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SEDIMENT DEPOSITS IN RESERVOIRS 10.7
Bhakra Dam, India
Delta Profile
FIGURE 10.5 Timewise pattern of delta growth upstream of Bhakra Dam, India. The rate of delta
advance slows with time because of reservoir geometry, which deepens and broadens in the
downstream direction.
may in
itially be very rapid. At Guanting Reservoir in China, the delta initially progressed
downstream at a rate of 3.0 km/yr and simultaneously proceeded upstream at a rate of 2.6
km/yr. As the delta progresses downstream into deeper and wider portions of the reservoir,
its rate of longitudinal growth slows. This timewise pattern of delta growth at the Bhakra
Darn in northern India is illustrated in Fig. 10.5.
As the delta advances downstream, the topset and foreset deposits prograde over previously
deposited bottomset beds of finer sediments. This process can create alternating lenses of
coarse and fine material as large events deliver coarse material deep into the impoundment,
which may be subsequently overlaid by finer material. In general, fully penetrating cores in an
extensively sedimented reservoir will reveal the finest sediment at the bottom, coarsening
until the surface level is reached.
Large volumes of sediment deposited in delta areas during periods of flood inflow can
be reworked during subsequent periods of drawdown. A remarkable example of delta
deposition and the subsequent reworking of these deposits was described at the 85.3-Mm
3
Kulekhani hydropower reservoir in Nepal by Galay and Okaji (1995). During July 1993, the
126 km
2
watershed was affected by a rainstorm with up to 540 mm of precipitation in one day.
Slope failures resulting from this storm affected between 4.0 and 9.2 percent of the entire
watershed surface area tributary to the dam, and 83 percent of all delivered sediment was
estimated to have originated from slope failures within the watershed. This storm
delivered about 4.8 Mm
3
of sediment to the reservoir, for a single-event delivery rate of
38,000 m
3
/km
2
. Of this, 2.9 Mm
3
was estimated to consist of fines and the remainder was
coarse material.
Coarse material including boulders created a massive delta deposit which extended
upstream as the pool level rose during the course of the storm. During drawdown in the
subsequent monsoon period, these deposits were reworked and moved downstream, prograding
over finer sediment (Fig. 10.6). An oblique aerial photo of the delta deposits exposed at low
water is presented in Fig. 10.7, along with a photograph taken on the delta which
illustrates the eroding deposits.
Reservoir deltas reflect the interaction between the inflowing stream and the
deposited bed material, and the fluvial process affecting deltas is similar to that of an
alluvial stream. The bed continuously adjusts to changes in water and sediment inflow and
changing base level (the reservoir water level). Stream channels crossing deltas can experience
fluvial processes such as meandering, levee formation, channel incision, and armoring. In
reservoirs with a fluctuating water level, the pattern of delta development will be heavily
influenced by the changing base level and alternating periods of deposition and erosion.