Experimental study on vertical velocity and ...

Labyrinth and Piano Key Weirs II – PKW 2013 – Erpicum et al. (eds) © 2014 Taylor & Francis Group, London, ISBN 978-1-138-00085-8

Experimental study on vertical velocity and submergence depth near Piano Key weir N. Sharma & H. Tiwari Department of Water Resources Development and Management, IIT Roorkee, India

ABSTRACT: Insufficient spillway capacity has been the cause of one-third of all dam failures. An innovative and effective way of increasing the spillway capacity is to use a Labyrinth weir in the modified form of PKW having range of specific flow from 3 to 1000 m3 /s/m. As the flow approaches towards the PK Weir, it was observed that the Z-component of the velocity of water in lower levels increases which enhances the flushing capacity. One of the important observations is that with decrease in discharge from higher flow to medium flow; Z component velocity is getting lowered by about 14 percent. Increase in discharge from lower to medium flow Z component velocity is getting increased by about 40 percent. It indicates that at lower discharge, upward velocity component is appreciably more which may be helpful for significant amount of sediment suspension at lower discharge also. For flood flow, it was found that there has been reduction of around 25–30% of submergence depth with respect to ogee spillway. Induced flow characteristics near PKW and tractive stress exertion by flowing water in the inlet cell are capable to flush out even the coarser sand particles.

1 INTRODUCTION Spillways play a major role in ensuring the flood safety of dams. Insufficient spillway capacity has been the cause of one-third of all dam failures (Schleiss, 2011). Spillways represent a substantial portion of total project costs and they play a major role in ensuring safety (Modi & Seth, 2011). An effective way of increasing the spillway capacity is to use a Labyrinth weir (Tacail et al., 1990; Tullis et al., 1995). The concept of the Labyrinth weir is to vary the plan shape of the crest to increase the effective crest length (Lempérière & Jun, 2005). A new concept of a Labyrinth weir has been proposed with a new shape like black and white Piano keys when viewed in plan, this new concept was called the Piano Key Weir (Lempérière & Ouamane, 2003). This design solves most of the problem presented by the original Labyrinth weir, and is also more efficient (Anderson, 2011). Some differences between traditional Labyrinth weir and Piano Key Weir are as follows: – Vertical walls founded on a flat area are replaced by lateral vertical walls and sloping slabs upstream and downstream of the crest. These slabs are partially a cantilever structure, upstream and downstream. Therefore the overall structure is self-balanced. – The Piano Key Weir can be positioned on the top of the crest of new or existing gravity dams. – Application can cover a wide range of specific flows, from 3 to 1000 m3 /s/m. – Piano Key Weir can increase by a factor about 1.50 to 4.00 times than the specific discharge capacity of straight sharp crested weir. – From a structural point of view, Piano Key Weir is extremely hyper-static structures, which are solid and simple. The total length of the Piano Key Weir is typically three to seven times the spillway width. The Piano Key Weir is particularly well suited for cases where the length of the structure has to be restricted or for the rehabilitation of existing spillways. A Piano Key Weir can pass large discharge at a relatively low head. Its advantages include relatively low construction and maintenance costs 93

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Figure 1. A typical view of Piano Key Weir (PKW).

and more reliable operation, compared with gated spillways. Construction of the first prototype PK weir, Goulours dam in France, was completed in 2006; construction of the second prototype PK weir, Saint-Marc dam in France, was completed in 2008 (Anderson, 2011). This new Piano-Key Weir technology has already been used by the Himachal Pradesh Power Corporation Ltd. (HPPCL) on Sawra Kuddu Barrage. 2 DEVELOPMENT AND FEATURES OF PIANO-KEY WEIR (PKW) An initial model investigations and behavior of Piano Key Weir was studied by Lempérière and Ouamane (2003) in terms of a magnification ratio of the Piano Key Weir against sharp-crested linear weir having the same channel width. The results of the test showed that the Piano Key Weirs are simple solutions as safe and easy to operate as traditional free flow spillways and much more efficient. They may improve the flood control by many existing dams. Behavior of Piano Key Weir was studied by Barcouda et al. (2006) in terms of a magnification ratio of the Piano Key Weir flow for a sharp-crested linear weir having the same channel width. The most significant parameters in determining the capacity of a weir are its height relative to the upstream depth, the crest shape and the crest length (Falvey, 2003 & Afshar, 1988). The results of the test show that the Piano Key Weirs are more efficient than the traditional Labyrinth weir and Piano Key Weirs can be an interesting solution for increasing the active storage of reservoir or for improving the safety of dam during extreme flood. It will also help in minimizing the sub-mergence effect. Flushing is vital for the preservation of long-term storage in reservoirs. However, downstream impacts such as turbidity act as a constraint in the planning and operation of sediment flushing. In this study velocity distribution of all the three components of the velocity at different points at up-stream of the weir and over the weir and Velocity vector diagram in up-stream of the weir as the flowing water approaches the P.K. Weir and over the weir as it spills across the crest of P.K. Weir is investigated through laboratory experiment which has a good scope regarding the flushing study in reservoirs. A typical view of PKW is given in Figure 1. 3 LABORATORY ARRANGEMENT FOR VELOCITY COMPONENT STUDY An overflowing tank of 240 cm × 240 cm was installed at the upstream head-end to ensure the supply of steady discharge into the experimental flume. The flume used was made of mild steel with side walls made of transparent perspex sheet. Flume has an in-built upstream tank of 150 cm × 180 cm dimensions. Diagrammatic scheme is given in Figure 2. The bed of flume is supported on angle 94


