MAINTENANCE PROGRAM ACTIVITIES
By Mark R. Hunter, P.E., Manager, Maintenance Program

The Urban Drainage and Flood Control District (District) Maintenance Program committed a total of $5.8 million in 2004 to maintaining drainageways in the Denver metropolitan area. Most of these drainageways have been improved sometime in the past as part of a subdivision development or a capital improvement project. The bulk of the drainageways we maintain are open channels with grass-lined banks, riprap erosion protection, and rock or concrete drop structures (grade control structures).

Our maintenance work covers the spectrum of drainageway work. It includes debris pick-up and mowing, localized repair to damaged and eroded channels or detention facilities, and consultant-designed rehabilitation to long reaches of deteriorated drainageways.

Mowing and Debris Pick-up
For the year 2004 we awarded eight contracts for mowing and debris pickups under our routine maintenance program. Seven of those contracts were awarded as renewals of the prior year contract. The value of each contract was adjusted to match the movement in the regional Consumer Price Index over the prior year. The other contract was awarded through a competitive bid process.

The mowing and debris pick-up work was done on 261 different sections of urban drainageways within the District's boundaries. The contractual value of the work was $702,144. The table below summarizes the miles of drainageways within each county in the District on which we performed regularly scheduled mowing and/or debris pickup maintenance

Restoration Maintenance
In 2004 $3,305,600 of work was performed under our restoration contracts. Restoration projects typically address isolated drainage problems where the construction will cost from a few hundred dollars up to $200,000. Ninety-five individual activities were completed during the year. Several major restoration projects are shown in the accompanying table titled "STATUS OF MAJOR MAINTENANCE PROJECTS." A major advantage of the restoration program is the ability to use it to react quickly to local drainage needs.

Rehabilitation Maintenance
Fifteen rehabilitation projects were at various stages of design or construction during 2004. Those projects are included in the accompanying table titled "STATUS OF MAJOR MAINTENANCE PROJECTS." Rehabilitation projects typically are designed by private consultants and built by private contractors. They are intended to correct severe problems that have occurred on a previously improved urban drainageway. By the end of 2004 the District will have spent about $1,842,300 on rehabilitative design and construction for the year.

Channel Repairs
In last year's Flood Hazard News we described a recent extension of Federal Boulevard in Westminster that crossed Big Dry Creek at 122nd Avenue. Erosion was threatening the Federal Boulevard right-of-way at a severe bend in the creek that included a 25-foot tall near-vertical bank. After consulting with the permitting agencies the solution was to relocate a short portion of the creek so the erosive energy of the flow was not directed toward Federal Boulevard. The project was delayed for three months during the spring nesting season while bank swallows made use of the steep bank. The completed project included a backwater wetland at the base of the steep bank. Keeping all the parties informed and coordinated required close management of the work by Mike Sarmento, Engineering Inspector.

The Montbello area of Denver is tributary to Montbello Drainage and to Irondale Gulch. Nearly 10 miles of concrete-lined drainage channels collect the storm runoff from Montbello. These channels were constructed over the last 25 years as the subdivisions were built. Their top widths vary from 12 feet up to 35 feet with depths ranging from 4 to 10 feet. For the last seven years we have been setting aside funds to replace the worst block-long sections. Our replacement concrete panels include weep holes and steel reinforcement while most of the original panels did not.

Much of the length of Grange Hall Creek has been improved over the last 15 years. One of the upper reaches of the creek is within the City of Northglenn and extends from Grant Street on the east to I-25 on the west. Some improvements had been constructed in the past, but they are now deteriorating or being buried by sediment. This corridor has enough width and enough change of elevation to allow a straight-forward design. However, the drainageway revealed a major surprise when surveying and utility locations were done. Buried in the sediment was a storm sewer outlet that drained stormwater from several blocks of residences. The pipe outlet is well below the proposed grade of the channel so the design team is now reviewing options for how to deal with the storm sewer.

Multi-purpose Projects
In 2004 we again joined forces with other local governments to fund multi-purpose projects. The result of this combined funding is projects that meet a variety of community needs. The City of Morrison wanted to improve the appearance, drainage function, and urban usefulness of Mount Vernon Creek at its confluence with Bear Creek. All these goals were met through a focused design and concentrated effort by Cindy Thrush, Senior Project Engineer, and Jeff Fisher, Engineering Inspector.

A similar multi-purpose project at the confluence of Cherry Creek and the South Platte River brought together funds from the Maintenance Program, the South Platte River Program, The City and County of Denver, and the Greenway Foundation. The project created an urban park at the edge of the two streams that are considered to be the birthplace of the city.

