SEDIMENTATION IN RESERVOIRS 201
A comprehensive guide to sedimentation in lakes and reservoirs has
been published by Unesco (Bruk, 1985), Morris and Fan (1998) and
Batuca and Jordaan (2000).
Sediment run-off in many rivers is continuously increasing – mainly
as a result of human influence. Sediment concentration in rivers fluctuates
greatly and is a function of sediment supply and discharge (Sections 8.2.3
and 8.4.5). In some rivers it can be extremely high, with hyperconcentra-
tions over 200 g l^1. In the Yellow River basin mudflows containing up to
1600 g l^1 have been recorded (Bruk, 1985), while concentrations of
5000 mg l^1 (ppm) are certainly not unusual on many Asian, but also some
other rivers (concentrations expressed in mg l^1 or ppm are effectively the
same up to about 7000 mg l^1 ; here ‘concentration’ is the ratio of the mass
of dry sediment to the total mass of the suspension). In many Indian reser-
voirs the annual loss of storage due to sedimentation is between 0.5% and
1%. At Tarbela on the Indus in Pakistan, the loss has been about 1.5% per
year. At the Three Gorges Project on the Yangtze River in China the sedi-
mentation problem is one of the key issues and subject of long-term and
on-going studies of various design controls; according to a report by the
China Yangtze Three Gorges Project Development Corporation (CTGPC)
the annual average sediment load entering the reservoir is at present
526 106 tons. However, the morphology of the reservoir is favourable for
sediment flushing (see below). It is estimated that worldwide over
30 km^3 /year of storage are lost by sedimentation; the total storage loss up
to the year 2000 is of the order of 570 km^3 , i.e. about 10% of the gross
stored volume.
The relative bulk density of the deposited sediment varies enor-
mously as consolidation takes place over the years, and can reach values of
up to 2.0 (usually about 1.2–1.6).
The detailed computation of the amount of sediment deposited in a
reservoir requires not only knowledge of the quantity and composition of
the incoming sediment but also of the reservoir operation and cross-
sections along the reservoir.
The ratio of sediment load, W, left in suspension at the end of a
reach of length L, depth of flow y, and velocity Vto the initial load W 0 can
be expressed as
W/W 0 e^ KL/yV (4.13)whereKis a constant which is a function of the sediment particle fall veloc-
ityws(Section 8.2.3). A further treatment of equation (4.13), which is best
carried out by mathematical modelling, is beyond the scope of this text.
For preliminary studies the use of trap efficiency curves is sufficient.
The most frequently used version is the graph constructed by Brune
(1953), plotting the percentage of trapped sediment versus the ratio of the
reservoir capacity (m^3 ) and the annual inflow (m^3 ) (Fig. 4.2). The graph