FLOW CHARACTERISTICS AND SCOUR PATTERN AT VERTICAL HYDRO-SUCTION PIPES

Abstract

Reservoir sedimentation is a global challenge, raising substantial concern about the long-term sustainability of reservoir management and meeting the growing water demands. Reduction in storage due to sedimentation, directly and indirectly, impacts water availability and reliability, especially during drought and flood events. An effective sediment management strategy is the need of the hour. Several investigators have studied the hydro-suction sediment removal system and found it effective and efficient. Brahme and Herbich (1986) studied the flow pattern and sediment movement below the suction pipe. Rehbinder (1976), Brahme and Herbich (1986), Hotchkiss and Huang (1995), Yang et al. (2020), and Asiaban et al. (2017) have analyzed the sediment lift-off during hydro-suction. The scour profile evolution was studied by Ullah et al. (2005), Su Chin et al. (2010), and Ke et al. (2016). A few investigators have also proposed empirical equations to determine scour profile (Ullah et al., 2005; Sadatomi et al., 2015) and critical suction inlet height (Brahme and Herbich, 1986; Ullah et al., 2005; Ke et al., 2016). Ullah et al. (2005), Ke et al. (2016), and Asiaban et al. (2017) have investigated the temporal evaluation of sediment concentration during hydro-suction. The literature review highlights the complexity of the hydro-suction process and the multifaceted interactions between different variables. This complexity underscores the need for an extensive investigation to bridge the identified research gaps. The research objective for the current study was planned based on the research gaps identified from the critical literature analysis. The current study delves into the intricacies of hydro-suction as a method for removing deposited sediment from water bodies. Hydro-suction involves the use of a suction pipe strategically positioned vertically relative to the sediment bed, and sediment removal is accomplished through suction. The objectives of the current study were to investigate sediment movement, flow profile below suction, temporal evolution of maximum scour depth and scoured sediment volume, critical suction velocity, scour profile evolution, and scoured sediment volume during hydro-suction. The central focus of this study was to experimentally examine the hydro-suction removal of cohesionless bed material from reservoirs and to develop empirical relations using the dataset. A comprehensive dataset of 252 records for scour profile and 328 records for critical suction velocity were meticulously collected from experiments encompassing various combinations of governing parameters. The study starts with the analysis of the flow profile below the suction pipe under various hydraulic conditions. Experimental and numerical analyses were employed to study the flow characteristics beneath the suction pipe under two conditions – sediment bed surface is at infinite (unbound condition), and sediment bed surface is near the mouth of the suction pipe (bound condition). Ansys Fluent computational software was used for numerical investigation, and the Acoustic Dolpper velocimeter was used for experimental study. The Unbound and bound conditions exhibited distinct flow characteristics beneath the suction pipe, revealing unique flow patterns. The experimental results for resultant velocities were found to be consistent with the numerical results. It was noted that the resultant velocity is maximum along the centerline of the suction pipe, decreasing while going below. The velocity vector lines along the centerline, having maximum velocity, follow a straight path penetrating to a deeper depth, responsible for the scour of particles just below the suction pipe. The other velocity vector lines follow a radial path, responsible for the erosion of particles for the periphery of the scour hole. The resultant velocity at any depth below is maximum at the centerline and decreases while going away from the centerline along the radial direction. The effects of parameters such as suction pipe diameter, suction inlet height, and suction discharge on the flow pattern were studied. Based on these observations, further experimentations on scour profile and critical suction velocity were planned. Empirical relations were developed using the observed flow profile to compute the resultant centerline velocity and flow profile. Notably, the proposed relation displayed a ±15% error margin to predict the values. The study on the scour profile revealed that sediment removal begins as a result of lift forces created due to suction and continues due to changes in the flow field that induce sediment resuspension. However, the suction process ceases when the suction inlet reaches a specific depth called as critical suction inlet depth. Within this critical zone, sediment removal commences, resulting in a scour hole on the sediment bed surface. During this process, sediments just below the suction pipe are removed first, resulting in scour hole; after this, sediments from the sides of the scour hole are hydrodynamically drawn towards the center, forming a small hump at the center and eventually getting removed depending upon the flow strength. Importantly, for densimetric Froude numbers equal to or less than 5.8, a central hump within the scour hole was observed. Above this densimetric Froude number, the central hump was washed off. At the equilibrium, sediment removal ceases. The results showed that an equilibrium scour profile is characterized by a symmetrical configuration resembling a semi-ellipsoidal shape. The investigation revealed that optimal sediment removal efficiency is achieved when the suction pipe is positioned just above the sediment bed and when the highest densimetric Froude number is maintained. The removal rate of scoured sediment was most pronounced in the initial few minutes, followed by a swift reduction, a subsequent smaller peak, and a gradual decline to zero at equilibrium. The thesis also proposed empirical equations for estimating maximum scour depth, scour radius, and the scour profile at equilibrium. These equations demonstrate predictive capabilities within an impressive ±10% error margin. An empirical relation for the scoured sediment volume was also proposed. A significant aspect of this research centers on determining. The study also assessed the critical suction velocity required to lift sediment off the bed and the influence of governing parameters. The results underscored the significant impact of these parameters on the critical suction velocity, with suction inlet height being the most influential factor. The interdependency among these parameters affecting critical suction velocity was examined statistically, leading to the development of an empirical equation for the computation of critical suction velocity having a ±10% error margin, reflecting a satisfactory result. The findings of this research not only contribute to the scientific knowledge of sediment removal but also offer valuable insights for designing efficient suction systems in various sediment-laden water environments. Hydro suction can be used for many other problems regarding sedimentation, such as irrigation canal blockage, waterways clear depth, wastewater sedimentation tank, etc. Nonetheless, it underscores the pressing need to unlock its full potential in addressing the sedimentation problem.