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 Manual
for Maricopa County

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Table 4.1
Surface Retention Loss for
Various Land Surfaces In Maricopa County

Surface Retention
Land-use and/or Loss
Surface Cover lA, Inches
(1) (2)
NaturaC
Desert and ranoeland, flat sloDe 0.35
Hillslooes, Sonoran Desert 0.15
Mountain, with veoetated surface 0.25
Develooed (Residential and Commerciall
Lawn and turf 0.20
Desertlandscaoe 0.10
Pavement 0.05
Aorlcultural
Tilled fields and irrioated oasture I 0.50

Infiltration can be controlled by percolation if the soil does not have a sustained
drainage capacity to provide access for more infiltrated water. However, before
percolation can be assumed to restrict infiltration for the design rainfalls being
considered in Maricopa County, the extent by which percolation can restrict infiltra-
tion of rainfall should be carefully evaluated. SCS soil scientists have defined
, hydrologic soil group D as:

"Soils having very slow infiltration rates when thoroughly wetted and consist-
ing chiefly of clay soils with a high swelling potential, soils with a permanent
high water table, soils with a daypan or clay layer at or near the surface, and
shallow soils over nearly impervious materia!."

This definition indicates that hydrologic soil groups A, B, or C could be classified
as D if a near impervious strata of clay, caliche, orrock is beneath them. When these
soils are considered in regard to long-duration rainfalls (the design events for many
parts of the United States) this definition may be valid. However, when considered
for short-cluration and relatively small design rainfall depths in Maricopa County,
this definition could result in underestimation of the rainfall losses. This is because
even a relatively shallow horizon ofsoil overlaying an impervious layer still has the
ability to store a significant amount of infiltrated rainfall.

For example, consider the situation where only 4 inches of soil covers an impervious
layer. If the effective porosity is 0.30, then 1.2 inches (4 inches x 0.30) of water can
be infiltrated and stored in the shallow soil horizon. For design rainfalls in Maricopa
County, this represents a significant storage volume for infiltrated rainfall and so
when using ..h""""'pe Ett.lQill& for Maricopa County that contain significant areas
classifi~ as hydr~ogic soil group D, the reason for that classification should be
detemuned. . ~&',~ ""'1'",,15

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