Investigation of Hydraulic Stability of Boulder Weir

Abstract

The Himalayan Rivers are steep and are characterized by highly turbulent flow transporting a lot of bedload and boulders during floods. Conventional concrete diversion structures face continuous damage from rolling boulders, necessitating frequent repair. The risk of hydro-geomorphic disaster is increasing in the Himalayan region due to climate change effects. Boulder weirs, constructed from riverine boulders, emerge as a cost-effective and disaster-resilient alternative for small to medium hydropower projects. Boulders, abundant in Himalayan riverbeds, offer superior resistance to abrasion compared to concrete, and in case of damage, swift rehabilitation can be achieved using the riverbed materials. In Nepal, the Khimti-I Hydropower Plant pioneered the boulder weir as a diversion structure, demonstrating over two decades of successful operation with minimal maintenance. Design of the weir, including boulder stability were determined by physical model testing. Many hydropower projects have adopted the boulder weir design, relying on empirical experience from Khimti-I. Guidelines for design are available for rock ripraps using crushed stones and dumped rounded stones, and, packed angular stones in embankment dam overtopping situations. However, boulder weirs are constructed by packing boulders having smoother surface, in a single layer. So, this study employs flume experiments to investigate hydraulic stability criteria for boulder weirs, considering variations in downstream slope ranging from 12.5% to 25% and stone diameters of 30 mm, 35 mm, 40 mm and 50 mm. A relationship between the stone-related Froude number and downstream weir slope was established which can be potentially used to design boulder sizes for a given flood discharge and downstream slope of a weir