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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Special Issue 3, February 2014) International Conference on Trends in Mechanical, Aeronautical, Computer, Civil, Electrical and Electronics Engineering (ICMACE14)

Importance of Desilting Basins in Run-of-River Hydro Projects in Himalayan Region M Z Qamar1, M K Verma2, A P Meshram3 1&2

Research Officer, Assistant Research Officer, 1, 2 & 3 Central Water & Power Research Station, Pune (India) - 411 024 3

1

[email protected] 2 [email protected] 3 [email protected]

Abstract— The natural resources of Himalayas in terms of hydro power generation are not only crucial for the Himalayan states of India, but very important for the whole country. These states seem to be keen in exploiting their vast hydro electricity potential for net revenue earning. However, due to weak geological conditions and steep slopes these rivers carry a huge quantity of sediment with them. The suspended part of this sediment causes problems after getting entry through trash racks of power intakes in hydropower projects constructed in Himalayan region. This paper describes the elimination of suspended sediment in run-of-river (ROR) hydro power projects by means of providing desilting basins.

II. TYPES OF HYDRO POWER PROJECTS

Keywords— Desilting chamber, settling efficiency, silt flushing tunnel, suspended sediment concentration, inlet transition, outlet transition.

Broadly speaking, there are three types of hydro power projects namely, storage or impoundment type, run-of-river or diversion type and pumped storage type.

I. INTRODUCTION Many ROR hydropower projects have been commissioned in Himalayan region and many more are being constructed / planned in India, Bhutan and Nepal. The suspended part of sediment load carried by Himalayan Rivers mainly consisting of quartz particles (hardness 7 on Mohs scale) enters into the water conductor system through power intake. These projects generally utilize a very high water head sometimes ranging from 700 to 800 m. If this water along with huge quantity of suspended sediment load is allowed in the power house with such a great velocity, it will cause lot of damage to the turbines and other under water parts due to abrasion and wearing effect. One such example of damage to the turbines due to sediment is shown in photo 1. To tackle this suspended sediment problem, some approach has to be planned during the design stage of the project. Provision of desilting basins in these projects is one of the widely used methods to deal with this problem.

Damaged runner New runner Photo 1: Damage to the runner due to sediment

A. Storage / impoundment type hydropower project The most common type of hydropower plant is with an impoundment facility. In this system a large reservoir is created by constructing a dam to store river water for major hydro power project. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water is released through power intake either to meet peak electricity demands or base load needs or through spillways to maintain a constant reservoir level. B. Run-of-river or diversion type hydropower project In run-of-river or diversion type hydro power projects, a small reservoir is created by constructing a dam / diversion weir to divert river flow through head race tunnel to an adjacent valley utilizing the available head for power generation. In run-of-river hydroelectric stations diurnal storage is used to meet peak electricity demands. These projects have small or no reservoir capacity.

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Special Issue 3, February 2014) International Conference on Trends in Mechanical, Aeronautical, Computer, Civil, Electrical and Electronics Engineering (ICMACE14)

C. Pumped storage type hydropower project When the demand for electricity is low, pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of peak electrical demand, the water is released through turbines and stored in the lower reservoir. This method produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water back into the higher reservoir.

through desilting basin and SFT is controlled by provision of control gates at the outlet. Generally, desilting basins are designed for 90 % removal of suspended sediment particles of size 0.2 mm and above. However, basins may be designed to eliminate particles finer /coarser than 0.2 mm which will increase / decrease length of the basin and in turn adds / reduces cost of the project. This can be decided by carrying out the comparative study considering aspects such as replacement / maintenance of underwater parts, revenue loss and local site conditions. A typical plan and longitudinal section of desilting basin is shown in figure 2 and cross

III. DESILTING BASIN Desilting basins have become an integral part of the water conductor system of ROR hydropower projects to minimize the impact of damage due to suspended sediment. Desilting basins are provided just after power intake and discharge is passed through them before entry into the head race tunnel. Desilting basins are huge and costly underground structures, generally, constructed inside the hills and sometimes size is as large as 525 m long 15 m wide and 27.5 m deep (Nathpa Jhakri). A typical layout of water conductor system for a runof river hydropower project is shown in figure 1. section in figure 3. Figure 2: Typical plan and L-section of desilting basin

