SEDIMENTATION IN RESERVOIRS 203
The various deposition stages in a reservoir are shown schematically
in Fig. 4.3.
Reservoir capacity can be preserved by (a) minimizing the sediment
input into the reservoir, (b) maximizing the sediment throughflow, or (c)
the recovery of storage.
Minimizing sediment inputis by far the most effective measure, and
can be achieved by optimal choice of the location of the reservoir, the pre-
vention of erosion in the catchment by soil conservation methods
(afforestation, terracing, vegetation cover, etc.), the trapping of sediment
in traps or by vegetative screens on the tributaries upstream of the reser-
voir, or by bypassing heavily sediment-laden flows during floods from an
upstream diversion structure to downstream of the dam; (in a diversion
tunnel a compromise has to be achieved between sedimentation and abra-
sion; a velocity of about 10 m s^1 is usually acceptable).
Maximizing sediment throughflowrequires flow regulation during
floods (sluicing) and/or flushing during a reservoir drawdown. Under
certain conditions the sediment-laden inflow does not mix with the water
in the reservoir but moves along the old river bed as a density current
towards the dam, where it can be drawn off by suitably located and oper-
ated outlets (density current venting). In principle, the development of
density currents requires a significant difference between the density of
the incoming flow and the water in the reservoir, a large reservoir depth,
and favourable morphological conditions (steep, straight old river bed).
The techniques used for assessing the effect of reservoir operation
on sedimentation and its numerical modelling are discussed, e.g., by
Atkinson (1998); for further information on the flushing of sediment from
reservoirs see White (2001).
The recovery of storagecan be achieved by flushing deposited sedi-
ment, a technique which is effective only when combined with a substan-
tial reservoir drawdown, by siphoning or dredging; in the latter case either
Fig. 4.3 Deposition stages in a reservoir