2011-12-16_REVISION - M1977572 ‎(71)‎

• Permit No: M1977572
• IBM Index Class Name: REVISION
• Doc Date: 12/16/2011
• Doc Name: Submittal
• From: VALCO
• To: DRMS
• Type & Sequence: TR2
• Media Type: D
• Archive: No

Flood Event
Percent Annual Chance
of Occurrence
Discharge (cfs)
1- Year
99.9
3,500*
2 - Year
50
5,000*
10 - Year
10
11,200
50 - Year
2
28,000
100 - Year
1
40,000
500 - Year
0.2
84,000
II HYDROLOGY
Available hydrologic information was compiled for use in the hydraulic analyses. The collected
data included flood hydrology and USGS gage data.
2.1 Flood Hydrology
Flood hydrology was obtained from the City of Lamar FEMA Flood Insurance Study (FIS) and is
presented In Table 2.1. The FIS provided 10 -, 50 -, 100 -, and 500 -year flood discharges for the Arkansas
River at Lamar. The 100 -year flood discharge of 40,000 cfs was utilized in all floodplain analyses.
Additional flood discharges for the 1- and 2 -Year events were determined by extrapolation.
The U.S. Geological Survey (USGS) maintains a discharge gage on the Arkansas River at Lamar,
CO (USGS Gage #07133000) located on the left bank just downstream of the US Highway 50 Bridge and
within the current study reach. Instantaneous discharge, daily average discharge, and peak annual
discharge data have been collected over the gage period of record extending from May 1913 to
September 1955 and again from April 1959 to the present. It should be noted that the historic data set
is influenced by the operation of John Martin Reservoir which became operational in October of 1948.
Peak annual discharge records were obtained from the USGS gage and are presented in
graphical form in Figure 2.1. Significant peak flood events occurred in 1949 and 1965 with a peak
discharge of 24,900 cfs and 73,800 cfs, respectively. The 1949 peak discharge is on the order of a 50-
year flood event. The 1965 discharge was considered to be close to a 500 -year flood event having a
severe impact on the Town of Lamar, Colorado. The lowest peak discharge recorded since 1949
occurred in 2003 with a discharge of 215 cfs.
COVALCOO1_Phase II Report_082211.docx
Table 2.1 Flood Hydrology [FEMA 1982]
*Values determined by extrapolation.
2.1 ANdERSON CONSULTING ENgiNEERS, INC.