عنوان مقاله [English]
The main characteristic of the flow in coarse-bed rivers are the development of an armor layer during low to mean flow and the increase of channel instability and sediment load during flood flows. Hence, the process of flow and sediment transport is different and more complex in coarse-bed rivers than in sand-bed rivers. The main aim of the present study is to evaluate different mode of sediment transport (i.e. bed load, suspended load, and total load) from hydrometric and hydraulic methods, in coarse-bed river reaches. Three river reaches in the Zab transboundary river basin (Drebkeh,Gerjal, and Briso) were selected as representatives of coarse-bed rivers. In each of these reaches, river survey and characteristics of surface and subsurface layers were determined. A sediment transport model (STM) was used to compute sediment load from different relationships. The flow characteristics were determined from the calibrated HEC-RAS flow model, as input to STM model. The sensitivity and reliability of the estimated sediment loads are examined under different flows and physical properties of bed materials. The results indicated that the sediment transport capacity is well predicted when the characteristics of the sub-surface bed layer are introduced. The relationship of Einstein (1950) is the best fitted for the calculation of suspended loads with an average error of -91% in the three river reaches. With the lack of bed-load data, the order of 40% to 80% for the ratio of bed load to total load in coarse-bed rivers is considered. For the prediction of bed load, the methods of Schoklitsch (1934), Einstein (1950), and Parker (1990); and for total sediment load, the relationships of Engelund and Hansen (1967), Karim and Kenedy (1990), and Einstein (1950) are found to be more reliable, respectively.