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I I I I I I I I I I I I I I I I I I I 9 Airport. The lOB-inch pipe serves the upper Westerly Creek watershed; the 63-inch pipe serves the local airport drainage area. The floodflow from a storm event of approximately B- year frequency or greater, will overtop the runway and will result in widespread shallow flooding at Stapleton International Airport. From the discharge of the lOB-inch pipe and the 63- inch pipe to Sand Creek (approximately 0.6 mile), Westerly Creek is a 30- to BO-ft wide open channel which can contain the floodflow of a 100-year frequency storm event. Roughness Coefficients Roughness coefficients, Manning's "n" values, were determined after field investigation of the study reach. An average condition for the channel and flood plain in each reach was used in the flood routing and water surface profiles computa- tions. Manning's "n" values used in this study are: Table 5 Roughness Coefficients Sand Creek to Stapleton Airport, local tributary Stapleton Airport, local tributary to north runway Stapleton Airport, north runway to south boundary Stapleton Airport, south boundary to Montview Blvd. Montview Blvd. to E. 14th Ave. E. 14th Ave. to E. 12th Ave. E. 12th Ave. to Richthofen Place Richthofen Place to E. 11th Ave. n 0.035 0.040 0.035 0.040 0.045 0.050 0.045 0.035 Water Surface Profile Computations During the water surface profile computation, no blockage of culvert or bridge was assumed and blockages of the waterway by houses and buildings were treated in several different ways. In a reach (between two cross sections used in the com- putation) where there are only a few houses or buildings, the blockage effect was neglected. In a reach where houses or buildings occupy a significant-area of the water way, only the open area in the cross section was considered effective for flow conveyance. Along the reach between 11th Ave. and Montview Blvd., the natural Westerly Creek Channel has been obliterated by development and encroachment. The houses and buildings are so close together and so regularly located along the reach between 11th Ave. and Montview Blvd., except the reach between 16th Ave. and 19th Ave., that a row of houses or buildings within one street block was treated as a bridge pier for the normal bridge routine in the backwater computation. A repre- sentative cross section within a street block was used as the average cross section of the block. The average cross section was transferred with increase or decrease in elevation to the four locations (two sections, one within and one outside the bridge pier, downstream and upstream of the average cross section) specified for the normal bridge routine computation in the HEC-2 computer program. The street (perpendicular to the flow direction) between two blocks was treated as an open channel between two bridges. Because the natural drainage way between 16th Ave. and 19th Ave. is curved, cross sections were