Lefthand Creek Restoration

By Hannigan Fox, Carter & Burgess

Terraces were constructed via grading to support the revegetation, then natural-fiber sod blocks were preplanted with rushes and sedge for rapid growth. The sod blocks also provided bank protection where the stream curved. The Colorado Div. of Wildlife specified sandbar willow and non-native reed canary grass plantings.

The Lefthand Creek Channel improvement project in Longmont, Colorado started as a spinoff of the 3.2-meter extension of Ken Pratt Boulevard Carter & Burgess was designing for the city.

It evolved into restoring a 1.4 kilometer channeled reach of Lefthand Creek to more natural conditions and creating a wetland mitigation site for the project and for the Ken Pratt Boulevard (State Highway 119) extension. Reconstruction of the artificially-wide 100-year floodplain allowed construction of the Ken Pratt Boulevard extension, a city recreation center, a museum and a Greenways bicycle trail.

The Longmont team included staff members of the Colorado Department of Transportation (CDOT), the Colorado Division of Wildlife (CDOW), and Carter & Burgess engineers, environmental planners, landscape architect, and wetland scientist. The design evolved from a trapezoidal concrete and riprap channel to a meandering creek with pools and riffles. The Denver Transportation Programs unit at Carter & Burgess, with John Griffin project manager, oversaw environmental permitting, engineering and landscape architecture design.

Completed in 2002, the new corridor satisfies Longmont?EUR??,,????'???s planning requirements, provides wildlife habitat, and is a scenic asset to the city. The project also garnered recognition from the professional community. The Colorado Chapters of the American Public Works Association and of the American Council of Engineering Companies presented the project with two first-in-category awards.

The first step in the 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.

Site Conditions

Since previous filling of the floodplain and straightening of the stream had created a 1.2-kilometer-wide 100-year floodplain, construction choices for the highway extension were to elevate the roadway 1.3 meters (at a cost of $650,000 ) or to modify the floodplain.

Lefthand Creek?EUR??,,????'???s altered conditions are typical of Colorado front range streams. The plains reaches have been heavily grazed, native cottonwood forests cut and the streambed channeled into a straight, steep-banked course to accommodate agricultural fields and bridge crossings. Additionally, irrigation flows are diverted and transferred at many points along the stream.

Channelization of flows in Lefthand Creek and its tributary, Dry Creek, had eliminated the natural floodplains. The active stream channel varied from 2.5 to 6.3 meters wide, and flows were typically 0.3 to 0.6 meters deep. Hydraulic modeling showed that Lefthand Creek conveyed only the 10-year flood of approximately 22 cubic meters per second. Carter & Burgess estimated a 100-year flood of 132 cubic meters per second would extend up to 480 meters north and 750 meters south.

Wetland delineations showed narrow (one to nine meter) bands of emergent and scrub/shrub wetlands. Dominant vegetation was native sandbar willow (Salix exigua) and peach-leaved willow (Salix amygdaloides), as well as nonnative reed canarygrass (Phalaroides arundinacea) and crack willow (Salix fragilis). Scattered native plains cottonwoods (Populus deltoides subsp. monilifera) were also present. Canada thistle (Breea arvensis), a Colorado noxious weed, infested much of the upper stream banks.

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 discovery of a Model T Ford buried in the silt adjacent to the unrestored channel during excavation work eliciting speculations about its demise. An accident? A bootlegger or gangster fleeing police? Or just a mundane case of dumping?

Design Process

As the first step in the Lefthand Creek channel improvement, the design team studied aerial photographs taken in 1949 and 1955.

?EUR??,,????'??Although the channel had been straightened by 1949, we could see evidence of the meanders and terrace marks from former flood events in the aerials,?EUR??,,????'?? observes Laura Backus, an environmental scientist with Carter & Burgess Inc., in Denver.

The plan evolved through several stages with the team considering and eliminating four alternatives before deciding on a natural restoration. Longmont officials embraced the opportunity to restore this damaged natural resource.

Top: Previous filling of the floodplain and straightening of Lefthand Creek and its tributary, Dry Creek, created a floodplain about half a mile wide. The active channel varied from about 8-20 feet wide, and only a few inches to a foot and a half deep.
Above: The creek is in Longmont, Colorado, elevation 4,979, located 37 miles from Denver and 16 miles from Boulder. The Rocky Mountains are in the background.

?EUR??,,????'??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,?EUR??,,????'?? remarks David Hollingsworth, city of Longmont project civic engineer.

