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

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channel beds, and unusual debris accumula-
tions. These all tend to decrease the amenity
value of a stream.

The second factor is the accumulation of arti-
facts of civilization in the channel and on the
flood plain: beer cans, oil drums, bits of lumber,
concrete, wire-the wbole gamut of rubbish of
an urban area. Though this may not impor-
tantly ,dfect the hydrologic function of the
channel, it becomes a detriment of what is here
called the hydrologic amenity.

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r--~Lag time
,

Hydrograph
of streamflow

________Center of mass
of ru no ff
~and of "Jinfall

TIME, IN HOURS ~

r--L=i~Lag time
I after urbanization

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{ \, ~Ollglnal
I, Y'
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II \ /After urbanization
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TIME, IN HOURS--->-

Figure 1.-Hypothetical unit hydrographs relating
runoff to rainfall, with definition>; of
significant parameters.

The third factor is the change brought on
by the disruption of balance in the stream
biota, The addition of nutrients promotes the
growth of plankton and algae. A clear stream,
_hen, may change to one in which rocks are
overed with slime; turbidity usually increases,
and odors may develop, As a result of increased
turbidity and reduced oxygen content desirable

game fish give way to less desirable speetes,
Although lack of quantitative objective data on
the balance of stream biota is often a handicap
to any meaningful and complete evaluation of
tbe eft'ects of urbanization, qualitative observa-
tions tend to confirm these conclusions.

AVAILABiLITY OF nATA ANn THE TECH"IQUE
OF ANALYSIS

Basic hydrologic data on both peak flow and
volume of runoff may be expressed in terms of
the characteristics of the unit hydrograph, that
is, the average time distribution graph of flow
from a unit or standard storm, The unit hy-
drograph shows the percentage of the total
storm runoff occurring in each successive unit
of time. The standard storm may be, for ex-
ample, a typical storm which produced 1 inch
of runoft' (fig. 1). Snch data are derived from
the study of individual storms and the asso-
ciated runoff graphs meawred at gaging
stations,

One factoI' stating the relation between the
sturm and the runoft' is lag time. This is defined
as the time interval between the center of mass
of the "torm precipitation and the center of
mass of t.he result.ant hydrograph. Lag time is
a function of two basin parameters-the mean
basin sl"pe and the basin length, These factors
empirically correlate with lag time if expressed
in the f0rm of the basin ratio (basin length L
divided by the square root of the mean basin
gradient., s). This basin ratio is also related to
drainage area. As drainage area increases, the
basin length increases and the average value
of slope generally decreases, Thus, natural
basin characteristics can be translated into
flood-flow characteristics,

Lag time may be materially altered by the
eft'ects of urbanization on the basin surface.
Water nms oft' faster from streets and roofs
than from natural vegetated areas. This tends
to decrease the lag time. The construction of
artificial channels, especially storm sewers,
also decreases lag time. As the time required
for a given amount. of water to run oft' shortens,
the peak rate of rnnoft' (flood peak) increases.

In addition to the basin ratio and lag time,
the regimen of a stream, however, can be de-
scribed in many other ways, including flood

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