A Comparison of Methods of Estimation of Suspended Sediment in Rivers

There presently exists a great need for information in regard to an understanding of why and the extent by which sediments are carried in water through rivers. This information is indispensable in an evaluation of: rate and pattern of erosion; physical and chemical weathering; sediment transport from the higher to lower slopes in a watershed; water pollution (particularly non local pollutions); sediment balance determination, and finaly proper management of catchment basins. Sediment discharge is many times a good criterion for evaluation ofbasin's yearly erosion.
Numerous methods have been presented for evaluation of suspended sediments but the results widely differ from each other. The most important impediment, however in all these methods of evaluation is a lack of dependable observational data. Availability of rather consolidated and consistent data in the hydrometric station of Gazogly located on the upper elevations of voshmgir Dam, .Gorganrud catchment basin provided the means to determine the accuracy and precision ofthe evaluation methods in suspended sediment surveys. By use of different ways of "systematic, random-systematic and plain
systematic" along with varied yearly frequencies (12, 26, 25, 104), and by taking into account flow classification, sampling was made from a population ofobservational data.
Suspended sediment was then determined for anyone of the samples by 21 different methods (different.with regard to rating curve and lor discharge). Finally, after calssification of the methods and by use of the two parameters of standard deviation of errors and evaluated error relative to consolidated observational data, the diffeent methods were compared. Results indicated that the method of "a combination of average daily discharge with the rated curve of calssess, (group's) median" was of more accuracy and precision, and therefore selected as the most proper method. Also in this method of sediment evaluation, randomized systematic sampling responded in a more appropriate way than the others. Altogether, the method of sampling from classified flow together with additional readings of high and medium flows reduces the error to and extent of 50% as compared to sampling method devoid of grouping (classification).