Laboratory investigation of bed roughness effects on velocity distribution in the body of gravity currents

Gravity currents form when a heavier fluid propagates into a lighter one in a predominantly horizontal direction. They are frequently encountered both in the environment and engineering applications. Gravity currents can be driven by density differences of the fluids involved, or by differential particle loading. In natural or human-made aquatic settings such as lakes, oceans or reservoirs, there is a myriad of possible contributors to these density differences including temperature differences, salinity contrasts, suspended material, both organic and inorganic, as well as combinations of these mechanisms. In water resources management, to prevent sedimentation and drain sediment from dam reservoirs, in most cases, is tried to remove fine sediments by using of hydrodynamic forces. One of these methods is removing sediment by the gravity current dynamics. The gravity currents are the most important effective events on reservoir's sedimentary processes (transmission, distribution and deposition of particles). Therefore, identifying the factors affecting on this type of flow is very important. Both bed slope and surface roughness are the most important parameters on gravity current dynamics and have a considerable effect on velocity distribution of the body.
This study investigates the effect of bed roughness on the gravity currents characteristics and obtaining relations between effective parameter on gravity currents and finding their influences in body velocity distribution on rough and slope beds. In this study to determine the bed roughness effect on the velocity distribution, a series of gravity currents experiments was performed on beds with the rough natural element sizes of 4, 8, 12 and 15 mm and slopes from 0 to 2.2%. The experiments of saline gravity currents were carried out in three inflow discharges with amounts of 0.7, 1 and 1.3 (lit/s) and with three concentrations of 10, 16 and 20 (gr/lit). All Experiments was done on a Plexiglas flume with the length of 10 meters and width of 35 cm and in hydraulic models laboratory in Shahid Chamran university of Ahvaz (Iran). In all experiment, the velocity profile in the body of gravity current was acquired by an Ultrasonic Doppler Velocity meter device.
The results of this study can be summarized as an investigation on variations of velocity profile characteristics and analysis of velocity distribution in tow (wall and jet) regions of gravity current's body. Determination of general equations for velocity distribution in gravity current's body on rough beds showed that by increasing bed roughness the height of maximum velocity rose up to a higher position and the coefficient of velocity distribution equation changed consequently. Based on results of this study, the relationship between relative height of maximum velocity location in the velocity profile ( ), versus relative roughness ( ), was rational type; in which, hm is the height of maximum velocity location, ks is the bed roughness and h is the thickness of gravity current. The relative height of maximum velocity location was equal to 0.3 when was near to zero and by increasing , the value of converged to the constant of 0.65. The investigation on acquired velocity profile showed that the bed roughness was one of the most important factors on velocity distribution. The similarity of the velocity distribution, in both jet and wall flow regions, by Semi-Gaussian and power distribution equations showed that the coefficient of velocity distribution was varied by increasing the bed roughness.
For the velocity profile in jet region, the coefficients of Semi-Gaussian distribution (α and β) were varied from 1.2 to 2 and 2.2 to 1.8, respectively, by increasing the bed roughness. Also, for the velocity profile in wall region, the coefficient of Power distribution (n) was changed from 0.2 to 0.6.Gravity currents form when a heavier fluid propagates into a lighter one in a predominantly horizontal direction. They are frequently encountered both in the environment and engineering applications. Gravity currents can be driven by density differences of the fluids involved, or by differential particle loading. In natural or human-made aquatic settings such as lakes, oceans or reservoirs, there is a myriad of possible contributors to these density differences including temperature differences, salinity contrasts, suspended material, both organic and inorganic, as well as combinations of these mechanisms. In water resources management, to prevent sedimentation and drain sediment from dam reservoirs, in most cases, is tried to remove fine sediments by using of hydrodynamic forces. One of these methods is removing sediment by the gravity current dynamics. The gravity currents are the most important effective events on reservoir's sedimentary processes (transmission, distribution and deposition of particles). Therefore, identifying the factors affecting on this type of flow is very important. Both bed slope and surface roughness are the most important parameters on gravity current dynamics and have a considerable effect on velocity distribution of the body.
This study investigates the effect of bed roughness on the gravity currents characteristics and obtaining relations between effective parameter on gravity currents and finding their influences in body velocity distribution on rough and slope beds. In this study to determine the bed roughness effect on the velocity distribution, a series of gravity currents experiments was performed on beds with the rough natural element sizes of 4, 8, 12 and 15 mm and slopes from 0 to 2.2%. The experiments of saline gravity currents were carried out in three inflow discharges with amounts of 0.7, 1 and 1.3 (lit/s) and with three concentrations of 10, 16 and 20 (gr/lit). All Experiments was done on a Plexiglas flume with the length of 10 meters and width of 35 cm and in hydraulic models laboratory in Shahid Chamran university of Ahvaz (Iran). In all experiment, the velocity profile in the body of gravity current was acquired by an Ultrasonic Doppler Velocity meter device.