Maintenance Program Activities
by
Mark R. Hunter, P.E., Chief, Maintenance Program

Routine Maintenance

Through the routine maintenance program $687,700 was spent in 2000 for mowing and debris pickups. This work was done on approximately 220 different sections of drainageways within the District boundaries. This equates to a total of over 100 miles of drainageways in the Denver area on which we performed scheduled mowing and debris pickup maintenance.

In the year 2000, some local governments continued calling for an increased level of mowing and debris pickups on many urban drainageways. Many of the more urban drainageways now receive four or five mowings and seven or eight debris pickups per year. In the early 1980s the sole purpose of the routine program was to pick up large debris that could otherwise contribute to blockages and flooding problems. At that same time the corridors surrounding the urban drainageways were not appreciated for the resource that they are today.

These days urban drainageway corridors are prized as neighborhood amenities. Along with that outlook comes the community desire for a level of drainageway maintenance that goes beyond our original flood control debris pickups. Three or four debris pickups per year is now inadequate on the more urban drainageways.

We understand that on some urban drainageways our routine maintenance crews are the only service the drainageway will receive. Also, it is not practical to split duties on a given reach with a second agency. With this in mind we are capable of increasing the frequency of debris pickups and mowings on non-irrigated drainageways where the local government sets that as a priority. For certain drainageways, the local government may believe it is worthwhile to increase the routine services in exchange for less funding being available for restoration or rehabilitation work.

All of our routine work is done on structured schedules by private contractors. We are not set up to carry out landscape maintenance, on-call work, or emergency services. In the past we have participated with other agencies by splitting maintenance duties along drainageways through improved parks. We have seldom been pleased with our performance in these areas. Recognizing this, we will resume our initial policy of not providing mowing and debris pickup on irrigated drainageway corridors.

For the year 2000 we awarded eight separate contracts for routine work. Three of those contracts were awarded through an internal review of proposals submitted by potential contractors. The remaining five were awarded through a direct competitive bid process. Over the last three years we have been transitioning away from awarding routine contracts based on an internal review of contractor proposals toward awarding them based on the competitive bid process. For the year 2001 all the routine contracts will be awarded through competitive bids.

Restoration Maintenance

In 2000 the restoration program completed $1,149,000 of work. Restoration projects typically address isolated drainage problems where the solution involves small-scale construction. Seventy-five individual activities were completed during the year. A major advantage of the restoration program is the ability to use it to react quickly to local drainage needs.

Nature dictates that as long as sediment is being carried by a stream some of that sediment will drop out when the stream expands into a quiescent pool. One of the on-going obligations created when detention ponds are built is to keep those ponds relatively free of accumulated sediment. We have removed sediment from two dozen detention ponds over the last two years. It takes a continual effort to keep these flood protection facilities functioning as intended. We envision that it will be more difficult to carry out this type of regular maintenance in the future as the Clean Water Act regulations become more restrictive.

An example of regular detention pond maintenance is Fairfax detention pond which is situated on Fairfax Tributary in Commerce City. The sediment is removed from this pond once or twice a year. It is in a popular well-maintained park so maintenance access must be coordinated with other park uses. We are always impressed with how much sediment accumulates in this pond even though the whole tributary basin is developed. Perhaps dirt and debris washed off the streets are the main contributors.

East and West Toll Gate Creeks join together west of Chambers Road in Aurora. The site is surrounded by open space characterized by the meandering streams with steep cut-banks. The project called for confining the lateral erosion of the creeks without impacting the natural high-plains character of the area. Bio-engineering techniques were used exclusively to stabilize the toes of the steep banks. Limited regrading was done at the toes of the eroding slopes to allow erosion prevention measures to be installed. The upper part of each eroding bank was left intact to preserve the quality of the open space.

In last year’s Flood Hazard News we reported on the re-naturalization of a portion of Boulder Creek near Highway 287 in eastern Boulder County. The main feature of the work was to return sinuosity to the creek alignment and to install "bendway" weirs to keep the low channel banks in place. A bendway weir redirects the flow by letting the main current of the stream go the direction it intrinsically wants to go as it flows over the weir. Other weirs, such as jetties and groins, forcibly divert the flow by creating an obstruction. The re-contoured stream appears to be thriving with the help of attentive revegetation efforts.

An example of a recovery zone on Cherry Creek.

For its last three miles, before it discharges into the South Platte River, Cherry Creek flows in a vertical concrete-walled channel through central Denver. Approximately 70% of the edge of the low flow channel through this reach is lined with boulders. At the gaps in the protection the creek can still threaten the concrete walls or the nearby trail. When we repair this type of erosion near the trail we also install a two-foot wide recovery zone at the edge of the trail. This will help warn persons that they are in danger of going off the edge of the trail. It also gives them additional width in which to regain the main trail surface.

