FLOOD10328

• County: Boulder
• Community: Lyons
• Stream Name: North Saint Vrain and Saint Vrain Creeks
• Title: Floodplain Information Report, North Saint Vrain Creek, Saint Vrain Creek, Floodplain Update, Town of Lyons, Boulder County, Colorado
• Date: 6/1/1998
• Prepared For: Lyons
• Prepared By: BRW, Inc
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Floodplain Information Report
K:\35600\004\FLOODPLAIN INFORMATION REPORTdoc

June

1998

Floodplain Information Report
K:\35600\004\FLOODPLAIN INFORMATION REPORT.doc

23

June

998

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22

Flows at the mouth of North St. Vrain Creek were calculated by determining the peak rate of
runoff per drainage basin unit area into Buttonrock Reservoir for the 10-, 50-, 100-, and 500-year
flows. These peak flows per unit area were then multiplied by the drainage basin area between
Buttonrock Dam and the mouth of the North St. Brain, and added to the spillway discharges
from Buttonrock reservoir corresponding to the various recurrence interval floods. An
operation analysis of Buttonrock conducted by the Corps of Engineer indicated a reduction of
peak discharge from 6,000 cfs to 3,100 cfs for the 100-year flood. These values correlated well
with the peak flood flows from the South St. Vrain and the flows calculated by the Corps of
Engineers immediately downstream of the confluence of the North and South St. Vrain Creekso

Big Thompson River at
mouth of canyon, near
Drake, Colorado

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Flood flow frequency data were developed for the South St. Vrain based on regional
relationships for statistical parameters of log-Pearson Type III distributions (Reference 7). The
regional relationships were developed through statistical analyses of streamflow records at ten
USGS gauging stations located in the upper St. Vrain Creeks and Big Thompson River basins
(References 8, 9, 10, 11, 12, and 13). Table 3 lists these gauging stationso Rainfall and snowmelt
floods were separated in the frequency analyses and then statistically combined to obtain the
composite flood flow frequency curves

10

9

067380

067365

Big Thompson River
below powerhouse,
near Drake, Colorado

North Fork Big
Thompson River at
Drake, Colorado

304

280

9,050

9,240

9,150

1927-1976

1918-1947

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The U.S. Army Corps of Engineers, Omaha District, has completed a hydrologic investigation
(Reference 14) for the entire St. Vrain Creek basin which includes the study reach. Discharges
for the 5-, 10-, 50, 100-, and 500-Year floods were developed by the Corps for Sto Vrain Creek
from just below the confluence of the North and South Sto Vrain Creeks in Lyons to the mouth
of the St. Vrain Creek near Platteville, Colorado

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7

6

067360

067330

067320

Big Thompson River at
Estes Park, Colorado

Glacier Creek near
Estes Park, Colorado

82.8

137

24.4

9,900

10,530

1947-1976

1944-1976

1942-1952,
1954-1957

South Boulder Creek
near Eldorado Springs,
Colorado

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4.2

The potential for flooding also exists in the rapid melting of heavy snow cover during late
spring. Flood flows resulting from snowmelt are characterized by moderate peaks, large
volume of runoff, and long duration of moderately high flows. Floods caused by snowmelt are
usually not as damaging as those resulting from rainstormso However, severe flooding may
occur when rainfall accompanies the snowmelto

H

5

4

067295

067255

Middle Boulder Creek
at Nederland, Colorado

St. Vrain Creek at
Lyons, Colorado

109

36.2

9,040

10,470

1921,1954

1945-1976

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Floods in the study area have usually been caused by general rainstorms and cloudburst storms
during the period May through September. Floods resulting from prolonged heavy rainfall
over the watershed are characterized by high peak flow of moderate volume and duration.
Cloudbursts produce high intensity, short duration rainfall which results in a flood of high peak
flow short duration, and small volume of runoffo

3

2

067240

067225

South St. Vrain Creek
near Ward, Colorado

Station Name
North Sto Vrain Creek
at Longmont Dam,
near Lyons, Colorado

212

14.4

8,880

11,140

1923-1973

1926-1927,
1929-1931,
1955-1972

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Identification
Number
1

Station
Number
067220

Drainage
Area (Square
Mile)
106

Mean Watershed
Elevation (Foot
MSL)
9,490

Period of
Record
1926-1953

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4.1

Hydrology used in the study was compiled by Boulder
Conservation Board based on other existing hydrologic studies of
the hydrology follows.

Flood Characteristics

County and the
the St. Vrain.

Colorado W
A summ

ater
ary of

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TABLE 3
USGS GAGING STATIONS

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SECTION 4

North St. Vrain & St.
Floodplain Update

HYDROLOGIC AND HYDRAULIC DETERMINATIONS

Vrain Creeks

Town of Lyons

North St. Vrain & St.
Floodplain Update

Vrain Creeks

Town of Lyons