside curves and excavated 3- to 4-ft.-deep pools. The excavated sand then was placed opposite the pools to create sandbars. Boulders and cobbles were placed strategically in the stream between each pool and bend to produce riffles.
To protect the aquatic ecosystem from silt and sediments stirred up by the construction activity, the contractor built a temporary sediment pond at the downstream terminus of the project area. Later this pond was removed, and the area was planted with wetland vegetation.
To both stabilize the channel and provide habitats for fish, the contractor placed log structures and root wads with trunks firmly anchored in the streambank and roots overhanging the water at bends in the thalweg, or the main-flow channel within the low-flow channel. Each root wad structure consists of a unique configuration of tree roots, imported boulders, and existing concrete rubble uncovered during excavation. Rod Van Velson, CDOW aquatic researcher and stream channel expert, recommended utilizing existing concrete rubble for streambank stabilization in addition to its inherent habitat value for fish communities. Van Velson spent eight hours a day in his waders in the stream the whole month of December 2001 directing the trackhoe operator as he sculpted the low-flow channel one shovelful at a time.
Geotechnical borings taken during the initial physical-analysis phase of the project identified a layer of natural river cobble from the former streambed at a 7- to 8-ft. depth. Years of silt accumulating in the streambed and historic agricultural activities had buried the natural cobble material. This geotechnical feature signified that additional material did not need to be imported for the streambed, saving the City of Longmont a significant amount of money on the project. Lefthand Creek could revert to its historic, natural streambed condition.
"While we had a hydraulic model for the floodplain and overall design, the meandering low-flow channel was designed a root wad, a rock, and a planting at a time, relying on the experience of our stream expert Rod Van Velson and Carter & Burgess project staff showing the stream building contractor the placement of each element," Griffith says.
Where Lefthand Creek goes under US Highway 287 at the western boundary of the project site, the design specifies the installation of an extended boulder weir to facilitate the nondetrimental turning movement of the creek. The project team developed an objective necessitating that potential storm flows could not scour or wear away the soil material constituting the streambank at this location. This curving wall of boulders varies in height from 3 to 4 ft. and was constructed at specific elevations so a larger storm event would overtop the weir and not damage the low-flow channel as it moved into the overflow channel. Other elements addressed in the hydraulic model include the diversion structures for the Bonus Ditch Company and the Beckwith Ditch Company, both of Longmont; the Ken Pratt Blvd. bridge; and mapping of abutment locations and utility sleeves for a new bridge at Martin St. to be built when development occurs.
Vegetation and Fish Habitats
The construction in the fall jump-started vegetation growth and the emergence of fish habitats along the banks of the new channel by transplanting existing vegetation. Under the guidance of Van Velson, the trackhoe operator excavating the thalweg also skillfully scooped up plugs of existing vegetation and transplanted them to new locations along the banks. Plants and seeds were ready to germinate with the spring runoff and warm weather.
"We lucked out by not having too much runoff the first year on the new vegetation," Van Velson observes.
Landscaping and wetland work began as the spring of 2002 approached. To ensure wetland planting success and provide a demonstration planting method, the project specified that Seattle-based Bitterroot Restoration's Wetland Rollsod, an erosion control–type mat preplanted with germinated wetland plants native to the area, be incorporated into the matrix of the fabric and placed in areas of potentially erosive streamflows. This product provides an immediately vegetated bank and necessary protection against erosion. To aid in the root development of planted willow brush-layer cuttings, the willows were dipped in mycorrhizal fungi. Willow cuttings were placed, Wetland Rollsod was installed, and 16,000 individual wetland plugs were hand-planted. Upland grass seed was drilled, and trees and shrubs were planted.
Wetland and upland plantings provide valuable erosion control, and developed root systems of willows and perennial grasses bind the soil. These species are especially important in floodways since their flexible stems bend during flood events instead of wash out, according to Backus. Similarly, silt and waterborne contaminants from adjacent development stormwater outfall channels were filtered sequentially through upland and wetland plantings into the overflow channel for several hundred feet before being discharged into the low-flow channel.
The original plan was to take the grade for the creekbed down to groundwater to assist with the plants' root development, but in 2001 Colorado still was experiencing drought conditions. That year rainfall totaled only 7.48 in., well below the average precipitation of 15.81 in. The groundwater table fell below the grading level of the excavated channel, making it difficult to establish plantings. With the slow release of meltwater from record snows deposited in the upper mountain reaches of Lefthand Creek during the blizzard of March 2003 and with subsequent spring rainfall, however, the groundwater table rose, allowing for seed germination, vegetation, and root development.
