FLOOD04722

• County: Las Animas
• Basin: Arkansas
• Title: Storm and Flood of July2-3, 1981 Frijole Creek Basin
• Date: 9/1/1982
• Prepared For: Colorado and Southern Railway
• Prepared By: Bishop Associates, Inc.
• Floodplain - Doc Type: Flood Documentation Report

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contained on the south side, however, and there was considerable over-

bank flow with water possibly combinipg with floodwaters from the

adjacent San Francisco Creek basin. High water elevations on this side

were measured using debris trapped in sagebrush and small bushes. The

measurement of 48,000 cfs does not include the overbank flow component,

The two methods of measurement are in reasonable agreement and

indicate a total peak discharge near the railway bridge site of approxi-

mately 50,000 cfs.

RUNOFF HYDROGRAPH

The hydrograph of the flood, or time distribution of runoff, is

illustrated in Figure 5. The hydrograph was based on discharge compu-

tations described above, observations of conditions at the bridge before

and after the washout, and empirical runoff analyses performed using

U. S. Soil Conservation Service (SCS) methodologies, including computer

modeling of the flood using the SCS TR 20 runoff hydrograph program.

FLOOD ATTENUATION

The flood discharge at the Colorado and Southern Railway bridge was

approximately 50,000 cfs, while at a site 8 miles downstream the dis-

charge was 28,400 cfs. The difference between these values is due to

the effect of normal flood peak attenuation during flood routing being

greater than the influence of the comparatively minor effect of addi-

tions to the flood through channels entering downstream of the railway

bridge. The combination of steep stream gradients abruptly decreasing a

short distance upstream of the railway bridge, as shown on Figure 2, and

intensive, short-duration rainfall concentrated in the watershed above

the bridge, as shown on Plate 1 and Figure 3, resulted in a flood of

short duration but great magnitude. The hydrograph of this flood shown

on Figure 5 illustrates the very rapid rise and fall of the flood wsters

and the peak intensity of about 50,000 cfs combined with a small total

flood volume (represented by the area under the hydrograph curve),

Downstream of the railway bridge, other tributaries enter Frijole
Creek and the water they carried on the night of July 2-3 added to the
total flood volume but did not increase the intensity of the flood for
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