Figure 1: Typical layout of water conductor system for ROR hydropower project

In case of desilting basins the cross sectional area of flow is increased so as to achieve a reduction in forward velocity which ultimately induces the settlement of the suspended sediment. The silt flushing tunnel (SFT) is provided below the desilting basin which is connected with main basin by provision of openings at the bottom slab of the desilting basin. This settled sediment is flushed out through silt flushing tunnel and discharged into the river downstream of the dam. An excess discharge of 15 to 20 % of design discharge is taken from the power intake into the desilting basin for flushing of the settled sediment from SFT. The discharge

Bottom slab SFT 1.5 m deep

Figure 3: Typical cross section of desilting basin

A. Types of desilting basins

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Special Issue 3, February 2014) International Conference on Trends in Mechanical, Aeronautical, Computer, Civil, Electrical and Electronics Engineering (ICMACE14)

Various types of desilting basins and their mode of classification are indicated in table 1.

TABLE 1 TYPES OF DESILTING BASINS

Basis of Classification

Type of basin

Mode of construction

Natural or artificial

Method of cleaning

Manual or mechanical or hydraulic removal of deposition

Mode of operation

Continuous or intermittent

Type of flow

Open channel or Pressure flow

Configuration / layout

Single or multiple unit

B. Design aspects of desilting basins The performance of desilting basins depends upon the reduction in velocity and turbulence, provision of adequate length of the basin for achieving the desired settlement and the skimming arrangements at the outlet [1]. However, the settled sediment is required to be removed periodically or continuously to maintain its settling efficiency. Thus, though the design of the desilting basin includes two main parts viz. i. Settling efficiency and ii. Flushing system

large velocity at the inlet has to mix satisfactorily in a desilting basin and a proper diffusion / dispersion is to be achieved. From the study of the mechanism of the dispersion of the jet in the water body, it has been seen that the region of the expansion of flow is the region of appreciable modification of mean flow pattern and the region of appreciable eddy motion. From the model studies of these basins a bed slope of between 2.0 and 2.3 was found to be satisfactory. 3) Size of the basin: The ideal horizontal settling basin as shown in figure 4, demonstrates the basic theory of sedimentation developed by Hazen [2]. The following assumptions are made: - uniform distribution of flow and suspended solids at entry to settling zone; quiescent flow; solids entering deposition zone are not re-suspended. Consider a sediment particle entering the basin at point x:

4) x

Following aspects are also required to be taken into consideration: 1) Location and orientation: Generally, desilting basin should be located as near the power intake as possible to achieve the desired control and to minimize the sedimentation in the approach channel. However, the location of the basin too near the intake would create a problem due to the turbulence downstream of the intake. The basin is also required to be properly oriented with respect to the alignment of the inlet tunnel on upstream to achieve satisfactory distribution of flow as naturally as possible. For this purpose, the basin may be located in the reach where at least a straight length equal to ten times the average width of the channel or diameter of the inlet tunnel is available on the upstream. 2) Inlet transition: The flow area in the desilting basin is required to be increased for reducing velocity to induce the settlement of sediment. This increase in area is achieved by suitable horizontal and vertical divergence. For obtaining the satisfactory distribution of flow, the flow with relatively

Figure 4: Concept of ideal settling basin

t s = d/w Retention time, t R = basin volume / discharge = d A / Q Settling time,

Where d = flow depth; A = mean plan area of basin; Q = discharge. For quiescent settling, all particles of settling velocity w are removed when retention time equals settling time: i.e.

d A / Q = d/w, or Q/A = w

In general for both ideal and real basins, the ratio wA /Q can be regarded as a dimensionless indicator of the physical ability of a basin of plan area A to remove particles of settling velocity w at supply discharge Q. In case of ideal settling basins, for discrete particles:

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Special Issue 3, February 2014) International Conference on Trends in Mechanical, Aeronautical, Computer, Civil, Electrical and Electronics Engineering (ICMACE14)



Removal is independent of basin depth and flowthrough velocity, For a given discharge and suspended sediment load, removal is a function of basin surface area.

and Standards of USSR i.e. TCaS [7], H.A. Einstein [8] and Hippola [9] are in use. These functions are based on gravitational, diffusion or probability theory of the sediment transport.

In case of real desilting basins the higher velocity incoming flow enters the main basin which causes turbulence in its initial reaches. Therefore, effect of turbulence is also being considered as opposed to ideal settling basins. Camp [3] based his classic approach to settling basin design of the work of Dobbins [4]. After making simplifying assumptions that (a) fluid velocity, and (b) the turbulent mixing coefficient are the same throughout the fluid, Camp derived a relation for settling efficiency :

5) Outlet transition: The centre line of outlet should coincide with the axis of desilting basin for uniform withdrawal / skimming of top layers of flow over the entire width of basin. The outlet should be as high and as wide as possible. Narrow outlets or outlets located on the side would result in a reduction in the effective length of the basin.



