For an article with a downloadable Excel spreadsheet template for making storm sewer design calculations, see " Stormwater Sewer Design using Excel Formulas in a Spreadsheet Template.
In order to calculate a value for peak runoff rate for a given drainage area, values are needed for the three parameters, A, C, and i. As you can see, determining a value for i is the most complicated part of using the Rational Method.
The runoff coefficient for this drainage area has been estimated to be 0. There are many tables of values for runoff coefficient for a variety of types of drainage areas in handbooks, textbooks and on the internet. The table at the right shows some typical ranges of values for runoff coefficient.
A greater slope leads to a higher runoff coefficient. Based on a specified design return period and the time of concentration of the drainage area, the design storm intensity has been determined to be 5. One use for the Rational Method is in calculating design storm water runoff rate for storm sewer design.
Each of these parameters will be discussed separately in the next three sections. It is best for areas less than acres, but is sometimes used for up to 2 mi2 areas. Once the design return period and duration are determined, the design rainfall intensity can be determined from an appropriate intensity-duration-frequency graph or equation for the location of the drainage area.
Once the boundaries are known, the area can be determined using the map scale. Values for the runoff coefficient, drainage area, time of concentration and design return period are needed.
It is an empirically determined constant, dependent on the nature of the drainage area surface. An impervious surface like a concrete parking lot will have a runoff coefficient of nearly one. What is the peak runoff rate from this area to be used for design of the storm water inlet?
In addition to the nature of the surface and the soil, the slope of the drainage area has an effect on the runoff coefficient. It may be necessary to first determine the boundaries of the drainage area using a contour map.
A very tight clay soil will also have a relatively high runoff coefficient, while a sandy soil would have more infiltration and a lower runoff coefficient. A map with a drainage area outlined is shown at the left.Urban Hydrology for Small Watersheds TR Chapter 2: Estimating Runoff . SCS Runoff Curve Number method .
Factors considered in determining storm runoff in a watershed. It also causes other problems that.
iii STORMWATER RUNOFF QUALITY AND QUANTITY FROM SMALL WATERSHEDS IN AUSTIN, TX EXECUTIVE SUMMARY The purpose of this report is to summarize stormwater data collected by the City of Austin with. ESTIMATION OF RUNOFF FOR SMALL WATERSHED USING WATERSHED MODELLING SYSTEM (WMS) AND GIS estimating runoff from a watershed for which there is records of precipitation and no Egypt programmed for conservation and development of natural resources management has necessitated the runoff.
• The peak discharge from a small rural watershed is usually caused by intense rainfall. drainage area for estimating peak discharge. – If potholes or marshland constitute more than Estimating Runoff • The runoff from a watershed may be.
A Method for Estimating Volume and Rate of Runoff in Small Watersheds K. M. Kent (retired), Chief.Hydrology Branch, discharge for a small watershed with unusual the high rates runoff from small areas are usually generated from summer thunderstorms. The Rational Method is a commonly used procedure for estimating the peak surface runoff rate from an urban watershed for use in design of storm water management facilities, such as storm sewers, storm water inlets, and storm water detention ponds.Download