Doctoral dissertation considers velocity field of water flow at sharp-crested rectangular side weirs and their discharge coefficients for subcritical flow. This work focuses on experimental results, obtained at various physical hydraulic models. A visualization method is employed, which - in contrast to more common approaches - allows non-invasive quantification of velocity fields. These are measured in separate horizontal planes of observed flow. Determination of vector field of velocities is based on the analysis of films displaying the movement of inserted pollutant, e.i. particles, dye or electrolysis-generated hydrogen bubbles. The method allows numerical calculation of components of velocity vectors with great spatial and time resolution. Results of measurements provide a new, more detailed insight into complexity of water flow at side weir, and - in contrast to one of the energy-approach assumptions - indicate the distinctively non-uniform distribution of considered velocities along the flow. Results are compared with published studies. As an additional contribution to science a new, phenomenological equation for a side weir discharge coefficient has been formulated on the basis of dimensional analysis and a power-law formulation. This equation is simple to use and is valid for fairly wide range of dimensionless parameters that give main geometrical and hydraulic characteristics of flow over side weirs.