=f

 wA w  ,    Q v* 

Where v* is the shear velocity and w / v* can be regarded as a dimensionless indicator of distribution of suspended sediment in vertical. Camp’s solution to equation is shown graphically in Figure 5.

6) Size and slope of the hopper: The slope of the hoppers is required to be steeper than the angle of repose of the suspended sediment to allow the sediment to slip into the openings at the bottom connecting to the flushing conduits/pipes underneath. In the case of narrow desilting basins, instead of individual rectangular hopper, a continuous hopper bottom side with sediment accumulation trench below is preferable. The spacing of the openings between the settling trench and flushing conduit is decided in such a way that the top of the dunes formed between the successive openings would not protrude in the settling zone above. Based on model studies, the preferable side slope of hopper is 40 0. 7) Size of silt flushing tunnel: Size of the flushing tunnel is required to be decided for efficient transport of the sediment. From the experience of studies carried out at CWPRS, Pune, it is seen that minimum velocity of 3.0 m/s is required for efficient functioning of the tunnels. In flushing system of desilting basins, the concentration is likely to be more due to higher settling efficiency. The flow in flushing system is a pressure flow since the sediment enters in flushing tunnel through the openings from main basin. The flushing discharge is controlled by a gate at downstream end.

𝜔𝐷1/6 𝑛𝑈 𝑔

Figure 5: Camp’s solution for settling basin efficiency

The shear velocity in the relation is given by: Shear velocity,

v*

=

gRS

Where R = hydraulic mean depth, and S = hydraulic gradient which is calculated from a boundary resistance equation such as Manning’s and essentially depends on flow through velocity. For dimensioning of desilting basins, various sediment removal functions such as proposed by C.P. Vetter – [5], T.R. Camp – [3], Hunter Rouse [6] Technical Conditions

8) Size and spacing of openings: The first opening from the desilting basin to flushing conduit is required to be larger to allow removal for higher rate of deposition and larger size of particles. Though, no definite criteria can be suggested, from experience of model studies for desilting basins for various projects, size of the first opening should be adequate to pass 20 to 30 % of the flushing discharge with a velocity of 3.0 m/s. The total area of the openings can be broadly estimated for passing the remaining discharge with velocity of 3.0 m/s. The size of the openings may decrease progressively towards the downstream as concentration and size of the sediment settling goes on decreasing towards downstream. This reduction could be done in steps on the

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Special Issue 3, February 2014) International Conference on Trends in Mechanical, Aeronautical, Computer, Civil, Electrical and Electronics Engineering (ICMACE14)

basis of the practical considerations. It has also been observed that the smaller size of the material settling near the outlet end forms a reverse ramp at the upstream edge of the skimming weir. The last opening should, therefore, be a little larger than the opening just on its upstream. IV. HYDRAULIC MODEL STUDIES In absence of any definite criteria, the design of desilting basins is based on many assumptions, broad guidelines and site specific conditions. Verification of these assumptions and adequacy of layout as well as other design aspects is therefore required to be tested by conducting hydraulic model studies. The basic aim of conducting these model studies is to judge the hydraulic performance of desilting basin in terms of settling efficiency and flushing efficacy of the settled sediment. Central Water & Power Research Station, Pune has conducted about thirty five physical model studies for desilting basin for various hydropower projects in India and abroad. On the basis of drawings supplied by concerned project authorities, the model of desilting basin is fabricated partly in fibre glass with transparent perspex windows and top dome to observe the flow conditions and sediment movement / deposition pattern. These models are fabricated to a geometrically similar scale ranging from 1: 20 to 1: 35 depending upon discharge, their shape and size and availability of water head. The inlet transition, outlet transition and silt flushing tunnel below the desilting basin are also fabricated in fully transparent perspex sheets. Generally, the model of desilting basin is tested for inlet sediment concentration of 5000 ppm or as otherwise indicated by the project authorities. A typical view of desilting basin in model is shown in photo 2 and inlet and outlet transitions in photo 3 and 4.

