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Sand Creek Stabilization11-01-00 | 16
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Aquatic and Wetland Company

As the population in the Front Range of Colorado continues to grow, it is becoming increasingly necessary to find ways to accommodate the needs of this population while minimizing or mitigating harm to the indigenous natural resources and protecting the little wildlife habitat that remains. Aquatic and Wetland Company (AWC), an ecologically-based design/build company based in Ft. Lupton, Colorado recently completed a stream restoration project that illustrates how it is possible to meet these two needs in a single project. The project, completed this past August, involved the 3,600 foot stretch of Sand Creek near the old Stapleton Airport. AWC was recognized by the International Erosion Control Association with the Contractor of the Year award, as the designer and contractor for the bank stabilization project. CDM Engineers and Constructors was the prime consultant on the project.

Bluff Lake Park is a 123-acre parcel that includes a lake, 30 acres of wetland areas, a 0.7 mile stretch of the Sand Creek corridor, short grass prairie and a remnant cottonwood gallery. The reach of Sand Creek that extends through the park near the old Stapleton Airport in Denver, Colorado presented particularly challenging erosion control issues. The soils along the entire corridor are highly erodible sands. Runoff to the stream has increased dramatically in recent years due to increased development throughout the watershed. The flows that Sand Creek must convey are extremely flashy and markedly higher than natural levels. The streambanks have to accommodate this overload. These flow conditions had created vertical banks that were over 12' high in some areas as well as channel downcut to 4'. Both these factors contributed to the degradation, and in many areas, lack of riparian habitat. In addition to the high flows and erodible material, the project presented other challenges, including existing structures in need of protection. The option of energy dissipation through floodplain widening was not economically or politically feasible. Therefore, the project team developed several erosion control techniques to address the issues at hand.

Increasing Stability

The goals of the project were to stabilize the highly erodible banks, lessen the channel gradient to a slope deemed stable for sand systems, restore riparian and upland vegetation along the creek corridor, augment wetland habitat, and maintain compatibility with the adjacent environmental education facility. Because of the combination of a high need for stability and the goal to restore biological functioning to the corridor, the design plan needed to creatively integrate stabilization techniques known to be effective for these different challenges. To meet these goals, Aquatic and Wetland Company took a design approach that embodied three main principles: integrate hard protection and soft, bioengineered treatments, restore biological function and create a meandering low flow channel within an armored flood channel.

A double terrace with boulder toe and brush layering was used to treat the high, eroded banks throughout most of the project. This treatment consists of a row of boulders placed on a rubble foundation at the toe and the top of the slope. Between the boulder toes, the bank is graded to a 3:1 slope to create a planting terrace. Brush layering is installed at an angle behind the lower row of boulders. The slope protection in this treatment comes primarily from the two rows of boulders and secondarily from the root structure, which will be created as the vegetation matures. The vegetation also serves to create riparian habitat and function. Amendments added with the plantings included slow release fertilizer, mycorrhizal inoculants and polymer. The brush layering will eventually grow out over the boulders, screening them from view to create a more natural look. Additionally, the brush layering provides shade and overhead cover for fish. By the end of the first growing season, some of the brush layering had grown almost 4 feet. AWC's designers and construction estimators completed a cost comparison for the double terrace with boulder toe and a single boulder toe with riprap. The bioengineered method proved to be more cost effective.

Constructing Willow Logs
To construct the willow logs, a rectangular piece of erosion control fabric was laid out flat. A strip of mulch and native soil mixed with fertilizer and moisture holding polymer was spread along one long side. A wide strip of willow cuttings was laid out along the other long side. Beginning with the mulch side, the fabric was rolled up burrito-style with the willows ending up on the outermost layer.

In some areas, such as low-risk inside bends, hard treatment was not necessary. In these areas a willow log designed specifically for the project was used for toe material. The logs were constructed by hand on site using available fill material from the project site with some imported mulch to supplement on site material.

To construct the willow logs, a rectangular piece of erosion control fabric was laid out flat. A strip of mulch and native soil mixed with fertilizer and moisture holding polymer was spread along one side. A wide strip of willow cuttings was laid out along the other side. Beginning with the mulch side, the fabric was rolled up burrito-style with the willows ending up on the outermost layer. The log was then staked into place at the toe of the slope. Willow wattling was installed behind the log. The intent of the specialized willow logs was to have the outer willow layer in the log producing stabilizing roots and overhead foliage which was augmented by the willow wattles to double the effect. Several months after installation, the treatment showed exceptional growth and resistance to high flow events.

