FLOOD09332

• County: Boulder
• Community: Boulder County
• Stream Name: North St. Vrain, St. Vrain Creeks
• Basin: South Platte
• Title: Floodplain Information Report
• Date: 12/1/1987
• Prepared For: Boulder
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION 4 - HYDROLOGIC AND HYDRAULIC DETERMINATIONS

analyses and then statistically combined to obtain the composite
flood flow frequency curves.

Hydrology used in the study was compiled by Boulder County and
the Colorado Water Conservation Board based on other existing
hydrologic studies of the St. Vrain. A summary of the hydrology
follows.

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. Vrain, 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 Creeks.

4.1 Flood Characteristics

Floods in the study area are usually 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 resul ts in a flood of high peak flow short
duration, and small volume of runoff.

The potential for flooding also exists in the rapid melting of
heavy snow cover during late spring. Flood flows resulting from
snow melt 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 rainstorms. However, severe flooding may occur when
rainfall accompanies the snowmelt.

As future floods will be attenuated by Buttonrock reservoir, and
as adequate streamflow records are unavailable to adequately
account for the effects of Buttonrock on peak flood flows through
statistical analysis, the Corps of Engineers modeled the St.
Vrain Creek Basin using the Environmental Protection Agency's
(EPA) Storm Water Management Model (SWMM) and the Missouri River
Division's version of Harder's diffusion routing model.
Discharge-Frequency relationships at the confluence of North and
South St. Vrain Creeks and at the mouth of the St. Vrain were
derived through a statistical analysis of the streamflow records
at these points. Since Buttonrock Dam has a controlling effect
on peak discharges at the Lyons gauge, only the period of record
prior to the construction of Buttonrock was used in the analysis.
The analytical methods presented in BUlletin No. 17 (Reference 7)
published by the Water Resources Council were used for the
statistical analysis. Parameters in the St. Vrain model were
then adjusted until the model accurately reproduced the peak
flows as derived from the statistical analysis of streamflow
records. Details of the calibration of the model may be found in
the Technical Addendum. After calibration of the model, the
modeled frequency-discharge relationship without Buttonrock Dam
fit the historic frequency-discharge relationship well.

4.2 HydrOlogic Analysis

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 St. Vrain Creek from just below the confluence of the
North and South St. Vrain Creeks in Lyons to the mouth of the St.
Vrain Creek near Plattevil1e, Colorado.

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 stations.
Rainfall and snowmelt floods were separated in the frequency

Buttonrock Dam was then inserted in the calibrated SWMM model of
St. Vrain Creek at the Lyons gauge. It was found that 6-hour
rainfall over South St. Vrain Creek and North St. Vrain Creek

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