Page 252 - Fundamentals of Geomorphology

FLUVIAL LANDSCAPES 235

( ) Ripples (F 1) ( ) Dunes with superimposed ripples (F 1)
b
a
Weak boil

coefficient
roughness
Manning roughness coefficient
Manning
0.017–0.028
0.017–0.028
( ) Dunes (F 1) ( ) Washed-out dunes (F 1)
d
c
Boil
Boil
0.018–0.035
0.018–0.035
f
e
( ) Plane bed (F 1) ( ) Standing wave antidunes (F 1)

0.011–0.015
0.012–0.016
0.011–0.015 0.012–0.016
() Antidunes (F 1) () Chute and pool (F 1)
h
g
Incipient breaking wave Breaking antidune wave
Accelerating flow

0.012
0.012 – 0.020
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. silt, and clay – washes out over the playa and settles as
They occur in closed basins of continental interiors, the water evaporates. The floor of the playa accumulates
which are called bolsons in North America. The bolsons sediment at the rate of a few centimetres to a metre in
are surrounded by mountains out of which floodwa- a millennium. As water fills the lowest part of the playa,
ters laden with sediment debouch into the basin. The deposited sediment tends to level the terrain. Playas typi-
coarser sediment is deposited to form alluvial fans, which callyoccupyabout2–6percentofthedepositionalareain
may coalesce to form complex sloping plains known as a bolson. Many bolsons contained perennial lakes during
bajadas. The remaining material – mainly fine sand, the Pleistocene.