FLOOD10347

• County: Fremont
• Community: Fremont County
• Stream Name: Oak Creek, Coal Creek
• Basin: Arkansas
• Title: Hydrologic Analysis - Oak Creek & Coal Creek
• Date: 9/1/2000
• Prepared For: CWCB
• Prepared By: USACE - Albuquerque District
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Time of concentration (Tc) in minutes
Basin lag in minutes

= 0.007S x (L 0.77 x S .0.385)
= 0.6 x (Tc)

L = Length of the watercourse in feet (from headwater to outlet, ft)
S = Average watershed slope, (ft/ft)

Table 3 lists the unit hydrograph parameters computed for each subarea in the Coal Creek watershed,
and Table 4 lists the unit hydrograph parameters computed for each subarean in the Oak Creek
watershed.

Infiltration

To determine appropriate infiltration rates, the Albuquerque District used the NRCS Curve
Number method. Soil types were determined from the soil survey maps developed for Fremont
County and Custer County provided by the NRCS; the NRCS National Engineering Handbook,
Section 4, Hydrology, dated November 1988; and the NRCS report, Runoff Curve Number
Computations, dated September 1989. Table 3 and Table 4 list the curve numbers used for each
subarea in the Coal Creek and Oak Creek watersheds, respectively. The Albuquerque District
assigned the urbanized portions of Florence the percent impervious for low residential areas in
accordance with the criteria listed in Table 5.

TABLE 3
UNIT HYDROGRAPH PARAMETERS AND CURVE NUMBERS
FOR COAL CREEK IN FLORENCE, COLORADO
SUBAREA AREA SLOPE TIME OF BASIN LAG CURVE
(Sq.mi.) (Feet/Feet) CONCENTRAION (Hours) NUMBER
(Minutes)
SUBAREA 1 1.40 0.0074 78.66 0.78 76
SUBAREA 2 1.46 0.0133 45.32 0.45 74
SUBAREA 3 0.20 0.010 30.87 0.31 81
SUBAREA 4 2.53 0.0188 45.85 0.46 80
SUBAREA 5 1.62 0.022 69.68 0.70 75
SUBAREA 6 2.10 0.0168 74.30 0.74 . 71
SUBAREA 7 1.16 0.028 24.09 0.24 80
SUBAREA 8 3.34 0.0784 51.79 0.52 57
SUBAREA 9 3.62 0.075 51.18 0.51 70
SUBAREA 10 0.64 0.0551 34.79 0.35 75

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