Environmental Engineering Reference
In-Depth Information
After the completion of the Fenshuba Dam fish harvest has been much greater than the fishery within the
river before the dam.
Assessments of impacts on fish, shortly after impoundment, may not provide an indication of the
long-term effects, because the fauna will require time to readjust, and to recover after the initial impact of
dam-closure and reservoir-filling. Post-impoundment studies of Lake Kainji, Nigeria, for example,
suggested that the diverse, abundant and commercially important Mormyridae —highly specialized for
lotic habitats—of the Niger system were radically affected by impoundment, being reduced from about
20% (Banks et al. , 1965; Motwani and Kanwai, 1970) to less than 5% (Lelek and El-Zarka, 1973; Lkwis,
1974) of the fish caught. Some species were wiped out, others adjusted rapidly to the change to lacustrine
conditions, while yet others may recover but at a slower rate (Blake, 1977). For example, H. pictus increased
in number to comprise over 70% of the total mormyrids caught, whereas M. senegalensis declined to 0.5%.
Moreover, despite the sudden decline in total abundance during the first post-impoundment year, an
increasing trend was subsequently apparent and as the species-composition adjusted to the changed
conditions, the abundance of the Mormyridae recovered towards pre-impoundment levels.
7.2 Reservoir Sedimentation Management
7.2.1 Reservoir Sedimentation
7.2.1.1 Loss of Reservoir Capacity Due to Sedimentation
Sediment deposition always occurs in reservoirs, which causes not only loss of storage capacity but also
environmental impacts. At the end of the 1950s, the United States made an investigation into the
sedimentation in 1100 reservoirs. Of them, data from 66 representative reservoirs selected by L.G. Gotts
are listed in Table 7.8. Table 7.9 lists the reservoir sedimentation in China (IRTCES, 1985).
A wide range of sedimentation related problems occur upstream of dams as a result of sediment
trapping. Because of storage loss the functions of the reservoir reduce for flood control, power generation,
and water supply. Sediment can enter and obstruct intakes and greatly accelerate abrasion of hydraulic
machinery, thereby decreasing its efficiency and increasing maintenance costs. Sediment deposition in
the delta region in the reservoir may affect navigation and impact the ecology. Dam construction is the
largest single factor influencing sediment delivery to the downstream reaches. The cutoff of sediment
transport by the dam can cause stream bed degradation, accelerate the rates of bank failure, and increase
scour at structures such as bridge piers. Table 7.10 lists the main impacts of reservoir sedimentation on
the environment.
T able 7.8 Reservoir sedimentation and capacity loss in the USA (after IRTCES, 1985)
Loss of storage
capacity (%)
Region
Number of reservoir
Number of years
Northeast
3
30
24.7
Southeast
10
18.6
15.1
Middle West
11
16.5
14.0
Middle South
12
17.2
8.8
Northern Great Plain
9
23.1
29.6
Southwest
15
29.8
15.7
Northwest
6
23.1
7.0
Whole country
66
22.1
15.6
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