Response of Density Currents to Sudden Change of Bed Roughness

Document Type : Research Paper

Abstract

Because of buoyancy effect, the response of density currents to sudden change of bed roughness is different from that at the water flows in open-channels. In this study, hydraulic behavior of sediment-laden and saline density currents on the bed with conic and cylindrical suddenly changing roughness was investigated experimentally. Velocity and concentration profiles were measured for different flow sections and bed effective roughness, and then bed shear stress and drag coefficient were calculated. Results showed that buoyancy had significant effect on the bed shear stress and effective roughness, as with the sudden change in the bed roughness these parameters changed gradually, especially in the presence of lifting phenomenon. According to the results, logarithmic velocity law had the highest accuracy in the intervals from the bed, located at the 90% and 75% of the maximum velocity distance from the bed for saline and sediment-laden density currents, respectively.

Keywords


ترابی­پوده ح، فتحی­مقدم م، قمیشی م و شفاعی ­بجستان م، 1386. بررسی سرعت جبهه و شدت اختلاط جریان­های غلیظ ورودی به یک مقطع واگرا. مجله تحقیقات منابع آب، جلد 3، شماره 1، صفحه. 56-67.
Alavian V, 1986. Behavior of density currents on an incline. J Hydrau Eng ASCE 112(1): 27-42.
Antonia RA and Luxton RE, 1971.The response of a turbulent boundary layer to a step change in surface roughness Part 1. Smooth to rough. J Fluid Mech 48(4): 721-761.
Bhuiyan F, Rajaratnam N and Zhu DZ, 2010.An experimental study of mounds formed by dumping coarse sediment in channel flow. J Hydrau Res IAHR 48(3): 283-291.
Chen X and Chiew YM, 2003. Response of velocity and turbulence to sudden change of bed roughness in open-channel flow. J Hydrau Eng ASCE 129(1): 35-43.
Dallimore JD, Imberger J and Ishikawa T, 2001. Entrainment and turbulence in saline underflow in lake Ogawara. J Hydrau Eng ASCE 127(11): 937-948.
Ead SA and Rajaratnam N, 2002. Hydraulic jumps on corrugated beds. J Hydrau Eng ASCE 128(7): 656-663.
Fang X and Stefan HG, 2000. Dependence of dilution of a plunging discharge over sloping bottom on inflow conditions and bottom friction. J Hydrau Res IAHR 38(1): 15-25.
Islam MA and Imran J, 2010. Vertical structure of continuous release saline and turbidity currents. J Geophysical Res 115: 1-14.
Monaghan JJ, Merieux C, Huppert HE and Mansour J, 2009. Particulate gravity currents along V-shaped valleys. J Fluid Mech 631: 419-440.
Nourmohammadi Z, Afshin H, and Firoozabadi B, 2011.Experimental observation of the flow structure of turbidity currents. J Hydrau Res IAHR 40(2): 168-177.
Ohey CD and Schleiss AJ, 2007. Control of turbidity currents in reservoirs by solid and permeable obstacles. J Hydrau Eng ASCE 133(6): 637-648.
Roscoe R, 1952. The viscosity of suspensions of rigid spheres. Br J Appl Phys 3(8): 267–269.
Schlichting H, 1979. Boundary-Layer Theory. Seventh Edition McGraw-Hill press U.S.A.
Turner JS, 1973. Buoyancy Effects in Fluids. Cambridge University Press U.K.