Figure 2.

Laboratory Arrangements (PKW).

Figure 3.

Layout of grid in the space up-stream to the Piano Key weir.

iron sections. The water flowing in the flume, after it passes over the Piano-Key weir, falls into a down-stream tank installed with V-notch discharge measuring devices, which is connected to down-stream of V-notch storage tank. From the down-stream of V-notch storage tank, tank water is lifted with the help of a two 10 HP pumps. 10 cm diameter pipes carry water from the storage tank to the upstream constant head tank. The discharge is regulated with the help of a gate valve placed between the constant head tank and the inbuilt upstream tank. Laboratory Arrangements for vertical velocity measurement was given in Figure 2. An imaginary 3D mesh-grid was formed in the space available in the up-stream of the Piano-Key weir. This grid was divided into 5 sections along x-direction, 7 layers along the y-direction and 15 levels along the z-direction. The velocity readings were taken on the nodes of this mesh-grid using Acoustic Doppler Velocimeter (ADV). The ADV was fitted with a trolley which can move along the movable hand rail and across the fixed side rails of the flume. The hand rail can also move along the side rails so as to change to the desired position. The position along the z-direction was adjusted with the help of the vernier gauge, through which ADV was connected to the trolley. The position along the y-direction was adjusted by moving the trolley along the movable hand rail. The position along the z-direction was adjusted by a moving handrail itself along the fixed side (also along the direction of flow). The layout of the plan and section of the grid formed is as shown in the Figure 3 and Figure 4. As illustrated by Wang et al. (2011), the velocity measurements obtained with the 95


Figure 4.

Depths (Levels) of nodes at which velocity readings were taken (@ 69, 50, 31 lps). Table 1. Basic Preliminary Design Data (Kharagprasad Village, Odisha (India)). River Bed Level Lowest Water Level High Flood Level (HFL) Average Bank Level Upstream Reach Length Design Discharge for High Flood Design Discharge for Low Flow Water Way Width Average Channel Slope

53.5 m 54.0 m 58.0 m 62.0 m 20 km 2818 cumec 54 cumec 500 m 1:2000

Table 2. Preliminary Design Analysis of Piano Key Weir Type Barrage. Height of Piano Key Weir Crest Level of Piano Key Weir Head Over the Crest on P. K. Weir For High Flood Discharge Maximum Water Level on P. K. Weir Head Over the Crest on P. K. Weir For Low Flow Minimum Water Level on P. K. Weir

4.5 m 58 m 1.4 m 59.4 m 5.56 m 58.05 m

SONTEK ADV, is at 5 cm below the acoustic sensor (of the ADV) with a probe volume of 0.09 cm3 . With a frequency of 25 Hz, a recording time of 40 sec was taken for 1000 measurements required for satisfying time average conditions. A filter condition of Signal to Noise Ratio (SNR) greater than 15 dB and correlation (COR) greater than 70 percent was used as suggested by Volugaris and Trowbridge (1998), and the filtered data only are retained. 4 CASE STUDY FOR BRAHMANI RIVER IN LABORATORY Table 1 explains field data available and Table 2 give details about design criteria. Model in laboratory was run for high flood as well as low flow condition. This model was also run for 96


Figure 5.

Plan and Sectional View of Piano-Key Weir laboratory arrangement.

