120th Avenue Extension Project: A Case Study for Bridging the Gap Between Engineering and Ecology



The 120th Avenue Extension Project in Adams County, Colorado is a partnership between Adams County and the Colorado Department of Transportation (CDOT). URS Corporation, an international environmental consulting firm, was hired to work in concert with Adams County and CDOT personnel to conduct the environmental studies necessary for the project to comply with local, state, and federal regulations. Once complete, the project will provide a crucial east-west link between I-25 and Denver International Airport by constructing a new 2.5-mile section of 4-lane roadway with a new bridge over the South Platte River. Project construction began in late 2003 and is expected to be completed mid-2006.

The South Platte River is the largest river in the region and historically had a very large and dynamic floodplain with a braided channel that was free to migrate. Today, the river is very constrained and encroached upon by various types of development including gravel-mining, residential communities, agricultural, and various kinds of industrial and commercial businesses. Nonetheless, the river still provides a riparian corridor that is unmatched in the region. Although this corridor has been badly degraded by various land uses, it is still home to mule deer, white-tailed deer, red-tail hawks, great-horned owls, coyotes, red foxes, belted kingfishers, great blue herons, northern leopard frogs, and hundreds of other small mammals, birds, reptiles, amphibians, fish, and insects.

Maintaining the South Platte River riparian corridor was important to the project team. During the planning for the new roadway, URS ecologists conducted numerous field studies to identify and quantify the types of habitats present in the corridor. These studies revealed a riparian corridor that contains a mosaic of open water, sandbars, and small wetlands, intermixed with riparian grassland, shrubland, and woodland. Unfortunately, most of these habitats are very weedy and dominated by many non-native, invasive species like reed canarygrass (Phalaris arundinacea), Siberian elm (Ulmus pumila), Canada thistle (Cirsium arvense), leafy spurge (Euphorbia esula), and diffuse knapweed (Centaurea diffusa).

In order to mitigate the loss of these riparian habitats as a result of constructing the new bridge and roadway, Adams County committed to replacing the riparian habitat around the new bridge at a 1:1 ratio. An unusually large footprint of disturbance was required to protect the new bridge from damage or loss during a major flood event. The result was the required replacement of 18 acres of riparian habitat as part of the project. Although most of the impacted areas were already degraded and contained many non-native species, Adams County was dedicated to replacing them with higher-quality vegetation communities.

The design of the mitigation areas involved a team effort between the project ecologists and the project engineers to fit over 18 acres of riparian habitat into the existing right-of-way. This required creativity and open-mindedness to reconfigure a stormwater detention pond to accommodate the riparian plantings, turn a borrow-pit for roadway construction into a second riparian planting area, and transform a portion of the lower floodplain that was to be re-graded for flood control purposes into valuable habitat.

Due to limited space for the mitigation, the engineers and ecologists worked together to modify the design of a stormwater detention pond so it could double as a riparian woodland area. The team designed the pond to be excavated such that the newly planted trees and shrubs could access groundwater for at least a portion of the growing season. In order to determine the appropriate depth of excavation, URS installed shallow groundwater monitoring wells. These wells were monitored bi-weekly prior to planting and provided the information needed to properly excavate the pond.

In addition to the groundwater, the stormwater captured by the pond will provide a secondary source of water. Using the data the engineers compiled from computer modeling to determine how much water these ponds would detain (and for how long), it was possible to design a pond that could also function as riparian habitat. The final design of the pond included a series of small ponds that trap incoming sediment and debris from stormwater runoff. These small ponds are situated within a larger pond that only floods during larger storm events. Again, the ecologists and engineers worked together to design the larger pond to resemble a natural swale and to plant the site with native trees and shrubs. The larger pond was designed to require minimal maintenance and function as wildlife habitat, whereas the smaller ponds were designed to be easily accessible for regular maintenance and dredging by highway personnel.

In order to provide the proper density of trees and shrubs in the riparian mitigation areas, URS ecologists conducted a tree inventory of the impact area associated with the bridge. All the trees in the floodplain greater than 3-inches in diameter at breast height (DBH) within the impact area of the proposed bridge had to be counted. A total of approximately 800 trees were identified within the impact area and well over half of these were non-native, mostly Siberian elm. The few native species that were identified included a large grove of young (generally less than 10 inches DBH) plains cottonwood (Populus deltoides), and scattered peachleaf willow (Salix amygdaloides) and box elder (Acer negundo).

In addition to the 800 replacement trees, approximately 3,300 shrubs will also be planted in order to create more functional habitat. Rather than purchasing all of the plant materials, the ecologists worked with the project engineer and the contractor to adjust the project schedule to attempt a mass tree transplanting effort. As a result, over 150 plains cottonwood trees were saved and were left standing in the middle of an active construction work zone for several months until transplanting could occur. The remaining trees and shrubs will have to be imported to the site. Once all of the plantings are well established, the area will provide cover and forage for resident and migrating wildlife.

As part of the riparian mitigation plan, it was important to address the noxious weeds in the construction area. For this, URS ecologists created a detailed map of all the weeds along the 2.2-mile project corridor. This involved field work with a Global Positioning System (GPS) accurate to less than a meter to identify exact locations of existing weed populations. This map, along with the supporting documentation, enabled the construction team (including engineers and ecologists) to manage the weed populations before and during construction so that the weed problem was not worsened. The ecologists recommended not using specific portions of the lower floodplain for topsoil salvage so that the weed seeds would not be moved into areas not previously infested. Other badly infested areas were purposely buried under at least 4 feet of soil to preclude reestablishment.

Although the construction of the 120th Avenue Extension Project is not yet complete, it is a good example of how, with the support of the project proponents (Adams County and CDOT), the project engineers, ecologists and contractors can work together to solve environmental problems while being sensitive to engineering needs. Andy Herb is an ecologist working for URS Corporation in Denver, Colorado. He has been involved in over 75 projects in the region that have dealt with wetland and riparian issues.