Photo 3: View of Inlet Transition in Model

Silt Flushing Tunnel

Photo 4: View of Outlet Transition in Model

For simulation of suspended sediment crushed and sieved walnut shell powder is used. This is a light weight material with specific gravity of 1.32. The material is injected into the desilting basin as per designed inlet concentration along with flow at the inlet. The simulation of sediment between model and prototype is done by fall (settling) velocity criteria. V. DISCUSSIONS The known volumetric quantity of sediment on the basis of inlet discharge and sediment concentration is injected in the model. There are two sediment collection chambers constructed at the downstream of the model one each for head race tunnel and SFT respectively. The sediment deposited in desilting basin and flushed through SFT is collected in SFT collection chamber and measured volumetrically. The settling efficiency of the desilting basin is found out as follows: ( )

Photo 2: View of Desilting Basin Model

The settling efficiency obtained by above equation is the overall settling efficiency for entire range of particle sizes of inlet gradation curve. However, the objective is to find the settling efficiency of 0.2 mm sediment particle size. To find out the efficiency for particle size of 0.2 mm, calculations are

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Special Issue 3, February 2014) International Conference on Trends in Mechanical, Aeronautical, Computer, Civil, Electrical and Electronics Engineering (ICMACE14)

carried out using the T.R. Camp’s criteria and these analytical results are compared with physical model study results to obtain settling efficiency for 0.2 mm particle size. As indicated earlier, desilting basins are designed for 90 % removal of sediment particle size of 0.2 mm and above. If the efficiency is considerably less or more than 90 % then length of basin is increased or decreased and again tested on the model. The efficacy of flushing system is judged by visual model observations. In case there is some sediment deposition on bed of inlet transition or in SFT, the design is slightly modified and again tested on the model. An example of sediment deposition at the inlet transition is shown in photo 5.

various parameters viz. inlet discharge, inlet sediment concentration & gradation, gross head, desired settling efficiency and flushing efficacy. Moreover, design of desilting basins is based on various assumptions and broad guidelines. Therefore, in spite of proper site planning and designing, their hydraulic performance is required to be tested on a hydraulic model to obtain optimum design for desired settling efficiency of suspended sediment for each project. Acknowledgements The authors sincerely thank Mr. S. Govindan, Director, CWPRS for his constant encouragement, guidance and kind permission for publishing this paper. The authors are also thankful to the various project authorities for providing the necessary financial support and data for conducting the model studies in absence of which it would not have been possible to conduct so many model studies at CWPRS, Pune. References [1] CWPRS: “Guidelines for Design of Desilting Basins (Pressure Flow)”, 2005. [2] Hazen, A. on Sedimentation, Trans ASCE, Vol LIII, 1904, p 63.

Sediment deposition

[3] Camp, T.R. Sedimentation and the Design of settling tanks, Trans ASCE, Vol 111, 1946, Paper No.2285. [4] Dobbins, W.E. Effects of turbulence on sedimentation, Trans ASCE, Vol 109, 1944, p 629.

Photo 5: Sediment deposition on bed of inlet transition

On the basis of experience of CWPRS on findings of model studies, a Technical Memorandum titled “Guidelines for design of desilting basins (pressure flow)” was published in year 2005. Another Technical Memorandum titled, “Guidelines for operation of desilting basins” has been published in year 2008 for practicing hydro engineers and designers for efficient and trouble free use of these devices on prototype. These memoranda are the only guidelines available at present for the designers and are being immensely used for the design and operation of desilting basins by various agencies like NHPC, NTPC, SJVNL, Central Water Commission and WAPCOS etc.

[5] Vetter, C.P.: Technical aspects of silt problem on Colorado river Civil Engineering Vol.10, No.11, Nov.1940, pp 698-701. [6] Rouse Hunter: "Engineering Hydraulics", John Wiley and Sons Inc. New York-1949, pp 811-814. [7] T. CaS: Technical Conditions and Standards for designing settling basins of hydropower stations- Moscow 1949. [8] Einstein, H.A.: Final report spawning ground’ University of California Hydraulic Engineering Laboratory 16 p, 2 tables 10 figs., 1965. [9] Hippola, U.T.B.: Influence of suspended sediment distribution on settling basin design’ International symposium of river mechanics Bangkok Jan.1973, pp 277 to 288.

VI. CONCLUSIONS Desilting basins are integral part of water conductor system of ROR hydropower projects in Himalayan region and are huge and costly underground structures. Once put into operation, it is very difficult to maintain and repair them. On the other hand, each project has its own site specific design considering Tamizhan College of Engineering and Technology (ISO 9001:2008 Certified Institution), Tamilnadu, INDIA

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