Geoscience Reference
In-Depth Information
Fig. 3.7
Photograph of typical fluvial overbank sediments, with gravel base and fining upward sequence. Length of tape measure is 1.4 m.
(high flow stages) play a major role in eroding
and depositing fluvial sediment, and modify-
ing river channels and floodplains (Knighton
1998). During large floods bank erosion up-
stream can result in downstream overbank
deposition of fine-grained sediments as thin
sheets over surfaces of previously-formed ter-
races, or of coarser-grained gravel and sand
as sheets, lobes and splays. During relatively
moderate floods that do not exceed bank full
conditions, coarse to fine material is transported
downstream and eventually deposited as bars
or overbank units on lower elevation terraces
(Fig. 3.7). The accumulation and preservation of
sediments within river systems also depends on
the factors outlined in the previous paragraph,
and particularly on the availability of space and
the deposition and erosion rates (Schumm &
Lichty 1963). Deposition on floodplains depends
on flood characteristics such as frequency, dura-
tion and suspended sediment concentrations
(James 1985). The length of time that sediment
remains stored in a part of the river system is
variable, ranging from 1 to 10
4
years (Fig. 3.8).
Bridge (2003) has pointed out that all dis-
charges that result in sediment transport can
affect river channel geometry. He defines
dynamic
equilibrium
as adjustment of the channel to
relatively consistent flood discharges over decadal
time periods, and
disequilibrium
as a major and
regional channel adjustment to a significant event
(or events), such as an extreme flood or anthro-
pogenic activity (e.g. mining). These are similar
concepts to
passive dispersal
and
active trans-
formation
, defined by Lewin & Macklin (1987)
for mining-contaminated river systems (see Case
Study 3.2). Bridge's (2003) dynamic equilibrium-
disequilibrium concepts are well-illustrated in
Fig. 3.9, where a system, previously at equilib-
rium, is affected by increase in discharge that
