تعيين روابط تجربي براي برآورد زمان تمركز حوضه هاي آبخيز كوهستاني

Identifying emprical equations for time of concentration in mountainous watersheds

Time of concentration is the most important factor in choosing design rainfall of watersheds. There are several equations presented by different researchers to estimate time of concentration which are usually limited to a particular study area and it is not reliable to use them in other areas with different climatic, topographic and morphologic conditions. Due to the importance of time of concentration in estimating flood and the role of floods in damaging water structures, residential areas, etc. It is an important task to estimate it in irrigation and sewerage systems, dam construction and urban hydrology projects. In this study, 11 mountainous watersheds in Tehran, Mazandaran and Isfahan with 16.1 to 712 Km2 area and main channel slope of 0.77 to 13.8% provinces, equipped with rainfall and runoff estimated using flood hydrograph and rainfall hyetograph and compared with 14 empirical equations and the calibrated equations were derived using SAS software. The study showed that all of these equations donʹt have sufficient accuracy for the selected watersheds except for Pilgrim-McDermott equation. Thus, to get the required accuracy, eight effective variables on time of concentration were applied for final equation and 248 new equations were derived combining these factors and omitting variables of the watershed with minimum slope. From all this, eight equations which consist of two equations with one variable, three with two variables and three with three variables with sufficient accuracy where derived among which was the Pilgrim-McDermott equation. At least, the equations were selected to be the best time of concentration equations for mountainous watersheds.