Operation of dams, especially small hydro-electric power station entails many technical problems. One of them is the deposition of material carried by the water on the grating. The task is purpose of the grating is the water pump or turbine against solid contaminants flowing with the water. When selecting the grating it is important to determine the hydraulic losses, which depend inter alia on the width, shape and spacing of lattice bars, and the water flow rate. The total amount of loss can be defined taking into account: the input losses, losses on trellises, the losses caused by the presence of side cavities. These losses can increase when organic material carried by the flow of water accumulates on the trellises and increases the speed and flow resistance. The paper analyzes the hydraulic losses and demonstrates the impact on trellises of the deposited organic material.

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