FLOOD03016

• County: El Paso
• Community: Colorado Springs
• Stream Name: Arkansas River and Tributaries
• Basin: Arkansas
• Title: Areawide Urban Runoff Control Manual
• Date: 9/28/1979
• Prepared For: Pikes Peak Area Council of Governments
• Prepared By: Gilbert Meyer & Sams
• Floodplain - Doc Type: Miscellaneous

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basin. This procedure assigns a curve number to each of four
soil groupings and then modifications are made to account for
the other variables. The four hydrologic soil groups, according
to their infiltration and transmission rates, are:

GROUP A Low runoff potential. Soils having high infiltration
rates even when thoroughly wetted. These consist chiefly
of deep, well to excessively drained sands or gravel.
These soils have a high rate of water transmission in
that water readily passes through them.

GROUP B Soils having moderate infiltration rates when thoroughly
wetted. These consist chiefly of moderately fine to
moderately course textures. These soils have a moderate
rate of water transmission.

GROUP C Soils having slow infiltration rates when thoroughly
wetted. These consist chiefly of soils with a layer
that impedes downward movement of water or soils with
moderately fine to fine texture. These soils have a
slow rate of water transmission.

GROUP D High runoff potential. Soils having very slow infiltra-
tion rates when thoroughly wetted. These consist chiefly
of clay soils with a high swelling potential, soils with
a permanent high water table, soils with a claypanor
clay layer at or near the surface, and shallow soils
over nearly impervious material. These soils have a
very slow rate of water transmission.

Appendix A lists the hydrologic soil types for El Paso and
Teller Counties. Maps and other information showing soil series
are available for viewing at local SCS offices.

For design purposes, the normal antecedent moisture condition
(AMC II) shall be used. Wet and/or dry antecedent moisture
conditions (AMC III and AMC I respectively) should only be used
for purposes of reproducing historical events. The curve numbers
given in this manual are for AMC II only. Figure 111-7 and 111-8
show curve numbers corresponding to various conditions of cover
for range and urban soil cover complexes. Similar information
is presented in a tabular form in Tables 111-4 and 111-5. A
weighted CN for a given basin can be determined in a similar
manner as the composite C in the rational method.

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CN =

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where:

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