by Blair Hurst, P.E. Walsh Environmental
|Photo 1 – Pre-project view of south bank. Note unvegetated bank of approximately 15 feet in height depositing sediments in to channel. Also note existing bedrock cascade in background.|
The new St. Anthonys Hospital in Lakewood, Colorado is scheduled to open in the spring of 2011. Although not yet helping patients, the hospital has already brought considerable benefit to the 900- foot section of McIntyre Gulch within its boundaries, as voluntary enhancements of this urban stream were completed as a part of the site remediation and hospital construction project. Walsh Environmental Scientists and Engineers, LLC (Walsh) provided assessment, design, permitting, construction documentation preparation, and construction support services.
Pre-project conditions in the Gulch were typified by a deeply incised and actively eroding corridor, including a 180- foot section of near vertical bank with heights reaching up to 15 feet. Such conditions posed a potential safety threat, increased fine sediment loading to downstream waters, and limited both floodplain function and public access to the gulch. Historic channelization of a meandering portion of the reach, and placement of construction debris in the cut-off meander left waste south of the Gulch, and right up to the channel in some locations. Both the understory vegetation and riparian canopy were intermittent and sparse due to a lack of sustainable hydrology, and were primarily comprised of upland grasses and weeds. Further compromising the stability of the reach, the confluence of the south branch of McIntyre Gulch with the main stem was constructed decades ago at a hydraulically unfortunate 90 degree angle.
Constraints and challenges were numerous for this small stretch of urban stream. Physical constraints included the narrow and deeply incised corridor, as well as lateral constraints imposed by remediation requirements and proposed land uses. The project employed multiple-objective management to fulfill its widely varied goals of landfill remediation, stream corridor enhancement, and providing safe accessible recreation opportunities for all.
|Photo 2 – South bank and north bank overview immediately following planting of trees and grass plugs. Note bank layback, preserved willows and cottonwoods, and bedrock cascade in photo for reference.|
To accomplish the multiple goals, Walsh utilized a creative approach that mimics natural features to improve the health and function of this stream reach, as well as its safety and opportunities for public enjoyment. All waste adjacent to the Gulch was removed and banks were regraded to mild slopes in a manner that maximized preservation of isolated mature cottonwoods and willow stands (refer to Photos 1, 2, and 3). Walsh designs applied varied grading such as low planting benches and small boulder walls to break up the uniformity of the channelized section, restore floodplain function, and provide unique local diversities in soil and water conditions to enable specialized plantings. Natural bedrock formations were exposed during waste removal, prompting the upgrade of a proposed grouted sloping boulder drop structure at the south branch confluence to a sculpted concrete cascade that fits the natural setting of the gulch, provides increased stability to the reach, and ensured preservation of the largest streamside cottonwood tree (refer to Photos 4, 5, and 6). The cascade was shaped and stained to match the existing bedrock and included innovative micro-grading to create “precip pools” to hold water for birds and other wildlife. All enhancements were designed to be consistent with UDFCD guidelines and standards to ensure maintenance eligibility for the project reach. The project has already served as a model for other projects, as UDFCD has recommended it as an example for a downstream section of the gulch. The project stayed within budget even with the upgraded drop structure, as the sculpted cascade naturally accommodated the steep gulch sideslopes and eliminated the need for expensive grouted boulders.
Further successes include accommodating several proposed land uses, such as vehicular and pedestrian crossings, a new bike path paralleling the north bank, and surface water spillways and pipe outlets in to the Gulch, in a way that would impart the most natural function and appearance on this stream system. The vehicular crossing is a free span, open bottomed structure that reduces impacts on stream hydraulics and the natural channel bed. Spillways were vegetated with native grasses and shrubs.
Vegetative improvements, such as removal of weed tree species acting as a seed supply, and extensive planting of a diverse mix of native trees, shrubs and grasses replenish what was lost in this urban area – the invaluable “green line” of a healthy and functional riparian corridor.
|Photo 3 – South bank in channel view of trees, grass plugs, and preserved willows and cottonwoods.||Photo 4 – View of natural bedrock formations at bedrock cascade exposed from vertical bank during waste removal.|
|Photo 5 – Staining of sculpted concrete cascade to match natural bedrock formations.||Photo 6 – View of completed sculpted concrete cascade. Note preserved cottonwood trees.|