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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Rainfall Losses

Hydrologic Design Mar.ual
for Maricopa County

Hydrologic Soli Group, Soli Texture
A sand
loa
B slit loam
loam
C sandy loam
silt
o clay loam
silty clay loam
sandy clay
silty clay
cia

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This grouping of soils is based on the four hydrologic soil groups as defined by SCS
soil scientists, with limits for each group established by the minimum Infiltration
rate as defined by Musgrave (1955). This classification system assumes that the
hydraulic conductivity (XKSAT) of the Green and Ampt equation corresponds to
the minimum infiltration rate (Ie).

Oassification of soil according to hydrologic soil group involves some large scale
lumping of soils. For example, silt loam Is placed in hydrologic soil group B based
on soil texture classification, whereas using particle size percentages (and percent
organic matter) can place silt in any of the four groups. The A and 0 soil groups
are most nearly invariant with respect to soil texture classification, and the Band C
soils are less definitive in regard to soil texture. This classification indicates that the
SCS hydrologic soil groups are not uniquely related to soil hydraulics and
hydrologic properties; but it does indicate that Green and Ampt equation
parameters can be estimated with some degree of confidence and reproducibility
from readily available soil properties and from an estimate of CN.

Brakensiek, Rawls, and Stephenson (1984) extended this general classification of
soils into a procedure for estimating hydrologic soil groups and CN bas~on soils
data. Their analysis resulted in a procedure to relate CN to saturated hydraulic
conductivity of the soil. This procedure has been modified so that hydraulic
conductivity for the Green and Ampt equation can be estimated from the CN for
the 5011-cover complex and percentage of vegetation cover. This il; shown in Figure
4.11,and this figure can be used to estimate hydraulic conductivity from an estimate
ofCN. Capillary suction (PSIF) Is usually inversely related to the value of hydraulic
conductivity (Xl(SAT), as illustrated in Figure 4.12. Figure 4.12 can be used in
conjunction with Figure 4.11 to estimate the Green and Ampt equation parameters.
DTHETA should be selected from Table 4.2 based on the assumption of initial soil
moisture and estimated XKSAT and PSIF.

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