| dc.description.abstract | There are many reservoirs which can no longer serve their design
functions because much of their life storage volume have been
filled by sediment. The need, therefore, to consider reservoir
sedimentation in respect of both existing and proposed
reservoirs, topredict the loss of available storage isparamount
as a function of time, the distribution of deposition, trap
efficiency and discharge - sediment relationship. The practical
problems arising from reservoir sedimentation can be 1 . rI eXp.1.alneu
by reference to examples and include;
1} loss of storage and consequent loss of yield;
2} increased upstream flooding;
3} blockage of intakes;
4} abrasion of turbines; and
5) impact on downstream river regime.
The study of reservoir sedimentation was carried out with
reference to Masinga darn, a project of Tana and Athi River
Development Authority (TARDA), Ministry of Land Reclamation,
Regional and Water Development, Government of Kenya. The life of
a project is likely to be seriously affected during the early
part of its useful life as a result of disruption of fLow
equilibrium resulting from damming of a channel. This may lead
to aggradation in the reservoir at e rate more than predicted
during design period and hence reduce the storage
capacity of the project thus shortening its life period.
Aggradation may also lead to increased upstream flooding and
therefore endanger not only the existing stru
capacity of the project thus shortening its life period.
Aggradation may also lead to increased upstream flooding and
therefore endanger not only the existing structures but also
human and animal life. It is inview of the many problems arising
from rapid sedimentation that this project was chosen to
investigate the changes insediment accumulation rate which might
have resulted from the reservoir impoundment eight years ago.
The study revealed that;
1) The rate of sediment accumulation in the
reservoir is of the order of 4.21x105 m3j annum.
This rate is higher than that used during
design of , ~ 3.0x10o m"j annum. Therefore, the
reservoir life will be 370 years assuming the
current rate of siltation persists. This is less
than 500 years used as a design value. Thus the
high rate of siltation has the effect of
reducing the life span of the project by 130
years.
2) The reservoir trap efficiency ranges from -28.82
to 99.9 % .
3) Aggradation has taken place more significantly at
the sections where the channel enters the
reservoir.
4) Rivers TanajSagana and Maragua are heavy sediment
laden with naturalfbackground levels reaching
high levels of 1500 tonnes/day for Sagana and 100
tonnes/day for Maragua.
5)
During any given runoff event, suspended sediment
concentrations on the rising limp of a storm
hydrograph are usually greater than those measured
at equivalent flows on the falling limp. | en |
