The Vegetation Influence on the Hydraulic Characteristics in a Lowland River – A Case Study of the Uppear Biebrza River

Hydraulic and hydrodynamic models that allow to compute and describe the river flow are frequently used as a tool supporting decision making process in environmental management, especially in the valuable natural areas. Preparation and setup of such models requires to identify friction coefficients and active cross-section flow part that determine discharge conditions. In the case of small, low-dynamic lowland rivers, friction factors are determined by the macrophyte development. Macrophytes may root in the bottom of the river and in its banks, but the submersed parts of the vegetation influences the whole cross-section of the river at whole range of depths. In our study we analysed the results of hydrometric measurements done in the Upper Biebrza River (NE Poland; Catchment area 14.12 km², length of the river from the spring to the downstream-most profile is 10.8 km). It is a meandering stretch of the river. River width varies from 3 up to 6 m, depths vary from 0.7 up to 1.5 m. Water table slope is low and varies from 0.00016 to 0.00024. In the growing season (May-October) the river channel overgrows with macrophytes (e.g., Berula erecta, Potamogeton sp., Saggitaria saggitifolia). Species and density of macrophytes were sampled throughout the seasons along all of the stretches where flow velocity measurements were done, so the seasonal variability of the vegetation could have been recorded and related to the measured variability of flow velocities. Automatic water level and water temperature sensors were used do provide additional data. The measurements were done in selected cross-sections in seasons with macrophytes fully developed (June-August) and without macrophytes (November). We measured flow velocities with electromagnetic current meter and measurements were done every 0.1 m in the verticals located 0.25 m from each other. Flow velocities did not exceed 0.15 m/s.

On the basis of these measurements we managed to determine the cross-section active flow areas and Manning’s friction factors. Results of the measurements were presented as interpolated fields of flow velocity isolines. Developed maps allowed to delineate active and inactive fields of river flow. Calculated Manning’s coefficients, varied in the range of 0.082 – 0.336 [m^-1/3×s]. Conducted measurements of water levels variability and parameters of macrophytes’ biomass variability allowed to derive correlations between environmental variables and river flow conditions. Results of the study provide insights for macrophyte management in the river.