Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 374

below 323 m, the capacity of 1.05 billion m 3 is available for controlling medium floods in the flood season
whenever necessary. About 3 billion m 3 of the reservoir capacity below 330 m has been kept for use in the
event of an extremely large flood. In the normal operation of the reservoir, the floodplain in the reservoir
would be inundated only during extremely large floods; the major task of operation is the detention of
such floods. The detained sediment would be eroded in the next several years. However, once the direct
backwater effect reaches the confluence area above Tongguan during a flood event and lasts for a certain
period of time, sediment deposition on the floodplain would be inevitable and a part of the reservoir
capacity would become unavailable for use in the later periods. It is obvious that for preserving a usable
capacity in the reservoir, one of the guiding principles of the reservoir's operation is to prevent the direct
backwater effect from exceeding Tongguan as much as possible.
Fig. 6.43 Variations of accumulated deposition and reservoir capacity in Sanmenxia Reservoir: (a) accumulated
deposition; (b) reservoir storage capacity
As shown in Fig. 6.44 the longitudinal and transverse profiles in the reservoir have been varied with
the changes of operational conditions. The figures show that during the impoundment up to October
1961, i.e., the first year of operation, deposition was in the form of a delta with a topset slope of 0.00015
-0.00017, nearly half the original river bed slope, and a foreset slope of 0.0006-0.0009. The apex of the
delta was near section 31, and the cross section was raised evenly in the transverse direction, making no
distinction between the main channel and floodplain. During the wet year of 1964, the annual water and
sediment inflows were 69.7 billion m 3 and 3.06 billion tons, respectively. The reservoir was severely
silted because the outlet capacity was too small and the sluice holes were located too high. About 1.95
billion tons of oncoming sediment was deposited in the reservoir during the flood season, which was
70% of the total incoming sediment. This was the most serious year of siltation. Longitudinal deposition
was in the form of a cone, and a channel-floodplain configuration was formed. This can be seen from the
transverse section, in which the floodplain rose simultaneously with the main channel. In 1973, the
deposition occurred with the main channel being eroded because the reservoir was used for flood
detention only during the flood season and the outlet discharge was enlarged after reconstruction. The
figure reveals that the main channel had been lowered by erosion with basically no changes in the
floodplain. The longitudinal bed slope of the main channel was 0.0002-0.00023 and that of the floodplain
was 0.00012. A high floodplain and deep main channel had been formed; about one billion m 3 of channel
storage below Tongguan was recovered.