Active Flow Zones Distribution Along the Upper Vistula River Established with the Use of 1D and 2D Modelling

2D numerical modeling of hydraulic flow conditions was performed and calibrated on the Vistula River sector between Zawichost and Słupia Nadbrzeżna. The results of the modeling were used to calculate the active cross-section flow zone according to the Pasche method. The range of the active flow zone was used in the 1D modeling to compare the results. Values of flow, water velocities, and unit discharge were used to analyze active flow zones and energy exchange between the main river channel and floodplain. As a result, the size of the active zone was compared, and the effectiveness of the method was established by indicating similarities and differences from the 2D model. The study shows that all relations among the three main determining parameters of Pasche’s method were found. Dimension of the active zone, relations between depth in the river channel and on the floodplain, and roughness contrasts appear in all versions. The analyzed river’s part can be as wide as the accessible space between banks and dikes or almost nonexistent caused both by humans and nature. The floodplain can be low and accessible from the river or high and hardly available for the energy exchange. A positive correlation between the calculated and real active zone size is always reached when spatial changes in the floodplain are taken into account along the river. Active zone size can be established using both 1D and 2D models; however, the 1D model in the presence of the connected oxbow lakes cannot guarantee full accuracy.