Summary:

Vegetation growing in the River Bed is strongly influencing flowing condi-tion. Effective flow section is reduced, velocity field is disturbed and flow resistance is increased. Its consequence is change of water level and depth in comparison with flow in river bed without vegetation.
Equations of momentum conservation in river bed with trees based on
a concept of whirly viscosity proposed by Rowiński and Kubrak [2002] are one of mathematical description of vegetation hydraulic influence. Above equations were solved using implicit and explicit differential scheme to determine flow velocities distribution in vertical profile. Accuracy of proposed algorithms was tested by comparing numerical solutions with detailed analytical solution obtaining very good conformity. Velocity distribution calculated on the base of the model was used to determine volume of the flow.
To evaluate possibilities of using developed model for calculation concern-ing small rivers with vegetation, some filed investigation on one section of Ślęganina river were carried out. Despite of geodetic measurements, some profiles of local flow velocities for vegetated and free of plants sections were made. Field measurements results were used for verification of mathematical model based on equations of momentum conservation.