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Rational Method Runoff Coefficients
One of the parameters in the Rational Method equation ( Q = CiA ) is the runoff coefficient, C. The other parameters are A, the area of a watershed; i, the design rainfall intensity for a storm of specified recurrence interval and duration equal to the watershed time of concentration; and Q, the peak storm water runoff rate due to a storm of intensity i, on a watershed of area, A, and with runoff coefficient, C.
The article, "Use of Excel Formulas for Design Rainfall Intensity Calculation," contains information about design rainfall intensity, and a downloadable Excel spreadsheet for making the calculations. The other articles in this series also provide useful related information. This article provides information about watershed runoff coefficients, factors that affect their value, and tables that can be used to obtain values for runoff coefficients.
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Factors Affecting Watershed Runoff Coefficients
The major factors affecting the rational method runoff coefficient value for a watershed are the land use, the soil type and the slope of the watershed. The physical interpretation of the runoff coefficient for a watershed is the fraction of rainfall on that watershed that becomes storm water runoff. Thus the runoff coefficient must have a value between zero and one.
Land Use: Surfaces that are relatively impervious like streets and parking lots have runoff coefficients approaching one. Surfaces with vegetation to intercept surface runoff and those that allow infiltration of rainfall have lower runoff coefficients.
Slope: All other things being equal, a watershed with a greater slope will have more storm water runoff and thus a higher runoff coefficient than a watershed with a lower slope.
Soil Type: Soils that have a high clay content don't allow very much infiltration and thus have relatively high runoff coefficients, while soils with high sand content have higher infiltration rates and low runoff coefficients. The U.S. Soil Conservation Service (SCS) has four soil group identifications that provide information helpful in determining watershed runoff coefficients. The four soil groups are identified as A, B, C, and D. Classification of a given soil into one of these SCS groups can be on the basis of a description of the soil characteristics or on the basis of a measured minimum infiltration rate for the soil.
The descriptive characteristics of the four SCS soil groups are summarized in the following list:
- Group A: Deep sand; deep loess; aggregated soils
- Group B: Shallow loess; sandy loam
- Group C: Clay loams; shallow sandy loam; soils low in organic content; soils usually high in clay
- Group D: Soils that swell significantly when wet; heavy plastic clays; certain saline soils
Following are the minimum infiltration rates in inches/hr for each of the SCS soil groups:
- Group A..............0.30 - 0.45 in/hr
- Group B..............0.15 - 0.30 in/hr
- Group C..............0.05 - 0.15 in/hr
- Group D.................0 - 0.05 in/hr
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Rational Method Runoff Coefficient Table
The images in this section show two rational method runoff coefficient tables for a variety of land uses, watershed slopes, and SCS soil types. Part I of the table, which is on the left, gives information for SCS soil groups A and B. Part II of the table, which is on the right, gives information for SCS soil groups B and C. Land use and watershed slope are also factors in determining the runoff coefficient from these tables. The source for the information in the tables is the first reference below, the Knox County Tennessee Stormwater Management Manual. Similar rational method runoff coefficient tables are available in both of the other references.
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References for further information:
1. Knox County Tennessee, Stormwater Management Manual, section on the Rational Method
2. McCuen, Richard H., Hydrologic Analysis and Design, 2nd Ed, Upper Saddle River, NJ, 1998.
3. Bengtson, Harlan H., Hydraulic Design of Storm Sewers, Including the Use of Excel, an online, continuing education course for PDH credit.
The Rational Method for Calculating Peak Storm Water Runoff Rate
- Spreadsheets for Design Peak Storm Water Runoff Rate and Peak Rainfall
- Calculating Design Rainfall Intensity for Use in the Rational Method
- Runoff Coefficients for Use in Rational Method Calculations
- Calculating Watershed Time of Concentration
- The Rational Method for Calculation of Peak Storm Water Runoff Rate