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<br />North St. Vrain & St. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />North St. Vrain & St. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />SECTION 4 - HYDROLOGIC AND HYDRAULIC DETERMINATIONS
<br />
<br />4,1 Flood Characteristics
<br />
<br /> TABLE 3
<br /> USGS GAGING STATIONS
<br /> Drainage Mean Watershed
<br />Identification Station Area (Square Elevation (Foot Period of
<br />Number Number Station Name Mile) MSL) Record
<br />1 067220 North St. Vrain Creek 106 9,490 1926-1953
<br /> at Longmont Dam,
<br /> near Lyons, Colorado
<br />2 067225 South St. Vrain Creek 14.4 11,140 1926-1927,
<br /> near Ward, Colorado 1929-1931,
<br /> 1955-1972
<br />3 067240 51. Vrain Creek at 212 8,880 1923-1973
<br /> Lyons, Colorado
<br />4 067255 Middle Boulder Creek 36,2 10,470 1945-1976
<br /> at Nederland, Colorado
<br />5 067295 South Boulder Creek 109 9,040 1921,1954
<br /> near Eldorado Springs,
<br /> Colorado
<br />6 067320 Glacier Creek near 24.4 10,530 1942-1952.
<br /> Estes Park, Colorado 1954-1957
<br />7 067330 Big Thompson River at 137 9.900 1944-1976
<br /> Estes Park, Colorado
<br />8 067360 North Fork Big 82.8 9,150 1947-1976
<br /> Thompson River at
<br /> Drake, Colorado
<br />9 067365 Big Thompson River 280 9,240 1918-1947
<br /> below powerhouse,
<br /> near Drake, Colorado
<br />10 067380 Big Thompson River at 304 9,050 1927-1976
<br /> mouth of canyon, near
<br /> Drake. Colorado
<br />
<br />Hydrology used in the study was compiled by Boulder County and the Colorado Water
<br />Conservation Board based on other existing hydrologic studies of the St. Vrain, A summary of
<br />the hydrology follows.
<br />
<br />Floods in the study area have usually been caused by general rainstorms and cloudburst storms
<br />during the period May through September. Floods resulting from prolonged heavy rainfall
<br />over the watershed are characterized by high peak flow of moderate volume and duration,
<br />Cloudbursts produce high intensity, short duration rainfall which results in a flood of high peak
<br />flow short duration, and small volume of runoff,
<br />
<br />The potential for flooding also exists in the rapid melting of heavy snow cover during late
<br />spring, Flood flows resulting from snowmelt are characterized by moderate peaks, large
<br />volume of runoff, and long duration of moderately high flows. Floods caused by snowmelt are
<br />usually not as damaging as those resulting from rainstorms, However, severe flooding may
<br />occur when rainfall accompanies the snowmelt.
<br />
<br />4,2 Hydrologic Analysis
<br />
<br />The US, Army Corps of Engineers, Omaha District, has completed a hydrologic investigation
<br />(Reference 14) for the entire 5t. Vrain Creek basin which includes the study reach, Discharges
<br />for the 5-, 10-. 50, 100-, and 500-Year floods were developed by the Corps for St. Vrain Creek
<br />from just below the confluence of the North and South St, Vrain Creeks in Lyons to the mouth
<br />of the St. Vrain Creek near Platteville, Colorado,
<br />
<br />Flood flow frequency data were developed for the South St. Vrain based on regional
<br />relationships for statistical parameters of log-Pearson Type III distributions (Reference 7). The
<br />regional relationships were developed through statistical analyses of streamflow records at ten
<br />USGS gauging stations located in the upper St. Vrain Creeks and Big Thompson River basins
<br />(References 8, 9, 10, 11, 12. and 13). Table 3 lists these gauging stations. Rainfall and snowmelt
<br />floods were separated in the frequency analyses and then statistically combined to obtain the
<br />composite flood flow frequency curves.
<br />
<br />Flows at the mouth of North St, Vrain Creek were calculated by determining the peak rate of
<br />runoff per drainage basin unit area into Buttonrock Reservoir for the 10-, 50-, 100-, and 500-year
<br />flows, These peak flows per unit area were then multiplied by the drainage basin area between
<br />Buttonrock Dam and the mouth of the North St. Brain. and added to the spillway discharges
<br />from Buttonrock reservoir corresponding to the various recurrence interval floods, An
<br />operation analysis of Buttonrock conducted by the Corps of Engineer indicated a reduction of
<br />peak discharge from 6,000 cfs to 3,100 cfs for the 100-year flood. These values correlated well
<br />with the peak flood flows from the South St. Vrain and the flows calculated by the Corps of
<br />Engineers immediately downstream of the confluence of the North and South St. Vrain Creeks.
<br />
<br />Floodplain Information Report 22
<br />Ko\35600\004IFLOODPLAIN INFORMATION REPORT,doc
<br />
<br />June 1998
<br />
<br />Floodplain Information Report 23
<br />K135600\OO4IFLOODPLAIN INFORMATION REPORT.doc
<br />
<br />June 1998
<br />
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