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

frequency, flow duration, mean annual flood,
disc harge at bankfull stage, and frequency of
bankfull stag-e. This is evidenced in past studies
of the effeets of urbanization on the hydrology
of an area. :VTany different techniques of re-
lating- rainfall to runoff have been used, along
\vith ,'al'ious parameters to lneasure the degree
of urbanization. In order to evaluate our pres-
ent knowledge, it is necessary to express the
results Df these studies in some common de-
nominator.

Most reports on hyorologic effects of urbani-
zation present the conclusions in a form which
is more Llseful to the hydrologist than to the
urban planner. This circular will attempt to
interpret the hydrologic conclusions of these
studies in terms that are meaningful to the
planner. Two forms of presentation will be
used.

The first is a slight modification of a method
previou~ly used by several investigators, es-
pecially D. G. Anderson (1968) and L. D.
.James (1965). The percentage of an area
se\Vered is plotted against the percentage of the
area rendered impervious by urbanization;
isopleth lines (Jines of equal value of the ratio)
on the ,~'l'apb show the ratio of peak discharge
unoer urbanized conditions to the peak dis-
charge lInoer rural or unurbanized conditions.
Such a graph will be different for different
orainage area sizes and for different flow fre-
quencies.

The second method utilizes a relationship be-
tween the degree of urbanization and the fre-
quency elt which the original channel capacity
would be exceeded.

Table 1 is an interpretation and summary of
the effeets of urbanization on peak discharges
based on pre\'ious studies. Results of the
studies were interpreted and extrapolated to a
common oenominator of 1 sq mi (square mile),
a practical unit of size for planning.

Carter (1961) developed a technique that
followed the reasoning previously used by
Snyder (1938) and that showed lag time as a
function of basin characteristics. For 20
streams in the vicinity of Washington, D.C.,
Carter developed this relation for natural

basins, for partly sewered, and for comPletel~
sewered basins. As in most studies the diffi-
culty comes in translating these descriptive
terms to quantitative measures of urbanization.
From data presented by Carter, values were
read for a basin ratio of 0.12 representing a
l-sq-mi area having an estimated length of 1.2
miles and an average slope of 100 feet per mile.
H was further assumed that in Carter's study,
"partly sewered" is equivalent to 50 percent
sewered and 20 percent impervious. These
conditions provide some of the data shown in
table 1.

Table l.-IncrcQsc in discha-rgc as a ,;'csult of urban-
ization in a i-sQuare-mile area

[D~~~ha~'c:;(' is mean annual flood: Tf'CUrrcnce int('Tvlll is 2.3 y..nrs.
Datn 111'(' ~'xpr('_~~ed ao> l'lltio of diseharge after urbanization to
,li~charw' \lnde~ previous condition~. Data from James (1!J6,;) have
nu super~criptl

P0rrenUu:!;e of area Percentage of area made impervious
,~el"ve{l by Etorm snv('rage

20 50 80
{ 1.0 11.2 11.8 ]2.2
0 '1.3 21.7 22.2
1.3 1.6 ~
20 [ 1.1 31.9 1.8 2.
1.4
{ 1.3 42.1 13.2 ]4,7
12.8 2.0 2.5
50 53.7
62.0 2.5 :14.2
1.6
80 1.6 1.9 3.2
{ 1.7 13.6 '4.7 45.6
100 2.0 2.8 16.0
3.6
And~'rSfln (1968). :;Wiitala (1961).
Mnrlens (1%6). d Espey, Morgan, and Masch
'i~.lili\on (1966). 0%6).
em'ter (1[l6l).

As an indication of the change in impervious
area resulting from urbanization, Harris and
Rantz (1964) showed that an area near Palo
Alt0, Calif., changed from 5.7 percent to 19.1
percent impervious in a 10-year period.

One of the most complete analyses of urban-
ization effects was made by D. G. Anderson
(1968) in his study of the urbanization in
Fairfax County, Va., near the metropolitan
complex of the Nation's Capital. Anderson's
analysis follows the procedure suggested earlier
by Carter, but Anderson included a larger
array of data from 64 gaging stations. Ander.
son closely confirmed the conclusions of Carter,
but he .,arried the analysis further in a plot of

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