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.
Highlights
Studies on the impact of aquatic vegetation on the flow conditions in watercourses are extremely complex
The occurring systems of plants are characterized by high species diversity, heterogeneity of occurrence and density, and - what is important for this work – in seasonal variability [3]
The analyses carried out have unequivocally confirmed the impact of plants on seasonal changes in hydraulic conditions in the overgrown lowland river bed
Summary
Studies on the impact of aquatic vegetation on the flow conditions in watercourses are extremely complex. It is influenced by the amount of possibility of interactions between plants, hydrological and fluvial conditions as well as morphology of the riverbed [1, 2]. Seasonal variability of vegetation does not occur in the whole world due to the climatic conditions. It is most visible in temperate zones with distinct seasons of hot and cold year, where the temperature drops below the border to favourable vegetation conditions (depending on plants, 5–15°C). Together with the distinct vegetative and non-vegetative season, the resistance induced by vegetation is significantly different due to the morphological and biomechanical characteristics of plants [4, 5]
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