Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

RUNOFF DRAINAGE CRITERIA MANUAL (V. 1)
DRAINAGE CRITERIA MANUAL (V. 1)
RUNOFF
Table RO-7—Recommended Horton’s Equation Parameters
NRCS Hydrologic Infiltration (inches per hour) Decay
Soil Group Initial—f i Final—f o Coefficient—a
A 5.0 1.0 0.0007
B 4.5 0.6 0.0018
C 3.0 0.5 0.0018
D 3.0 0.5 0.0018
To calculate the maximum infiltration depths that may occur at each time increment, it is necessary to
integrate Equation RO-8 and calculate the values for each time increment. Very little accuracy is lost if,
instead of integrating Equation RO-8, the infiltration rate is calculated at the center of each time
increment. This “central” value can then be multiplied by the unit time increment to estimate the
infiltration depth. This was done for the four NRCS hydrologic soil groups, and the results are presented
in Table RO-8. Although Tables RO-7 and RO-8 provide recommended values for various Horton
equation parameters, these recommendations are being made specifically for the urbanized or urbanizing
watersheds in the Denver metropolitan area and may not be valid in different meteorologic and climatic
regions.
Table RO-8—Incremental Infiltration Depths in Inches*
NRCS Hydrologic Soil Group
Time in Minutes** A B C and D
5 0.384 0.298 0.201
10 0.329 0.195 0.134
15 0.284 0.134 0.096
20 0.248 0.099 0.073
25 0.218 0.079 0.060
30 0.194 0.067 0.052
35 0.175 0.060 0.048
40 0.159 0.056 0.045
45 0.146 0.053 0.044
50 0.136 0.052 0.043
55 0.127 0.051 0.042
60 0.121 0.051 0.042
65 0.115 0.050 0.042
70 0.111 0.050 0.042
75 0.107 0.050 0.042
80 0.104 0.050 0.042
85 0.102 0.050 0.042
90 0.100 0.050 0.042
95 0.098 0.050 0.042
100 0.097 0.050 0.042
105 0.096 0.050 0.042
110 0.095 0.050 0.042
115 0.095 0.050 0.042
120 0.094 0.050 0.042
* Based on central value of each time increment in Horton's equation.
** Time at end of the time increment.
Figure RO-10—Slope Correction for Natural and Grass-Lined Channels
RO-22 2007-01
Urban Drainage and Flood Control District
2007-01 RO-27
Urban Drainage and Flood Control District