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On the south side of Clear Creek, downstream of Pecos Street to the irrigation dam, commercial infilling has occurred following a recent Letter of Map Revision (LOMR), while upstream of Pecos the gravel operations have created significant changes to the topography. We were unable to replicate the LOMR floodplain and conveyance zones in this reach, and there is also a small amount of overtopping of Pecos Street on the north side of Clear Creek at the intersection with West 64th Avenue. After an extensive 4 /MES ,",SS::lClATES Rating curves were developed for evaluating the flows passing between the Main Channel and the North Overflow under 1-76 at Tennyson Street and Lowell Boulevard and the spill direction and values were incorporated into the model. The water surface elevations for the main and the North Overflow at Tennyson were essentially the same so no cross flow was considered to occur at Tennyson. At Lowell Boulevard, significant flows would occur to the north from the main flow to the I In the reach between 1-25 and Federal Boulevard, the floodplain remains mostly undeveloped up to the irrigation dam downstream of Pecos Street. On the west side of Broadway, north of Clear Creek, a commercial strip has been filled and developed. This was discovered during a field inspection and the floodplain modified to reflect the filled area based on field observations. . Clear Creek Main then crosses under 1-76 western crossing and from this point downstream to the eastern crossing of Clear Creek Main under 1-76, the flows are divided and the water surface elevations modeled separately with the flows recombining between the eastern crossing of 1-76 and the Rio Grande Railroad I 1-25 to Federal Boulevard Between Washington Street and 1-25, there are two ramps with bridges that cross Clear Creek well above the 1 DO-year floodplain. In this area the floodplain is contained between Colorado Highway 224, (East 70th Avenue), and 1-76. CDOT has replaced a dangerous single drop structure in the reach under 1-25 with 3 smaller, safer drop structures. These structures were incorporated into the hydraulic model using field- surveyed data for each structure . Downstream of Sheridan Boulevard to the 1-76 western crossing, a split flow line was established based on the topographic divide between Clear Creek Main and the North Overflow floodplain, and a lateral weir was modeled in HEC-RAS to determine the flows diverting to the North Overflow (flowing at a significantly lower elevation) through this portion of the reach. I . Clear Creek flows begin separating upstream of Sheridan Boulevard so two additional cross sections were added upstream of Sheridan Boulevard to assist in analyzing the distribution of flows overtopping Sheridan Boulevard. I From 1-270 to Washington Street, the 1-76 embankment on the south side of the creek contains the flood plain, and the north side spills across Colorado Highway 224. At Washington Street, the flood plain is contained between Colorado Highway 224 and the 1-76 embankment. Washington Street has 1 DO-year capacity and is not overtopped. . Upstream of the Rio Grande Railroad to Sheridan Boulevard, 1-76 splits the Clear Creek flows in two separate flow paths that rejoin between 1-76 and the Rio Grande Railroad. The two paths are called the Clear Creek Main flows and the North Overflow. The analysis of the split flows proceeded as follows 'I -I At York Street, flows overtop the street north and south of the bridge, but the bridge structure is not overtopped. Upstream of York Street, on the south side of Clear Creek, a regional park-and-ride parking area has been constructed and an interchange connecting the newly extended 1-270 to 1-76 is being completed. The extension of 1-270 added a two-bridge overpass crossing Clear Creek and except for the support piers, the bridge decks are well above the 1 DO-year flood elevation in this area. The backwater effects from Federal Boulevard on Lake Sangraco were analyzed and will raise the water surface elevation in Lake Sangraco to 5216.9 msl This elevation will not inundate the existing houses around the lake. I I York Street to The flooding along the lower part of Clear Creek in Adams County from the South Platte River to York Street is primarily confined to lowlands and gravel pits. No residential areas or commercial establishments, except for the gravel mining operations, are threatened in this reach 1-25 Federal Boulevard is overtopped for its entire length where it crosses the floodplain. Flows from upstream of the Rio Grande Railroad divide at the Rio Grande Railroad bridge with the majority passing under the railroad to Federal Boulevard. A significant portion of the flow is diverted easterly along the south side of the railroad to Federal Boulevard where the flow passes under the railroad at the Federal Boulevard underpass, returning to the Clear Creek floodplain downstream of Federal Boulevard. This flow diversion was analyzed and incorporated into the hydraulic model. The Rio Grande Railroad bridge is not overtopped I I South Platte to York Street The 1 DO-year floodplain limits and water surface profiles are shown on the Flood Hazard Area Delineation Maps and Profiles appended to this report. The computed elevations at each reference point are tabulated in Tables 1 and 2. A description of the extent of flooding is discussed in the following paragraphs. Federal Boulevard to Sheridan Boulevard Downstream of Federal Boulevard, there is an area of shallow flooding around commercial structures on the north side of Clear Creek resulting from the overtoping of Federal Boulevard. I Flooded Areas Upstream of Pecos Street at the Colorado & Southern Railroad bridge the outfall for Little Dry Creek has been relocated upstream of the railroad bridge. The railroad overtops for most of the floodplain width and a small spill occurs south of Clear Creek where the railroad crosses under 1-76. This inundates an area south of 1-76 with some of the spill outletting to the east, with the potential for sheet flows on Pecos Street. I I I I Flood magnitudes greater than the 1 DO-year magnitude can and will occur. Land improvements adjacent to the 1 DO-year floodplain limits should also consider the chance for possible flood damage of a 1 DO-year event. The peak flows for the 1 O-year event should be used in the planning and engineering of improvements where the higher risk of failure or damage is economically feasible and the hazard to life and property is lower or nonexistent. comparison of the LOMR model with the model developed for this FHAD, we believe that differences in Manning's "n' values for the channel and revised topographic information contributed to the higher water surface elevations predicted in this FHAD. Our delineation on the south side of Clear Creek downstream of Pecos Street was based on field-surveyed data.