Bedforms in Unidirectional Flow
Sedimentary layer thicknesses span several orders of magnitude, from mm to
more than a meter
That continuum is arbitrarily subdivided: anything <1 cm is called a lamina, >1
cm is a bed
Although beds can be planar or contain parallel laminae, turbulent flow in the
boundary layer combined with sediment deposited from traction often
produce bedforms (three-dimensional features on the bed)
Kennetcook River, Nova Scotia
Flow velocity decreases close to the bed surface, due to frictional
interaction between the water and the sediment
This interval of reduced flow is called the boundary layer
Boundary layer divided into turbulent sublayer (10s of cm to m thick) and
laminar sublayer (basal mm or less of the boundary layer)
Constant average velocity
Turbulence controlled by relative
importance of inertial vs. viscous forces
(Reynolds number Re)
ruD
Re =
m
Density, velocity, depth
Viscosity
Re > 2000
Laminar
Sublayer
Re < 500
Ripples are the smallest bedform, typically a few cm tall and 10-20 cm wavelength
Erosion
Deposition
Due to flow reattachment on stoss side of ripple, laminar sublayer of
boundary layer is compressed and flow velocity is higher – leads to erosion
Low velocity in eddy on lee side leads to deposition
Dunes have a similar appearance to ripples (gentle stoss slope, steep lee side),
but are larger
Height: 10 cm to 10 m, spacing: 60 cm to 100s of m
Are dunes just big ripples?
Dunes form from large-scale turbulence; ripples related to laminar sublayer
Dune size scales with flow depth; ripples scale with grain size instead
Height and wavelength distribution of dunes and ripples are separate
Upper plane bed produces parallel laminations with low-relief ridges and
grooves (called “parting lineations”) parallel to flow on the bed surface
Parallel laminations
Parting lineations
Parting lineations form because turbulent boundary layer develops
longitudinal regions of higher and lower velocity flow
High-velocity bursts disrupt laminar sublayer and erode sediment
Antidunes are unusual in typical river flows, but are broad, slightly asymmetrical
bedforms that migrate upstream (!) in most cases
Form where supercritical flow produces standing waves
What is supercritical flow?
Subcritical and supercritical flows are defined by a Froude number, the ratio
of flow velocity to the speed of wave propagation (wave celerity)
Fr =
u
gD
Fr < 1: subcritical – velocity
less than wave celerity
Fr > 1: supercritical – velocity
is greater than wave celerity
Supercritical
Hydraulic Jump
Subcritical
When Fr < 1, the water surface perturbation
is out of phase with the bed perturbation
High velocity
flow
Low velocity

Forms regular dunes

High shear stress
Erosion
Low shear stress
Deposition
Low velocity
When Fr > 1, the water surface perturbation
is in phase with the bed perturbation
High velocity
flow
Forms antidunes


Low shear stress
Deposition
High shear stress
Erosion