Geology Reference
In-Depth Information
Convex floodplains
more rapidly than suspended load, and deposition is more
frequent in and near to the channel than it is in overbank
sites. In consequence, the channel banks and levees grow
faster than the flood basins and may stand 1-15 m higher.
The low-gradient floodplains of most large rivers, includ-
ing those of the Rivers Mississippi, Amazon, and Nile,
are broad and have slightly convex cross-sections, the
land sloping away from the riverbank to the valley sides
(Figure 9.8a). The convexity is primarily a product of
sedimentation. Bed load and suspended sediment are laid
down in the low-water channel and along its immediate
edges, while only suspended materials are laid down in
the flood basins and backswamps. Bed load accumulates
Flat floodplains
The majority of small floodplains are flat or gently
concave in cross-section (Figure 9.8b). On these flat
floodplains, natural levees are small or absent and the
Box 9.1
POOLS AND RIFFLES
River channels, even initially straight ones, tend
to develop deeper and shallower sections. These
are called
pools
and
riffles
respectively (Plate 9.6).
Experiments in flumes, with water fed in at a con-
stant rate, produce pool-and-riffle sequences, in
which the spacing from one pool to the next is
about five times the channel width (Figure 9.6).
Continued development sees meanders forming with
alternate pools migrating to opposite sides. The mean-
der wavelength is roughly two inter-pool spacings
of ten channel widths, as is common in natural
rivers.
Plate 9.6
Riffles and pools in a straight section of the River Dean, Adlington Hall, Cheshire, England. A pool may
be seen in the foreground, a riffle to the right of the middle-ground bar, with other pools and riffles beyond.
(
Photograph by David Knighton
)