Boulder County has a very different problem with Rock Creek which enjoys a broad open-space corridor on the west side of Highway No. 287. This section of the creek has seen very little direct impact from humans. The exception is that in-stream erosion has damaged the natural habitat of the corridor. The close coordination of multiple interests has been guided by David Bennetts, Senior Project Engineer. We will participate by funding the drop structures to limit the down-cutting of the creek while Boulder County will contribute toward the bank shaping and restoration of the habitat.

Detention Ponds and Sediment Control
In remote areas such as the Canyonlands of Utah or the Grand Canyon of Arizona erosion and sedimentation are normal occurrences. Even if the resultant changes are severe they are a natural and expected component of the environment. Urban settings are far less tolerant of aggradation and sedimentation. A change in elevation of a few feet can have a dramatic impact of the flow characteristics and the associated floodplain of an urban stream. In the past year the maintenance Program carried out significant sediment removals on Harvard Gulch from University Boulevard to Colorado Boulevard, on Grange Hall Creek from 108th Avenue to Colorado Boulevard, and at the sediment trap on Willow Creek south of Dry Creek Road. Additional sediment removals were also done on 10 to 15 other channels and detention ponds within the District. Concrete-lined channels can be even more sensitive to aggradation where even a couple inches of sediment is visible and can support undesirable vegetation.

A detention pond was built in the late 1990s on the Long's Way Tributary near it's confluence with Cherry Creek in Parker. At the downstream end of this pond a lowered micro-pool had been installed to trap sediments and provide a maintainable feature that would improve the quality of the water discharged from the pond. The five foot elevation drop from the trickle channel into the micro-pool was made of graded earth. When the pond would detain water the elevation difference was drowned out by the water and little or no erosion occurred. However, at lower flow rates erosion was occurring as the water dropped into the micro-pool. Our solution was to restore the micro-pool by creating a sculpted concrete drop structure with a concrete cutoff wall to separate the main detention pond from the micro-pool.

Drop Structures
Drop structures can fail when water flows through, under, or around them due to an inadequate cutoff wall. We repaired several drops in 2004 that suffered from poor cutoff walls. Grouted boulder drops were built on Greenwood Gulch north of Orchard Road in Greenwood Village in 1999. The excavated trench for the original grout curtain simply did not extend to the depth called for in the plans. We repaired 4 of these drops a couple years ago and repaired an additional one in 2004. We were able to clean up the upstream side of the boulders and then drive sheet pile and grout it in place to bind the whole structure together.

Westerly Creek is one of several drainageways in Aurora that have been constructed as trapezoidal concrete channels. Vertical concrete drop structures were installed where necessary. Some of the drops did not have satisfactory foundations to support the vertical wall of the structure. The weak foundation allowed differential movement among the concrete panels which, in turn, allowed water to flow under and around the structure and pick up soil particles. The result was that the concrete panels near the drop structure became undermined and eventually collapsed. Last year we rebuilt one of these drops north of Mississippi Avenue. This year we reconstructed the structure that is southeast of Mississippi Avenue and Moline Street. The redesigned drops have also improved upon the existing underdrain systems which protect the foundations of the drop structures by controlling the ground water.

Over the last 15 years the City of Boulder has used rounded river boulders to build attractive drop structures in Boulder Creek. Those drops that were not grouted have experienced movement of some of the boulders. As the boulders tumble out of their configuration the drop structure looses its integrity. We re-used the rounded boulders and locked them in place with a grouted cutoff wall. We also restored any damaged stream banks in the vicinity of the drops.

Photo Gallery

Willow Creek sediment trap at Dry Creek Road in Arapahoe County
The pond is full with 800 to 1000 cubic yards of sediment. One of our indicators for when it is time to remove the sediment is when people start having picnics on our "beach."	Left side of photo shows the Willow Creek channel. The freshly cleaned sediment trap is on the right. The diversion boards are still in place to keep water out of the sediment trap while it is being cleaned.
Long's Way Tributary in Parker Recently completed sculpted concrete drop structure within a detention pond. The five-foot tall drop replaced an eroding headcut.  Note the sediment already accumulating in the micro-pool in the left side of the photo.
Greenwood Gulch west of Holly Street in Arapahoe County
The site has been graded. The sheet pile cutoff wall is hidden behind the grouted boulder drop structure.	Greenwood Gulch west of Holly St. Sheet pile is being installed to provide a cutoff wall that will prevent water from eroding around and under the existing boulder drop structure.
Mount Vernon Creek at confluence with Bear Creek in Morrison, CO
Looking downstream toward Bear Creek. March, 2004. Note concrete rubble.	Looking upstream from Bear Creek. March, 2004.
Looking downstream toward Bear Creek. December, 2004. New bridge and trails.	Looking upstream from Bear Creek. December, 2004. Box culvert was extended and new boulders and buried riprap were installed.

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