Figure 6. Variation of z component of velocity for 0.069 cumec discharge (layer 4).

a discharge of 20347.22 cumec which was the flood condition when upstream hydraulic structure releases excess water. This PKW at the proposed site is not yet commosioned. Plan and Sectional View of Piano-Key Weir laboratory arrangement details in Figure 5. 5 RESULTS AND DISCUSSIONS Z component velocity (cm/s) variation which plays important for sediment suspension as well as flushing point of view is shown in Figure 6, Figure 7 and Figure 8 for discharge conditions of 0.069, 0.050 and 0.031 cumec. 97




As the flow approaches towards the PK Weir, it was observed that the Z-component of the velocity of water in lower levels increases. From this it can be concluded that water gets some uplift as it approaches towards PK weir so as to pass through the inlet cell and get spilled in the downstream side. This observed flow characteristic not only enhances the discharge capacity of the weir but also helps in flushing of the sediment through the inlet cell of the Piano-Key Weir. Due to flow characteristics near PKW and tractive stress exertion by flowing water in the inlet cell is flow competence is enabled to flush out even the coarser sand particles of size up to 2 mm. One of the significant observations is that with a decrease in discharge from to higher flow (0.069 cumec) to medium flow (0.050 cumec); Z component velocity is getting lowered by 14 percent. Increase in 98


Figure 9.

Submergence Depth for PKW compared with Ogee spillway.

discharge from lower flow (0.031 cumec) to medium flow (0.050 cumec); Z component velocity is getting increased by 40 percent. It indicates that at lower discharge upward velocity component is quite more and may be helpful for a significant amount of sediment suspension at lower discharge also. From Figure 9, study on the geometry of Piano-Key weir, it can be observed that for a given design flood discharge 20,347.22 cumec (high flood condition), the head over the proposed PianoKey weir (4.2 m) was much lesser than that of a conventional ogee weir (6.173 m) of the same height. Some of the experimental results are given below for the case study of Brahmani River. – – – – –

Length of backwater due to incident afflux with P. K. Weir in position (High Flood) = 16 km Length of backwater due to incident afflux with P. K. Weir in position (Low Flow) = 9.1 km Volume of In-Stream Storage in Low Flow = 10.15 Million Cubic Meter (MCM) Water Requirement for Industry/Drinking Water Purpose = 2.2 Cumec If minimum flow ceases to exist, the above In-stream storage will cater for 54 days

Because of this, the submergence of the area upstream to the weir can be significantly reduced. So increasing discharge capacity and increment of flushing capability enhances the structural safety as well as reservoir sedimentation. It has also advantages of cost effective solution and it can be installed over existing dams.

6 CONCLUSIONS At lower discharge ranges, relatively higher vertical velocity component implies presence of sediment in suspension. The use of PKW can significantly reduce the upstream submergence area in comparison to conventional weir or barrage. Induced flow characteristics near PKW and tractive stress exertion by flowing water in the inlet cell, the flow competence is enabled to flush out even the coarser sand particles. REFERENCES Afshar, A. 1988. The Development of Labyrinth Weir Design, Water Power and Dam Construction, 40(5): 36–39. Anderson, R. 2011. Piano Key Weir Head Discharge Relationships, Dissertations for Graduate Studies, School of Master of Science, Utah State University.

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Barcouda, M., Cazaillet, O., Cochet, P., Jones, B.A., Lacroix, S., Laugier, F., Odeyer, C. & Vigny, J.P. 2006, Cost effective increase in storage and safety of most dams using fusegates or P. K. Weirs, in proceedings of 22nd ICOLD congress, CIGB/ICOLD, Barcelona, Spain, Q86, R3. Falvey, H. 2003. Hydraulic Design of Labyrinth Weirs. ASCE Press, USA. Lempérière, F. & Ouamane, A. 2003. The Piano Keys weir: a new cost-effective solution for spillways. Hydropower and Dams 5, 144–149. Lempérière, F. & Jun, G. 2005. Low Cost Increase of Dams Storage and Flood Mitigation: The Piano Keys Weir. Q.53 R. 2.06 International Commission on Irrigation and Drainage – Nineteenth Congress, Beijing. Modi, P.N. & Seth, S.M. 2011. Hydraulics and Fluid Mechanics including Hydraulics Machines. New Delhi. Standard Book House. Schleiss, A.J. 2011. From Labyrinth to Piano Key Weirs – A historical review. Proc. Intl. Conf. Labyrinth and Piano Key Weirs Liège B, 3–15. CRC Press, Boca Raton FL. Tacail, F., Evans, B. & Babb, A. 1990. Case study of a labyrinth weir spillway. Canadian Journal of Civil Engineering. 17(1): 1–7. Tullis, J.P., Amanian, N. & Waldron, D. 1995. Design of Labyrinth Spillways. Journal of Hydraulic Engineering, 121(3), 247–255. Volugaris, G. & Trowbridge, J.H. 1998. Evaluation of acoustic Doppler Velocimeter (ADV) for turbulence measurements. Journal of Atmospheric and Oceanic Technology 15(1):272–289. Wang, X.Y., Yang, Q.Y., Lu, W.Z. & Wang, X.K. 2011. Experimental study of near-wall turbulent characteristics in an open channel with gravel bed using an acoustic Doppler velocimeter, Springer-Verlag.

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