Fluvial Processes

1. Unit 3 – Gradational Processes Fluvial Processes

2. Introduction • Hydrological Cycle and Routes of Water to the Channel

3. Introduction • Canadian Drainage Basins • Delineates the surface drainage catchment (runoff) areas and groundwater sources (base flow)

4. Introduction • Spatial Distribution of Stream Flow • mean annual flow

5. Introduction • Discharge (Q): volume of water passing a point in the stream per unit time Q = wdv where Q is discharge (m3/s) w is stream width (m) d is stream depth (m) v is stream velocity (m/s)

6. Introduction • Stream Velocity • The ability of a stream to do work is strongly controlled by velocity • Velocity is not uniformly distributed within a channel. In general, velocities are higher where: • channel bed and banks are ‘smooth’ • channel gradients are steep • channel width to depth ratio is low

7. Introduction • At any point in a channel, the velocity will be lowest at the bed and banks (due to friction) and highest just below the surface of the flow. • At the surface, friction with the air tends to reduce velocity slightly

8. Introduction • A narrow deeper channel has a higher velocity compared to a wider shallow channel (at a fixed gradient and roughness) • Within any given channel, the width, depth, gradient and bed materials (roughness) will change, thus we would expect velocity to be highly variable • To understand how this affects sediments we must consider the flow properties

9. Channel Processes • Stream Boundary Conditions • Laminar and Turbulent Flow

10. Channel Processes • Stream Boundary Conditions • Laminar Flow • water molecules move in a series of parallel paths that slide over one another with relatively little mixing • Turbulent Flow • water molecules move along highly variable paths with considerable mixing

11. Channel Processes • As turbulent flow moves over the materials that are exposed on the bed and banks there is shear stress (force) exerted on the particles • It is this force that acts to move the sediments • The velocity at the bed is called the shear or drag velocity and denoted by the symbol u*

12. Channel Processes • As the shear velocity increases the shear stress increases • As the hydraulic radius (or depth) and slope increase the shear stress increases

13. Channel Processes • Sediment Entrainment • What determines when sediments in the bed and banks will become detached and entrained within the flow? • Sediments become mobilized when the forces (mainly shear stress) acting to move the materials exceed the resisting forces • There is a critical shear stress τcrthat isrequired to entrain materials

14. Channel Processes • Sediment Entrainment • What physical properties of the sediments would influence the τcr(critical shear stress) required to mobilize the materials? • Grain Size • Density of Particles • Shape and Packing of Grains

15. Channel Processes • Sediment Entrainment • Thus, we see several variables that influence entrainment, a partial list: • Properties of the flow: shear velocity and shear stress • Properties of the channel: slope, hydraulic radius (depth) • Properties of the sediment: grain size, shape, packing, density

16. Hjulstrom Diagram • Shows the mean flow velocity (at 1 m above the bed) required to mobilize particles of given sizes, assumes a flat uniform bed Entrainment Deposition

17. Channel Processes • Sediment Transport • Suspension • Silts, clays • Saltation • sands • Traction (Rolling, Sliding) • gravels • Solution

18. Channel Processes • Sediment Load • Suspended Load • Silts, clays • Bedload • Materials moving by saltation and traction • Solute Load or Dissolved Load • Materials in solution

19. Channel Processes • Erosion in Bedrock Channels • Corrosion • solution of bed and banks by water • Corrasion (abrasion) • scour of bedrock by materials in the flow • Hydraulic Plucking and Cavitation • fracture of weak rock • shock and fracture of rock in highly turbulent flow