Estimating Storm Water Runoff

John Poullain, P.E.

Course Outline

This two-hour 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.

Learning Objective

At the conclusion of this course, the student will:

Intended Audience

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 presented.

Course Introduction

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).

Course Content

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

Course Summary

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.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.