Hazardous Waste

One of the most challenging areas of bank work in the project was the presence of a hazardous waste burial site. Prior to the stabilization project, the burial site was treated with loose rubble and grouted riprap. While this treatment provided the needed protection, it did not meet the habitat, aesthetic and environmental education goals of the project. The previous work also included a boulder toe. The toe was reworked to achieve a tighter configuration and to allow installation of a continuous willow brush layer behind the toe. Where possible, the toe was manipulated to allow some reduction in slope for the bank behind it.

Overall mild slope reductions were achieved, however existing fence line and the need to maintain channel conveyance greatly limited the effort. A geocellular confinement combination treatment was used to create biological functionality in this section. A shallow layer of soil from the site was spread over the riprap to create a smooth surface. The 6" geocellular confinement material was installed over this and was backfilled with topsoil. The area was seeded and mulched and covered with erosion control fabric. The seed mix was a combination of native grasses and forbs that are indigenous to the area. Additionally, annual grass seed was planted to create short-term, immediate coverage. By the end of the first growing season, the treatment exhibited full vegetative cover, improved aesthetics and enhanced biological function.

Reducing Flow

The erosion control solution for Sand Creek was not entirely in the treatment of the streambanks. In order to reduce the velocity of flows through the corridor and consequently reduce the erosive capacity of the water, the channel gradient was lessened utilizing two drop structures. The structures were constructed in a step-pool design with 18" drops and 3' deep pools. The size of the drops was determined to be a maximum of 18" to allow for fish passage and to create a natural aesthetic.

In order to prevent flows from cutting under the drop structures, sheet piling was driven to refusal across the channel. Additionally, buried hard points built with broken concrete rubble were extended out to the limits of the 50 or 100 year floodplains at each drop structure. The stabilized channel invert promotes a stable water table with localized increases, which helps to sustain and augment the revegetated riparian corridor.
Bluff Lake Park is a 123-acre parcel that includes a lake, 30 acres of wetland areas, a 3/4 mile stretch of the Sand Creek corridor, short grass prairie and a remnant cottonwood gallery. The integrated erosion control project called for the use of various equipment including excavators, front-end loaders, a dump truck, scrapers and a flat bed truck for delivery only.

Aquatic and Wetland Company secured the necessary 404 permits for the project. Mitigation under the permit required the creation of 0.14 acres of wetland habitat to compensate for impacts incurred during construction. The wetland area is designed to accommodate effluent from the Aurora Wastewater Treatment plant but will not be dependent on the plant. The wetland is sustained hydrologically by flows in Sand Creek. Willow logs were used again to create a check structure for the inflow of effluent.

Indigenous Plantings

All of the vegetation, including seed mixes and potted transplants that were grown at the AWC nursery facility, were comprised of species indigenous to the Sand Creek corridor. The species were selected and laid out to replicate the communities found in undisturbed riparian corridors in the Colorado Front Range. These included cottonwood galleries, riparian scrub/shrub and transitional shrub zones, wetland zones, riparian perennial zones and shortgrass prairie zones. The planting plan showed sensitivity to the project's location within an environmental education facility.

The diversity of habitat has already proven to be attractive to a variety of wildlife. Great blue heron and black crowned night heron have been sent hunting along the creek and in the new wetland area. A duck family has been routinely observed near the new wetland. Other wildlife at the park include fox, coyote, Swainson's hawk and great horned owl.

A portion of the project was installed by volunteers from Friends of Bluff Lake, the stewardship group for the environmental education center, and Volunteers for Outdoors Colorado. Plant material and field supervision were provided by the contractor. The volunteers planted willows and applied seed along an overflow side channel and sand bar.

Project Recognition
A wide range of materials were used for the Sand Creek Stabilization Project. Rounded quarry rock was utilized for the creation of the boulder toes, while concrete rubble was used for the foundation. The team used Type II crushed angular gravel for the bedding, along with other materials such as Types M, MG, VH, HG riprap, BonTerra CF7 erosion control fabric, 4" Terracell cellular confinement system, Webtec mulch and straw.

The Sand Creek Stabilization Project has also been recognized for environmental excellence by the Colorado Contractors Association and the Association of Landscape Contractors of Colorado. Most of the project can be seen by walking the creekside trail at the Bluff Lake Environmental Education Center in Denver. The park is accessed from Havana, south of I-70.

The project is a good example of the integration of hard and soft erosion control techniques and the necessity for both. Combined with the sound application of engineering principals, bioengineering can be an appropriate solution for managing erosive floodwaters. The reach of Sand Creek that runs through Bluff Lake Environmental Education Center will mature to fulfill its obligation as an educational resource that provides functional wildlife habitat while reducing threats to existing structures and adjacent property.

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