By S. Alam

Keynote speech by S. Alam
Water lndia - 4
Delhi, February 3 -4, 2004 .

Introduction

"Among the many sessions ofthe Third World Water Forum held in Kyoto, Japan in March 2003, there was one titled 'Sedimentation Management Challenges for Reservoir Sustainability'. Two main messages emerged from that session:

Whereas the last century was concerned with reservoir development, the 21 st century will need to focus on sediment management; the objective will beto convert today's inventory ofnon-sustainable infrastructures for future generations

The scientific community at large should work to create solutions for conserving existing water storage facilities in order to enable their functions to be delivered as long as possible, possibly in perpetuity."

We must say that the above messages summarizes very weil the challenge that all engineers involved in storage reservoir sedimentation management should keep in mind and work towards finding reasonable solutions.
For new projects this would mean:

Having good knowledge of the watershed sediment yield and ifnecessary and/or possible propose long­term solutions which would either maintain the present sediment yield if considered satisfactory , if not, take actions which will gradually reduce the sediment yield from the watershed area.

Confirm predicted sediment yield by actual field measurement of the total sediment load (sand, silt and clay) that is being transported by the river.

Amongst other criteria used for determining the proposed dam and reservoir characteristics also include the impact of the expected sediment load on short and long-term evolutions of the project and possible remedial measures.

Analyze reservoir sedimentation management strategies by using numerical rnodel like RESCON proposed by the World Bank¹.

Incorporate in the dam design possibilities of future structural modifications or retro fitting of structural arrangements to alleviate the problems that might be created by a reservoir full of sediment.

For existing projects with severe reservoir sedimentation, it will be necessary:

To develop solutions which will enable to stop its further degradation and carry out remedial measures which will enable restorations of ample power generation in a manner permanent.

In this respect we think that the very small scale physical modelling technology developed for studying the diversion concepts ofthe bulk of the Mississippi River sediment load into the wetlands in its delta could eventually be used for:

* Reproducing the actual known historical sedimentation process of the reservoir

* Develop practical and viable alternative solutions ofreservoir rehabilitation by flushing and
hydro suction.

* Verify the potential for optimization of water consumption for the combined flushing and
hydro-suction operation

* Limitations of flushing operation during the flood flows and hydro-suction during the average
or low flows.

We believe that such design procedures have great potential for reducing the initial construction cost, increase the project life and defer costs for structures not necessary at the project inception and eventually completely eliminate structures like the de-sanding structures for projects with a large reservoir. Where it is not required when the reservoir is free of sedimentation and with the reservoir full of sediment the concentrations during the flood flows are so high, that the de-sander is no longer capable of trapping sand particles adequately to prevent turbine abrasion, and plant shutdown becomes often necessary .

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1 Quoted from the foreword by lan Johnson, Vice President, Sustainable Development, World Bank for the World Bank Publication 'Reservoir Conservation by Alessandro Palmieri, Farhad Shah, George W. Annandale and Ariel Dinar, June 2003.

1. Watershed sediment yield assessment and its management

During the project design phase thorough investigation of the project watershed regarding the parameters such as nature of the soil, intensive useofland for agriculture, pastures, systematic de-forestation, intensity of rainfall etc., should be carried out and documented. Based on the distribution of the above parameters over the entire watershed area its long-term annual sediment yield in tons/km²/year may be established. As general information we may indicate that the areas with highest sediment yields may produce more than 10,000t/km²/yr and the minimum value could be less than 50 t/km²/yr. The precise assessment of the sediment yield of a watershed is therefore not an easy task. Expert advice and in situ measurements may help to obtain more reliable indications.

Another way of estimating watershed sediment yield from a given particular geographical area will be to carry out regular and precise bathymetric survey by using multi-beam echo sounding technique in existing storage reservoirs and by collecting sediment samples at selected locations within the reservoir. This way it will be possible to have fairly complete information on the average annual or periodic volume and/or weight of sediment load transported by the river into the reservoir. At the same time it will be possible to obtain information such as: Sediment particle gradation, their mineralogical composition and sand particle form coefficient.

