Influence of Aquatic Vegetation on Dispersive Parameters as a Part of Hydrodynamic Conditions in Natural Streams

Abstract

Submerged and emergent aquatic vegetation is a natural and organic component of natural rivers and streams. It plays an important role in all physical, chemical and biological processes in the stream biocoenosis. This type of vegetation has also a non-negligible impact on flow conditions. It influences the discharge, hydraulic roughness, velocity as well as other hydraulic parameters. Important part of the river hydrodynamic processes are also dispersive processes and its parameters, which defines the speed and intensity of the dispersive processes in the natural stream. This paper analyses these aspects of the stream hydrodynamics, which are influenced by aquatic vegetation and analyses the influence of the submerged and emerged vegetation on mixing processes in a river. Presented results are findings of hydrometric measurements and tracer experiments at the Šúrsky kanál stream, located in south-west part of Slovakia. The Šúrsky kanál stream is a typical lowland stream, where significant changes in the vegetation are present during different periods of a year. The hydrometric and tracer experiments were performed on 1700 m long straight reach of the stream with a relatively prismatic cross-section profile during the growing as well as non- growing season of a year. The results show, that the level of vegetative growth has a significant influence on the hydrodynamic parameters of the stream, as well as on the dispersive process. The dispersive process is influenced not only by the velocity and concentration gradients, but also by the fact that the vegetation forms in the stream so-called dead zones. Such dead zones modify the velocity profiles of a stream and affect dispersive mass transport within the stream by collecting and separating parts of the tracer from the main current. Subsequently, the tracer is slowly released and incorporated back to the main current in a stream. This process deforms the shape of the tracer concentration distribution in time. All these facts were confirmed by the experiments results described in this paper, which contains also the analysis of the dead zones effect on the dispersive process.