Studies on the threshold density of Phragmites australis plant concentration as a factor of hydraulic interactions in the riverbed

The paper features the results of numerical analyses and field survey concerning the density threshold for concentration of Phragmites australis plants in the aspect of its impact on hydraulical flow conditions in the river. Simulations were performed for a typical vegetation concentration mapped from representative natural rivebed of Sleza River (Lower Silesia/Poland). The numerical model was constructed, which was calibrated on the basis of field survey conducted with use of hydrometric current meter and an acoustic Doppler current profiler (ADCP). For given fixed spatial configuration (of the area) of analysed riparian plant community, in numerical model, the community density was altered by i.a increase of stems distribution density. Such boundary measure of plant concentration (density) was sought, that after it is exceeded, the flow through the plant community is negligibly small in comparison to dominant stream circumfluenting the plant,obstacle. The analysis was also conducted on variable and dependent on vegetation density hydraulic interactions between the vegetation zone and water stream in river. Knowledge of boundary density for particular vegetation species in riparian zones can be used in ecological engineering, especially in proecological river regulation. The conservation practices can be performed in the riverbed that allow to keep vegetation communities, interfering only its density and by that there is an opportunity to control the riverbed processes.