Urban Drainage and Flood Control in the metro area of Denver

Denver Metro Area

In 25 years, we have seen the District’s population in the Denver Metro area grow by about 850,000 people, along with all of the structures needed to support that population. However, the number of structures located in identified 100-year floodplains is approximately 4000 fewer than 25 years ago. This is the result of the District’s long standing policy of correcting past mistakes through the planning, design, construction and maintenance of flood mitigation projects while preventing new development in floodplains through the Floodplain Management Program. Of course none of this could have happened without the participation of our local government partners.

Flood Control Project Tested by Design Storm 

In 1988, the District and the City of Broomfield completed $370,000 in flood control improvements along Basin 3207 Drainageway (also known as Nissen Reservoir Channel) between E. 10th Ave. and Ash St. in Broomfield. This project involved the construction of two detention facilities (Ponds 6 and 7), which effectively reduced the 100-year discharge downstream by more than half (from 1090 to 480 cfs). This peak flow reduction resulted in a regulatory floodplain confined to the street and front yards along E. 7th Ave, thus removing more than 60 residential properties from the 100-year floodplain.

One decade later on Saturday, July 25, 1998, these improvements returned dividends when a thunderstorm produced in excess of three inches of rain over significant portions of Basin 3207.

At Pond 6, the July 25 storm produced 2.76” of rain and resulted in a peak stage of 38.6 feet, equaling the 100-year design flood according to the consulting engineers’ design report prepared by Sellards & Grigg, Inc. A data plot showed the stage hydrograph and 30-minute rainfall amounts between 7/25 noon and 7/26 midnight. A resident at the intersection of E. 7th Ave. and Birch St. measured 3.45” of rain. The storm hit the Basin 3207 area shortly after 5 p.m., with the first inch of rain falling within the first 20 minutes, causing major street flooding. Runoff quickly filled both detention ponds to capacity. The Pond 6 peak occurred at 6:54 p.m., cresting at a depth of 19” over the spillway and releasing 470 cfs.

Measurements at Pond 6 were made by an automated gauge that was installed as part of the flood control improvements. Prior to the July 25 storm, the largest recorded event occurred on May 17, 1995, with the water surface reaching a maximum stage of 35.7 feet. It is suspected that this stage may have been exceeded on July 19, 1997, but no data was available for this event thanks to the work of vandals on the preceding day. The Basin 3207/Pond 6 gaging station is one of 143 ALERT stations operated by the District.

During and following the July 25 storm, local officials received reports concerning flood problems at a number of locations throughout the City, but the actual damages were relatively low considering the magnitude of the event. Five homes in the 900 block of Birch St. and three in the Eagle Trace Subdivision had water backup in their basements from sanitary sewers. The City later determined that this problem was caused by some unsealed manholes and property owners were compensated for their losses. No sewer backups were reported along E. 7th Ave. At least one resident along E. 7th Ave. did report two-inch deep water in her basement, presumably from seepage or poor site drainage. She also said that her property had been flooded five times in the past 26 years and this was the first high water since the flood control improvements were
completed.

An Engineering Department official noted that storm drainage facilities at the new Broomfield Town Center along 120th Ave. (US 287) were flowing full and performed well on July 25. If the storm had been worse, businesses in this area may have sustained significant damages. Recent drainage improvements along City Park Drainageway were credited with preventing damages on July 25. The City official also noted that a smaller event had just occurred a few days earlier that nearly flooded the U.S. West Communications building located in the floodplain at 120th and Sheridan. The July 22 flooding was aggravated by a construction project that partially obstructed the City Park Drainageway channel. The problem was immediately remedied, which proved fortunate just three days later.

Restoration Maintenance

In 1998 the restoration program completed $1,428,000 of work. Restoration projects typically address isolated drainage problems where the solution involves small scale construction. One hundred individual activities were completed during the year. A major advantage of the restoration program is the opportunity to use it to react quickly to local drainage needs.

An example of reacting to a drainageway maintenance need occurred in Brighton, Colorado during the summer of 1998. City staff informed us that Line B, also known as South Urban Channel, needed repairs. Line B was originally improved by the City of Brighton and the District Design and Construction Program about 20 years ago. Changes in the upstream reaches of the creek coupled with natural processes caused sediment deposition to occur in the improved section. What was originally intended to be an urban passive-recreation corridor was becoming a marshy and mosquito ridden area. The channel was wide and flat-bottomed with a riprap-lined trickle channel. The deposition was occurring in and around the trickle channel due to the frequency of the smaller storm events and the roughness of the riprap. Our work included removing the sediment as well as reshaping and resetting the riprap for much of the length of the trickle channel. Not all the problems were solved, however. This section of Line B is still awaiting an improved outlet to the South Platte River.

A similar opportunity to react arose in mid-1998 on what is called the Pinehurst Tributary to Bear Creek in southwest Denver. At a rear-yard location, overland flow was captured by an inlet and pipe system. Because of its setting, the pipe inlet frequently became plugged with debris. The result was that runoff could not enter the pipe and would back up enough that the water, in its obligation to seek the lowest point, would sweep around and through several homes. The final solution to this problem was not in a maintenance project but in capital improvements that would ultimately remove the homes from the floodplain. Such improvements had yet to be planned, designed and built. Recognizing that it could be years before such improvements would be made project planners hoped to make short term changes to help the neighborhood. It was recommended that the inlet to the pipe be improved to increase the amount of water it let into the pipe. This fell within the work the Maintenance Program could perform. The inlet design and construction were completed within a couple months. While the development of the master plan for flood control improvements is still underway, the improved inlet will now provide better water carrying capacity for the neighborhood than it had before.