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

develop a deeper, more permeable soil zone, some
structures are built on the channels which stabilize
the gradients and retard the flood flows, This ex-
plains in part the smaller increase in the peak dis-
charges in the Dallas area compared to Houston.

The watersheds in the Houston region have fla~tcr
slopes and the streams have flatter grad~en:s. ~s
urbanization progresses, land around the bU.1ld.1ngs .1S
filled in and built up and street drainage is developed.
The street drainage is discharged into either natural
or constructed drainage channels, The larger inc~8~se
in the flood peaks may be explained on the basis of a
local increase in the relief as concerns the overland
flow and a subtle increase in the drainage "':'eilsity
accompanying the development of streets and street
drainage. Thus the slope is increased on a micro
scale in addition to the c.onstruction of a denser
drainage network.

Van Sickle (1962) found that the peak discharge
of the unit hydro graphs for Brays Bayou at Houstol1 in-
creased three times after urbanization had taken place.
Utilizing a much more extensive data base, Johnson ffild
Sayre (1973) found that complete urbanization increased
the magnitude of the 2-year flood by nine times and
that the 50-year flood was increased by five times.
On the other hand, Dempster (1974) found that the
flood discharge increased by 1.35 times for the 2--year
flood while the peak discharge for the 50-year flood
increased by 1.16 times for a similar increase in the
imperviousness of the watershed. The results are not
entirely comparable because Brays Bayou is a relatively
large watershed and even in 1970, only 18 percent of
the watershed was impervious. Some of the smaller
watersheds in the region have up to 35 percent imper-
vious watersheds (Stoney Brook Street Ditch in Houston,
and Turtle, Coombs and Cedar Creek in Dallas). The
comparison of the flood runoff in these two.regions
does demonstrate the importance of the relatlve con-
veyance capacity of the drainage network in the in-
crease in the flood peaks for a given recurrence
interval; however, imperviousness per se is not the
key factor.

Ve.cJtea.6e. in Re6pon6e. T.<.me.. Espey and Winslow
(1968) analyzed data obtained from the lIouston network
during the period 1964 to 1967. They analyzed data
from 17 Houston watersheds of which 6 ""ere rural ""a~er-
sheds and 11 were urban watersheds. Espey and Winslow
presented a definition of a channelization factor
which takes into account relative efficiency of the
storm drainage network, Espey's channelization fact~r
was proposed as a second coefficient which w~en mult.1-
plied by the coefficient in Carter's equatlon for a
pristine watershed produced the rise time for the
urban watershed:

TR

(L)a b
20.S. IS I

where TR is the rise time,
. is Espey's channelization factor,
1 is the percent of impervious wat ershed,
a is an exponent a 0,29,
b is an exponent b . -0,6.

Channel and StOkm s~~ Netwv~k.-- Espey and
Winslow (1968) found that in the Houston watersheds,
the channelization factor ~ could have two parts:

· = ., + .2

The first part, $1' relates to storm sewer-drainage

network, The $1 coefficient should have a constant

value as long as these was no change in the drainage
density. The second part of the coefficient, $2'

relates to a part of the channel resistance which may
change during the year. Espey and Winslow found that
in the Houston region, the growth of vegetation in the
drainage channels retarded the flow of water which in-
creased the watershed response time. Typical values
of the Espey channelization factors are given in
Tables 3 and 4.

During the period 1945 to 1949, the Louisville
District, U. S. Corps of Engineers made measurements
of rainfall and runoff hydrographs in storm drains in
Louisville, Kentucky. Six urban watersheds in which
the storm runoff was disposed of through stopm sewers
provided data from which S-minute unit hydrographs
were derived. Eagleson (1962) used these data in an
analysis of the unit hydro graph characteristics of
urban watersheds. The Louisville watersheds contained
a larger proportion of storm sewers when compared with
the Texas urban watersheds. Eagleson found that the
discharge hydrographs from the smallest watershed
(0.096 sq, mi.) were so sensitive to storm rainfall
variations over the watershed, that consistent unit
hydro graphs could not be real ized. Of the remaining five
urban watersheds (varying from 0.22 to 7,52 sq. mi, in
size), 27 flood events were used in his data base.

Carter (1961) found that the degree of impervi-
ousness of the watershed could be used to define a
family of lines which were parallel to similar lines
for a natural watershed and he suggested that the pro-
cess of urbanization could be quantified on the basis
of the percent of imperviousness. Carter proposed an
urbanization factor, K . which is computed from the
percent of imperviousness in the watershed, I. The
use of the impervious factor always had a value great-
er than 1,0. In addition the use of this factor also
allowed the use of a variable local coefficient to
account for those impervious roof areas which drain
onto a grassed area and do not result in immediate sur-
face runoff. Dempster also used a variation of the same
imperviousness factor:

K

0.3 + 0.0045
0.3

(Carter)

K = 1

(Dempster)

+ ,015 I .

Carter introduced the term length-slope parameter used
by Dempster in the analysis of the Dallas, Texas data.
A similar parameter had been used previously by Snyder
(1958) and Kirpich (1940),

L

IS

(Carter)

Le

IOn .l..
IS

(Snyder)

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