Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 422

flow (MWR, 2007). About 8.4 million tons of fine sediment, or 36% of inflowing sediment, was released
during the turbidity currents because the sedimentation reservoir bed approached to the bottom outlets.

7.2.2.6 Sediment Releasing Efficiency
The sediment releasing efficiency, E , can be defined in two ways:

volume ~ of ~ released ~ se dim e t

E (7.6)

volume ~ of ~ income ~ se dim e t

time ~ needed ~ for ~ dim
time ~ to ~ flush ~ the ~

se entation

E (7.7)

se dim e t

For the reservoir sedimentation management with pressure flushing, density current, and storing the
clear and releasing the turbid the sediment releasing efficiency is calculated with Eq. (7.6). However, for
empty flushing the sediment releasing efficiency is calculated by using Eq. (7.7). The sediment releasing
efficiency is: 30%-100% for storing the clear water and releasing the turbid water, 6%-36% for turbidity
current, varies in a range of several percent to more than 100% for pressure flushing. For empty flushing
the sediment releasing efficiency may be as high as 2400%-5500%.
For the Hengshan Reservoir the efficiency was 24.7 during the empty flushing in 1974 and was 23.9 during
the empty flushing in 1979. For the Zhuwo Reservoir, the efficiency was 55 during the empty flushing
experiment. Empty flushing has the highest sediment releasing efficiency, but the flushing may cause
high ecological stress on the downstream reaches. Suspended sediment adsorbs pollutants from flowing
water in rivers and deposits in the reservoirs. A study showed that the concentration of heavy metals (Cr,
Cd, Hg, Cu, Fe, Zn, Pb and As) were highest in the sediment and lowest in the water. Benthic invertebrates
had higher concentrations of heavy metals in their tissues due to their proximity to contaminated sediments
and fish had lower concentrations of heavy metals (Yi et al., 2008). Empty flushing might release the
pollutants from the sediment and increase sharply the concentration of pollutants in water. The ecological
system downstream of the Zhuwo Reservoir consists of numerous species of aquatic plants, benthic
invertebrates and fish. Empty flushing can seriously impair the ecology. The Beijing government decided
to dredge the Zhuwo Reservoir rather than implement an empty flushing technique. This had a higher
economic cost but a much lower ecological cost.
In conclusion, density current has low sediment releasing efficiency; empty flushing may achieve very
high sediment releasing efficiency but may cause a great disturbance to the stream ecology; dredging is
used only as an auxiliary method for reservoir sedimentation management; the strategy of storing the
clear water and releasing the turbid water is the best sedimentation management strategy because it may
achieve relatively high releasing efficiency and only slightly affect the stream ecology.

7.3 Dam Failure and Dam Removal

7.3.1 Dam Failure

7.3.1.1 Dam Failure Events
Throughout history, dam incidents and dam failures have inflicted tremendous loss of lives, as well as
great property damage. Dam failure has occurred to dams built according to accepted engineering standards
of design and construction at the time and also to dams built without application of engineering principles.
Dam failure may be caused by extreme events due to intense rainfall from thunderstorms and from extreme
weather such as hurricanes, massive landslides, and landslide induced waves in the river, volcanic
eruptions, and fires which could damage the outlet control structure and make the operation of the