Little Dry Creek crosses under the Highline Canal south of Orchard Road in Arapahoe County. Downstream from the irrigation canal crossing steep-banked erosion on the creek is threatening private property. The owner of the canal and the private property owner are both willing to participate with us in funding the project to accelerate the erosion control work. This is a good example of cooperation among multiple parties to address a common problem. We enjoy doing this kind of project because the result is very often a multi-purpose improvement with many pleased parties.

Rehabilitation Maintenance

Twenty-two projects were at various stages of design or construction during 2000. Those projects are listed in the accompanying table titled "STATUS OF MAINTENANCE REHABILITATION PROJECTS". Rehabilitation projects usually take the form of consultant-designed repairs that are intended to address severe problems that have occurred on a previously improved urban drainageway. By the end of 2000 the District will have spent about $3,077,100 on rehabilitative design and construction for the year. A few of the unique projects are discussed below.

Shaw Heights Tributary joins Little Dry Creek at Sheridan Boulevard and 76^th Avenue in Westminster. Upstream from that point Shaw Heights Tributary passes through a small park. Above that it is compressed into a narrow corridor between a railroad track and residential backyards. Construction of drop structures and a geocellular confinement system to provide bank stabilization is nearing completion.

We have learned that there are drawbacks to designing multiple phases of a lengthy project and then building those phases over several years as funds become available. As the plans sit on the shelf new design concepts are developed, neighborhood concerns change, and political directions are amended. This frequently results in a construction plan that is obsolete before it is built. The goal of the channel reconstruction on Dutch Creek east of Pierce Street was to correct bank erosion near a pedestrian bridge and to stop a headcut from eroding through a wetland. The original design had been done seven or eight years ago. Much of that design had to be redone this year as a result of the factors mentioned above plus the topographic changes that had occurred in the project area.

Southwest of Federal Boulevard and Colfax Avenue Lakewood Gulch flows through the rolling landscape of Sanchez Park. The original design for this work was begun 10 years ago. This design also had to be revised, more than once, for the same reasons mentioned in the paragraph above. The work, which is currently under construction, will install drop structures and boulder edging in select locations on the low flow channel to arrest the erosion and protect the corridor’s Cottonwood trees.

Construction is now being completed on the West Side Tributary to the South Platte River northeast of 6^th Avenue and I-25. This project sat on the shelf for five years waiting for funding and because of a conflicting sewer line construction project that needed to be completed before this project could begin. While waiting for its turn this project was also redesigned to updated standards. But, that was not the last change as construction of this project revealed another dilemma. The channel drained an industrial area from early Denver. The water was contaminated with residual petroleum products which in turn had contaminated the soil throughout the length of the project. Dealing with the tainted soil in the proper manner caused a 56% cost overrun over the original bid.

Cleaning out the Willow Creek sediment trap.

In the previous two issues of Flood Hazard News we have reported on the sediment trap that was constructed on Willow Creek at Dry Creek Road in Arapahoe County. It has been in operation since early summer of 1999. In early fall of 1999 we removed nearly 1000 cubic yards of accumulated sediment from the facility. We have done two sediment removals so far this year with each being about 800 cubic yards. It is a small sediment trap with a holding volume of about 0.6 acre-feet serving a drainage basin of over eight square miles. It is not capable of capturing the fine sediments but, it has served well in catching the sandy and granular material.

Channel erosion on Sulphur Gulch below the drop structure.

Flowing from east to west through the center of downtown Parker is sandy-bottomed Sulphur Gulch. In 1994 a grouted sloping boulder drop structure was built on Sulphur Gulch about two thousand feet upstream from its confluence with Cherry Creek. There are no grade control structures on the gulch downstream of the drop structure. The consequence has been rapid and destructive erosion of the channel bed below the drop resulting in the collapse of the lower edge of the structure. A construction contract to repair the structure is now underway.

In the early and mid-1980s the Maintenance Program rehabilitated six drop structures on Cherry Creek between University Boulevard and Holly Street in Denver. We rebuilt all six of them as sloping riprap drop structures making use of the existing sheet pile as the cutoff wall for our new drops. Each of the rebuilt drops starting failing almost immediately. Areas of the riprap appeared to be undersized and, in general, riprap is only as strong as its weakest area. With the completion of the reconstruction of the structure west of Colorado Boulevard in May, 2000 we have now re-rebuilt five of them as grouted sloping boulder drop structures.

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