Geoscience Reference
In-Depth Information
25.6.4.2 Information Needed
The following represents a brief list of the parameters needed to compute the peak discharge of a
watershed using the
TR-55
graphical peak discharge method:
1. The drainage area, in square miles
2. Time of concentration (
t
c
) in hours
3. Weighted runoff curve number (RCN)
4. Rainfall amount (
P
) for specified design storm, in inches
5. Total runoff (
Q
), in inches
6. Initial abstraction (
I
a
) for each subarea
7. Ratio of
I
a
/
P
for each subarea
8. Rainfall distribution (Type I, IA, II, or III)
25.6.4.3 Design Parameters
The
TR-55
peak discharge equation is
q
p
=
q
u
×
A
m
×
Q
×
F
p
(25.3)
where
q
p
= Peak discharge (cfs).
q
u
= Unit peak discharge (cfs/mi
2
/in., csm/in.).
A
m
= Drainage area (mi
2
).
Q
= Runoff (in.).
F
p
= Pond and swamp adjustment factor.
All of the required information has been determined earlier except for the unit peak discharge (
q
u
)
and the pond and swamp adjustment factor (
F
p
).
The unit peak discharge (
q
u
) is a function of the initial abstraction (
I
a
), precipitation (
P
), and the
time of concentration (
t
c
) and can be determined from the unit peak discharge curves in
TR-55
. The
unit peak discharge is expressed in cubic feet per second per square mile per inch of runoff. Initial
abstraction, as discussed previously, is a measure of all the losses that occur before runoff begins,
including infiltration, evaporation, depression storage, and water intercepted by vegetation, and can
be calculated from empirical equations or Table 4-1 in
TR-55
. The pond and swamp adjustment fac-
tor is an adjustment in the peak discharge to account for pond and swamp areas if they are spread
throughout the watershed and are not considered in the
t
c
computation. Refer to
TR-55
for more
information on pond and swamp adjustment factors.
The unit peak discharge (
q
u
) is obtained by using
t
c
and the
I
a
/
P
ratio with Exhibit 4-I, 4-IA, 4-II,
or 4-III (depending on rainfall distribution type) in
TR-55
. As limitation number 5 above indicates,
the ratio of
I
a
/
P
must fall between 0.1 and 0.5. The engineer must use the limiting value on the
curves when the computed value is not within this range. The unit peak discharge is determined
from these curves and entered into the above equation to calculate the peak discharge.
25.6.5
tR-55
t
abular
h
ydrograph
m
ethod
The tabular hydrograph method can be used to analyze large heterogeneous watersheds. The tabular
method can develop partial composite flood hydrographs at any point in a watershed by dividing
the watershed into homogeneous subareas. The method is especially applicable for estimating the
effects of land use change in a portion of a watershed. The tabular hydrograph method provides a
tool to efficiently analyze several sub-watersheds to verify the combined impact at a downstream
study point. It is especially useful to verify the timing of peak discharges. Sometimes the use of
detention in a lower sub-watershed may actually increase the combined peak discharge at the study
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