Based on the preliminary findings during the design stage it may either be concluded that the sediment yield from the watershed is compatible from the stand point of long term reservoir sedimentation rates and the project life or that in order to assure a reasonable project life it might be possible to create appropriate vegetal cover which will reduce the watershed erosion characteristics, but this may not be possible for some projects.

2.Sediment sampling

Generally the gauging stations used for discharge rating and sediment sampling are located in the same area where the river flows are fairly uniformly distributed and the reference channel section is constant over time. Discharge measurement and sediment sampling are generally carried out at regular intervals, often every 15 days for mobilization and cost reasons.

In the past we have discussed the difficulty of assessing sediment data sufficiently accurately by using the conventional depth integrated sediment sampling method at periodic intervals. Regular sampling intervals combined with a fairly small number of verticals (Fig. 1) may in our opinion induce some error in the assessment of total sediment load. Perhaps this is one of the reasons for which the actual reservoir sedimentation rates are often much higher than that predicted during the initial evaluation of the reservoir sedimentation rates and the ultimate Project life.

Fig.1 - Typical depth integrated sediment sampling verticals in a river cross section

Figure 2 shows a typical correlation between the river discharge variations in a fairly large river like the Mississippi River at Old River Control and the corresponding total sediment load and sand load variations based on daily sediment sampling. This figure shows clearly that if the sediment sampling is carried out every 15 days interval that is say December 1 and 15 the average sediment load estimated will be very different than if the samplings were carried out on December 10 and 25.

To avoid such uncertainty a sediment sampling arrangement has been developed at Old River Controlon the Lower Mississippi River where samples are taken twice a day and at three different structures: Sidney A. Murray Hydroelectric Station and two US Army Corps of Engineers flow diversion structures. In the past a paper has been published to give full details of this system.²

We will therefore only briefly describe the main concept ofthe system and the need for knowing the daily variations of the sediment concentrations as shown in Figure 2 and its importance in assessing correctly the total sediment load.

Mixture of water and sediment is pumped from the highly turbulent flow areas such as: From the top of the turbine runner chamber and the energy dissipation areas of the flow diversion structures and hand samples are collected twice a day from easily accessible installations adjacent to the structures. Experience has shown that the vertical mixing of the total sediment load is complete and samples collected represented the total sediment load including the coarsest sand particles found on the river bead.

Fig. 2 - Correlation between the river discharge and the sediment load variations

These sampling stations have now been in operation for 13 years and have proven that their operation is simple safe and very reliable. Only one technician is required to collect and analyze all the samples and carrying out sediment analysis using standard United States Geological Survey (USGS) laboratory procedures.

It is also very important ta know the mineralogical composition of the sand particles. Sediments with very high content of Quartz sand (85% or more) are very detrimental to the turbines. Himalayan rivers often transport sand with high Quartz content, so particular attention must be given in the design of the structural arrangements, which will reduce the risksas much as possible entrainment of such material into the turbine flow. This aspect is of prime importance ifthe project designers intend ta use de-sanding structures for this purpose.

It would be of interest to mention that at the Jhimruk Run-of river Hydroelectric Station in Nepal, due to the very high concentration of Quartz sand in the river flow (maximum sediment concentration recorded 23,760 PPM or 23.76 kg/m³) has produced very severe abrasion of turbine cover, guide vanes and blades short time after starting the project operation.³ ln this case the de-sander was designed to retain sand particle sizes equal or greater than 0.09 mm. This would tend to indicate that the use of de-sander in certain cases is not a viable solution. The project designers should therefore weigh carefully the need and the cost benefit ratio of such structures before retaining them as necessary project component.

For information it is interesting to know the average annual sediment discharge of the 10 major rivers of the world⁴

"Geology, slope, climate, drainage density, and patterns of human disturbance all affect sediment yield, and no single parameter or simple combination of parameters explain the wide variability in global yields."

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