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I I I I I I I I I I I I I I I I I . I basin contains 711 acres (1.11 square miles) and extends from the confluence to 1-25 just north of 112th Avenue. The sub-basin lies entirely within the City of Northglenn. The major features of the basin are Washington Street, 112th Avenue, Irma Drive, the Union Pocific Railrood embankment, ond the Northglenn Municipal facilities. The water course above the Union Pacific Railroad consists of overland, street, and storm sewer flow. Below the Union Pacific Railroad a well-defined open chonnel carries the storm flows to Grange Hall Creek. C. Basin Hydroloqy The next step in the hydrologic analysis was to define the runoff peak flows and volumes for the existing and future developed conditions for eoch of the sub-basins and then combine the flows to define the design runoff at the desired hydrologic points. These design flows were subsequently used to define the different flood plains and thence potential droinage problem areas. I. Sub-basin Characteristics 5. Tributary SE The smallest of the sub-basins (that is part of the major droinageway plonning) is Tributary SE, which is a right bank tributary to Grange Hall Creek approximotely one- quarter mile above Riverdale Rood. The sub-basin contains 141 acres (0.22 squore miles) and extends from the confluence to just east of York Street along 104th Avenue. The area above the Union Pacific Railroad lies within Tharnton. From the Union Pacific Railroad The entire study area was divided into 44 sub-basins ranging in size from 37 acres to 395 acres (see Figure 111-1 and Table III-I). The rather detailed basin division was considered necessary to adequately define the runoff at the various hydrologic points and to account for the effects of basin development, channelization, and stream detention on the runoff characteristics in the various channels using the U.D. and F.C.D. runoff modeling techniques. to Colorodo Blvd. the sub-basin lies within Adams County. Below Colorado Blvd. the sub- For each of the sub-basins the area, length, centroid length, slope, percent imperviousness, Cp and Ct values were determined for the existing and future development conditions with existing drainage facilities. The parameters for the existing development were determined fro'n aerial photos provided by the District and Northglenn, and from field inspection of the basin. The future development para:neters were defined basin ogoin comes under the jurisdiction of Thornton. The major features of the bosin are the Union Pacific Railroad, Colorado Blvd., Brewer Reservoir, and the Colorado Agricultural Canal. The channel above the Union Pacific Railroad is mainly overland flow due to the lack of development. Between the Union Pacific Roilroad and Colorado Blvd. the woter course consists of broad V-type channel. Just to the east of Colorado Blvd. Brewer Reservoir has been constructed with a by utilization of the future land use map. high embankment. Between the pond and the Colorado Agricultural Canal there exists a brood V-type channel, which essentially disoppears below the canal. The actual woter In addition to the above-listed characteristics, the storm sewer drainage density was determined for each sub-basin. The storm sewer drainage density is defined as the lineal feet of storm sewer per acre of land, and is used to determine the degree that the basin is towards 104th Avenue, then east along 104th Avenue and into the Colorodo Agricultural Conal. The overflow from the canal then flows northeasterly into Grange Hall Creek. sewered so that the Cp and Ct parameters con be modified. The amount of sewer length in each sub-basin was determined from the storm sewer maps of Northglenn and field course is not within the channel, since the emergency overflow from the pond is south investigations. 9