Recent Trends in the Design of Auxiliary/Additional Spillways for Flood Management—Some Hydraulic Considerations

Auxiliary spillways are generally provided to supplement the main or service spillways for augmenting the discharging capacity and will be operated when floods in excess of the capacity of the service spillway are to be encountered. Though they serve as secondary spillways, the functions of auxiliary spillways extend to flushing of floating debris, floating ice formations from the reservoir, especially in Himalayan regions where ice formation is expected due to climatic conditions. The energy dissipators for these spillways are different from that of the main spillways and are governed by site specific conditions. Overtopping has been the most common cause of failure of embankment dams due to inadequate spillway capacity in most of the cases. Failure of dams thus becomes a major concern in providing safety assurance and rehabilitation. It is of utmost important to estimate the design outflow flood/PMF accurately with the latest available methods/techniques to design the spillways for flood management. Providing additional spillway/auxiliary spillway with proper design along with the service spillway at the conceptual stage of the project itself perhaps will avoid the problem of reassessing the spillway capacity of existing dams and providing alternative solutions to manage the exceptional flood events as is being done in recent times for existing projects. Provision of additional spillway/auxiliary spillway needs hydraulic model testing for adopting the most efficient and techno-economically feasible design. This paper describes the hydraulic model studies conducted at CWPRS, Pune, for few hydro power projects wherein various hydraulic aspects considered for evolving safe and efficient design of auxiliary/additional spillways that forms part of the main spillways and efficacy of the energy dissipators of these spillways for managing the flood.