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I I I I I I I I I I I I I I I I I I I SECTION I SUMMARY MAJOR DRAINAGEWAY IMPROVEMENTS This report contains the preliminary drainageway plan for Direct Flow Area 3207, known also as the Nissen Reservoir Basin. The plan and profile draw- ings at the end of this report graphically illustrate the recommended im- provements, chosen by the Cities of Broomfield and Westminster following evaluation of alternative plans. Section I provides a synopsis of the 'recommended drainageway improvements presented in this report. Specific . direction for the preliminary plan was received from the Urban Drainage and Flood Control District, and a copy of the selected plan items is reproduced at the end of Section VI. The existing flood hazard areas in the basin have been the focus of the master pl anning effort for Direct Flow area 3207. Under Phase A of the master plan effort, various alternative plans were developed as solutions to the existing flood problems and presented to the Urban Drainage and Flood Control District (UD&FCD) and local governments for review. Following review, a plan was chosen by Westminster, Broomfield the UD&FCD which recog- nizes the potential development in the basin, is cost effective, reduces the hazard to life and property and provides drainageway planning for future development within the basin. The plan utilizes storm water detention, street crossing improvements, minor channelization, and floodplain regulation. With the except ion of the proposed detention facil ity, all master plan improve- ments are designed for the 100-year event with future basin development. The improvements include: THE STUDY AREA The Direct Flow Area 3207 drainage basin, identified also as the Nissen Reservoir Basin, is a 1,660 acre tributary to Big Dry Creek. The basin lies predominantly within the City of Broomfield with approximately 30 acres lying in the City of Westminster. The basin .is approximately 40 percent developed with primarily single-family residential and some commercial and industrial development. A single tributary drains the area surrounding and including Nissen Reservoir, and combines with the main stream at a point east of Main Street and north of 10th Avenue. Major transportation routes are Main Street, Lowell and Sheridan Boulevards in the north-south direction, and East 10th Avenue and Midway Boulevard in the east-west direction. 1. Installation of a flow separation structure at Community Ditch, and re- moval or renovation of the abandoned small dam northwest of Daphne Street. 2. Construction of new double box culverts at Main Street and East 10th Ave- nue. 3. Construction of a detention facility with volume equal to that of the 5-year flood event upstream of Ash Street. 4. Replacement of the existing culverts at Sheridan Boulevard with a double box culvert. 5. Selected channel improvements at the mobile home park northwest of Perry Street. The basin is rapidly being developed as the City of Broomfield grows, and was thus singled out as a priority basin for master drainageway planning. Hazar- dous floodplain encroachment has occurred along 7th Avenue between Birch and Ash streets, and at the mobil e home park north-west of Perry Street. Deve 1- opment has al so occurred in the Broomfield Country Cl ub adj acent to Ni ssen Reservoir. The developer has combined stormwater detention with conduits and drainageways to modify the 100-year floodplain so that the new development is not within the regulatory floodplain. 6. Construction of a new double box culvert at Perry Street, and a new triple box culvert at Lowell Boulevard. 7. Throughout the basin, the existing 100-year floodplain is to be regulated until such time as improvements are installed, after which the residual 100-year floodplain is to be regulated. Floodplain regulation will be according to the cities' ordinances in conjunction with the officially adopted Flood Hazard Area Delineation Report (Ref.21). 8. Encouragement of the purchase of flood insurance by those residents lying in flood hazard areas. g. Operation and maintenance of existing and proposed channels, pipes, cul- verts, and detention basins.