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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below Buttonrock Dam generated higher peak discharges at Lyons
than did rainfall over the entire St. Vrain basin above Lyons.
The adopted frequency curve at Lyons therefore does not reflect
any contribution to peak discharges from the 99 square miles of
drainage basin above Buttonrock. The adopted frequency curve at
the mouth was modeled by applying 6-hour rainfall to 924 square
miles of contributing basin area. This frequency cUrve was
developed for existing conditions and with Buttonrock Dam in
place. Table 2 summarizes the results of the hydrologic analysis
of flood flows within the study reach.

For a detailed description of the hydrologic analysis, refer to
the Technical Addendum.

TABLE 2
PEAK DISCHARGES AT SELECTED LOCATIONS
Drainage Peak Discharge (cfs)
Location Area 10-YR 50-YR 100-YR 500-YR
(Sq.Mi. )
MIDDLE ST. VRAIN
Peaceful Valley (State
Hwy 72 bridge) 18.7 470 860 1,180 2,320
at mouth 32.4 590 1,430 2,000 4,070
SOUTH ST. VRAIN
immed. D!S of confluence
with Mid. St. Vrain
Creek 66.7 1,220 2,790 3,990 8,560
at mouth 92.0 1,400 3,750 5,430 11,900
NORTH ST. VRAIN
at mouth 119.0 1,000 2,850 4,310 10,630
Buttonrock spillway 99.0 680 2,000 3,100 7,600
Buttonrock inflow 99.0 1,600 4,200 6,000 15,000
ST. VRAIN CREEK
confluence of No. &
So. St. Vrain 211.0 2,040 5,570 8,880 20,260
Lyons Gauge 212.0 2,030 5,560 8,830 20,140
No. 85th St. (D!S
study limit) 241. 0 3,160 6,890 9,580 19,680
Boulder-Weld
County line 351.0 5,520 10,950 14,850 28,670
at mouth 924.0 5,350 12,120 16,520 40,080
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