Although the original goal was to create 3.64 ac. of wetland mitigation for the Ken Pratt Blvd. extension and the Lefthand Creek restoration, runoff from the March 2003 blizzard would prove the value of the design. Lefthand Creek experienced so much runoff from snowmelt that it flooded several times in the 2003 growing season. With these favorable weather conditions, the project exceeded expectations and grew to 7.25 ac. of wetland. On September 25 and 28, 2003, Backus conducted a wetland delineation documenting the establishment of wetland conditions on 7.25 ac. in the floodway channel west of the Ken Pratt Blvd. bridge.
Taking pits, she discovered the soils were saturated to the surface and driftlines were evident from spring and early-summer flooding. She notes in her report, "The wetland soils are in the process of development following grading of the new floodway channel in 2002. Soils from 0 to 5 in. were very dark brown sandy loam, and soils from 5 to 12 in. were sand and gravel."
To maintain the high-quality wetland and wildlife habitat, she recommended ongoing weed control. Canada thistle, one of the top 10 noxious weeds in Colorado, was well established in adjacent areas and likely to infest the edges of wetland sites. Another weed, Russian olive, was beginning to invade the wetland meadows. In the plant delineations, she also identified two threatened species: the Colorado butterfly plant and the Ute orchid. As further evidence of the wetland habitat, she discovered a beaver dam and a pond in the new channel.
As previously mentioned, Lefthand Creek is one of only two watersheds in the state to shelter common shiners. To protect habitats for this endangered species, CDOW participated in the planning and execution of the Lefthand Creek restoration. In September 2001, CDOW Aquatic Biologist Randy Van Buren shocked the common shiners and transported them to the fish hatchery in Alamosa, CO. Five hundred common shiners spawned from the captured fish and were released in the new portion of the channel on August 21, 2003. Van Buren also moved other fish to a holding pond in the new channel. Postconstruction sampling showed that all 13 species of small fish found in the old creek are thriving in the new fish habitat in the restored creek.
Resulting Win-Win Project
The city entered the project in the Colorado Chapter of the American Public Works Association Project of the Year Awards and received first-place recognition in the Utility, Drainage, and Environmental category for medium communities. Longmont also proudly entered the project in the 2003 Colorado Association of Stormwater and Floodplain Managers' awards program.
"We are truly pleased for the City of Longmont," says Griffith. "This multiuse project was genuinely rewarding in how local, state, and federal agencies worked together to create an environmentally sensitive restoration of the channel."
The engineering community also applauded the restoration. Carter & Burgess submitted the project to Colorado's American Council of Engineering Companies' (ACEC) prestigious Excellence Awards competition in the Water Resources category, and the entry captured first place. Graphics created to illustrate the beauty of the restored creek also earned the top prize in graphic design at the ACEC Excellence Awards.
"This project had so many positive aspects, everybody is a winner," says Hollingsworth, adding, "We were fortunate this opportunity came to the city this time as so many times [projects go to] the developers. The cooperation among technical people, such as the floodplain managers and designers and the park planters out in the field, made this work."
While the Colorado region is still in the drought, Longmont is monitoring the project for sustainability. The redefining and confinement of the floodplain have removed 320 ac. that now can be developed. Longmont already has built its new recreation center and a museum in this area. Future plans call for a community-college campus in the area.
Longmont continues to monitor development on the Lefthand Creek corridor and requires developers to construct holding swales to capture water draining off the property. The upstream portion that was untouched to preserve fish habitats has a parallel overflow channel. The drainage runoff from the adjacent property is designed to flow into a new grass swale in the overflow channel and use the water-quality enhancement of the swale prior to discharging into the creek. Drainage flows on the other side of the creek will be designed to be piped downstream of the sensitive nature area before being released into the creek.
"I submitted the two award applications and participated in the ACEC award application because I'm really excited about the asset this project brings to our community and feel the teamwork that made it happen deserves recognition," Hollingsworth concludes. "Longmont is pleased to share information on our successful erosion and flood control project."
Guest author Christopher P. Ricciardiello, RLA, is a senior designer with Carter & Burgess's Urban Design Group.