Geoscience Reference
In-Depth Information
of the major measures used to control flood in many rivers. River training works, such
as spur-dikes, weirs, and bank revetments, are often constructed to control the flow
and protect banks and levees. Flood storage areas, such as reservoirs, lakes, deten-
tion ponds, and floodplains, help detain the flood propagation speed and reduce the
peak of the flood. Diversion areas or channels are usually designated for emergency
purposes when flood threatens the safety of backbone structures and key areas. As a
new technology, flood forecasting and warning systems have been established in many
regions to mitigate the flood disaster.
Reservoir sedimentation
Sediment deposition reduces the storage capacity and life span of reservoirs. With
time, the deposition will extend upstream and submerge more land, while sediments,
especially coarse particles, will be detained by reservoirs, causing erosion in down-
stream channels. The deposition and erosion processes and the ultimate equilibrium
profiles in reservoirs and downstream channels are topics of concern. After reservoirs
reach equilibrium states, their efficiency in terms of flood control, power genera-
tion, and sediment detention may be significantly reduced, and then problems with
dam decommission and rehabilitation and their impacts on the environment become
important.
Sediment control in low-head hydro-projects
Low-head hydro-projects include low dams, sluice gates, spillways, power generation
facilities, water diversion structures, water intake structures, and navigation facili-
ties. Because the reservoirs formed by low dams are small, sediment transport and
morphological evolution in the reservoirs and downstream channels reach new equi-
librium states relatively quickly. The appropriate design of sluice gates, spillways, and
power generators can prevent coarse sediments from entering into turbines. In princi-
ple, navigation and water intake structures should be placed at locations such as the
outer bank of the channel bend where less sediment deposition occurs. Flows around
hydro-projects should be controlled with certain river training works. Sometimes it
may be necessary to dredge and flush the deposits.
River restoration
Because of the impact of human activities or the variation of natural environment con-
ditions, river systems change their forms through bed aggradation, degradation, and
bank migration. These changes may be undesirable. For example, channel meander-
ing and main flow displacement may cause land loss, bridge failure, levee breach, and
difficulty in water intake. Serious erosion and deposition may impair aquatic habi-
tats. Once adverse impacts occur, training, mitigation, and restoration are needed to
change river systems to more favorable stable states.
Protection of structure foundations
In-stream structures, such as bridge piers, abutments, spur-dikes, andweirs, change the
flow significantly and may induce considerable erosion. Erosion also occurs due to jet
