Comparing experimental methods and analyzing flood hydrograph in estimating time of concentration, case study: Atashgah Watershed, Ardabil Province

Time parameters are adopted in most hydrological and hydraulic models. The most common known time parameter is time of concentration. The present study aimed to choose the best method for estimation of time of concentration in Atashgah Watershed in Ardebil Province. The values of times of concentration in the present study were obtained by fourteen experimental relations and SCS method in WMS and HEC-Geo-HMS softwares. Mean weight of main channel’s slope and Manning’s roughness coefficient were measured by field operation. In order to evaluate efficiency of the method by using rainfall and hydrometry statistics, three flood events which were equal to rainfalls in terms of time were chosen and time of concentration was determined for these events by graphical method. Estimated and observed times of concentration (2.28 h) were evaluated by relative error percentage and remained mean error. Determination of the relations adopted in the present study indicated that area plays a pivotal role to estimate time of concentration properly. Evaluation of Hata-Vay relations, Kirpich, California, and Yen and Chow hydrographs showed that these methods do not consider area in order to estimate time of concentration; therefore, their estimations are considerably different from observed values. On the contrary, the estimations performed by Passini and Kerby relations were closer to observed values. Results revealed that adoption of more parameters would result in more acceptable results. So, total loss, land use, correction operation, hydrological status and soil hydrological group in addition to parameters as length and slope of the main channel are used to estimate time of concentration in SCS relation. HEC-GeoHMS and WMS models, which are used to estimate time of concentration by SCS method, adopt raster data, which cause higher accuracy compared to traditional methods such as manual method. It is concluded that in Atashgah Watershed, SCS method is suitable for estimation of time of concentration using WMS software with minimum values of remainder mean error values and relative error percentage being 0.05 h and 2.1%, respectively.