FLOOD08489

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Determination of Urban Watershed Response Time
• Date: 12/1/1974
• Prepared By: E.F. Shulz and O.G. Lopez
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

For practical reasons, it is desirable to devise
a single measure of channel slope to represent the
slope of the whole watershed channel. The problem
arises then to define the slope such that the defined
slope bears the most meaningful relationship to the
flood characteristics of the watershed. The channel
selected to represent the whole watershed is usually
the longest collector in the watershed; although the
most significant channel is probably the channel hav-
ing the longest transit time. The simplest slope
expression is the fall in the watershed between the
headwaters and the outlet divided by the length of the
longest collector:

51 = H/L .

This definition may be faulty because too great empha-
sis may be placed on the steep slopes in the headwaters
region which are hydraulically quite far removed from
the outlet. Another method of defining the average
channel slope was described by Reich (1962) and later
incorporated in the Colorado State University small
watershed flood data file, Laurenson et a!. (1963).
The slope quantity is the slope of a straight line
joining the elevation of the outlet on the profile of
the mainstream with the average elevation of the
actual stream profile. Nash and Shaw (1966) have
given this equation for finding this particular slope:

52

2ILiZ;
(ILi)2

where

is the distance along the mainstream
tween successive contours,

be.

L,
1

is the average elevation above the outlet
for each reach of length Li.

These are shown on Fig. 1. It is apparent that
the area under the stream profile diagram is equal to
the area under the straight slope line.

Z,
1

A simpler definition of the stream slope is given
in Laurenson et al. (1963) which had been suggested
earlier by Benson (1959). The greatest bias is placed
on the 75 percent of the channel length (longest chan-
nel extended to the watershed divide) which, in most
watersheds, collects the majority of the flood runoff:

s = Elevation at O.85l - Elevation at D.ll
3 O.75L

Data on these various methods of defining stream
slope and other watershed parameters have been assem-
bled in the Small Watershed Data File at Colorado
State University. Using the data assembled, several
types of multivariate analyses were made to attempt
to select a significant difinition of the various
pertinent watershed parameters. Yevjevich et al.
(1972) reported that for the data in the esu flood data
file, it appeared that the third definition of channel
slope listed previously was the more satisfactory
way of defining channel slope for a natural water-
shed.

Whatever definition of channel slope is employed,
the effect of the slope is that the watershed response
time is inversely related to the square root of the
slope. This was demonstrated by Kirpich (1940), USBR
(1965), Taylor and Schwarz (1952) and many others.

L ; 2: l i

1

H

./
Lii r.........../

~
.....1
.....

LlH

f
Zi

Fig. 1. Definition of Average Channel Slope

Straight line is drawn such that the area under
line is equal to area under the profile diagram.

5 = ^H = ^H
2 L ZC
1
and area under line is:

1/2 ^H(Ll;)
and area under profile is:

E(L ,Z,)
11
Equating these two areas:

1/2 ^H(EL,) = E(L.Z,)
1 11
Eliminating ~H using the Slope

definition

^H 52 ILi

1/2 S2(Lli l

2I(L.Z, )
11
(EL. )2
1

,(L,Z. )
11

S2

CHARACTERISTICS OF FLOODS FROM URBAN REGIONS

Savini and Kannnerer (1961) traced the stages of
urban development and classified the effects of this
development on the hydrologic regimen. Their classi-
fications were broadly divided into the effects on
water quality and water quantity. The changes were re-
lated to the hydrologic processes--evaporation. trans-
piration, infiltration, groundwater and flood flow.
The problem of storm runoff was investigated in the
urban Rochester, New York area by Kuichling (188~,
in the St. Louis area by Horner and Flynt (1936), in
the Los Angeles area by Hicks (1944) and in the
Chicago region by Tholin and Kiefer (1960). The
Procedures developed by these investigators are sum-
marized in Chow (1964).

Tucker (1969a) assembled lists of urban water-
sheds having rainfall and runoff data. One of the
earlier systematic programs for gaging urban water-
sheds began in 1948 at Johns Hopkins University under

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