Two District Maintenance Projects Profiled

By Dave Bennetts, Senior Project Engineer, Design, Construction & Maintenance Program

Rock Creek Restoration Project
The Rock Creek channel restoration project is located within the Carolyn Holmberg Preserve at Rock Creek Farm in Boulder County. Rock Creek Farm is public open space owned and operated by Boulder County Parks and Open Space. As a response to urbanization upstream, Rock Creek has experienced significant increases in runoff volume and frequency of discharges. Due to these changes, the channel has become deeply incised and is experiencing severe stream bank erosion resulting in the loss of vegetation and wetlands.

The primary goal for the project was to provide channel stability and enhancements while blending them into the natural environment. In order to accomplish this, two innovative techniques were used, sculpted concrete drop structures and low-flow channel soil wrapped lifts.

Drop Structures
Six drop structures were constructed along a two mile reach to stabilize the channel bottom and prevent further degradation. Sculpted concrete drop structures were used instead of sloping grouted boulder drop structures since rock is not naturally found in the lower part of the watersheds in Boulder County. Sheet pile cutoffs were installed at the upstream and downstream edges of the drop structures. The subgrade was shaped and compacted to the desired drop structure configuration, steel was shaped to fit the drop contours and tied into the sheet pile, a weep drain system was installed, and the structure was poured in one full depth pour.

The sculpted concrete was colored and shaped to mimic sandstone outcroppings. New to the process was texturing the wet concrete with a rubber stamping process that closely matched the patterns found in the rock outcroppings in the area. Six sculpted structures were constructed varying in height from two feet to eleven feet. On several of the structures, planting wells were installed and planted with native shrubs to soften the appearance of the structure.

Soil Lifts
In an effort to soften the vertical banks and create more riparian habitat, a bioengineering technique was chosen over buried and revegetated riprap. Soil wrapped lifts were selected because they could withstand the higher stream velocities and provide a place for plantings that would further stabilize the lifts. The soil wrapped lifts were constructed with a multi-layer erosion control fabric made with both biodegradable and permanent materials. The lifts were filled with imported topsoil instead of the onsite material to avoid the introduction of noxious weed seeds from the area. The lowest lift was placed 1-2 feet below the final invert of the channel to prevent undercutting. The lifts were an average of 1 foot in height and placed at a 3:1 slope. In the steeper areas, 8 to 10 lifts were used to transition to the upper bank.

Severe stream bank erosion and incised channel in Rock Creek

Revegetation and Restoration
Once the drop structures were constructed, channel grading completed, and soil lifts installed, extensive revegetation and restoration efforts were completed. Russian olive trees were removed and approximately 90 plains cottonwood and peachleaf willow trees were planted. Riparian shrubs and trees were also planted to improve wildlife habitat along Rock Creek corridor. In addition almost 9000 native plants were planted, not including the native seeded areas. These native plants were placed into the soil lifts and along the lower reaches of the channel. This work was accomplished by volunteers coordinated through Boulder County. Most of the plant material was hand watered during the first summer season to ensure establishment.

The project was completed in the spring of 2005 at a final cost of $800,000 and has been closely monitored by Boulder County Parks and Open Space and the District. Site maintenance has included selective weed control and re-seeding of upland areas that did not establish after the first growing season.  Since the project was completed, the drainageway has seen several significant runoff events with no damage. The vegetation has established itself well and the site has flourished.  The District will continue to monitor the project to evaluate the long term success of these two techniques.

Sanderson Gulch Project
The Sanderson Gulch project is located within the City and County of Denver approximately 1 mile upstream of the confluence with the South Platte River.  The projects reach meanders through a park in a residential neighborhood established in the 1960’s.  Rapid urban runoff had caused channel degradation and bank failure along the 1000 foot channel reach.  Several gabion basket drop structures built in the early 1970’s had also failed.  In addition to the drainageway problems, the Denver Parks Department wanted to upgrade the irrigation system and make several other improvements to the park.

They teamed up with the District and were a funding partner on the project. The District’s project goals included building two new drop structures, constructing headwalls at the upstream and downstream ends of culvert going under Florida Avenue to improve the hydraulics and safety, regrading the entire reach of the channel, and extensive tree and wetland planting efforts.  The parks department project goals included 1100 feet of trail, a new trail crossing, an upgraded irrigation system, and adding several other park improvements such a park benches and picnic tables.

Several aspects of this project were unique.  The two new drop structures upstream and downstream of Florida Avenue were constructed using precast concrete blocks with exposed black aggregate in lieu of boulders.  The drop structure upstream of Florida Avenue was constructed as an improved inlet with the concrete blocks forming the approach and drop into the inlet.   The drop structure downstream of Florida Avenue used the precast concrete blocks to form a cascading stair approach to the plunge pool.  Large boulders of varying sizes and shapes were placed to stabilize the structure for larger events.  The boulders placement created pocket areas that were then planted with nativeIncised channel upstream of Florida Avenue shrubs and trees.  Wing walls were constructed on both ends of the culvert with an architectural rail on top.

A 10-foot wide channel through the park was constructed to provide more sinuosity, reduce bank erosion, improve the channel hydrology, and to increase areas of vegetation for water quality.  The outside bends of the channel were graded to a 6:1 slope to create pocket wetlands and were reinforced with coir biologs pre-planted with wetland plant material.  This was the first project the District had used the pre-planted biologs. The plant material was fully rooted in the biologs that were then placed in small groves in the channel bottom.  This process allows the plant material to immediately root and establish itself.  Approximately 1320 lineal feet of biologs were installed as part of this project.  The inside bends of the channel were graded to a 4:1 slope and protected with a biodegradable erosion control blanket planted with native grasses and shrubs.

Extensive planting was completed once the final grading was complete.  This included 112 trees, 325 shrubs, 4570 plant plugs and willow whips, and upland seeding.  The upgraded irrigation system installed by the parks department was used to help establish the new vegetation.  The project was completed in the spring of 2006 at a cost of $940,000.

New drop structure and headwall downstream of Florida Avenue