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<br />I <br /> <br />I <br /> <br />Floodplain Information Report <br />K:\35600\004\FLOODPLAIN INFORMATION REPORTdoc <br /> <br />24 <br /> <br />June <br /> <br />998 <br /> <br />Floodplain Information Report <br />K:\35600\004\FLOODPLAIN INFORMATION REPORT.doc <br /> <br />25 <br /> <br />June <br /> <br />1998 <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Analyses of the hydraulic characteristics of the stream in the study areas were carried out to <br />provide estimates of the water surface elevations of floods of the selected recurrence intervals <br />along each stream. The water surface elevations were computed through use of the Corps of <br />Engineers' HEC-RAS step backwater computer program. Cross sections for the backwater <br />analysis of the North St. Vrain Creek and the main stream of the St. Vrain Creek were obtained <br />by digitizing cross sections obtained from the topographic maps of the study reach. Additional <br />cross sections were field-surveyed at close intervals above and below bridges and culverts in <br />order to account for possible backwater effects of these structureso The locations of the cross <br />sections used in the flood profile computations are reference on the flooded area maps (Plates <br />13 through 19) and the flood profiles (Figures 1 through 6) <br /> <br />Channel roughness coefficients (Manning's "n") for the flood profile computations were <br />assigned on the basis of field inspection of the channel and floodplain through the study reach. <br />The range of "n" values for channel and floodplain are summarized below in Table 5: <br /> <br />I <br /> <br />line <br /> <br />at <br /> <br />mouth <br /> <br />924.0 <br /> <br />5,350 <br /> <br />12,120 <br /> <br />16,520 <br /> <br />40,080 <br /> <br />I <br /> <br />4.3 <br /> <br />For a detailed description of the hydrologic analysis, <br />produced in 1987 by Engineering Professionals, Inc. <br /> <br />Hvdraulic Anal <br /> <br />sis <br /> <br />No. 85th St. (D/S study <br />limit) <br />Boulder-Weld County <br /> <br />351.0 <br /> <br />241.0 <br /> <br />5,520 <br /> <br />3,160 <br /> <br />10,950 <br /> <br />6,890 <br /> <br />14,850 <br /> <br />9,580 <br /> <br />28,670 <br /> <br />19,680 <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />refer <br /> <br />Buttonrock Dam was then inserted in the calibrated SWMM model of St. Vrain Creek at the <br />Lyons gaugeo It was found that 6-hour rainfall over South St. Vrain Creek and North Sto Vrain <br />Creek below Buttonrock Dam generated higher peak discharges at Lyons than did rainfall over <br />the entire St. Vrain basin above Lyons. The adopted frequency curve at Lyons therefore does <br />not reflect any contribution to peak discharges from the 99 square miles of drainage basin above <br />Buttonrock. The adopted frequency curve at the mouth was modeled by applying 6-hour <br />rainfall to 924 square miles of contributing basin area. This frequency curve was developed for <br />existing conditions and with Buttonrock Dam in placeo Table 4 summarizes the results of the <br />hydrologic analysis of flood flows within the study reach. <br /> <br />to the <br /> <br />Technical <br /> <br />Addendum <br /> <br />St. <br /> <br />Buttonrock inflow <br />ST. VRAIN CREEK <br /> <br />at mouth <br />NORTH ST. VRAIN <br /> <br />confluence of No. & SOo <br />Vrain <br />Lyons Gauge <br /> <br />Buttonrock spillway <br /> <br />at <br /> <br />mouth <br /> <br />211.0 <br />212.0 <br /> <br />99.0 <br /> <br />119.0 <br /> <br />9900 <br /> <br />2,040 <br />2,030 <br /> <br />1,600 <br /> <br />680 <br /> <br />1,000 <br /> <br />6,670 <br />5,560 <br /> <br />4,200 <br /> <br />2,000 <br /> <br />2,850 <br /> <br />8,880 <br />8,830 <br /> <br />6,000 <br /> <br />3,100 <br /> <br />4,310 <br /> <br />20,260 <br />20,140 <br /> <br />15,000 <br /> <br />7,600 <br /> <br />10,630 <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />As future floods will be attenuated by Buttonrock reservoir, and as adequate streamflow <br />records are unavailable to adequately account for the effects of Buttonrock on peak flood flows <br />through statistical analysis, the Corps of Engineers modeled the St. Vrain Creek Basin using the <br />Environmental Protection Agency's (EPA) Storm Water Management Model (SWMM) and the <br />Missouri River Division's version of Harder's diffusion routing model. Discharge-Frequency <br />relationships at the confluence of North and South St. Vrain Creeks and at the mouth of the St. <br />Vrain were derived through a statistical analysis of the streamflow records at these points. <br />Since Buttonrock Dam has a controlling effect on peak discharges at the Lyons gauge, only the <br />period of record prior to the construction of Buttonrock was used in the analysiso The analytical <br />methods presented in Bulletin No. 17 (Reference 7) published by the Water Resources Council <br />were used for the statistical analysiso Parameters in the St. Vrain model were then adjusted <br />until the model accurately reproduced the peak flows as derived from the statistical analysis of <br />streamflow recordso Details of the calibration of the model may be found in the 1987 <br />Engineering Professionals, Inco Technical Addendum. After calibration of the model, the <br />modeled frequency-discharge relationship without Buttonrock Dam fit the historic frequency- <br />discharge relationship well. <br /> <br />Irnrned. D/S of <br />confluence with Mido St. <br />Vrain Creek <br /> <br />at mouth <br />SOUTH STo VRAIN <br /> <br />MIDDLE STo VRAIN <br /> <br />Location <br /> <br />Peaceful Valley (State <br />Hwy 72 bridge) <br /> <br />TABLE 4 <br />PEAK DISCHARGES AT SELECTED LOCATIONS <br /> <br />9200 <br /> <br />Drainage <br />Area (Sq. Mio) <br /> <br />66.7 <br /> <br />32.4 <br /> <br />18.7 <br /> <br />1,400 <br /> <br />10- YR <br /> <br />1,220 <br /> <br />590 <br /> <br />470 <br /> <br />3,750 <br /> <br />Peak Discharge (cis) <br />50- YR 100- YR <br /> <br />2,790 <br /> <br />1,430 <br /> <br />860 <br /> <br />5,430 <br /> <br />3,990 <br /> <br />2,000 <br /> <br />1,180 <br /> <br />11,900 <br /> <br />500- YR <br /> <br />8,560 <br /> <br />4,070 <br /> <br />2,320 <br /> <br />I <br /> <br />I <br /> <br />North St. Vrain & St. <br />Floodplain Update <br /> <br />V rain Creeks <br /> <br />Town of Lyons <br /> <br />North St. Vrain & St. <br />Floodplain Update <br /> <br />Vrain Creeks <br /> <br />Town of Lyons <br />