The Soil and Water Assessment Tool (SWAT) was used to model runoff and sediment in the Beheshtabad (3860 km 2 ) and Vanak (3198 km 2 ) watersheds in the northern Karun catchment in central Iran. Model calibration and uncertainty analysis were performed with sequential uncertainty fitting (SUFI-2), which is one of the programs interfaced with SWAT, in the package SWAT-CUP (SWAT Calibration Uncertainty Programs). Two measures were used to assess the goodness of calibration and uncertainty analysis: (a) the percentage of data bracketed by the 95% prediction uncertainty (95PPU) (P factor), and (b) the ratio of average thickness of the 95PPU band to the standard deviation of the corresponding measured variable (D factor). Ideally, the P factor should tend towards 1 with a D factor close to zero. These measures together indicate the strength of the calibration-uncertainty analysis. Runoff and sediment data from four hydrometric stations in each basin were used for calibration and validation. The P factor for Beheshtabad stations ranged from 0.31 to 0.86, while those for Vanak stations were between 0.71 and 0.80. The D factor for Beheshtabad ranged from 0.3 to 1.1, and for Vanak it was 0.77-1.16. These measures indicate a fair model calibration and accounting of uncertainties. The predicted runoff values were quite similar to those for discharge.
Local scour around the obstacles such as bridge pier is the result of complex interaction between turbulent flow and sediment particles at the mobile bed. The entrainment of sediment particles from the bed is stochastic in nature and it is strongly influenced by instantaneous shear stresses of the bursting process. The focus of this study is the investigation of turbulent flow and analysis of contribution of each bursting events to the Reynolds shear stress to find the dominant bursting events and also the flow structure around the circular bridge pier. The velocities around the bridge pier were measured in three dimensions using an Acoustic Doppler Velocimeter. These velocities were measured at different positions around the bridge pier and at different depths. Quadrant analysis is used to recognize the susceptible regions for sediment entrainment and deposition. According to quadrant analysis sedimentation is the dominant effect in the scour hole whereas at higher levels the erosion force becomes more important. In downstream of the pier, sediment particles are put in suspension and transported downstream due to sweep. Our results indicate that the secondary currents are more dominant in downstream of the pier, as compared to the upstream of the pier. Consequently the maximum velocity in downstream of the pier takes place in a location closer to the bed. In upstream of the pier the streamwise component of velocity becomes positive for which the universal log-law turns out to be valid.
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