FLOOD09481

• County: Statewide
• Community: State
• Stream Name: All
• Basin: Statewide
• Title: Practices in Detention of Urban Stormwater Runoff
• Date: 1/1/1974
• Prepared For: American Public Works Association
• Prepared By: American Public Works Association
• Floodplain - Doc Type: Educational/Technical/Reference Information

torrential rainfall in urban and suburban areas
of northeastern lllinois in August, 1972, The
areas hardest hit were communities located
west of the City of Chicago in Cook, DuPage
and Will Counties. The storm started Friday,
August 25th at about 5 p.m. and lasted until
I a,m. Saturday morning. Total rainfall was
6.88 inches (unofficially) in one locality and
2.49 inches (officially) several miles away.
Over $23 million in damages were reported in
preliminary estimates. Electricity was cut off
from 40,000 customers and phone service to
45,000 homes was interrupted for days by
problems resulting from the flooding. Many
homes were flooded with up to II feet of
water. Some individual well water supplies
were polluted, although public water supplies
were tested and found to have no
contamination.
In a r100ding incident such as this, on-site
detention facilities would have been of vast
help by delaying the entry of stormwater into
the sewer systems and rivers, The stormwater
could have been released from storage
facilities over a long period of time to reduce,
if not eliminate, much of the flooding that
resulted from sewer surcharging and high
stages in rivers, creeks and ditches. It would
not have been possible to detain all of the
excess runoff from all affected areas, but an
area-wide system of detention facilities may
have reduced peak runoff discharges to levels
that the receiving streams and sewer systems
might have been capable of handling.
Pollution: Pollution of stormwater
runoff comes largely from four major sources:
I. contact with municipal and industrial
wastes in combined sewers;
2. litter, debris and chemicals on parking
lots, streets and alleys;
3. silt resulting from soil erosion; and
4. pesticides and fertilizers washed off
vegetation.
The pollution from combined sewers is
obvious. The high flow rates in such sewers
during storm periods carries wastes deposited
in the sewers, during low flows, to the outfall.
Sanitary waste commingled with
stormwater is forced out of sewer inlets and
manholes, and is also discharged to receiving
waters during unavoidable overflows from

intercepting sewers. The problem of pollution
from combined sewer overflows has received
considerable attention by the Federal
Government as evidenced by its extensive
funding of research in this field.
The pollution aspects of urban runoff
from street debris are becoming more and
more obvious. In his paper, Treatment of
Urban Runoff I, FJ. Condon describes a
hpyothetical situation by comparing the
pollution from urban runoff with the pollution
from sanitary sewage and secondary
treatment plant effluent. He states that the
effects of this pollution, especially tlfe heavy
metals, is not adequately defined to
determine its seriousness, but that it is
obvious that storm water runoff is highly
polluted. Mr, Condon notes in his paper that
data on the pollution load of stormwater
runoff is often contradictory. In some studies,
for example, it was found that antecedent
rainfall or street cleaning more than 24 hours
prior to the rainfall had no effect on water
quality, Other investigators found that
antecedent events up to 10 days previous to
the rainfall had measurable effects on water
quality. Nevertheless, the possible need for
treatment of runoff to control pollution of
receiving streams from drainage of streets and
other urban areas is recognized. In the future,
discharge of untreated stormwater from
separate sewers into lakes or rivers may no
longer be acceptable.
Soil erosion is a third area of pollution of
stormwater. In developed areas, soil erosion
can be very minor; but land under
development can contribute large amounts of
sediment through erosion. Soil erosion is
estimated at over I billion tons of soil a year,
with land under construction contributing
from 50 to 40,000 times the erosion of
undeveloped or developed land. Erosion is
considered by some to be the largest single
source of stream pollution. The application of
on-site detention of storm water runoff to
control erosion and accompanying siltation is
covered in detail in Chapter 6.
Another significant source of storm water
pollution can be pesticides and fertilizers
washed off vegetation.
Beneficial Use of Storm water: As many

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