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 2005 we awarded nine contracts for debris pickups and native-grass mowing under our routine maintenance program. Five contracts were awarded through a competitive bid process in March. The remaining four contracts were awarded as renewals of the prior year contract. The value of each of these renewed contracts was adjusted to match the movement in the regional Consumer Price Index over the prior year.

The mowing and debris pick-up work was done on 270 different sections of urban native-grass-lined drainageways within the District's boundaries. The contractual value of the work was $657,770. 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.

Construction Activities
In 2005 $4,942,000 of work was performed under our various construction contracts. The smaller projects typically address isolated drainage problems where the construction will cost from a few hundred dollars up to $200,000. Within this cost range seventy-nine individual activities were completed under our restoration contracts. Projects in this category of work are directly awarded to a single contractor or bid among all contractors from our group of six contractors who have won open-ended contracts with the District. A major advantage of work under the open-ended contracts is the ability to use them to react quickly to local drainage needs.

When project construction will cost more than $200,000 the work is designed by a private consultant and then put out for public bid to be built by a private contractor. These larger projects typically address severe problems that have occurred on previously improved urban drainageways. Fifteen large projects were at various stages of design or construction during 2005. Our major projects for the year are summarized in the accompanying table titled "STATUS OF MAJOR MAINTENANCE PROJECTS."

It used to be that when the last piece of construction equipment left the work site the project was finished. Permit regulations now often require that the revegetation of a completed project be monitored and documented for several years. The goal is to ensure that vegetation and habitat are restored to pre-construction levels. These annual site inspections and reports require our on-going attention plus the services of qualified vegetation specialists.

Channel Repairs
In last year's Flood Hazard News we described a project on Grange Hall Creek between Grant Street and I-25 in the City of Northglenn. Some improvements had been constructed in the past, but they were deteriorating. A contributing factor appeared to be inadequate energy dissipation as the creek discharged from the culvert under I-25. A drop structure near I-25 has been rebuilt and others added in order to control the erosion. The channel has also been regraded and a trail has been added.

High energy at culvert outlets appears to contribute to erosion at other sites as well. On Kettle Tributary to Willow Creek west of Yosemite Street in Centennial we rebuilt the outlet area and regraded the area downstream. Similarly, on Little Dry Creek between Hoyt and Kipling in Arvada, the erosion we repaired was worse immediately downstream of roadway crossings.

The low flow channel of Cherry Creek is confined between vertical concrete walls for its lower three miles before it joins the S. Platte River. Over the past 20 years we have placed thousands of linear feet of boulders to reduce bank erosion. Recently we have also installed wrapped soil lifts to control bank erosion of the low flow channel. Getting the vegetation to take hold in the soil lifts requires determination, but so far the installation has performed well.

Multi-purpose Projects
In 2005 we again joined forces with other local governments to fund multi-purpose projects. Drainageways and parks often share the same corridor. Dave Bennetts, Senior Project Engineer, coordinated with staff from Denver Parks to design improvements to West Harvard Gulch at Clay Street, West Harvard Gulch at the South Platte River, and Sanderson Gulch at Florida Avenue. Each site had unique hydraulic and right-of-way problems and different residential/recreational needs. Sanderson Gulch is now under construction and is being administered in the field by Mike Sarmento, Senior Construction Manager.

Two projects in Lakewood made use of combined funding to repair severe erosion to residential backyards. Stacked boulders were used to limit the channel width and still maintain the flow capacity of McIntyre Gulch west of Holland Street. Boulders and riprap were used to make repairs to Lena Gulch east of Youngfield where the contractor had to deal with difficult access conditions. On both projects the City of Lakewood managed easement acquisition in addition to participating in the project funding.Multi-purpose projects occasionally take us into irrigated bluegrass parks. In Broomfield we used wrapped soil lifts reinforced with vegetation to provide low flow bank protection on City Park Drainageway south of Midway Boulevard. Since we were in an improved park a narrow concrete "mow strip" was installed behind the upper soil lift to provide an edge for the bluegrass and a solid surface for mower wheels.

Detention Ponds and Sediment Control
The hydraulic designs of urban drainageways are generally intolerant to sedimentation and aggradation. A change in bottom elevation of just a foot can have a dramatic impact on the flow characteristics and the associated floodplain of an urban grass-lined stream. Concrete-lined channels can be even more sensitive to aggradation where even a couple inches of sediment is visible and can support undesirable vegetation. In the past year the maintenance Program carried out significant sediment removals on Grange Hall Creek in Thornton from 108th Avenue to Colorado Boulevard, and at the sediment trap on Willow Creek south of Dry Creek Road in Centennial.

We have removed sediment several times from the low flow channel of an on-stream detention pond on Wildcat Ridge Tributary to Big Dry Creek in Douglas County. We are assessing the upstream sediment sources to determine if we should construct a sediment trap which would make maintenance much easier.

Making changes to drainageway configurations in urban areas is often a balancing act. Flowing water has a mind of its own and if you try to confine it too much in one area it may respond by popping out in a nearby area. An off-channel detention pond was constructed for Goldsmith Gulch at Iliff Avenue in Denver several years ago. The pond is immediately upstream of a long box culvert that goes under a shopping center parking lot. Each runoff event brings with it its own set of stream flow characteristics. Ideally, flow in the box culvert will be maximized before stormwater starts spilling into the detention pond. Also, the detention pond should fill with water before any flow overtops the box culvert and flows through the parking lot. Goldsmith Gulch has often disregarded this logic and as a result we are participating in a study to reconfigure these sensitive elements of the drainageway.

Lena Gulch in Lakewood between 20th Avenue and Youngfield Street. Boulder edge channel built within a confined backyard drainagewayeasement

Drop Structures
Little Dry Creek north of 80th Avenue in Arvada. This grouted boulder drop structure has stabilized the channel within the park and protects a sanitary sewer line that had been completely exposed by erosion.

Last year we mentioned that drop structures can fail when water flows through, under, or around them due to an inadequate cutoff wall. Drop structures can also be damaged in another way. When the channel bottom downstream from a drop is steeper than its equilibrium slope nature will try to flatten the slope by eroding upstream. If enough elevation is available for the stream the erosion can eventually undermine the foundation of a drop structure. In the Town of Parker we added two drop structures on Sulphur Gulch south of Main Street to flatten the longitudinal slope and reduce the threat to a recently built sculpted concrete drop structure. A similar structure was built on Happy Canyon Creek at Oak Hills Drive in Douglas County to stop erosion that was undermining the energy dissipation basin of a drop structure built several years ago. The close coordination of multiple interests on both projects was guided by Cindy Thrush, Senior Project Engineer. Jeff Fisher, Senior Construction Manager kept all the parties informed and coordinated while overseeing the construction.

A large stand of cattails and other wetland plants exists on Dry Creek No.2 east of 55th Street in Boulder. This environment was created by three wide drop structures made up of large stacked concrete blocks. These blocks have shifted enough that water now flows through the structures and has caused an undesirable combination of sedimentation and erosion. Our project will rebuild the drop structures to restore the water table, reduce the sediment deposition, and better manage the low flows and stormwater flows.