10.5.4. No-impact Cisterns Hydraulic impact is possible only when a supercritical flow meets a subcritical flow. In low falls and falls with large submergence, hydraulic impact may not be possible. Hence, other means of energy dissipation are adopted. A properly designed baffle wall along with suitable roughening devices are useful means of energy dissipation for such cases. The depth of no-impact cisterns cannot be calculated from theoretical considerations. However, it is useful to depress the cistern floor below the bed of the downstream channel as much as is economically feasible (1). This results in a larger cistern volume and, hence, permits retention of water for a longer period. Besides, it necessitates the construction of an upward slope (at the exit of the cistern) which helps in suppressing large-scale turbulence.
