FLOOD10506

• County: Statewide
• Basin: Statewide
• Title: State of Colorado - 2006 Flood Documentation Reports
• Date: 6/1/2007
• Prepared For: CWCB
• Prepared By: ICON Engineering
• 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

the roadway is approximately 3,1 OO-cfs. This indicates a total peak flow (culvert and
weir) of about 4,300-cfs.

Using the same methodology, the total flow upstream at the Highway 50 Bridge was
approximately 5,200-cfs.

Regression equations, which relate the frequency of flood events to a flooding source's
drainage area, are reported in the water resources investigation report entitled, "Analysis
of the Magnitude and Frequency of Floods in Colorado," dated 2000 as prepared by the
USGS in cooperation with the Colorado Department of Transportation and the Bureau of
Land Management. According to this report, the Penrose area is located in the Plains
region and therefore, the applicable regression equation for the 1 OO-year recurrence
interval is:

Q = 1640 (A/.388

Where A, the drainage area, is expressed in terms of square miles.

At the Highway 115 bridge, the above regression equation indicates that the anticipated
100-year flow would be about 4,342-cfs. This compares favorably to the calculated flow
based on high-water marks of about 4,300-cfs, indicating that the July flood event at the
Highway 115 Bridge was equivalent to the 100-year flood. Although rainfall depths
were reported in excess ofthe 100-year event, it is believed that these were likely
localized rainfall amounts which were not necessarily indicative of the average rainfall
that occurred over the entire drainage basin.

Estimated Flood Dama2:es & Anv Special Factors Affectin2: the Flood
Flood damage from this event occurred to a number of areas including:

· Erosion and loss of shoulders along County Road 67, also known as Phantom
Canyon Road.
· Erosion and loss of shoulders along an 8-mile stretch of upper County Road 132.
· County Road 132 Culvert crossing washed out at Eightmile Creek crossing.
Damage was also reported to buildings upstream of this crossing.
· County Road 123 Bridge over Eightmile Creek overtopped damaging the
abutments. A residence upstream of this bridge also sustained damage.
· Highway 50 Bridge crossing at Eightmile Creek had erosion and debris damage
with minor shallow utility damage.
· Highway 115 Bridge crossing at Brush Hollow Creek had major roadway damage
causing the closure of the highway. Erosion and debris damage with minor
shallow utility damage also occurred here.
· Damage to local commercial buildings, primarily an apartment complex near
Broadway Street and Highway 115.

ICON Engineering, Inc

11

September 2006