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<br />North St. Vrain & St. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />North St. Vrain & SI. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<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 SI. 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 SI. Vrain Creeks and at the mouth of the SI.
<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 analysis. The analytical
<br />methods presented in Bulletin No, 17 (Reference 7) published by the Water Resources Council
<br />were used for the statistical analysis, 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 records. Details of the calibration of the model may be found in the 1987
<br />Engineering Professionals, Inc, 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 />TABLE 4
<br />PEAK DISCHARGES AT SELECTED LOCATIONS
<br />
<br />Location
<br />
<br />Drainage
<br />Area (Sq, Mi.)
<br />
<br />Peak Dischar e (cfs)
<br />10- YR 50- YR 100- YR
<br />
<br />500- YR
<br />
<br />MIDDLE ST. VRAIN
<br />
<br />Peaceful Valley (State
<br />Hwy 72 bridge)
<br />
<br />18,7
<br />
<br />470
<br />
<br />860
<br />
<br />1,180
<br />
<br />2,320
<br />
<br />at mouth
<br />SOUTH ST, VRAIN
<br />
<br />lmmed, DIS of
<br />confluence with Mid. SI.
<br />V rain Creek
<br />
<br />32.4
<br />
<br />590
<br />
<br />1,430
<br />
<br />2,000
<br />
<br />4,070
<br />
<br />66.7
<br />
<br />1,220
<br />
<br />2,790
<br />
<br />3.990
<br />
<br />8,560
<br />
<br />92,0
<br />
<br />1,400
<br />
<br />3,750
<br />
<br />5,430
<br />
<br />11,900
<br />
<br />Buttonrock Dam was then inserted in the calibrated SWMM model of SI. Vrain Creek at the
<br />Lyons gauge, It was found that 6-hour rainfall over South St, Vrain Creek and North St. 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 place, Table 4 summarizes the results of the
<br />hydrologic analysis of flood flows within the study reach.
<br />
<br />at mouth
<br />NORTH ST. VRAIN
<br />
<br />at mouth
<br />
<br />119,0 1.000 2,850 4,310 10,630
<br />99,0 680 2,000 3,100 7,600
<br />99.0 1.600 4,200 6,000 15,000
<br />211.0 2,040 6,670 8,880 20.260
<br />212.0 2.030 5,560 8.830 20,140
<br />241.0 3,160 6,890 9.580 19,680
<br />351,0 5,520 10,950 14,850 28,670
<br />924,0 5,350 12.120 16,520 40,080
<br />
<br />Buttonrock spillway
<br />
<br />Buttonrock inflow
<br />ST. VRAIN CREEK
<br />
<br />For a detailed description of the hydrologic analysis, refer to the Technical Addendum
<br />produced in 1987 by Engineering Professionals, Inc,
<br />
<br />confluence of No. & So.
<br />St. V rain
<br />Lyons Gauge
<br />
<br />4,3 Hydraulic Analysis
<br />
<br />No. 85th SI. (D IS study
<br />limit)
<br />Boulder-Weld County
<br />
<br />line
<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 SI. 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 structures, 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 />at mouth
<br />
<br />Channel roughness coefficients (Manning's UnU) 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 UnU values for channel and floodplain are summarized below in Table 5:
<br />
<br />Floodplain Information Report 24
<br />K:\35600\OO4\FLOODPLAIN INFORMATION REPORT,doc
<br />
<br />June 1998
<br />
<br />Floodplain Information Report 25
<br />K:\35600\OO4\FLOODPLAIN INFORMATION REPORT.doc
<br />
<br />June 1998
<br />
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