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<br />, <br /> <br />~ydraulic Analysis <br />Water surface elevations for the 10., 50-, 100-, and 500-year floods <br />werecooputed by the U.S. Army Corps of Engineers, ()nana District using <br />the U.S. Army Corps of Engineers' standard step backwater computer <br />program, HEC-2 (Reference 11). This program was also modified in 1979 by <br />tile Omdha District, Corps of Engineers to include an optional bridge <br />routine not found in the standard HEC-2 program. This optional bridge <br />routine enables the use of the Bureau of Public Roads method of computing <br />bridge losses. <br />Manning's "n" values ranged from .040 to .045 in the channel, and <br />from .050 to .1 on the overbanks. The results obtained from the computer <br />model compared favordbly to stream gaging records at U.S. Highway 287 <br />nearlongmont, Colorado. However, these g<lgerecords <Ire limited by the <br />f<lct that the highest disch<lrge of record is only slightly above the <br />magnitude of the 10-yearflood. This gage may also be affected by flows <br />on St. Vr<lln Creek because of its proximity to the mouth of left hand <br />Creek. 8ec<luse of these factors, Manning's "n" values were selected <br />based upon a combination of engineering judgment, Inspection of the <br />flOildplain, and the U.S. Ge010ylcal Survt'y ~tream gaging station. No <br />high water marks were located whiCh could be used to assist calibration <br />of the hydraulic model. <br />Starting water surface elev<ltions were determined from data contained <br />in a report entitled Floodplain Information, lower St. Vrain Creek, <br />Volume III, Boulder County, Color<ldo (Reference 12). <br />Bridges inthi; study were analyzed assuming no loss of <lrea beneath <br />the bridge due to debris blockage. Both the normal bridgeroutinefrOlTl <br />HEC-2, and the Bureau of PUblic Roads bridge routine were used to <br />determine water surface losses through bridges. <br /> <br />The 100- and 500-year flood boundaries were determined by use of the <br />topographic mapping referenced earlier. <br />A possibility exists that flooding could be more severe than is <br />indicated in this report downstream of Pike Road. The Pike Road crossing <br />creates a floodflow obstruction which temporarily impounds a conSiderable <br />amount of flood water. Failure of this roadway embankment could produce <br />discharges that are higher than used in this report in the reach between <br />Pike Road and the mouth. Additional flood hazard also exists near <br />irrigation ditches which usually run near capacity during floods. <br />Failure of these ditches could result in flooding in areas not covered by <br />this study. <br /> <br />7 <br />