International Journal of Civil Engineering and Technology (IJCIET), ISSN (Print), AND TECHNOLOGY (IJCIET)

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1 INTERNATIONAL ISSN (Online), Volume JOURNAL 5, Issue 4, April (2014), OF pp. CIVIL IAEME ENGINEERING AND TECHNOLOGY (IJCIET) ISSN (Print) ISSN (Online) Volume 5, Issue 4, April (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJCIET IAEME LOCATION OF AIR INCEPTION POINT FOR DIFFERENT CONFIGURATIONS OF STEPPED SPILLWAYS Najm Obaid Salim Alghazali 1, Salam M. Jasim 2 (1) Corresponding author, Asst. Prof. Doctor, Civil Engineering Department, Babylon University, Iraq (2) M.Sc. Student, Civil Engineering Department, Babylon University, Iraq ABSTRACT Twelve stepped spillway models have been manufactured with three downstream slope angles: 25, 35 and 45, and four numbers of steps: 5, 10, 15 and 20. The results of experimental work emphasize that for the same model air inception point location moved downstream as the discharge increased. For the same discharge, the location of air inception point was closer to the crest for larger step heights and lesser slope angle. The location of air inception point was closer to the crest in pooled steps compared with flat steps; this location was farther than flat steps when the gaps between end sills and step rises were filled with gabions. Twelve empirical equations for air inception point distance on stepped spillways were suggested based on the experimental results. The experimental results for flat steps were compared with the results of three relationships: Matos et al. (2000), Chanson (2001) and Boes and Hager (2003) for the prediction of the location of air inception point in the limits of this study. The comparison showed that Boes and Hager relationship results (2003) were the closest to the experimental results. Keywords: Air Entrainment, Gabions Steps, Inception Point, Pooled Steps, Stepped Spillway. 1. INTRODUCTION The stepped spillway is a spillway whose face is provided with a series of steps from near the crest to the toe, they have gained popularity with modern construction techniques including roller compacted concrete (RCC) and gabions [1], [2], [3], [4]. A peculiar aspect of the flow on a stepped spillway is the large aeration occurs at all nappe, transition and skimming flow regimes. Modern stepped spillways are designed to operate with a 82

2 skimming flow regime [5].In the skimming flow regime, the determination of the exact location at which air entrainment starts on stepped spillway is very important due to its significant effect on energy dissipation rate, cavitation risk and training wall height [6], [7]. Air entrainment starts at the location where the turbulent boundary layer reaches the free surface [1]. The presence of air within high-velocity flows may prevent or reduce cavitation damage. On stepped spillway with skimming flow regime, the reduction of flow velocity and the resulting increase of flow depth reduce also the risks of cavitation [8]. Uncertainty in this notion has perpetuated conservative design practices [9]. The zone near the inception point of air entrainment is critical in terms of the risk of cavitation damage. For increasing flow rates, the non-aerated region of the spillway will increase and larger velocities will be reached that could cause unacceptable pressure fluctuations. Downstream the inception point, the presence of an adequate percentage of air in the mixture near the solid surfaces is expected to prevent cavitation damage [10]. In the last two decades, there has been an increasing interest in the stepped spillways in various laboratories around the world [11]. The stepped spillway design is not limited to flat uniform steps, some prototype stepped chutes were designed with pooled steps (e.g. Sorpe dam, Germany), alternate sills (e.g. Neil Turner stepped weir, Australia), and weir structures designed with gabion steps, etc.[3], [12]. Alternative stepped designs are poorly understood [3].In the recent years, the airwater flows on pooled stepped spillways were researched in a few studies[13].also, the hydraulics of gabions stepped spillway has received less attention due to the complexity to evaluate the flow patterns and flow resistance [14]. The advantages of gabions are: low cost, ease of installation, flexibility, and ease of maintenance [15]. With proper construction practice, spillways having a stepped downstream face built of gabions can withstand floods of up to 3 m 2 /s without damage [16].Stone size and shape have little influence on the energy loss and flow velocity as compared to the increasing effect of the weir slope [14]. Many relationships have been developed to predict the location of air inception point for conventional flat stepped spillway. The estimation of this point in other configurations is not yet well understood. The objective of this study is to suggest new empirical equations for three configurations (flat steps, pooled steps, pooled with gabions steps). 2. EXPERIMENTAL WORK Twelve stepped spillway models were made from wood and coated with varnish to avoid wood swelling of water and to increase its smoothness. The models have vertical upstream face with three downstream slope angles (25, 35 and 45 ). For each slope, four models were designed as ogee stepped spillway with 5, 10, 15 and 20 steps. All models have 0.3 m width and 0.3 m height (from the base to the upper point in the crest). The tests were carried out in a recirculating flume located at the fluid laboratory of Engineering College, Babylon University, Iraq (Photo 1). The flume is 10 m length and 0.3m width. It has transparent side walls with height of 45 cm. The flume has a pump with a discharge capacity of 30 l/s, a flow meter is installed on its pipeline for measuring the discharge of the passing flow. Two movable carriages with point gauges were mounted on brass rail at the top of flume sides, which have an accuracy of 0.1 mm. The experiments were conducted for fifteen discharges runs ranging from 0.9 to 9.3 l/s (Table 1). This range was satisfying the need of this study. Photo 2 shows a sample of the observed locations of air inception points. Figure 1 explains the distance to the air inception point ( ) and the step roughness height (K s ). 83

