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I I I I I I I I I I I I I I I I I I I North 75th Street to U.S. Highway 287- The Union Pacific Railroad also parallels the southern boundary of the floodplain in this 5.4 mile study reach represented by Cross Section Nos. 36 to 86. Floodplain limits for this reach are shown on Sheets 6 through 12 and water surface profiles on Sheets 19 to 22. The floodplain in this study reach is characterized by gravel pits and agri- cultural uses. Average 100-year floodplain widths range from 2200 to 4200 feet. Floodplain and Floodway Data The Flood Hazard Area Delineation Maps show the outline of the 100- and 500-year flood events for present channel and floodplain conditions. The maps are supplemented by Tables III and IV. The stream flows in an easterly direction in this reach. Stream crossings include a private drive, North 95th Street, and U.S. Highway 287. Diver- sions for the Liggett Ditch, Lower Boulder Ditch, and the Boulder and Weld County Ditch are also located in this reach. Major confluences include Dry Creek and Bullhead Gulch. Table III, "Floodplain Reference Data", contains a listing of the 10-, 50-, and 500-year flood discharges and corresponding water surface elevations at cross sections used in the hydraulic model. The table also includes stream stationing and descriptive information. The floodplain is used primarily for agricultural purposes. The average 100- year floodplain widths range from 1500 to 3300 feet in this reach. U.S. Highway 287 to Boulder-Weld County Line - Table IV, "lOO-year Floodplain and Floodway Reference Data", contains a list- ing of the 100-year flood discharges, stream stationing, thalweg elevations, 100-year water surface elevations, and 100-year floodplain widths at cross sections used in the hydraulic model. Descriptive information is also provided to aid the user in locating data pertinent to specific floodplain areas. The floodway data in this table defines the flow area required to convey the 100-year event without causing significant increases in stage due to future floodplain development outside the established floodway. The FEMA criteria permitting a one-foot maximum increase in the 100-year water surface was used to calculate the floodway encroachment limits. The floodway is a high hazard area characterized by relatively deep water and high flow velocities and should remain free of any development which would obstruct flow. The flood- way data includes floodway widths to the right and left of the channel center- line, the total floodway width and the 100-year flood elevations resulting from flows being confined to the floodway. This 3.6-mile study reach flows generally in a northeast direction and is represented by Cross Section Nos. B-4l to 36. Floodplain limits for this reach are shown on Sheets 12 through 17 and water surface profiles on Sheets 22 and 23. Stream crossings in this reach include North 109th Street, Kenosha Road, an access drive, State Highway 52 (Mineral Road) and Northeast County Line Road. Coal Creek is the major confluence in this study reach. Cross Sections B-4l and B-42 are a continuation of the cross sections and floodplain modelling for Boulder Creek through Weld County to its confluence with St. Vrain Creek. The Omaha District COE also performed the hydraulic modelling for this lower reach. The floodplain limits for Cross Section Nos. B-4l through 6 extend into Weld County and are not shown on the mapping in this report. For additional information regarding the Lower Boulder Creek flooding in Weld County, contact the Omaha District COE. 11