Evaluation of Limitations of Rational Method for Runoff Calculation for Urban Areas of Islamabad

Abstract

Rational Method is generally used for estimation of peak discharges. Different authors mentioned different catchment area limitations for use of rational method with respect to different locations due to variation in precipitation rate and variation of ground characteristics. Mostly, the researchers stated that the results from this method could be acceptable upto area limitations of 200 acres. However, there is need to evaluate the applicability of rational method with respect to catchment area for Islamabad region to check the application and catchment limitations for the design of storm drainage system in future. Digital Elevation Modeling (DEM) along with other helping tools like Global Mapper, Sam-Sam water harvesting have been used to estimate runoff by rational method. Different points of interest developed with drainage areas of 663.8 acres, 805.4 acres, 1056.9 acres and 1529.0 acres. Then, corresponding value of runoff coefficient “C” is calculated with respect to type of drainage area. Discharge from rational method was estimated for all points of interest. Then, the results were compared with Soil Conservation Service (SCS) Curve Number method applied to same catchments. The results obtained from comparison showed that Rational Method approach could be applicable for larger areas with respect to characteristics of the ground and type of the area i.e. for developed areas. Readers will be able to evaluate the applicability of Rational Method approach after going through this research work.