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Showing 2 results for Time of Concentration
Investigating the Performance of SewerGEMS Software in Simulating Urban Runoff for Small Basins (Case Study: Minab City)
M. Ranjbari Hajiabadi, J. Abedi Koupai, M.m. Matinzadeh,
Volume 28, Issue 4 (12-2024)
Abstract
Urban runoff is a serious issue due to urbanization and climate change. Therefore, paying attention to rainfall-runoff simulation models is important to manage and reduce adverse consequences. In this research, the performance of the SewerGEMS software was examined by studying different modes based on the number and area of sub-basins. Two modes, consisting of nine and seventeen sub-basins, were evaluated with varying durations of rainfall of 6 and 12 hours. Additionally, the performance of three methods for calculating concentration time (Kerpich, Brnsby-Williams, Carter) was compared to simulate flood hydrographs in Minab City. The results showed that the total volume of produced runoff in the nine sub-basins was 4% higher than in the seventeen sub-basins. The maximum runoff peak flow in the nine sub-basins was also 20% higher than in the seventeen sub-basins. Furthermore, the Brnsby-Williams method exhibited the least software continuity error among the three calculation methods for concentration time. On the other hand, the Carter method had the highest continuity error. The concentration time calculated by this method in some sub-basins exceeded the 6-hour duration of rain. A t-test was performed to compare the peak discharge data obtained from the Kerpich and Barnesby-Williams methods. The results indicated a significant difference between the data from the two methods at a 95% confidence level (p<0.05). Considering that the Kerpich method is suitable for calculating concentration time in small basins, it was used to compare the nine and seventeen sub-basins. Based on the findings, it was observed that merging the sub-basins and reducing their number from seventeen to nine resulted in an increase in the total volume of produced runoff from approximately 123,839 cubic meters to 128,446 cubic meters, as well as an increase in the maximum peak flow of runoff from about 2.400 m3/s to 2.884 m3/s. This demonstrates an increase in both the total volume and maximum peak discharge of the runoff.

Comparison and Analysis of Empirical Equations for Estimating Time of Concentration (A Case Study in Khuzestan Province)
M. Tahvilian, S. Eslamian, A.r. Gohari, M. Jamali,
Volume 29, Issue 3 (10-2025)
Abstract
Time of concentration (Tc) is one of the key parameters in hydrological studies, playing a critical role in flood control structure design, runoff simulation, and water resource management. This study evaluates the performance of seven empirical equations—Bransby-Williams, California, Giandotti, Kirpich, Pilgrim, Rational Hydrograph (SCS), and Carter—in estimating Tc across 35 sub-watersheds in Khuzestan Province, Iran. To assess the accuracy, six sub-watersheds with reliable rainfall-runoff data were selected, and observational Tc values were calculated. The estimated results from the empirical formulas were then compared with observed data using statistical indices such as RMSE, ME, and the Nash–Sutcliffe Efficiency (NSE). The findings revealed that the Kirpich equation provided the most accurate and reliable estimates, with RMSE = 2 hours, ME = 0.44 hours, and NSE = 0.91. Subsequently, all seven models were applied to estimate Tc for the remaining sub-watersheds. Finally, a concentration time zoning map was generated, which can serve as a practical tool for hydraulic design, flood risk analysis, and optimal water resource planning in Khuzestan Province.

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