Seasonal changes in hydraulic flow conditions in overgrown lowland river

Wroclaw University of Environmental and Life Sciences, The Faculty of Environmental Engineering and Geodesy, Institute of Environmental Engineering, ul. Norwida 25, 50-375 Wroclaw, Poland

^* Corresponding author: krzysztof.wolski@upwr.edu.pl

Abstract

The paper presents the results of numerical research on the impact of seasonal changes in vegetation on the hydraulic conditions prevailing in the bed of overgrowing lowland river on the example of Ślęza River (Poland). Hydrological characterization of the analyzed section of the river was made on the basis of data from the years 2006–2017 from the Ślęza water gauge. Based on them, the growing season and characteristic flows for modelling were determined. A two-dimensional hydrodynamic model (2HD) was built in two scenarios corresponding to the beginning and the end of the vegetation period for an about 50 m long section of the river. The bathymetric data of the bed and the plant distribution were measured in the field for both scenarios. The models were calibrated using a coefficient of roughness based on the velocities and surface elevations measured directly in the field. Various characteristic flows were then tested on calibrated and verified models for both scenarios. Changes in hydraulic conditions in both scenarios were compared by means of velocities and shear stress occurring in the riverbed. The obtained results indicate a significant impact of plants on the hydraulic conditions in the riverbed. The existence of plants causes the accumulation of backwater and change of local velocity distributions while maintaining medium and maximum velocities in the riverbed.