FLOOD03281

• County: Boulder
• Community: Boulder County
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Hydrology Addendum to FIS Boulder County and Incorporated Areas
• Date: 10/1/2000
• Prepared For: Boulder County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
'I
I
I
I
I
I
I
I

ENGINEERING METHODS

Hvdrolol!ic Analvsis
The Boulder County and Incorporated Areas PIS presents detailed and approximate floodplain
delineations, Hydrologic infonnation used for the FEMA approximate floodplain delineations are
not available or were not documented by FEMA. The CWCB has developed acceptable
hydrologic infonnation for undeveloped basins using regional regression methodologies for
computing IOO-year flood-flow values,

The CWCB methodology is fully described in a publication by the CWCB entitled "Guidelines
for Detennining loo-year Flood Flows for Approximate Floodplains in Colorado", The South
Platte Northern Foothills Subregion (SPL-4) was used for streams north of Lefthand Creek, while
the South Platte Central Foothills Subregion (SPL-3) was used for streams south of Lefthand
Creek, including the Lefthand Creek basin, The CWCB equations are as follows:

Sub-Region
10 Sub-Re ion Name

SPL-3 South PlaUe, Central Foothills

Drainage Basin
Limitations s ,mi.

1 to 175

SPL-4

South PlaUe, Northern Foothills

Q,oo = 800.8IA)...7.

1 to 445

For the equations shown in Table 2, the Q100 is the l<!O-year peak flow in cubic feet per second,
and A is the upstream (contributing) drainage area in square miles. The regression equations are
only valid for the range of drainage areas indicated,

The 1 DO-year peak discharges for approximate floodplains in Boulder County were estimated by
utilizing the regional regression equations shown above. The estimated loo-year peak discharges
are shown in Table 3.

Table 3. Estimated Discharaes for Boulder County
Discharge Drainage l00-year
Stream Name, Location Area (ml') DISCharge (cfs)
North SI. Vrain Creek at Apple Valley Road 117.68 7,800
at west Lyons corporate limits 121.6 7,900
Geer Canyon at mouth 6.41 2,100
Sunshine Canyon Trib. at mouth 0,28 Outside Bounds
Mount Santas Rd,)
Sunshine Canyon immediately ufs of Tributary 1.51 1,000
at mouth 1.89 1,100
Gregory Creek at mouth 1.94 1,100
Bluebell Canyon Creek at Anderson Ditch Extension 0.45 Outside Bounds
Kings Gulch at Anderson Ditch Extension 0.41 Outside Bounds
Skunk Creek at Anderson Ditch Extension 0.82 Outside Bounds
Bear Canyon Creek at NCAR Road 2.94 1,400

8