SCOUR IN SEDIMENTARY ROCK UNDER WATER JETS

Abstract

Scour in sedimentary rock due to a vertical jet is a significant phenomenon in hydraulic engineering. Scouring in rock can have serious consequences for the stability of foundations downstream of weirs and dams. This process involves the removal of layers of rock by water jets, which can lead to significant erosion and potential structural failure. Water jets can penetrate the rock bed in structures such as ski-jumps, free overfall spillways, and jet spillways, resulting in a highly turbulent zone. Scour caused by turbulent water jets can impact many hydraulic engineering projects. This study aims to investigate the scouring process and its characteristics in sedimentary rock formations when subjected to vertical water jets. The research focuses on understanding the factors influencing scour development, such as flow velocity, nozzle diameter, nozzle discharge and rock properties. Experimental investigations were conducted using sedimentary rock samples with varying grain sizes (like conglomerate rock, sandstone rock), textures, and porosity. A vertical water jet with the nozzle diameter of 8mm, 20mm and 27.5 mm was directed on the rock surface, and the scour depth were measured over time. The critical stream power, representing the minimum power required to initiate scour, was also calculated for each sample by Annandale (1995) formula. The results revealed that scour characteristics varied among the sedimentary rock samples. Despite similar erodibility indices and critical stream power values calculated using empirical formulas, differences in grain size, texture, and unconfined compressive strength affected the observed scour behaviour. Samples with higher porosity and weaker binding materials displayed more significant scour development, even when the applied stream power was lower than the predicted critical stream power. The findings show the complex nature of scour in sedimentary rock and highlight the importance of considering the inherent variability in rock properties and the influence of geological features. Further research is required to develop more accurate prediction models and design guidelines for scour in sedimentary rock formations, taking into account the specific characteristics of the rock and the hydraulic conditions. This knowledge will contribute to better understanding and managing scour. The experiments were carried out at the Hydraulics Laboratory of the Department of Civil Engineering at the Indian Institute of Technology Roorkee. The experimental setup consisted of a circular steel tank with a diameter of 1.18 m, which served as the testing apparatus. To generate the impinging jet, a nozzle was installed at the end of a circular supply pipe with a diameter of 0.0254 m. The height of the jet above the sediment bed could be adjusted using a specially designed arrangement. The discharge of the jet was accurately measured using a Calibrated Venturi meter that was incorporated into the supply pipe.