Maintenance Program Activitiesby
Mark R. Hunter, P. E.
Chief, Maintenance Program
Through the routine maintenance program $716,500 was spent in 1997 for mowing and debris pickups on approximately 210 different sections of drainageways within the District boundaries. This equates to a total of nearly 100 miles of drainageways in the Denver area that were given routine maintenance.
The amount of money spent on routine work this year is up substantially over last year. This is the result of an increased frequency of mowing on certain drainageways. Some of the more urban drainageways now receive four to five mowings per year. Three to four mowings per year was inadequate for effective weed control and for overall appearance.
Other drainageways we maintain are more rural in character. On portions of some of these drainageways we have taken the opportunity to reduce or eliminate our mowing activities. This has been done to encourage habitat and leave a more natural character in the drainageway corridor.
In 1997 the restoration program completed $841,000 of work. Restoration projects typically address isolated drainage problems where the solution involves small scale construction. Ninety-four individual activities were completed during the year.
In last years Flood Hazard News we described a site on Lena Gulch at the confluence with Clear Creek where, through natural processes, Clear Creek had captured the lower 300 feet of Lena Gulch. The result was a 50-foot wide pool of slow-moving water in Lena Gulch that has experienced some sedimentation and has contributed to insect populations and to uncontrolled vegetation. On the other hand our assessment has been that the changed conditions on Clear Creek and Lena Gulch have not elevated the 100-year floodplain. Thus, there has been little incentive to move rapidly to reconstruct the confluence of these two creeks.
In contrast to the Lena Gulch site described above two other sites have also had increased sediment and vegetation, but they have also experienced a loss of flood-carrying capacity. Tucker Gulch in Golden was improved about five years ago. Through earlier land-use decisions the channel was already limited to a confined right-of-way pinched between streets and residences. During the design the project participants made the decision to not expand the right-of-way. That resulted in a narrow concrete-lined channel with little or no surplus flood-carrying capacity. When sediment accumulated and vegetation took root it became necessary for maintenance crews to remove to material to restore the intended capacity to the creek.
The other similar site is on South Boulder Creek on the east side of Boulder, Colorado. As with Tucker Gulch, South Boulder Creek has experienced sedimentation and dramatic vegetation growth in the 20 years since it was improved. The resultant flooding threat was recognized by nearby landowners and city officials. To restore the channel capacity a plan was developed to remove the sediment and vegetation.
The excess material needed to be removed from the channel cross-sections of both Tucker Gulch and South Boulder Creek in order to maintain them in a manner that was consistent with the intent of their original designs. Accomplishing this maintenance work caused some discomfort for the involved parties. First, it was a time-consuming process to acquire federal permits to carry out this kind of local maintenance work. A second issue was that some of the neighbors objected to the removal of the material. To them the vegetation was a welcome addition to otherwise stark drainage channels.
Twenty-eight projects were at various stages of design or construction during 1997. 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 drainageway. By the end of 1997 the District will have spent about $2,178,000 on rehabilitative design and construction for the year. A few of the unique projects are discussed below.
East of Irma Drive at 104th Avenue in Northglenn there is a detention pond on Grange Hall Creek. The pond serves as both stormwater detention and as a surface water re-use program run by the City of Northglenn. The existing 108-inch outlet passes under an unused 30-foot high railroad embankment. The outlet pipe had deteriorated to the point that it was being held up with railroad timbers. Through a combined project with the Districts construction program and the City of Northglenn the pipe and outlet systems are being replaced. The contractor is laying the pipe in an open cut and is backfilling the conduit with products called "flash the conduit with products called "flash fill" and "flow fill" to prevent water movement along the pipe.
Since we reported on it last year our project on Niver Creek in Adams County near the South Platte River has made slow but steady progress. Adams County was successful in acquiring enough land to expand the Niver Creek trail-head park. The design is now 95% complete. With funding in place from several different sources this project will soon begin construction. Deteriorated pipe, concrete lined channel and eroded channel will be replaced with a new bridge over Niver Creek, a restored open channel for Niver Creek and a trail-head park facility.
In Arapahoe county, just south of Dry Creek Road, Willow Creek has cut a 25-foot high vertical bank. About 2,000 feet downstream the creek enters the flood detention pool behind Englewood Dam. The flood pool has been experiencing aggradation for several years. The Districts construction program has a design underway to solve the severe erosion problem. Coupled with that design the maintenance program is having the same consultant design a sediment trap upstream of the flood pool. This will reduce the amount of large sediments being deposited in the detention pond and improve the quality of the stormwater. The sediment trap should also help reduce the on-going muck removal that has been necessary to keep open the pedestrian trail under Dry Creek Road.
To the west of Holly Street in Greenwood Village Greenwood Gulch flows through a wide area of wetlands. East of Holly Street the gulch has eroded a steep and narrow channel behind several homes. The maintenance program has combined with the Districts construction program and the City of Greenwood Village to fund a single project to address these problems. The wetland is soon to be re-established using low grade control structures. The eroded channel will be rehabilitated with grouted boulder drop structures. Extensive vegetation planting will complete the project.
In last years Flood Hazard News we reported that two large projects were underway on Goldsmith Gulch in the City of Denver. Both projects are situated on park land and both have benefited from a design partnership with participants from nearby neighborhoods, the Denver parks department, the consultant and the District maintenance program.
Within Bible Park Goldsmith Gulch has eroded a vertical-sided channel ranging from three to ten feet deep. The gulch flows through a broad natural area. This allowed the design partnership the opportunity to recreate a relatively natural riparian corridor for the stream. The rehabilitation project has recently been bid and will create a meandering stream which will repair the erosion damage and make the stream more accessible and aesthetically pleasing to the park users.
Within Cook Park the erosion in the low flow channel of Goldsmith Gulch has created a steep-sided channel that is three to six feet deep. Although this is not as severe as in Bible Park the setting in Cook Park is an improved blue-grass multi-use area. This dictates that the channel configuration for Goldsmith Gulch be rehabilitated to be less of a threat to existing facilities such as pedestrian bridges and play areas. The design for this project is well underway. Construction will take place in 1998.
STATUS OF MAINTENANCE REHABILITATION PROJECTS