92    PRACTICAL METHODS FOR DETERMINING
STORMWATER RUNOFF RATES AND QUANTITIES AT
INDUSTRIAL SITES
George D. Biemel, Director, Environmental Studies Section,
Burgess & Niple, Limited
Columbus, Ohio 43220
INTRODUCTION
Determining stormwater runoff rates and quantities is a necessary step in several phases of industrial plant operations. First, flow rate estimates are needed for initial design of stormwater runoff
collection and conveyance system. Secondly, runoff rates and quantities are required for design and
installation of stormwater detention/retention facility design and stormwater runoff treatment plant
design. Finally, runoff rates must be reported as a part of the stormwater permit application requirements as established by the United States Environmental Protection Agency (U.S. EPA).
DEFINITION OF TERMS
Drainage Area
The drainage area or tributary area is the area, including ground surfaces, buildings, and paved
areas, that contribute stormwater runoff to the point of interest. Drainage areas are developed by
determining drainage boundaries from topographic maps, grading and site drainage plans, and field
inspections. From this information, drainage boundaries are marked on the site map and the drainage
area (in acres or square miles) is determined by planimetering around the drainage boundary on the
map.
Time of Concentration
The time of concentration is the time in minutes or hours that is required for runoff to reach the
point of interest from the most distant point in the drainage area. This represents the time at which the
entire drainage area is contributing runoff to the point of interest. The time of concentration is
composed of two parts: first, the travel time of overland flow and, second, the travel time as pipe or
channel flow. Overland flow travel time is a function of the ground slope and the ground cover, such
as forested, grassed, or paved land. Pipe and channel flow is related to the bed slope and the cover or
material of construction. Cover and materials of construction affect the pipe or channel friction
forces which may slow down the speed with which runoff leaves the drainage area.
Flow Hydrograph
A typical flow hydrograph is illustrated on Figure I, The hydrograph represents the flow rate,
usually in cubic feet per second (cfs) or gallons per minute (gpm), as a function of time at the point of
interest for a given storm event. Several pieces of information are shown on a hydrograph. First, the
peak flow can be read directly from the hydrograph. The time to peak can be read, representing the
time from the beginning of runoff to the time when the peak flow occurs. The area beneath the
hydrograph represents the total runoff volume from the tributary area. This volume is the total storm
rainfall less interception by plants, infiltration into the soil, evaporation, and storage in local ponds
and depressions. The rainfall that caused the runoff may also be shown in the upper left hand corner
on the plot.
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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