The Himalayas have the largest concentration of glaciers outside the polar region. In the face of global warming, there is clear evidence that Himalayan glaciers have been melting at an unprecedented rate in recent decades. These in turn give rise to an increase in the potential threat of glacial lake outburst floods occurring. Because of fragile geology and steep Slopes, soil erosion rate is high during snowmelt and monsoon. Sudden discharge of large volumes of water with debris from the lakes potentially causes glacial lake outburst floods (GLOFs) in valleys downstream. These in turn give rise to an increase in the potential threat of glacial lake outburst floods occurring.
The rapid altitudinal variations with swift flowing perennial rivers have made Himalayan region a natural haven for large-scale hydro power production. Considering the experience of silting of dams and damages to power plants, the trend has changed to design the Himalayan hydropower projects as run-of-the-river schemes utilizing the stream flow as it comes without any permanent storage. Innovative designs of orifice spillways in the form of low-level sluices in the body of the dam or a spillway with a breast wall have been evolved based on the concept of reservoir flushing with minimal impact to the environment. A number of hydroelectric projects in India, Bhutan and Nepal are being planned in the Himalayan regions. It has become necessary for the project planners and designers to account for the GLOF also along with the Probable Maximum Flood (PMF) due to rainfall for deciding the spillway capacity of projects.
Special considerations are required for design of suitable energy dissipator, since the spillway has to surpass both the flood and the sediment. Ski-jump bucket is found to be the most suitable form of energy dissipator because of its obvious advantage during flushing operation. The sediment passes down the spillway with supercritical flow without deposition and churning in the bucket. Fortunately, steep bed slopes of the rivers result in low tail water depth permitting this type of energy dissipator. If the geological conditions are not favourable, a hydraulic jump stilling basin may have to be adopted. Because of the requirement of passing silt-laden flows, use of energy dissipating appurtenances like chute and baffle blocks is not advisable. Cylindrical end-sills are generally preferred for easy movement of sediment out of the basin. A concrete apron downstream of the end sill is required to protect the spillway against undermining due to scour during transition action from hydraulic jump to flip action and vice versa. Provision of roller bucket is generally avoided as an energy dissipator due to likelihood of abrasion damage of the bucket due to churning of sediment.
The present paper describes various hydraulic design considerations while designing the spillway and energy dissipator for run-of-the river projects in Himalayan region. It is also emphasized that although no design procedures are readily available for such type of spillways, experience from the hydraulic model studies and prototype to evolve suitable layout of spillways and energy dissipators provide useful guidelines.
