FLOOD03304

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Colorado Flood Hydrology Manual
• Date: 1/1/1995
• Prepared For: State of Colorado
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

An S-curve hydrograph may be computed by tabulating a series of identical unit
hydrographs, arranged with origins spaced progressively one unit-rainfall duration apart of
the time scale and accumulating the ordinates for specific times. With the unit hydrograph
values known, the S-curve hydrograph would be computed by steps. During the first
period of unil-rainfall duration, the unit hydrograph and S-curve values are identical. The
S-curve values for the first period are added to unit hydrograph discharges for the second
interval to obtain corresponding S-curve ordinates. The process is continued until the S-
curve discharge rate is equal to the basic rate of rainfall excess. The operations involved
are more apparent if the reverse procedure is considered. assuming that the S-curve
hydrograph values were originally known. Discharge values represent the rates of runoff
that would resuit from a uniform continuous rate of rainfall excess of 1 inch per unit-rainfall
duration beginning at time zero.

In accordance with the unit hydrograph principle, the ordinates of an S-curve
hydrograph representing runoff from a uniform continuous rainfall excess rate of 1 inch per
12 hours may be muitiplied by 2 in order to obtain values applicable to a rail1fall excess
rate of 1 inch per 6 hours. Accordingly, S-curve hydrographs developed from runoff data
for unit storms of various durations may be adjusted to apply to any unit rainfall duration
desired, within practical limits.

In addition to the applications referred to above, the S-curve procedure is useful
in modifying unit hydrographs to represent more conservative peak values, or to reflect
moderate changes in rainfall distribution.

4. SNYDER'S METHOD.

The empirical relations developed by Franklin F. Snyder have proven to be
particularly useful in the study of runoff characteristics of drainage areas where streamflow
records are not available, as well as in modifying or supplementing available runoff records
to serve speci!ic purposes. The following terms (Chow, 1964) are used in the equations:

t = lag time from midpoint of unil rainfall duration, t , to peak of unit hydrograph,
p . h '
In ours.
t, = unil rainfall duration equal to 4 in hours. 5.5'
t R = unil rainfall duration other than standard unit, t R' adopted in specific study.
in hours.
t pR = lag time from midpoint of unit rainfall duration, t R' to peak of unit hydrograph,
in hours.
q p = peak rate of discharge of unil hydrograph for unit rainfall duration, t " in
c.f.s.lsq. mi.
q pR = peak rate of discharge of lInit hydrograph for unit rainfall duration, t R' in
c.f.s.lsq. mi.
Q p = peak rate of discharge of unil hydrograph, in c.f .s.
A = drainage area in square miles.
L oa = river mileage from the station to center of gravity of the drainage area.
L = river mileage from the given station to the upstream Iimils of the drainage

Colorado Flood
Hydrology Manual

7-39

a:w=r