Experimental Study On The Flow And Deposition Of Turbidity Currents With Multiple Slope Transitions

As the an important sediment transport process on our planet,turbidity currents are widely found in nature.Channel slope bed determines the driving force of a turbidity current.At present,there are not many experimental study on the flow and deposition of turbidity currents with multiple slope transitions.The slope of the ocean shelf varies in a wide range,so it is necessary to explore the flow and deposition of turbidity currents with the multiple slope transitions,which has important theoretical significance and practical application value.We use a variable-slope gravity flow tank device designed with the support of the National Laboratory for Natural Science Foundation to study the turbidity currents with steady sustained inflow.The turbidity currents experiments of three slope beds of3°,6°,and 12°were carried out in a variable-slope gravity flow flume equipment.The3°and 6°turbidity currents experiments in this study were carried out three times,and the 12°turbidity currents experiment was carried out twice.The inflow volume concentration C₀=0.006 in the first and second turbidity currents experiments at 3°,6°.The inflow volume concentration C₀=0.006 in the first turbidity currents experiment at 12°.In the third turbidity currents experiment at 3°,6°and 12°,the inflow volume concentration C₀=0.02.In the first experiment of each slope,the acoustic doppler flowmeter?ADV?was used to measure the velocity of each cross-sectional of the turbidity flow.The concentration of each cross-sectional of the turbidity flow was measured by a siphon and other instruments.The turbidity depth average,specific energy,density Frederian number,and bulk Richardson number were calculated according to the measured concentration and velocity.The particle size distribution of each cross-sectional in the vertical direction was measured using a particle size analyzer?LS-C-IIA?.After 3°,6°second,third and 12°second turbidity currents experiments,the bottom shape and thickness of each turbidity flow were observed and measured.The particle size distribution of the peaks and troughs of the sediments were measured by a particle analyzer,the following conclusion are reached.?1?Under the same inflow and volume concentration,the depth average velocity and specific energy increase with the increase of slope,and the depth average concentration decreases with the increase of slope.The maximum speed at 2 m of each slope increases with the increase of slope.?2?The turbidity currents experiment of each slope has subaqueous water jump phenomenon at the slope transition.The comparative analysis shows that the subaqueous water jump intensity with the increase of the slope.?3?At the same slope,inflow and volume concentrations,the average thickness of the slope deposit is less than the average thickness of the subsurface sediment.Under the same inflow and volume concentrations,the average thickness of the sediment on the slope decreases with the increase of the slope.In the turbidity currents experiment with volume concentration of C₀=0.006,the average thickness of the sediments on each slope were 1.5 cm,1 cm and 0.5 cm,and the average thickness of the sediments under the slope were 2 cm,2.5 cm and 1.5 cm.When the volume concentrations is C₀=0.02,the average thickness of the sediment on on each slope were 2.5 cm,1.5 cm,1 cm,and the average thickness of the sediment below the slope were 4 cm,3 cm and 2 cm.?4?As the slope increases,the wave height of the sediment on the slope decreases and the wavelength increases.In the low-slope 3°turbidity currents experiment,the wave height and wavelength of the sediment on the slope are in the range of 1² cm and 8⁹ cm.The wave height and wavelength of the sediment on the slope of 6°mid-slope are in the range of 0.3¹ cm and 9¹0 cm.The wave height and wavelength of the sediment on the slope of 12°high slope are in the range of 0.2⁰.5 cm and 11¹2 cm.?5?At the same slope,inflow and volume concentrations,the mean peak size of sediment peaks in each cross-sectional decreases along the path.In the low slope 3°turbidity experiment,the average peak particle size decreased from 21?m to 19?m.The mid-slope 6°is reduced from 28?m to 24?m.The high slope of 12°is reduced from 33?m to 28?m.?6?The average particle size of the wave peaks on the slope at the same position gradually increases with the increase of the slope.In the low-slope 3°turbidity currents experiment,the average peak particle size at the 2 m,3 m,and 4 m cross-sectional are 21?m,20?m,and 19?m.In the mid-slope 6°turbidity currents experiment,the average peak size of the peaks at 2 m,3 m,and 4 m cross-sectional are 28?m,25?m,and 24?m.The average peak particle size of the high-slope 12°turbidity currents experiment at the 2 m,3 m,and 4 m cross-sectional are 33?m,30?m,and 28?m.?7?In the high gradient?12°?turbidity currents experiment,the particle size of the lower layer of the sediment at the slope transition is larger than that of the upper layer.?8?The specific energy of each slope decreases only as the average thickness increases at the slope transition.The change in the specific energy seems to increase with the increase of the slope.This provided a first experimental verification on specific energy of turbidity currents across different slope transitions revealed recently by numerical simulations.The results of these experimental studies provide an important data set for the exploration of oil and gas resource based on sedimentary topography,seismic scale,drill core datga etc,and the protection of seabed disasters as well as the study of the migration and transformation of pollutants.