The U.S. Army Corps of Engineers permitted the project under Nationwide Permit 27 Streams and Wetland Restoration Activity. CDOW voiced concerns that since the St. Vrain River near its confluence with Lefthand Creek supported populations of two rare fish species, common shiner (Notropis cornutus) and stonecat (Noturus flavus), they were likely present also in Lefthand Creek. These small fish species require moderate gradient streams with clear water, gravel bottoms and shade from shrubs and trees.

?EUR??,,????'??The Division of Wildlife, as a condition of the Colorado Senate Bill 40 Certification, stipulated that we preserve the upper reach of Lefthand Creek in natural state for fish habitat, so we had to create the overflow channel in the floodplain for the major flood flows which was bit of a challenge,?EUR??,,????'?? Griffith explains. Also, the project had to remain in the right-of-way obtained from adjacent property owners.

Redtop (Agrostis stolonifera), barnyard grass, reed canarygrass and rabbitfoot grass were among the plant species introduced to the Lefthand Creek mitigation wetlands.

Final design specifications were a 75-meter top width channel, 1.8 to 2.1 meters deep to convey the 100-year flood. To achieve the ecological project objectives, the channel length was slightly increased and included a three-meter wide meandering low-flow channel with pools excavated every seven to nine stream widths. Where possible within the constraints of the project corridor, the existing stream channel minimum depth of the low-flow channel would be maintained for undisturbed fish habitat. Grading would construct a low wetland terrace with an average nine-meter width and a higher riparian terrace with an average 18-21 meter width and 4:1 slopes.

The widened floodway channel would support tree and shrub plantings on stream terraces.

The geotechnical borings had identified a layer of natural cobbles from the former streambed approximately two meters below the surface. The design returned the creek to its natural bottom saving Longmont the cost of importing streambed materials.

Options for the highway extension (right) were either to elevate the roadway about 3.5 feet, an expensive option that was rejected, or to modify the floodplain, the result of which is seen here.

Construction

The engineers timed channel grading during low-flow conditions from October 2001 to January 2002. Under the direction of Duran Excavating, heavy earth moving equipment including scrapers and front-end loaders, graded the new channel configuration.

Rod Van Velson, a CDOW aquatic researcher and stream channel expert, spent eight hours a day in his waders in the stream during December directing the backhoe operator as he sculpted the low flow channel one shovel full at a time with a flexible ?EUR??,,????'??thumb.?EUR??,,????'?? The backhoe placed root wads, the irregular root fan still attached to the trunk, at expected scour sites with the root mass extending into the creek a slight angle and the trunk buried and protected with boulders selected for natural appearance. Within the meanders the contractor tilted the bed to the inside curves excavating one-meter deep pools, then placed the excavated sand placed opposite the pools to create sandbars. Boulders and cobbles are strategically placed in the stream between each pool and bend to produce riffles.

Where Lefthand Creek dives under U.S. 287 at the west end of the project, the contractor built a boulder weir to keep normal flows in the current channel and to allow flood waters to over top the boulder line into a wide overflow channel south of the stream. The contractor added utility sleeves to facilitate future development on adjacent lands without necessitating future construction in the channel.

The native plants thriving along the stream include cattails (Typha latifolia); sandbar willow; switchgrass; yellow foxtail; slender wheatgrass; bulrush and spikerush.

Channel Vegetation and Fish Habitat

Revegetation strategies included direct transplanting of sod blocks on the outside banks of curves in new streambank locations to provide additional bank protection. Species were mainly sandbar willow and nonnative reed canary grass specified by CDOW for immediate growth of wildlife habitat structure and rapid soil stabilization. Sod blocks of natural fiber mat (Wetland Rollsod??????oe, supplied by Bitterroot Nursery in Montana) were preplanted with Baltic rush, soft stem bulrush, and Nebraska sedge for rapid development of wetland vegetation at streambank stress points. Prior to planting, willow brush layer cuttings were inoculated with AgBio-Ectos, a blend of ectomycorrhizal fungi used to help developing root systems better absorb nutrients.

The landscape architect timed initial native landscape plantings for fall 2001 to stabilize newly exposed soils with a nurse crop as well as appropriate wetland and upland species. Seeding rates were doubled in wetland areas. Tall upland grass species were added to the seed mix to produce quickly growing wildlife cover. Prior to seeding, noxious weed infestations (mainly Canada thistle) were spot sprayed with glyphosate. In spring 2002, following development of the new site hydrology, in the second vegetation effort workers planted riparian trees and shrubs and plugged wetland species. Aquatic and Wetland Company provided the seed and nursery stock.

