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I I I I I I I I I I I I I I I I I I I STUDY AREA DESCRIPTION Direct Flow Area 0054 is a west bank tributary to the South Platte River. From its origin in the hills along the west bank of the PIa tte River, the basin slopes to the east and consists of several short tributary channels which either combine or flow directly into the Platte. The basin is situated predominantly in T 67 N, R 2 W, of the 6th Prime Meridian, between Grange Hall Creek and Brantner Gulch. Drainage Basin Characteristics The 0054 drainage basin is triangular in shape, approximately 2.1 miles along the South Platte River and 2.8 miles from the South Platte to the western-most point of the drainage basin boundary. It drains approximately 4.3 square miles. Except for occasional interruptions by private land, Holly Street generally constitutes the easternmost Thornton City limits. Approximately 610 acres of the basin are within the City of Thornton and 2,140 acres are in Adams County. The basin is also intersected by Riverdale Road, the Lower Clear Creek Canal and Colorado Agricultural Canal, all of which including Holly Street, are directly impacted by the drainage patterns within the basin. From the Platte River westward to Riverdale Road, there are three tributaries which drain DFA 0054 and run roughly parallel to each other. The southern and longest tributary basin, 0054-1, rises at an elevation of 5,270 feet above mean sea level and falls about 240 feet in its 2.4 mile course. It is fed indirectly by two small tributaries, the middle and northern branches of 0054-1. The northern most tributary, 0054-3, has two main branches that merge below Riverdale Road to a common drainage swale that continues to the South Platte River. The basin of the middle tributary, 0054-2, is bounded by the basins of 0054-1 to the south and 0054-3 to the north, and is the smallest of the three. The development status of Direct Flow Area 0054 has been classified as "c" by the Denver Regional Council of Governments (DRCOG) in their 1971 Project REUSE, meaning the basin has undergone partial development and has been evaluated to have a moderate to low probability of future development. As it stands now, however, the basin west of Holly Street in Thornton is expected to experience nearly total development as single family residential communities. East of Holly Street, the land use is still rural residential and agricultural and this status is not expected to change. For the purposes of developing runoff hydrographs, the land use patterns shown on the "Future Land Use" map, sheet 22 of the Drawings, were assumed. - 4- Study Reach Description Typical of South Platte River tributaries, the channels in Direct Flow Area 0054 are not well defined and can contain only minor flows. Development in the floodplains and agricultural activies have encroached into the channel area partly because there has never been well defined flow areas, and the channels have been cut off by roads, stock watering or irrigation ponds, and irrigation canals. Until the development west of Holly Street, there was no regular low flow from the area. All tributaries originate in the hills just to the west of Riverdale Road, where the swales are fairly well defined. Between the Platte River and Riverdale Road, however, all of the study channels traverse irrigated farmland and become little more than slight depressions in the fields. The Lower Clear Creek Canal flows northeasterly across the basin and intersects each of the three tributary channels. Since no wasteways or diversion channels have been constructed, storm waters are usually carried by the canal until it overflows and spills back into the fields, usually downstream from the original point of interception. Between Riverdale Road and Holly Street are the uplands of the drainage subareas. The channels are grassy swales with no regular low flow except when irrigation runoff occurs. In several places, small dams have been constructed across the drainageways to retain water for agricultural purposes. Much of the storm runoff is intercepted by the Colorado Agricultural Canal as it traverses the area above Riverdale Road. Culverts beneath Riverdale Road at each of the tributary crossings consist of 12", 15" or 18" corregated metal pipe culverts. The southernmost tributary of the drainage area extends to the west of Holly Street into Thornton. As the development has occurred, right-of-way has been dedicated to the City of Thornton for the drainageway, but the only completed improvements to date are underground storm sewers servicing completed residential developments west of Cherry Drive and two small detention ponds above Holly Street. Proposed improvements consisting of detention dams, improved inlets and culverts, and channel improvements are in the planning stages at the writing of this report.