3 The used end sills are of height equals to half step height i.e., he = 0.5h; he is the end sill height and h is the step height. Four thicknesses of end sills are used: 0.5, 1, 3 and 5 mm for the models having number of steps: 5, 10, 15 and 20 steps respectively. Photo 1: The used flume (Civil Engineering Department, Engineering College, Babylon University, Iraq) Table 1: Discharges used in the 15 runs Run No. Q (l/s) q (l/s.m) Run No. Q (l/s) q (l/s.m)

4 International Journal of Civil Engineering and Technology (IJCIET), ISSN (Print), Photo 2a Photo 2b Photo 2: Sample of the observed locations of air inception points a) Side view. b) Top view. Figure 1: Schematic representation of the measured The gabion dimensions are h e (l-t) 0.3 m, where h e is the end sill height, l is the step length and t is the end sill thickness. During testing runs, the gabions were placed into the steps and removed alternately. Wire mesh of rhombus shape with side length of 0.68 cm and diagonals of 0.65 and 1.2 cm has been used. The wire mesh boxes were filled with gravel of size cm and porosity of 41%. These types of wire mesh and gravel were selected taking into account that the filled material should be larger than 1.5 times the wire mesh opening and the porosity values between 38 and 42 are preferable as suggested by previous studies (such as Stephenson (1979) and Kells (1993) cited in [17]). 3. DATA ANALYSIS 3.1 Suggested Relationships The suggested empirical equations for the location of air inception point for steps and pooled with gabions steps for all the slope angles 25, 35 and 45 Table 2. All symbols are defined in appendix 1. flat steps, pooled are presented in 85

5 Table 2: Suggested empirical equations. θ Equation R 2 Case 45 K s = Fr * K s = Fr * Flat steps 25 K s = Fr * For all slopes K s = sin θ Fr * = Fr * = Fr * Pooled steps 25 = Fr * For all slopes = sin θ Fr * θ Equation R 2 Case 45 = Fr * = Fr * = Fr * Pooled steps with gabions For all slopes = sin θ Fr *

6 International Journal of Civil Engineering and Technology (IJCIET), ISSN (Print), 3.2 Comparison with Previous Relationships For flat stepped spillways, the selected equations to be compared with the results of the present study are presented in Table 3. Comparisons between the obtained data and the calculated ones for the three slope angles (45, 35 and 25 ) are shown in the Figs. 2, 3 and 4. Table 3: The studied relationships Researcher Equation Chanson (2001) [1] Matos et al. (2000) (cited in [18]) Boes and Hager (2003 3) [19] Figure 2: /K s and Fr * scatter on 45 slope angle flat models 87

7 Figure 3: / and Fr * scatter on 35 slope angle flat models Figure 4: /K s and Fr * scatter on 25 slope angle flat models 4. CONCLUSIONS It can be concluded that for the same model the location of inception point of air entrainment moves downstream as the discharge increases. For the same discharge, the location of air inception point is closer to the crest for larger step heights and lesser slope angle. The location of inception 88

