Greenwood Gulch project enhances recreation area

In a project that may serve as a model for erosion control initiatives in other cities, Greenwood Village, Colo., has stemmed a severe erosion problem at Greenwood Gulch, a stream running through town. Moreover, the project has had a significant additional benefit — preservation of a recreational and wildlife habitat area.

The 2.5-mile-long stream, along with its floodplain and associated wetlands, had become a major recreation area. The 55-acre Horseshoe Park, 28-acre Greenwood Gulch Park and about 4 miles of pedestrian and equestrian trails are located along Greenwood Gulch. Urbanization in the gulch basin during the past 15 years had produced changes in the area’s natural balance. Higher rates of runoff caused by rapid development had worsened erosion in the gulch, which damaged public and private property, deposited sediments and other stormwater pollutants in the gulch, and transformed wetlands into uplands with noxious weeds.

Greenwood Village’s initial proposal to control the erosion relied upon a traditional “hard channel” design. The stream channel would be regraded with a 6-foot-wide bottom and side slopes, and the channel would be lined with rock or concrete.

That design would have been effective in controlling the erosion and would have contained the normal streamflows in a relatively narrow channel. However, it would have eliminated wetlands as well as riparian vegetation and habitats for birds, fish and animals.

Because of the adverse ecological and recreational impact and its high cost ($2.5 million), the village rejected that option. Instead, it elected to develop a “soft channel” that would control erosion, enhance wetlands, improve the natural habitat, and preserve the character of the park and trail systems in an otherwise urban setting. The concept included: * removing 9,830 cubic yards of accumulated sediment in the floodplain; * constructing 10 sculptured rock drop structures to stabilize the stream channel bottom and to create pool and ripple patterns to enhance the aquatic habitat; * restoring 5 acres of wetlands in the floodplain. (Decreasing downstream flooding, treating urban stormwater runoff, enhancing wildlife habitat and improving the recreational value of the trail system were the goals); * regrading about 12,000 feet of stream channel; * relocating and repairing 3,000 feet of pedestrian, bicycle and equestrian trails; * reconstructing three stream trail crossings; and * relocating the trail system from an at-grade crossing to a below-grade, streamside crossing to improve trail safety and foster the public’s interaction with the trail system.

Regrading the channel and placing the sculptured rock drop structures flattened the slope of the stream. That slowed velocity enough to eliminate the bank erosion. It also provided the opportunity to plant vegetation in riparian zones and improve the aquatic habitat. The sculptured rock drop structures in the lower half of the project are 60 to 100 feet wide and span virtually the entire width of the 100-year floodplain. Consequently, the entire base and polluted stormwater flow in a shallow pattern only a few inches deep through a 5-acre wetlands zone.

The wetlands vegetation, primarily cattails and grasses, filters the streamwater to remove sediments, phosphorous, heavy metals and other urban pollutants. The wetlands will provide an important buffer zone to help protect the quality of downstream ponds. They also will create a wide riparian corridor of dense vegetation that will form excellent animal and bird wildlife habitats usually not available in an urban setting.

The plan, undertaken last year, originally was budgeted at slightly more than $1.2 million. However, because of the favorable bids received on the project, the improvements were extended further upstream to include repairs to a major bridge. Consequently, the overall cost reached nearly $1.7 million.

Greenwood Village paid just over half the cost, while the remainder was paid by the Urban Drainage and Flood Control District, a property tax-funded, regional agency that constructs and maintains flood control projects in metropolitan Denver.