FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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Hydrologic Design Manual
for Maricopa County

Rainfall Losses

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4.4.2 Initial Loss Plus Uniform Loss Rate (IL+ULR)

This is a simplified rainfall loss method that is often used, and generally accepted,
for flood hydrology. In using this simplified method it is assumed that the rainfall
loss process can be simulated as a two-step procedure, as Illustrated in Figure 4.13.
First, all rainfall is lost to runoff until the accumulated rainfall is equal to the initial
loss; and second, after the initial loss is satisfied, a portion of all future rainfall is
lost at a uniform rate.

According to HEC-1 nomenclature, two parameters are needed to use this method;
the initial loss (STRTI.) and the uniform loss rate (CNSTL). The initial loss (STRTI.)
is the sum of all losses prior to the onset of runoff and is made up of surface retention
loss (IA) and an initial amount of infiltration (IL); therefore,STRTI. = IA + IL. Values
of the infiltration component (IL) ofSTRTI. for bare ground according to soil texture
classification are shown in Columns (3) through (S) in Table 4.3. These values have
been derived from the Green and Ampt infiltration equation and parameter values
that are shown in Table 4.2.

The value of IL "Dry" should be used for soil that is usually in a state of low soil
moisture at or near the wilting point for vegetation. This is a reasonable assumption
for most nonirrigated lands in Maricopa County because of the infrequency of
rainfall and because of the rapid drainage of these soils after rainfall. The value of
IL "Normal" should be used for soil that is usually in a state of moderate soil
moisture such as occurs for irrigated lawns, turf, and pennanent pastures. The
value ofIL "Saturated" is used for a soil maintained in a state of high soil moisture,
such as in irrigated agricultural lands.

Values of IL for bare ground that have been classified according to hydrologic soil
group are shown in Table 4.4. These values within each hydrologic soil group have
been derived from the data in Table 4.3 for the various soil texture classifications.

The uniform loss rate (CNSTL) represents the long-term, equilibrium infiltration . >tt
capacity of the soil. The values of CNSTL shown in Column (2) of Table 4.3 for soils c/tH.,'A
according to soil texture (IF 'fi.;alislI are equivalent to the hydraulic conductivity
at natural saturation (XK5AT) as determined for the Green and Ampt equation
(Table 4.2). The values of CNSTL for soils classified according to hydrologic soil
groups are shown in Table 4.:zfrhese values within each hydrologic soil group have
been selected from inspection of XKSAT values in Table 4.2 for the various soil
texture classifications. Values of CNSTL shown in Table 4.4 are consistent with
general infonnation available for estimating CNSTI. as shown in Table 4.5. Figure
4.11 can be used to estimate CNSTL based on an estimate of CN if adequate soils
data is not available.

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