Dear Hashitha,

The followings are the methods used to calculate discharge of a catchment area using rainfall data:

Runoff Computation Methods

As described in the Stormwater Management Rules, the NJDEP has specified that one of two general runoff computation methods be used to compute runoff rates and volumes. These are the NRCS methodology, which consists of several components, and the Rational Method (and the associated Modified Rational Method), which are generally limited to drainage areas less than 20 acres. A general description of each method is provided below.

1Rational Method

The Rational Method uses an empirical linear equation to compute the peak runoff rate from a selected period of uniform rainfall intensity. Originally developed more than 100 years ago, it continues to be useful in estimating runoff from simple, relatively small drainage areas such as parking lots. Use of the Rational

Method should be limited to drainage areas less than 20 acres with generally uniform surface cover and topography. It is important to note that the Rational Method can be used only to compute peak runoff rates. Since it is not based on a total storm duration, but rather a period of rain that produces the peak runoff rate,

the method cannot compute runoff volumes unless the user assumes a total storm duration. Complete descriptions of the Rational Method can be found in many hydrology and drainage textbooks. 

Rational Equation Method 

The Rational equation is the simplest method to determine peak discharge from drainage basin runoff. It is not as sophisticated as the SCS TR-55 method, but is the most common method used for sizing sewer systems.

Rational Equation:  Q=ciA

The Rational equation requires the following units:

Q = Peak discharge, cfs

c = Rational method runoff coefficient

i = Rainfall intensity, inch/hour

A = Drainage area, acre

Note that our calculation allows you to use a variety of units.

The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. A simplified table is shown below. See the references at the bottom of the page for more complete tables including impact of slope.

The Rainfall intensity (i) is typically found from Intensity/Duration/Frequency curves for rainfall events in the geographical region of interest. The duration is usually equivalent to thetime of concentration of the drainage area. The storm frequency is typically stated by local authorities depending on the impact of the development. A 10-yr, 25-yr, 50-yr, or even 100-yr storm frequency may be specified.

2-Modified Rational Method

The Modified Rational Method is a somewhat recent adaptation of the Rational Method that can be used to not only compute peak runoff rates, but also to estimate runoff volumes and hydrographs. This method uses the same input data and coefficients as the Rational Method along with the further assumption that, for the

selected storm frequency, the duration of peak-producing rainfall is also the entire storm duration. Since, theoretically, there are an infinite number of rainfall intensities and associated durations with the same frequency or probability, the Modified Rational Method requires that several of these events be analyzed in

the method to determine the most severe. Similar to the Rational Method, there are several urban hydrology and drainage publications that contain descriptions of the Modified Rational Method, including Appendix A-9 of the Standards for Soil Erosion and Sediment Control in New Jersey published by the New Jersey State Soil

Conservation Committee. Use of the Modified Rational Method should also be limited to drainage areas less than 20 acres with generally uniform surface cover and topography.

http://www.lmnoeng.com/Hydrology/rational.php

http://drdbthompson.net/writings/rational.pdf

http://ocw.unesco-ihe.org/pluginfile.php/493/mod_resource/content/1/Urban_Drainage_and_Sewerage/7_System_Components_and_Design/Chapter_7-System_components_and_Design_part2_for_PDF.pdf

http://www.njstormwater.org/bmp_manual/NJ_SWBMP_5%20print.pdf

3-NRCS Methodology

The USDA Natural Resources Conservation Service (NRCS) methodology is perhaps the most widely used method for computing stormwater runoff rates, volumes, and hydrographs. It uses a hypothetical design storm and an empirical nonlinear runoff equation to compute runoff volumes and a dimensionless unit

hydrograph to convert the volumes into runoff hydrographs. The methodology is particularly useful for comparing pre- and post-development peak rates, volumes, and hydrographs. The key component of the NRCS runoff equation is the NRCS Curve Number (CN), which is based on soil permeability, surface cover,

hydrologic condition, and antecedent moisture. Watershed or drainage area time of concentration is the key component of the dimensionless unit hydrograph.

Several runoff computation methods use the overall NRCS methodology. The most commonly used are the June 1986 Technical Release 55 – Urban Hydrology for Small Watersheds (TR-55), the April 2002 WinTR-55 – Small Watershed Hydrology computer program, and Technical Release 20 – Computer Program for Project

Formulation: Hydrology (TR-20) published by the NRCS. The computer programs HEC-1 Flood Hydrograph Package and HEC-HMS Hydrologic Modeling System published by the U.S. Army Corps of Engineers’ Hydrologic Engineering Center also contain components of the NRCS methodology. A complete description

of the NRCS methodology can be found in the NRCS National Engineering Handbook Section 4 – Hydrology (NEH-4).

TR-55 Peak Discharge and Runoff Calculator

Hydrologic calculation for Peak Discharge, Runoff Depth, Runoff Curve Number, Time of Concentration, and Travel Times. Based on TR-55 (1986): Urban Hydrology for Small Watersheds

TR-55 peak discharge calculation is mobile-device-friendly as of August 14, 2014

The United States Soil Conservation Service (now called the Natural Resources Conservation Service), division of the USDA (U.S. Department of Agriculture) has worked for decades developing equations and conducting experiments to determine reliable models for predicting peak discharge from storm events. Relying upon extensive research, Technical Release 55 (TR-55: SCS, 1986) presents a methodical and reliable approach to predicting peak discharge due to a 24-hr storm event. (This web page uses TR-55 and SCS (1986) interchangeably; they are the same document.) TR-55 is valid for watersheds that have a time of concentration from 0.1 to 10 hr. Such watersheds are considered small. Our calculation uses the equations and graphs (coded into equations) in TR-55 chapters 1 thru 4 to solve for peak discharge. Chapter 5 (titled Tabular Hydrograph Method) also solves for peak discharge but models more complicated watersheds - watersheds that have several main channels requiring channel hydrograph routing techniques.

Though the TR-55 document mentions specific units (all English) for its equations, our calculation allows a variety of input and output units (English and metric). We have tried to make the calculation useful for the international community. Unfortunately, TR-55 only presents rainfall distribution maps for the USA. Therefore, non-USA users need to determine whether a typical 24-hr rainfall resembles a Type I, IA, II, or III distribution and determine 24-hr rainfalls from local sources.

http://www.lmnoeng.com/Hydrology/hydrology.php

http://www.njstormwater.org/bmp_manual/NJ_SWBMP_5%20print.pdf

Hoping this will be helpful,

Rafik

thank you very much Rafik Karaman.

this details are very very helpful for me. (Y)

You can try the RORB software to calculate the outlet discharge based on IDF data. Its free. You can refer to the following link:

http://www.harconsulting.com.au/software/rorb/

Although RORB is based in Australia, but you can try this out. You can use your own catchment and IDF (intensity rainfall) data in the software and calibrate the parameters to get the desired output and compare with the actual output.

Good luck.

thanks sajal

i want to know about manual methods.

Just as a compliment. What Rafik mentioned is only a small sample (allthough may be one of the most widely used) of rainfall - runoff models. There is a much larger number of them, allthoug less known. Very roughly the methods used to estimate discharge may be grouped into the following:

1. Regionalization and Regression Methods (Empirical)

2. Rainfall - Runoff Conceptual based

3. Rainfall - Runoff Physically based

The ones proposed by Rafik are of the second cattegory and in the case of the rational method, perhaps the most simple method at all. But you have dozens of other options. Be aware that the methods have limitations so you may want to read about them. Just as a simple example you cannot directly apply the Rational Method to large areas (beyond 10 km2). So depending on your goals it may be a good idea to take a deeper look on the subject.

thanks Rafeel and Arash