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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.ease in number of flows equal to or exceeding
natural channel capacity. An auxiliary scale is
shown at the top of figure 5 to facilitate this.

For example, under natural conditions it is
expected that a 10-year record would show
about seven flows equal to OJ' exceeding 67 ds,
or channel capacity. But if the average annual
flood were increased 1.5 times (from 7[, to 112
cfs) corresponding to 20 percent sewered and
20 percent jmpel'vjous~ the ne'y frequency CUfye
indicates that 14 flows of 67 dg or greater
would occur in a ] O-year period, or a twofold
increase in nnmber of flows. Similarly, the
ratio of number of flows exceeding bankfull
capacity was read from the intersection of the
other curves in figure" with the ordinate value
of 67 dB to obtain the ratios plotted in figure 6.

Figure 6 shows that with an area 50 percent
sewered and 50 pel~cent imperyious, for ex-
ample, the number of flows equal to or exceeding
bankfull channel capacity would, over a perio,]
of year." be increased nearly fourfold.

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.OCAL STORAGE TO C01IPENSATE FOR PEAK
FLOW INCREASE

Urbanization tends to increase both the flood
volume and the flood peak. But the increase
can be compensated so that the discharge
through channels downstream is maintained to
any degree desired within the range which ex-
isted prior to urbanization. It is obvious that
reservoir storage is installed on a dyer in order
to reduce the magnitude of peak discharge by
spreading the flow over a longer time period.
Channels themselves provide temporary storage
and act "s if they were small resen-oiril. Over-
bank flooding on to the flat flood plain is a way
that natural rivers provide for temporary stor-
age and thus decrease flood peaks downstream.
This effect of storage has been fully investi-
gated and described (for example see Leopold
and Maddock, 1954, especially p. 36-49).

The provision of flood storage upstream,
then, will decrease flood peaks and compensate
for the increase caused by urbanization. This
storage could take many forms including the
following:
1. Drop inlet boxes at street gutter inlets.
. Street-side swales instead of paved gutters
and curbs.

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3. Check dams, ungated, built in headwater
swales.
.L Stol'nge volume.s in basements of large
buildillgs receiving \-vater from roofs 01"
g'utters and emptying into natura]
strearns or swales.
5, Off channel storage volumes such Cl.S arti-
ficial ponds, fountains, or tanks.
6. Small res'2l'\'(li]'~ in 8trcam channels such as
those built for farm ponds.

Yariolls types of storage yo]urnes could be
used simultaneously In various mixes. The
effectiwness depends on the volume oj' storage
relative to the volume of inflow during a storm
peak period. Design criteria to guide city engi-
neers and developers are needed.

SEDDlEYf PIWDITTIOK

The basic clata anlilable for analyzing the
effect of urbanization on sediment yield, though
sparse, haye been sumnlarized to some extent
in the litemture. Especially valuable is the re-
port by Wolman (196'1) who summarized not
only the data obtained from sediment sampling
stations in streams in Eastern United States
but also studied the sediment yield from build-
ing t'Onstruction activities. Sediment yields
from urbanized or developing areas ranged
. from 1(1)0 to more than 100,000 tons per square
Inile pel' year.

It should be recognized that sediment yield
pel' square mile decreases with increasing
drainage area, but neyertheless it is apparent
that unurbanized drainage basins yield 200 to
500 tons per square mile per year, on the aver-
age. These figures are slightly higher for the
farmed Piedmont lands, which may be expected
to produce sediment yield of 500 tons per square
mile per year, such as the Watts Branch basin
near Rocln'ille, :\Id.

The data on urbanized areas studied by
Wolman are plotted in figure 7 together with
data from suspended load sampling stations of
the U.S. Geological Survey as summarized by
Wark and Keller (1963).

In the graph (fig. 7) three bands or zones are
labeled A, C, and UC. Wolman and Schick
(1967) differentiated the following types of
activity: ,ct, agricultural or natural; C, under-

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