Estimating Storm Water Runoff
John Poullain, P.E.
online course provides general guidelines and techniques for estimating storm
water runoff from development areas. Storm water management is necessary to
control erosion and sediment from construction activities. Estimating stormwater
runoff is the first step in designing a stormwater management system. The rational
method is primarily discussed in the course. Water quality objectives must be
met to avoid discharging pollutants into waterways, creeks and rivers. The necessary
data and terms describing design storms such as hydrographs, time of concentration,
lag time, duration, manning's n and runoff coefficient C are discussed. Comparisons
between predevelopment and post development conditions on a sites hydrograph
will show the importance and benefits for stormwater management. Stormwater
management can reduce the peak runoff and increase the time of concentration
so that erosion and sedimentation are better controlled.
This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
At the conclusion of this course, the student will:
This course is intended for civil engineers and planners.
Benefit to Attendees
The student will
become familiar with methods and techniques used to estimate stormwater runoff
from small drainage areas. The purpose of estimating the runoff is to aid in
controlling erosion and sediment and to manage the stormwater runoff of pollutants
into downstream water or streams. Stormwater can contain such contaminants as
volatiles, soluble organic, corrosive acids and alkalis. The terminology of
stormwater runoff, typical hydrographs, design storm and procedures for using
the rational method properly along with several example calculations for estimating
runoff quantities are presented. The types of water flow through a drainage
area are described and differentiated. The assumptions, misconceptions and limitations
associated with the rational method are presented and steps to compensate are
This course covers
the rational method used for estimating stormwater runoff from drainage or development
sites. Migration of contaminants and runoff erosion can contaminate the subsoil,
groundwater, water wells and nearby surface water unless properly managed. In
order to properly manage the stormwater at a development or construction area
the predevelopment conditions must be estimated to use for comparison with effects
of future developments. A construction area must be investigated for a wide
range of conditions, including average slope, type of terrain, land use, groundwater
level and surface drainage. The course is based on methodology and data used
by the State of Florida but is similar to that used by other states, all of
which have variances in formula values and methodology used in applying the
formula. Hydrological data can be obtained for different geographical regions
from Technical paper No. 40 of the Weather Bureau or from local sources.
The rational equation was developed from simplified runoff analysis using isochrones, lines of equal travel time. It is the simplest method to determine peak discharges from an area to culvert or other points of interest. It is not as sophisticated as the SCS TR-55 method which can be used for much larger drainage areas (up to 20-sq. mi.) but has commonly been used for sizing sewers. The rational method uses a coefficient (C) based on the soil type, developments and drainage basin slopes. The rainfall intensity can be found from intensity/duration/frequency (IDF) curves for rainfall in the geographical region being analyzed. Local governments usually determine the storm frequency depending on the impact of development. The subject of stormwater drainage is complicated, sometimes controversial and accuracy is dependent on an individuals judgment and experience. Some of the factors that are involved besides rainfall for estimating stormwater runoff are as follows.
a. The rate and quantity of surface runoff are most affected by the size, shape and slope of the drainage area. The entire drainage area is assumed contributing to the runoff.
b. Runoff is affected by size in two ways. A large area will have a longer period of time over which runoff will occur and the peak runoff rate will be less per acre than for a smaller area if the total runoff per acre remains the same. The maximum intensity of rainfall for a given frequency varies inversely with the area covered by the storm.
c. The shape of a drainage area governs the rate of runoff because it controls the length of overland flow.
d. The average slope of a drainage area controls the time of overland flow. As the average slope increases, overland flow speeds up, the runoff rate increases and risks for erosion increases. Since the losses that occur take place over the entire area an average slope must be determined that is representative of the entire area.
e. The size and boundary of the drainage area is defined by the drainage flow pattern of the water in the general vicinity of the site. It can be delineated and divided into sub areas according to ridges and high points on saddles using a topographical map. Flow is perpendicular to contour lines and away from high points. Flow paths should be determined so that they do not cross over delineation lines that mark the drainage boundaries. The lines of separation for sub areas are based on the overland flow of different portions of the drainage area. Analyses of complex watersheds require a methodical approach to consider all variables and the delineation's of the drainage area itself.
f. There are three types of water flow. Sheet flow is water flowing as a blanket, more or less equal in depth, across a somewhat uniform surface such as a grassed field, surfaced road pavement, parking lot or roof. Generally water begins to channelize or collect into ditch like flow when it flows over unsurfaced areas or terrace and bermed areas. This occurs in about 300 feet or sooner in case of rough, steep terrain.
g. The length of sheet or channel flow is the horizontal distance measured along the representative paths of flow.
h. Retardance or "n" designates resistance of water flows caused by various surface conditions such as vegetation, woods and other surfacing (concrete, gravel, and asphalt).
This course is based primarily on Chapter 3, Florida Department of Transportation, Drainage Removal Manual, "Estimating Stormwater Runoff", (1994 Edition, 26 pages), PDF file and Table 3-1 of US Army Corps of Engineers EP 1110-1-16, 1 page, PDF file.
The link to the Florida Department of Transportation, Drainage
Removal Manual is "Estimating
Stormwater Runoff, Chapter 3" and Engineers Manual Table 3-1 "Runoff
Calculation Methods, Selection Criteria"
You need to open or download above documents to study this course.
Text Errors and Corrections
The student should note the following corrections to the above documents.
Pg. 3-9 Unlabeled Table on page is Table 3-1
Pg. 3-15 Table
4-2 changes to Table 3-2
For C-D, Pw =0.015 changes to Pw=28.2
Pg. 3-16 Figure 3-3 changes to Plate 3.5a
Pg. 3-20 Table
4-1 changes to Table 3-1
Figure 4-6 changes to Plate 3.5d
Pg. 3-23 Table 3-1 changes to Table 3-6
Page 3-23 - change Table 3-6 to Table 3-7
This course considers
the techniques and methods used to estimate stormwater runoff. Stormwater runoff
may be contaminated with volatiles, soluble organics, corrosive acids and alkalis
as well as sediments from site erosion. The terminology of stormwater runoff
and the rational method for calculating peak rate of runoff (Q) are described
and the factors affecting losses are considered. Procedures for using the rational
method properly are described along with examples and calculations for estimating
stormwater drainage. Data and terms for hydrographs, TC, L time, rainfall intensity,
duration, manning's n, runoff coefficient C and other terms are explained.
For additional technical information related to this subject, please refer to:
Information and software calculations for the rational equation and SCS TR-55 formula.
Software developed for modeling small watersheds and an overview of the rational method and procedures are described.
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.