Influence of the Length of River Bed Section and the Boundary Condition on the Accuracy of the Calculated Water Surface Level for Designing Structure in the River Bed

When designing structures in the watercourse, it is necessary to determine the design water levels in the closer or wider object zone. The river flow is a complex phenomenon that depends on the river bed characteristics that vary over time (hydrological, meteorological, geological, psamological, hydraulic, morphological and others). In order to calculate the relevant hydraulic values (water surface level, depth and velocity), it is necessary to prepare adequate basis and create a representative mathematical model. The model reliability is directly affected by the length of computational section and posed boundary conditions. On a short computational section boundary condition has dominant impact on calculated hydraulic values, while on a too long section the cost of basis data preparation get unnecessary increased. This paper presents hydraulic analysis of the South Morava River in the Grdelica Gorge in Southeastern Serbia, with the aim of the Graovo bridge protection against flood flows. The analyzed river section has expressed river bed characteristics variability with a particularly large narrowing downstream of the bridge. Therefore, it is necessary to set the calculation section and the boundary condition correctly, in order to obtain unbiased results on the model in the area of river bed training works. HEC – RAS software was used for modeling and analysis.