Page 128 - Biosystems Engineering

Soil and W ater Conservation 109

depth = volume/area). The approximation I = 0.2S was obtained by
a
analyzing data from many small watersheds. SCS further defines a
CN variable ranging from 0 to 100 related to S with the relationship

S = 1000 − 10 (3.22)
CN
The unit for S obtained using this relationship will be in inches. The
CN is higher for less permeable soils, approaching 100 for totally
impervious surfaces and lower for more pervious soils. The TR-55
handbook has comprehensive tables listing CN values for various
soil- and land-cover combinations under varying hydrologic condi-
tions. These values are for normal antecedent moisture conditions
denoted as CN . The CN values for dry (CN ) and wet (CN ) condi-
2 1 3
tions can be computed from CN using the following relationships:
2
42 . CN 2 23 CN 2
CN = 10 − 0 058 CN CN = 10 + 0 13 CN (3.23)
3
1
.
.
2 2
Dry/wet conditions are determined based on the total rainfall that
fell during the previous 5-day period (P ). If P < 0.5 in in a dormant
5 5
season or < 1.4 in in a growing season, then it is a dry condition. If
P > 1.1 in in a dormant season or P > 2.1 in in a growing season, then
5 5
it is a wet condition. Otherwise, it is a normal condition.
3.5.4 Rational Method
Peak flow rates are required for design of culverts, drainage works,
soil conservation works, spillways of farm ponds, and small bridges.
The rational method is the most widely used and the simplest tech-
nique in estimating peak flows from small watersheds (<10 km ):
2
C i A
Q = 0 278. ⋅ ⋅ ⋅ (3.24)
p
where Q is the peak flow rate in m /s, i is the rainfall intensity (mm/h),
3
p
A is the drainage area in km , and C is the runoff coefficient varying
2
from 0 to 1. The runoff ratio represents the ratio of runoff to rainfall.
This method assumes that rainfall continues at a uniform intensity
with a duration equal to the time of concentration, which is the time
required for water to travel from the remotest point of the watershed to
the outlet. The runoff ratio C is the least precise variable of the rational
method. In reality, C should be function of percent imperviousness,
slope, soil characteristics, antecedent moisture condition, proximity of
water table, and vegetation cover. For practical purposes, C is often
tabulated as a function of few of these variables. At a minimum, C is
listed as a function of land use type and the return period of a storm.
C is higher for areas with high imperviousness and for high return
periods. McCuen (1998) lists C values for type of land use, slope,
hydrologic soil group, and storm return period.