Geology Reference
In-Depth Information
through fluvial processes. Global estimates of the
quantities vary considerably: one study gave a range of
24-64 billion tonnes per year of bulk sediments, depend-
ing on the scenario used (Stallard 1998); another study
calculated that as much as 200 billion tonnes of sediment
move every year (Smith
et al
. 2001).
all 45,000 registered reservoirs is at least 4-5 billion
tonnes per year, or 25-30 per cent of the total. There
is an additional but unknown impact due to the still
smaller 800,000 or so unregistered impoundments. The
study shows that river impoundment is a significant
component in the global fluxes of water and sediment.
Changes in streamflow and sediment transfer caused
by dams lead to downstream changes in channel form.
The degradation of rivers downstream of dams is a con-
cern around the world. It has proved difficult to gen-
eralize about responses of channels downstream of
dams. Figure 9.13 displays expected responses over
a timescale of about 50 years to a reduction in sediment
load (Figure 9.13a) and a reduction in flood magnitude
(Figure 9.13b). Figure 9.13c shows the special case in
which a tributary confluence is involved. In all cases, a
change in a single process may produce any one of four
channel responses.
River channels and dams
Dams impose changes in streamflow and the transfer
of sediment. A study of the impacts of 633 of the
world's largest reservoirs (with a maximum storage capac-
ity of 0.5 km
3
or more), and the potential impacts of
>
the remaining
44,000 smaller reservoirs reveals the
strong influence of dams on streamflow and sediment
flux (Vörösmarty
et al
. 2003). It uses the residence
time change (the time that otherwise free-flowing river
water stays in a reservoir), in conjunction with a sedi-
ment retention function, as a guide to the amount of
incoming sediment that is trapped. Across the globe,
the discharge-weighted mean residence time change for
individual impoundments is 0.21 years for large reser-
voirs and 0.011 years for small reservoirs. The large
reservoirs intercept more than 40 per cent of global
river discharge, and approximately 70 per cent of this
discharge maintains a theoretical sediment-trapping effi-
ciency in excess of 50 per cent. Half of all discharge
entering large reservoirs shows a local sediment trap-
ping efficiency of 80 per cent or more. Between 1950
and 1968, global sediment trapping in large reservoirs
tripled from 5 per cent to 15 per cent; it doubled to
30 per cent between 1968 and 1985, but then stabilized.
Several large basins such as the Colorado and Nile show
almost complete trapping due to large reservoir construc-
tion and flow diversion. From the standpoint of sediment
retention rates, the most heavily regulated drainage basins
lie in Europe. Large reservoirs also strongly affect sed-
iment retention rates in North America, Africa, and
Australia-Oceania. Worldwide, artificial impoundments
potentially trap more than 50 per cent of basin-scale sed-
iment flux in regulated basins, with discharge-weighted
sediment trapping due to large reservoirs of 30 per cent,
and an additional contribution of 23 per cent from
small reservoirs. Taking regulated and unregulated basins
together, the interception of global sediment flux by
River modification and management
Fluvial environments present humans with many chal-
lenges. Many European rivers are complex managed
entities. In the Swiss Jura, changes in some rivers to
improve navigation destabilized the channels and a sec-
ond set of engineering works was needed to correct the
impacts of the first (Douglas 1971). Within the Rhine
Valley, the river channel is canalized and flows so swiftly
that it scours its bed. To obviate undue scouring, a large
and continuous programme of gravel replenishment is
in operation. The Piave river, in the eastern Alps of
Italy, has experienced remarkable channel changes fol-
lowing decreased flows and decreased sediment supply
(Surian 1999). The width of the channel has shrunk
to about 35 per cent of its original size, and in several
reaches the pattern has altered from braided to wander-
ing. In England, the channelization of the River Mersey
through the south of Manchester has led to severe bank
erosion downstream of the channelized section, and elec-
tricity pylons have had to be relocated (Douglas and
Lawson 2001).
By the 1980s, increasing demand for environmental
sensitivity in
river management
, and the realization that
hard engineering solutions were not fulfilling their design
life expectancy, or were transferring erosion problems
