Hydraulic model studies for optimising the design of two tier spillway – a case study

Abstract

Orifice spillways are provided in Run-of-the-river diversion dams where management of sediment along with flood is the main issue. However, accommodating the spillway in a narrow gorge with very high discharges is another aspect. Multistage spillways are provided in such cases to overcome the space constraint. Such designs are hydraulically complex as there is interaction of flow at the junction of exit of the lower and upper spillways. Design of these structures cannot be standardised and depend on site-specific conditions. Extensive physical model studies are required for optimising the design. Siang Lower H. E. Project is a run-of-the-river scheme located on Siang River in East Siang district of Arunachal Pradesh. The spillway arrangement envisaged is unique, consists of two-tier spillway comprising overflow ogee spillway on the top tier and sluice spillway in bottom tier. A common energy dissipator in the form of stilling basin has been provided for both overflow and sluice spillways due to high tail water. The present paper describes the hydraulic model studies for Lower Siang H.E. Project, Arunachal Pradesh carried out on 1:55 scale 2-D sectional model for optimising the design of two tier spillway and energy dissipater arrangement.