FLOOD09674

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Hydrology for Urban Land Planning - A Guidebook on the Hydrologic Effects of Urban Land Use
• Date: 1/1/1968
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

. Note in figure 3 that the largest flow in the
9-year record was nearly 10,000 cfs. The num-
ber 50 printed on the graph means that there
were 50 flows equal to or exceeding 500 cfs.
Once a year, on the average, a discharge value
of about 900 cfs will be equalled or exceeded.

A slightly different result would be obtained
if, instead of using the peak flow for each
storm, only the largest flow in each year were
included in the array. The principle involved
is similar. The arithmetic mean of the peak
flows for the nine annual events is the "average
annual flood." The statistics of this array are
such that the recurrence interval of this average
annual 1100d is the same regardless of the length
of record, which specifically is 2.3 years. That
is to say, a flood of that magnitude can be ex-
pected to be equaled or exceeded on an average
of once in 2.3 years, or 10 times in 23 years.

Studies of river channels have shown that
rivers construct and maintain channels which
will carry without overflow a discharge some-
.what smaller than the average annual flood. In
ct the recurrence interval of the bankfull
age in most rivers is a flow having a recur-
rence interval of about 1.5 to 2 years.

Urbal1ization tends to increase the flood po-
tential from a given basin. The channel then
will receive flows which exceed its capacity not
just once in 1.5 to 2 years on the average but
more often. It is now proposed to estimate how
much more often and to indicate the effect of
this increased frequency on the channel itself.

EFFECT OF URBANIZATION ON INCREASING
FREQUENCY OF OYERBANK FLOW

Taking the East Branch of Brandywine
Creek as an example, the flow-frequency curve
can be constructed for a typical subbasin hav-
ing a 1-sq-mi drainage area. Figure 4A shows
the relation of average annual flood to drainage
area, and figure 4B shows the flood-frequency
curve for annual peaks for basins in the
Brandywine area. The diagrams shown in fig-
ure 4 are similar to those published in the
nationwide series of flood reports, U.S. Geolog-
ical Survey Water-Supply Papers 1671-1689.

. From these curves a discharge-frequency re-
tionship is developed for a drainage area of

1 sq mi. The average annual flood is read from
the upper graph of figure 4 as 75 cfs, and the
lower graph is used to construct the frequency
curve in figure 5 pertaining to a 1-sq-mi basin
marked "unurbanized."

The arithmetic for the construction of the
curve is as follows:

Recurrence
intervfllof
Rnnual flood 1
(}'ears)

Ratio to
mean annual
flood ~

Recurrence
Discharge ~ interval duration
(cis) series 4
(years)

1.1 0.55 41 0.4
1.5 .75 56 .92
2.0 .90 68 1.45
2.3 1.0 75 1.78
5 1.45 110 4.5
10 1.9 145 9.5

1 Only the highest flood each year.
~ From figure 4B.
"Ohtained by multiplying ratios by 75 ds from figure 4A for a
drainage area of 1 sq mi.
I All peaks during the yeftT. The values in this column are mathe-
matically related to those in the first.

The graph marked "un urbanized" in figure
5 is constructed on semi logarithmic paper from
the data listed in the third and fourth columns
of the preceding table. The ordinate is the dis-
charge, and the lower abscissa is the recurrence
interval in the duration series." An auxiliary
scale gives the average number of floods in a
10-year period (calculated as 10 years divided
by the recurrence interval). Thus, the flow
expected to occur once in 10 years would be
about 145 cfs and the fifth largest would be 75
cfs. The latter would also be the average value
of the largest flows each year during the 10-
year record and thus would be the "average
annual flood." It would plot, therefore, at an
abscissa position approximately at 2.3-year re-
currence interval.

The effect of urbanization on the average
annual flood is shown in figure 2, which shows
the increase in average annual flood for differ-
ent degrees of urbanization as measured by the
increase in percentages of impervious area and
area served by storm sewers. For convenience
these are tabulated as follows:

Percentage Percentage Ratio to
of area oJ area average
sewered impervious annualftood
0 0 I
20 20 1.5
40 40 2.3
50 50 2.7
80 60 4.2
100 60 4.4

7