8 point of air entrainment is closer to the crest in pooled steps compared with flat steps; this location is farther than flat steps when the gaps between end sills and step rises are filled with gabions. The comparison of experimental results for flat steps with the results of three relationships: Matos et al. (2000), Chanson (2001) and Boes and Hager (2003) for the prediction of the location of air inception point in the limits of this study showed that Boes and Hager relationship results (2003) were the closest to the experimental results. REFERENCES [01] Chanson, H., The Hydraulics of Stepped Chutes and Spillways, Balkema, Lisse, the Netherlands, [02] Gonzalez, C. A., An Experimental Study of Free-Surface Aeration on Embankment Stepped Chutes, Ph.D. Thesis, University of Queensland, Australia, [03] Felder, S. and Chanson, H., Air Entrainment and Energy Dissipation on Porous Pooled Stepped Spillways, International Workshop on Hydraulic Design of Low-Head Structures (IWLHS), Aachen, Germany, (87-97), [04] Guenther, P., Felder, S. and Chanson, H., Flat and Pooled Stepped Spillways for Overflow Weirs and Embankments: Cavity Flow Processes, Flow Aeration and Energy Dissipation, IWLHS, [05] Gonzalez, C.A. and Chanson, H., Experimental Study of Turbulence Manipulation in Stepped Spillways. Implications on Flow Resistance in Skimming Flows, Proceeding of the 31 st IAHR CONGRESS, Soul, Korea, [06] Hunt, S. L. and Kadavy K.C., Inception Point Relationship for Flat-Sloped Stepped Spillways, J. Hydr. Eng., ASCE, 137(2): , [07] Jian-hua, W., Bin, Z. and Fei, M., Inception point of air entrainment over stepped spillways, ScienceDirect J. hydrodynamics 25(1):91-96, [08] Chanson, H., Hydraulics of Stepped Spillways and Cascades, International Conference on Hydraulics in Civil Engineering, University of Queensland, Brisbane, Australia: , [09] Frizell, K.W., Renna, F.M. and Matos, J., Cavitation Potential of Flow on Stepped Spillways, J. Hydr. Eng., ASCE, 139(6): , [10] Amador, A., Sanchez-Juny, M., and Dolz, J., Developing Flow Region and Pressure Fluctuations on Steeply Sloping Stepped Spillways, J. Hydr. Eng., ASCE, 135(12): , [11] Khatsuria, R.M., Hydraulics of Spillways and Energy Dissipators, Marcel Dekker, New York, U.S.A., pp , [12] Chanson, H. and Gonzalez, C.A., Stepped Spillways for Embankment Dams: Review, Progress, and Development in Overflow Hydraulics, Proc. Intl Conf. on Hydraulics of Dams and River Structures, Tehran, Iran, Balkema Publ., The Netherlands, pp , [13] Felder, S., Guenther, P. and Chanson, H., Air-Water Flow Properties and Energy Dissipation on Stepped Spillways: A Physical Study of Several Pooled Stepped Configurations, Research Report No. CH87, School of Civil Engineering, the University of Queensland, Brisbane, Australia, [14] Chinnaarasri, C., Donjadee, S. and Israngkura, U., Hydraulic Characteristics of Gabion- Stepped Weirs, J. Hydr. Eng., ASCE, 134(8): , [15] USACE, Use of Gabions in the Coastal Environment." CETN-III-31, 12/86, [16] Peyras, L., Royet, P. and Degoutte, G., Flow and Energy Dissipation over Stepped Gabion Weirs, J. Hydr. Eng., ASCE, 118(5): ,

9 [17] Salmasi, F., Sattar, M. and Pal, M., Application of data mining on Evaluation of Energy Dissipation over Low Gabion-Stepped Weir, Turkish Journal of Agriculture and Forestry No. 36: ,2012. [18] Sarfaraz, M. and Attari,J., Selection of Empirical Formulae for Design of Stepped Spillways on RCC Dams, World Environmental and Water Resources Congress, ASCE: ,2011. [19] Boes, R. and Hager, W., Two-Phase Flow Characteristics of Stepped Spillways, J. Hydr. Eng., ASCE, 129 (9): , Appendix 1: Symbols Symbol Unit Definition Fr * [ ] Roughness Froude number =q / g (sin θ)(h cos θ) 3 g [m/s 2 ] Gravity acceleration h [m] Step height h e [m] End sill height K s [m] Step roughness height perpendicular to the pseudo bottom [m] Equivalent roughness height perpendicular to the pseudo bottom [m] Distance from the upper point on the spillway crest to the inception point q [m 2 /s] Discharge per unit width R 2 [ ] Coefficient of determination y c [m] Critical flow depth above spillway crest θ Degree Downstream slope angle 90