Experimental Study of Hydraulic Parameters in Density Current Due to Channel Constriction

Introduction
Density currents are stratified flows caused by density differences between the inflow and the still water. The inflow could be moving under, through, or over the reservoir water depending on the density of each. The stratification is caused by differences in temperature, the presence of dissolved materials, or the presence of suspended solids. If the presence of suspended sediment in the inflowing water is the main or only cause of stratification, the density current is known as turbidity current. When turbidity currents enter a reservoir, they plungebeneath the fresh water traveling down the slope. If the turbidity current reaches the dam, it may be vented through the low-level outlets, thus preventing the deposition of sediment in the reservoir. There are many studies in the literature regarding the transport mechanism and characteristics of density currents
Methodology
The Hydraulic Laboratory of Shiraz University, Iran, was used to conduct density current tests. The intended flume is related to hydraulic single-phase open channels, which is transformed into a density current flume by modifying and adding accessories. Flume length is 8m, width 35cm and height 60cm. A 1000 liter tank containing sedimentary turbidity current was used by a 2-inch outlet pump with a maximum passing discharge of 35 m3/hr. A flow meter was used to control the density current inlet. The flume was filled with water using a pipe connected to a 20000 liter water tank. During the experiment, the inlet valve of this tank is disconnected and a 500 liter regulating water tank connected to the pump is used to control the water table in the flume. The channel has sloping capability. The velocity measured by a 2D electromagnetic flowmeter, a product of UK’s Valeport Company with a precision of ±5 mm/sec. The flowmeter includes a data logger and sensors to transmute measured current velocity and discharge time series, etc. to the computer for further analysis. A number of siphons with a diameter of 5 mm suction tube were used to remove the sediment. The tubes connected to the siphons were located at 10 points along with the direction of current depth, through which the average current concentration was measured. An experiments were performed on 3 model samples. Model 1 is a simple, sloping flume without obstacle. In model 2, the flume has a decreased section and continuous constriction at 4.5 m, which is the passing section of the half of channel. In model 3, the flume has a local constriction in 4.5 m, and the passing current through the sides is 10 cm. All records were conducted at 5 m distance from the channel. The parameters of velocity, concentration, and thickness of density current were measured at the desired position.
Results and Discussion
In the inner region below the maximum velocity, drag on the lower boundary is the main controller parameter, and the proper expression for the velocity profile is a power law distribution as follows in stratified currents. For the whole experiment, the variation of the coefficient n is almost large and varies from 3.027 to 5.33 in different experiments. High variation of this coefficient can be due to the effect of the shear in bed on the velocity profiles, and the coefficient α ranges from 0.13 to 0.84. The value of β coefficient also ranges from 1.185 to 1.758 in all experiments. From the general comparison of all the conditions for the dimensionless velocity profiles in wall region, the best coefficient n is equal to 3.86, which shows a high correlation of 0.878, and for the jet region, the best coefficient α and β are 0.412 and 1.343, with a correlation coefficient of 0.92.
Subsequent density current velocity increases and its concentration decreases by creating a constriction and this is due to the accumulation of current behind the obstacles. The constriction also causes an average increase of current velocity by 2.26 times, and the concentration of accumulated current behind the obstacles increases by 1.45 times. The Richardson criterion and the sediment trap efficiency were used in order to correlate accumulation of sediment with the hydraulic parameters of density current, the Richardson number is reduced with increasing trap efficiency, and the correlation coefficient is well established by 0.83 value in all the test modes. On average, about 29.8% from sediments of density current is accumulated behind obstacles for two models with all different states.
Conclusion
The dimensionless velocity profiles in the upper edge of current show greater dispersion due to the non-permanent behavior of the current in this region. The maximum current velocity in the wall region is greater than the jet region. The best coefficient n is 3.86 for dimensionless profile of velocity in wall region, which shows a high correlation of 0.878; the best coefficients α and β are 0.412 and 1.343, respectively, with a correlation coefficient of 0.92 for the jet region; the best coefficient α is 0.1 for the dimensionless profile of concentration in wall region; and the best coefficients β and γ are 1.15 and 0.8, respectively, with a correlation coefficient of 0.94 for the jet region. Also, the effect of local and continuous constriction showed that the constriction increased the velocity of density current by 2.26 times, and also increased the concentration of current sediments behind obstacles by 1.45 times, and trap efficiency rate of sediments was 29.8%.