Page 259 - Fundamentals of Geomorphology

242 PROCESS AND FORM

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