The role of the solid-transmitted bed shear stress of mobile granular material on cohesive bed erosion by unidirectional flow

• Main Author: Thompson, Charlotte E. L
• Format: Dissertation
• Language: English
• Published: University of Southampton 2003

Both the fluid- and solid-transmitted stresses to a cohesive bed have been investigated in both lab and field based flumes. The drag coefficient was determined over smooth and artificially roughened beds using a new method developed herein; flow deceleration. This method was compared to many of the existing methods of stress determination, and tested over naturally vegetated beds in the fields. This method was then used as the basis for investigating the solid-transmitted stress to a cohesive bed from mobile shells and sand. The value of C_D as determined by the flow deceleration method was found to converge to the constant value of 3x10⁻³ determined by Sternberg at intermediate velocities, and a reduction occurred at high velocities. Eight ways of determining bed shear stress under smooth bed conditions were compared: the Law of the Wall; the quadratic stress law; the turbulent kinetic energy method; Prandtl's seventh power law; the Reynolds stress calculation; hot film probes; flow deceleration; and numerical simulation. The best correlation was found between Prandtl's seventh power law and flow deceleration, the other methods over predicting the stress by up to a factor of 10. The shear stress over natural beds was found to increase with the presence of vegetation in the form of the seagrasses Cymodocea nodosa and Zostera noltii. The stress was affected by the bending of the seagrass blades under flow velocities exceeding 0.4ms⁻¹ reducing as the blades flattened. Stress was also reduced by an increase in levels of turbidity in the water column. Mobile disarticulated cockle shells moved as bedload, their onset velocity of motion (Ucr) was directly related to the settling rate (Ws) in still water. The fluid-induced stresses were unable to cause any detectable erosion of the clay bed. The addition of even a single shell induced significant erosion rates (Er) as the result of abrasion.