JWSS - Journal of Water and Soil Science

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  Continuous measurement of river discharge is a hard and expensive task in hydrology. To overcome this problem, the stage readings at hydrometric gauges are permanently taken and the discharge of any time at which the actual discharge is unavailable will be estimated through a relationship between discharge and stage. To study the stage-discharge relations and the capability of long-term data in establishing a permanent stage-discharge relationship, and also to determine the best time to measure the discharge of rivers, a study was conducted at the hydrometric station of the Zayandehrud regulatory dam using data from 1990 to 2003. The data were analyzed using simple regression analysis, the percentage of relative error and factor analysis. The results indicated that the best model to show the stage-discharge relation at the studied station is a power function model. Moreover, the model used for every year can only be used for that year. The results also showed that the most suitable times for the measurement of discharge are July, December and March.

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Direct measurement of discharge in rivers is time-consuming and costly, and sometimes, impossible under flood conditions because of the high speed of water, its transitory nature, and the existence of different floaters along the water. Therefore, the discharge-stage relation, known as Discharge Rating Curve is used. Moreover, to design hydraulic constructions, the maximum flood discharge and its maximum height are required. Therefore, to calculate the flood discharges, one should extend the discharge rating curve by using appropriate methods. In this study, in order to determine the best method for the extension of discharge-stage curve, and to estimate the corresponding discharge with high stages, the logarithmic method, the Manning method, the Chezy method, and the Area-Velocity method in 13 hydrometric stations at the Karkheh watershed in Lorestan province were compared. Data measured at each station were gathered for a ten-year statistical period. Results of calculating the Root Mean Square Error (RMSE) and the Mean Bias Error (MBE) for each method showed that the logarithmic method was more accurate than other methods, and it was more appropriate for the extension of the curve at the low average discharge stations. The Area-Velocity method, after the logarithmic method, especially at the stations with higher average discharge showed good results. The Manning and Chezy methods showed the least accuracy.

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Stage – discharge relationship is mainly developed from measured data in any hydrometry station. Measured data usually obtain in low to medium flow discharge, because in most cases it is very difficult to measure the flow discharge during flood. Therefore, the stage–discharge is extrapolated beyond the measured data to compute the flood which may estimate low or higher value. This is because during the high flow, the bed form is developed which causes the flow resistance to change. In order to establish a better stage – discharge relation, it is important to apply methods which consider the bed form resistance. In this study an attempt has been made to determine the best method for developing such relationship. To reach the goal, the required data such as river cross section, discharge and related stage and bed material gradation from Ghorbaghestan hydrometry station were measured for two years. Then a computer program was developed. Using this program and applying the measured data, the stage – discharge relationships were computed by five different methods. From the statistical comparison of the results of these methods with measured data, it was found that Shen, Brownlie, Engelund and White’s method overestimate the flow discharge. The best method was found to be the Einstein – Barbarossa’s method that provided the minimum absolute mean errors 0.31 and 1.468 m3/s and minimum root mean square error 0.112 and 0.466 m3/s for the two study years, respectively