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I I I I I I I I I 17th Street to the downstream Arapahoe Road bridge. A 100-year discharge of 12,000 c.f.s. was used for all reaches under investigation. While endorsing the concept of Plan II upstream from l7th Street, the City is concerned that the degree of environmental disturbance be minimized. A particular item of concern is the loss of trees l. Flood Plain and Floodway Data The limits of flooding anticipated during the lOO-year dis- charge of l2,000 c.f.s., as determined by the Corps of Engineers, are outlined on the attached drawings and tabulated in Table 1 in Appendix C, Flood Plain and Floodway Reference Data. The table lists potential 100-year flood elevations, flooding widths and reference line (thalweg) elevation for each cross- section within the Study area. The flood profile thalweg (reference) line and flood plain width at any point within the Study area is graphically presented in the drawings. and other vegetation. C. GENERAL APPROACH The existing 100-year flood plain and floodway were previously analyzed by the United States Corps of Engineers using the HEC-2 Water Surface Profile Computer Program and were not re-investigated for this Study. The HEC-2 program was developed by the Corps of Engineers at the Hydrologic Engineering Center in Davis, California, and has been accepted as one of the methodologies for calculating I I I I I I I I I I water surface profiles in natural channels. '. The Corps of Engineers have also determined the floodway for Boulder Creek under existing conditions. A floodway represents that part of the flood plain which is required to pass a 100- year flood event without rasing the flood profile more than acceptable amount and which represent hazards to personal safety and welfare. In this Study, the floodway represents that portion of the flood plain where the lOO-year flood velocities exceed two feet per second, where the depth exceeds two feet or where the water surface would not increase more than one-half feet after encroachment. Floodway widths, representing the limits of encroachment into the flood plain are tabulated in Table 1 of Appendix C. Water surface elevations anticipated after encroachment are also tabulated. The residual flood plain, the flood plain remaining after proposed master plan improvements have been implemented, was evaluated during the course of this study using the HEC-2 Water Surface Profi le Computer Program. Input to the HEC-2 program included discharge, cross-sectional data, data associated with drainage structures, obstructions such as buildings, vegetation and manmade embankments, and channel roughness coefficients. The Manning's roughness coefficients used in the water surface profile computa- tions varied from 0.037 to 0.055 in the vicinity of the main channel 0.045 to 0.078 for the overbank areas. Transition coefficients used to account for the effects of changing channel geometry on the energy losses associated with flow were 0.3 for expansion and O. 1 for contraction. 33