Experimental investigation on body velocity profile characteristics of turbidity current over a mobile Bed

Considering reservoir sedimentation phenomenon and turbidity current as the major factor of happening problems (such as reducing water storage capacity, water passages and gates clogging, depositing in water conveyance channels, reducing water resources quality and etc.) which cause environmental and financial damages, it is essential to study the various aspects of turbidity currents in creating sustainable management of reservoirs as the main source of water and energy supply. Almost all of the research studies, so far, have been conducted under experimental conditions on rigid beds. In this study, in order to achieve more realistic results regarding to natural conditions in rivers and reservoirs, the experiments were conducted on a mobile bed. Since the body velocity profile of turbidity current has an important effect on generation and destruction of different types of bed forms, characteristics of the body velocity profiles were analyzed on a rigid and erodible bed for different effective hydraulic conditions. In this study, at first according to Special Specifications of turbidity current (very low velocity in small experimental flumes), comprehensive study had been conducted on bed material properties and its required pre-experiments. Finally, the properties of the material used in this study were 450 µm of average diameter (d50), 1070 kg/m3 specific gravity (ρs) and 1.45 of standard deviation (σg). In order to simulate the turbidity current, an experimental model consisting of an inclined flume with length of 780 cm, height of 70 cm and width of 35 cm, was used. Bed material was situated at the bottom space of bed with height of 8 cm and length of 4.5 m. velocity profiles were determined in four cross sections (at the certain distances of 1.5, 2.5, 3.5 and 4.5 m from the gate) using acoustic Doppler velocity profiler (DOP 2000). Overall 48 experiments were conducted which included 36 main experiments on mobile bed changing flow discharge (1, 1.5, 2.5 and 3.5 l/s), bed slope (0, 1.5%, 2.5% and 3.5%) and volumetric concentration of sediment (15%, 20% and 25%) and 12 control experiments on rigid bed at flow discharge of 1.5 l/s.
According to the results, with increasing in the flow concentration, due to increase in flow momentum, in both rigid and mobile bed condition, flow velocity was increased. It is considerable that the percent increase in velocity with change of concentration, was less in mobile than rigid bed condition (about 50%) which could be due to the resistance of the bed material particle and especially bed forms roughness against the flow as long as the bed form is generated. Exactly in experiment condition with bed slope of 2.5 and 3.5%, in which the bed forms were washed out and destructed due to increase in bed shear stress, the increasing trend of flow velocity got improved. Also, in all the experiments, the values of maximum velocity in the mobile bed condition decreased specifically and the percent of decrease increased with increasing in the bed slope. Since the bed material particle are active in mobile bed condition, therefore the bed roughness and resistance increases so that acts as resistant force against the flow and results in decrease in flow velocity (up to about 30%).
Comparison of dimensionless body velocity profiles in all experiments showed that the maximum ratio of z/h (the boundary between clear water and a concentrated flow) was more in rigid bed condition (with average value of 2) than the mobile bed condition (with average value of 1.5). This decrease of z/h ratio in the mobile bed condition (about 25%) was due to decrease in average body velocity profile which caused increase in the elevation corresponding to the same velocity of rigid bed condition. The value of z/h ratio was more in the experiments without bed form which were in accordance with results. In addition, the maximum value of u/U in mobile bed condition was less than rigid bed condition. This confirmed the body velocity decrease in mobile bed condition which was more in the experiments with bed forms. The research data, especially dimensionless velocity profiles in supercritical flows, also were in good agreement with the results. In the present study generation of bed forms leaded to about 40% decrease in z/h ratio and 15% increase in elevation of the maximum of u/U location.
Consequently by the results, erodible bed and especially generation of bed forms leaded to decrease the body velocity of turbidity current up to about 30% and also to decrease the rate of increasing velocity, due to increase of current density up to about 50%. Finally, comparison of dimensionless velocity profiles showed that the relative position of the interface of turbidity current was decreased about 25% on erodible bed and even up to 40%, where the bed forms generated.