Geology Reference
In-Depth Information
( ) Ripples (
a
F
1)
( ) Dunes with superimposed ripples (
b
F
1)
Weak boil
Manning roughness coefficient
0.017-0.028
( ) Dunes (
c
F
1)
( ) Washed-out dunes
d
( )
F
Boil
0.018-0.035
( ) Plane bed (
e
F
1)
( ) Standing wave antidunes (
f
F
1)
0.011-0.015
0.012-0.016
()Chute and pool (
h
F
1)
()Antidunes
g
( )
F
Breaking antidune wave
Incipient breaking wave
Accelerating flow
0.012 - 0.020
Figure 9.7
Bedforms in a sandy alluvial channel change as the Froude number,
F
, changes. At low flow velocities, ripples
form that change into dunes as velocity increases. A further increase of velocity planes off bed undulations, and eventually
a plane bed forms. The plane bed reduces resistance to flow, and sediment rates increase. The channel then stands poised
at the threshold of subcritical and supercritical flow. A further increase of velocity initiates supercritical flow, and
standing antidunes form. Flow resistance is low at this stage because the antidunes are in phase with the standing waves.
The antidunes move upstream because they lose sediment from their downstream sides faster than they gain it through
deposition. At the highest velocities, fast-flowing and shallow chutes alternate with deeper pools.
Source:
Adapted from Simons and Richardson (1963) and Simons (1969)
salinas
in South America and
sabkhas
or
sebkhas
in Africa.
They occur in closed basins of continental interiors,
which are called bolsons in North America. The
bolsons
are surrounded by mountains out of which floodwa-
ters laden with sediment debouch into the basin. The
coarser sediment is deposited to form alluvial fans, which
may coalesce to form complex sloping plains known as
bajadas
. The remaining material - mainly fine sand,
silt, and clay - washes out over the playa and settles as
the water evaporates. The floor of the playa accumulates
sediment at the rate of a few centimetres to a metre in
a millennium. As water fills the lowest part of the playa,
deposited sediment tends to level the terrain. Playas typi-
cally occupy about 2-6 per cent of the depositional area in
a bolson. Many bolsons contained perennial lakes during
the Pleistocene.