The vegetation plantings in the wetlands and adjacent uplands provide valuable erosion control as the root systems of the willows and perennial grasses bind the soil, gravel and cobbles. These species also prevent floodway erosion as their flexible stems bend during flood events instead of washing out, according to Backus.

Although drought conditions persisted throughout 2002 spring and summer, when Backus conducted a survey in August, plant communities had begun in areas of standing water and sites adjacent to stream and stormwater flows of the new floodplain. Deer, beaver, toads, and waterfowl frequented the new flood channel. Wetland species along the creek included planted or seeded sandbar willow, softstem bulrush (Schoenoplectus lacustris subsp. creber) hardstem bulrush (Schoenoplectus lacustris subsp. acutus), three-square bulrush (Schoenoplectus pungens), sedges (Carex spp.), and spikerush (Eleocharis palustris). Volunteer sandbar willow and blue vervain (Verbena hastata) were also present. In the dry areas a cover of weedy wetland and nonwetland species including kochia (Bassia sieversiana), purslane (Portulaca oleracea), goosefoot (Chenopodium spp.), reed canarygrass, curly dock (Rumex crispus), rabbitfoot grass (Polypogon monspeliensis), puncture vine (Tribulus terrestris), barnyard grass (Echinochloa crus-galli), alfalfa (Medicago sativa), and teasel (Dipsacus fullonum) were developing. In most of the irrigated upland areas grasses and flowers were growing. Most planted trees and shrubs were vigorous.

In 2003 following the 5th wettest March on record, groundwater rose in the floodway channel, and germination of wetland species increased. The bank stabilization structures and plantings protected streambanks from erosion as spring runoff spilled into the new overflow channel. By the end of growing season, a site survey documented well-vegetated, high diversity wetlands in the new Lefthand Creek floodway channel and attainment of 1:1 wetland mitigation goals. Dominant plant species were native sandbar willow, switchgrass, foxtail barley (Critesiom jubatum), slender wheatgrass, cattail (Typha latifolia), bulrush, spikerush and nonnative reed canary grass, barnyard grass, redtop (Agrostis stolonifera), and lady?EUR??,,????'???s thumb (Polygonum persicaria). Numerous seedling cottonwoods, as well as weedy Russian-olive (Eleagnus angustifolia), had sprouted in the overflow channel south of Lefthand Creek. Soils were saturated to the surface, and drift lines were evident from spring and early summer flooding. Wetland boundaries are anticipated to fluctuate with area development and precipitation cycles.

In August 2003, CDOW released 500 Common Shiners bred from fish captured by electro-shocking before construction. Randy Van Buren, CDOW fisheries biologist, also released fish from a holding pond into the new channel. Postconstruction sampling showed that all 13 species of small fish found in the old creek are thriving in the restored creek.

To begin the natural restoration, the design team studied aerial photographs of Lefthand Creek taken in 1949 and 1955. The photographs provided evidence of the meanders and the terrace marks from flooding. Carter & Burgess constructed wetlands to mitigate wetland losses due to the bridge construction of State Highway 119 over Left Hand Creek and St. Vrain River.

Future Monitoring and Maintenance

The city is monitoring development in the Lefthand Creek corridor requiring developers to construct holding swales to capture water draining off their property. The untouched upstream portion has a parallel overflow channel. The runoff from adjacent property will flow into the grassy swale at the west end parallel to the overflow channel before discharging into the creek. Drainage flows on the other side of the creek will be piped downstream of the sensitive nature area.

Longmont committed to a three-year maintenance plan including temporary irrigation, spot re-vegetation and weed control, particularly of Canadian thistle and Russian olive.

?EUR??,,????'??The success of the project shows that, with teamwork, urban flood corridor projects can integrate natural stream design techniques to rehabilitate degraded streams and provide a scenic amenity to city populations,?EUR??,,????'?? Backus concludes.

?EUR??,,????'??This project had so many positive aspects, everybody is a winner,?EUR??,,????'?? exclaims Hollingsworth.

The designer of this winning project, Carter & Burgess, is consistently listed at the top of industry rankings for architectural/ engineering/construction management firms. Carter & Burgess is a full-service firm offering consulting services to the transportation, land development, public works, retail, telecommunications, energy, advanced technology and building industries. The firm has been in continuous professional practice for more than 60 years. During the past 10 years Carter & Burgess has expanded geographically to office locations in every region of the country. This feature presents work by the Denver office.