10.6. ROUGHENING MEASURES FOR ENERGY DISSIPATION Hydraulic impact is the best means of energy dissipation in most hydraulic structures including canal falls. Of the three possible types of impact cisterns, viz., vertical-impact, horizontal-impact, and inclined-impact cisterns, the vertical-impact cistern is the most effective while the inclined-impact cistern is the least effective. However, even in the case of an efficient vertical-impact cistern, the high turbulence persists downstream of the cistern and means for dissipating the residual energy are essential. In case of cisterns with no hydraulic impact, the roughening devices (provided on the cistern floor) are the only means to dissipate the surplus kinetic energy. Artificial roughness increases the actual wetted area which, in turn, increases the boundary friction. Besides, if correctly shaped and placed, the roughness increases the internal friction by increasing the interaction between high speed layers of the stream.
Provision of grids in a canal is impossible because of the presence of debris in the stream. As such, roughening has to be in the form of projections from the bed and sides of the channels. Projections are the most effective means of energy dissipation. The following roughening devices are generally used.
