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Urban Drainage & Flood Control District - Colorado 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 popula- tion. 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 ds). This peak flow reduction resulted in a regulatory floodplain confined to the street and front yards along K 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 por- tions 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, Ine A data plot showed the stage hydrograph and 30minute 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 ds, 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 actually 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 K 7th Ave. At least one resident along K 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 improve- ments were completed,