FLOOD05191

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Flood Characteristics of Urban Watersheds in the United States
• Date: 12/9/1983
• Prepared By: Sauer, Thomas, Stricker, and Wilson
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

area for this study was computed by various methods,
but primarily by the grid-overlay method.
The most significant index of urbanization that
resulted from this study is a basin development factor
(BDF). which provides a measure of the efficiency of
the drainage system. This parameter, which proved to
be highly significant in the regression equations, can be
easily determined from drainage maps and field inspec-
tions of the drainage basin. The basin is first divided
into thirds as described earlier in this report. Then,
within each third, four aspects of the drainage system
are evaluated and each assigned a code as follows:
I. Channel improvements.-If channel improvements
such as straightening, enlarging, deepening, and
clearing are prevalent for the main drainage channels
and principal tributaries (those that drain directly
into the main channel), then a code of I is assigned.
Any or all of these improvements would qualify for a
code of I. To be considered prevalent, at least SO per-
cent of the main drainage channels and principal
tributaries must be improved to some degree over
natural conditions. If channel improvements are not
prevalent, then a code of zero is assigned.
2. Channel linings.-If more than SO percent of the
length of the main drainage channels and principal
tributaries has been lined with an impervious mate-
rial. such as concrete, then a code of I is assigned to
this aspect. If less than SO percent of these channels is
lined, then a code of zero is assigned. The presence
of channel linings would obviously indicate the pres-
ence of channel improvements as well. Therefore,
this is an added factor and indicates a more highly
developed drainage system.
3. Storm drains, or storm sewers. -Storm drains are
defmed as enclosed drainage structures (usually
pipes), frequently used on the secondary tributaries
where the drainage is received directly from streets or
parking lots. Many of these drains empty into open
channels; however, in some basins they empty into
channels enclosed as box or pipe culverts. When
more than SO percent of the secondary tributaries
within a subarea (third) consists of storm drains,
then a code of I is assigned to this aspect; if less than
SO percent of the secondary tributaries consists of
storm drains, then a code of zero is assigned. It
sbould be noted that if SO percent or more of the
main drainage channels and principal tributaries are
enclosed, then the aspects of channel improvements
and channel linings would also be assigned a code of I.
4. Curb-and-gutter streets.-If more than SO percent of
a subarea (third) is urbanized (covered by residential,
commercial, and/or industrial development), and if
more than SO percent of the streets and highways in
the subarea are constructed with curbs and gutters,
then a code of I would be assigned to this aspect.

8 Flood Characteristic. of Urban Watenhed.

Otherwise, it would receive a code of zero. Drainage
from curb-and-gutter streets frequently empties into
storm drains.
The above guidelines for determining the various
drainage-system codes are not intended to be precise
measurements. A certain amount of subjectivity will
necessarily be involved. Field checking should be per-
formed to obtain the best estimate. The basin develop-
ment factor (BDF) is the sum of the assigned codes;
therefore, with three subareas (thirds) per basin, and
four drainage aspects to which codes are assigned in
each subarea, the maximum value for a fully developed
drainage system would be 12. Conversely, if the drainage
system were totally undeveloped, then a BDF of zero
would result. Such a condition does not necessarily
mean that the basin is unaffected by urbanization. In
fact, a basin could be partially urbanized, have some
impervious area, have some improvement of secondary
tributaries. and still have an assigned BDF of zero. As is
discussed later in this report, such a condition still fre-
quently causes peak discharges to increase.
The BDF is a fairly easy index to estimate for an
existing urban basin. The SO-percent gnideline will usu-
ally not be difficult to evaluate because many urban
areas tend to use the same design criteria, and therefore
have similar drainage aspects, throughout. Also, the
BDF is convenient for projecting future development.
Obviously, full development and maximum urban effects
on peaks would occur when BDF = 12. Projections of
full development or intermediate stages of development
can usually be obtained from city engineers.
A basin developmellt factor was evaluated for
each of the 269 sites used in this study. Approximately
30 people were involved in making these evaluations,
using guidelines similar to the ones described in the pre-
ceding paragraphs but somewhat less explicit. Tests
have not been made to see how consistently two or more
people can estimate the BDF for a basin. However, this
study indicates that fairly consistent estimates can be
made by different people. A relatively large group of
individuals made the estimates for this study and the
parameter was statistical1y very significant in the regres-
sion equations. If the results obtained by various indi-
viduals had not been consistent, it is doubtful that the
statistical results would be so significant.

Flood-Frequency Estimates

Two primary sets of flood-frequency estimates
(see appendix I) for selected recurrence intervals were
defined, in cubic feet per second, for each station. One
set represents an estimated flood-frequency relationship
for the urbanized basin during a period of constant
urbanization; another represents the estimated relation-