VOL. 27, NO. 40, June 20 , 2008 (Ashadh 06 2065 B.S.)

Opinion

AB Thapa

Sedimentation of storage reservoir has ever since been a big problem in development of water resources. Usefulness of the storage reservoir is almost totally diminished once the storage reservoir is filled up with sediments. Thus, we talk about the life of a reservoir or its half life which virtually means the life of the project.  Now Chinese experts have revolutionized the concept of the life of a  storage reservoir by boldly applying it in the design of  the  “Three Gorges Project”  which is  the world’s largest hydropower.  They are saying that the  life of the storage reservoir could be extended virtually in perpetuity if the dam is properly designed.  

In future Nepal would be facing serious sedimentation problems if our storage dam projects are not  properly designed. Certainly Chinese experience would prove to be immensely  valuable to our country.  

Finite Life of a Reservoir

Dr. A.N. Khosla, the then Chairman, Central Water Commission of the Government of India, who was a renowned   scientist  in the field of Hydraulic Engineering, had reviewed the work of reservoir sedimentation in Fifties based on data available for 200 reservoirs  all over the world including the USA, China and Africa.  He had developed curves for annual sedimentation rate for major as well as minor  catchments. He has concluded  that  silting is taking place in both types of reservoirs with slight variations. It was evident that after a certain period those reservoir would be filled up with sediments.  

Dr. K.L. Rao,  Minister of Water Resources of the India’s Central Government has written in his book  India’s Water Wealth  to caution that the useful life of a storage reservoir could be considerably shortened “ Silting of reservoirs reduces  their useful capacity. The deposition of silt in Nizamsagar, in the course of the last 40 years, has reduced the live capacity of the reservoir from 725 million cu.m. to practically half..”  

What Chinese Scientists are Now Saying.

The following is the conclusion of an article published by Jiahua Fan et al in the Journal of Hydraulic Engineering of the American Society of Civil Engineers: 

Experience in China has shown that reservoir sedimentation is neither an irreversible nor uncontrollable process.  Various design, operational, and reconstruction strategies can be used to balance sediment inflow and outflow across impounded river reaches,  thereby protecting a significant amount of useful storage against sedimentation.  The provision of properly sized low-level sluices is the key structural element for sediment management in reservoirs. The next major step in the development of this technology  is the refinement of  generalized computer simulation techniques for analyzing sediment management problems in reservoirs, and the testing of these techniques against field data from reservoirs where sediment management is being practiced.  

It is reported that the problem of reservoir sedimentation in China had been particularly acute in the Yellow River basin  which drains an extensive area of highly erodible  soils.

Sediment concentrations of as much as 941 kg/cum have been measured on the main stem of the Yellow River, and as much as 1,500  kg/cum on tributaries. The sediments are predominantly silt. Without control methods, reservoirs  fill  quickly with sediments in this environment. These conditions have necessitated to conduct for more than 30 years research works in China on stabilization of reservoir capacity.

Research on Sedimentation

The total capacity of the TGP reservoir amounts to 39.3 billion m3, only accounting for 8.7 % of the yearly runoff at the dam site, and its effective capacity 16.5 billion m3, accounting for 3.6 %. Therefore, the reservoir is of a seasonal regulation one with low run-off regulation capability.

Sediment is one of the major technical problems of the Three Gorges Project(TGP)  to be tackled. Since 1950's, many research institutes and experts at home have conducted extensive studies on this issue. The successful experience gained from the Gezhouba Project on the sediment management has laid solid foundation for the sediment study of the Three Gorges Project.

Comprehensive research methods, such as prototype observation, mathematical model computation, physical model tests, and analogue analysis on existing projects have been adopted for TGP's sediment study. The results show that the sediment problem of TGP is resolvable based on the examinations and authentication organized by sediment experts at home and aboard.

Sediment Load  - Based on the 40 plus years' observation at the Yichang Hydrologic Station, the sediment discharge of suspended load averages about 526 million tons per year, with a median diameter of 0.033 mm. The annual bed load including 0.76 million tons of cobbles is about 8.6 million tons, which takes up only a small proportion of the total sediment load.

The water discharge at TGP's dam site is large, amounting to 451 billion m3 annually with an average sediment concentration of only about 1.2 kg/m3, whereas  the sediment discharge at Sanmenxia Project on the Yellow River is 1.6 billion tons annually with an average sediment concentration of 37 kg/m3. The water discharge at TGP dam site is 10 times that at the Sanmenxia Project, while the annual sediment load is only one third of it. The sediment problem of the Yangtze River, therefore, is quite different in nature from that of the Yellow River.

In recent years, some experts deem that the sediment yield in the Yangtze River is in a tendency of increase based on the data of the early 1980's. However, it is not true' According to the long term observation and the checking results there from,  the sediment load in the Yangtze River changes periodically below or above the average level on a random basis, and there is no obvious tendency of increase. The amount of sediment load at the dam site was far below the average for several times in the late 1980's and early 1990's' This is an evident proof.

Physical Sediment Modelling -Owing to the complexity concerning hydraulics and sediment deposition problems in the dam area and in the fluctuating backwater region, which makes the computation of mathematical model almost impossible, 14 physical sediment models (5 for dam area and 9 for different sections in the fluctuating backwater region ) have been established for detailed study.

A great progress had been made in the physical sediment modeling in China during the construction of Gezhouba Project.  Now Gezhouba Reservoir has been in operation for 16 years, comparisons have been made on field observation vs. model testing results and these comparisons, generally speaking, are quite consistent. The physical sediment modelling of TGP has adopted basically the same methods for the Gezhouba Project with a model scale generally of 1 by 100 to 1 by 300.

Permanent Use of the Reservoir 

The Yangtze River is abundant in runoff' The Three Gorges Dam is to be equipped with 23 large  bottom discharge outlets at low elevation (90 m).  Meanwhile, the reservoir level in the flood season will be maintained at a fairly low elevation (145 m). These two characteristics allow the application of a  new strategy of reservoir operation Summed up in China as "discharging the turbid and impounding the clean”.  During the flood season from June to September, when the river carries 84% of the annual sediment load and 61 % of the annual runoff, the reservoir draws down to a low level(145m ) that creates a condition in favour of sediment sluicing, allowing a large discharge of sediment out of the reservoir from the bottom outlets. At the end of the flood season (October) when there is less sediment content in water, the reservoir will be impounded to NPL (175 m) for enhancement of power generation and navigation. In this operation manner, most of the sediment will be discharged, the large part of sediment deposited will remain in the dead storage of the reservoir, and most part of the effective storage can be preserved permanently.

The length of the TGP reservoir is over 600 km and its average width is only about 1, 100 m, looking like a ribbon in the plain. So it is still a river-like, rather than a lake--like reservoir. The whole reservoir is quite uniform in width and for the most part less than 1, 000 m, only one seventh of the reservoir is about 1, 000 to 1, 700 m wide. The morphology of the reservoir is favorable for the removal of sediment. Since the estimated width of the equilibrium channel is 1, 300 m corresponding to the hydrological conditions of the TGP reservoir, little flood plain is expected to be formed along the main channel in the TGP reservoir. Thus, large percentage of effective storage can be preserved permanently.

In Conclusion

The silt deposition in the TGP reservoir has been analyzed numerically by mathematical modeling.  According to the calculation results, after a century operation when the reservoir sedimentation already comes to the balanced state, the reservoir's effective storage capacity could still remain at 86--92 %.

It is believed that the conclusions are reliable, not only because the mathematical model is developed based on a large amount of research and observation and verified by many experts at home and abroad, but also because it is based on several successful engineering practices.