FLUVIAL PROCESSES

Unit 3 – Gradational Processes

Introduction Hydrological Cycle and Routes of Water to the

Channel

Introduction Canadian

Drainage BasinsDelineates the

surface drainage catchment (runoff) areas and groundwater sources (base flow)

Introduction Spatial

Distribution of Stream Flowmean annual

flow

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)

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

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

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

Channel Processes Stream Boundary Conditions

Laminar and Turbulent Flow

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

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*

Channel ProcessesAs the shear velocity increases the shear

stress increasesAs the hydraulic radius (or depth) and slope

increase the shear stress increases

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 τcr that is required to entrain materials

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

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

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

Channel Processes Sediment Transport

Suspension○ Silts, clays

Saltation○ sands

Traction (Rolling, Sliding)○ gravels

Solution

Channel Processes Sediment Load

Suspended Load○ Silts, clays

Bedload○ Materials moving by

saltation and traction Solute Load or Dissolved

Load○ Materials in solution

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