aeration on stepped spillways and its effects on

Seventh International Congress on Advances in Civil Engineering, October 11-13, 2006 Yildiz Technical University, Istanbul, Turkey

AERATION ON STEPPED SPILLWAYS AND ITS EFFECTS ON STREAM REAERATION Egemen ARAS, Mehmet BERKÜN Karadeniz Technical University, Department of Civil Engineering, Trabzon, Turkey

Abstract Dissolved oxygen (DO) is the most important parameter to control the water quality in streams and dam downstreams. DO effects the water quality directly. Spillways with their water-air controlling mechanisms are not only important for their structural properties but also from their effects on stream ecology. Spillways types also affect the efficiency of aeration. Decisions on the types of spillway should be made by taking the environmental conditions and flow rates into consideration. Gas transfer between air and water can be raised to high levels by aearation using the stepped spillways on dams. By this way, oxygen and nitrogen gas levels can be controlled at desired rates. Energy is gradually dissipated on the steps and this makes the stilling basins more economical with the size reduction. Also, by this way, expected levels of cavitations on the spillways can be reduced. In this study; the purpose of using stepped spillways and effects of aeration occurring on these structures on the stream reaeration was investigated. An experimental study results on stepped spillway also given. Key Words: Stepped Spillway, Aeration, Dissolved Oxygen, Stream Reaeration.

Introduction Recent advances in technology have permitted the construction of large dams, reservoirs and channels. These progresses have necessitated the development of new design and construction techniques, particularly with the provision of adequate flood release facilities.

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Chutes and spillways are designed to spill large water discharge over a hydraulic structure without major damage to the structure itself and to its environment. Because stepped spillways can significantly reduce the depth and size needed for a stilling basin at the toe of a dam and lead to great economic benefit, stepped spillways are increasingly attractive in engineering. But, at present, except for some formulas for the calculation, the study of stepped spillways is based only on physical models. Furthermore, most studies have focus on factors related to energy dissipation. Thanks to the technological advances in construction of roller compacted concrete (RCC) dams over the past two decades, stepped spillways for discharging excess flood water have gained significant interest and popularity among researchers and dam engineers, both for new dams for armouring of existing embankment dams. The use of stepped spillways has enhanced the performance and economy of many RCC dams where the concrete placement in lift allows an economic and fast construction of spillway steps on the downstream dam faces (Frizell, 1992).

Flow Regimes on the Stepped Spillways A stepped chute consists of an open channel with a series of drops in the invert. The flow over stepped spillways can be divided into two regimes; nappe flow and skimming flow. Nappe Flow Regime The nappe flow regime is defined as a succession of free falling nappes. The flowing waters bounce from one step to the next one as a series of small free falls. Nappe flows on horizontal steps are characterised typically by the presence of hydraulic jumps. A nappe flow without hydraulic jump might occur for relatively large discharge, before the apparition of skimming flow. Along a chute with horizontal steps, a typical nappe flow situation consists of a series of free-fall jets impinging on the next step and followed by a hydraulic jump. The flow energy is dissipated by jet break up in air, by jet impact and mixing, on the step and by the formation of a hydraulic jump on the step. Stepped channels with nappe flows can be analysed as a succession of drop structures (Fig 1). Skimming Flow Regime For large discharges, the waters flow down a stepped channel as a coherent stream, skimming over the steps. In the skimming flow regime, the external edges of the steps form a pseudo-bottom over which the water skims. Beneath the pseudo-bottom, recirculating vortices develop, filling the zone between the main flow and the steps. The vortices are maintained through the transmission of shear stress from the fluid flowing past the edges of the steps. In addition small scale vorticity is generated continuously at the corner of the step

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bottom. Most of the flow energy is dissipated to maintain the circulation of the recirculation vortices. On stepped chutes with skimming flow regime, the flow is highly turbulent and the conditions for free surface aeration are satisfied. The free surface aerated flow region follows a region where the flow over the chute is smooth and glassy. Next to the boundary, however, turbulence is generated and the boundary layer grows until the outer edge of the boundary layer reaches the surface. When the outer edge of the boundary layer reaches the free surface, the turbulence initiates natural free surface aeration. The location of the start of air entrainment is called the point of inception. Downstream of inception point, a layer containing a mixture of both air and water extends gradually through the fluid. Far downstream the flow will become uniform and for a given discharge any measure of flow depth, air concentration and velocity distributions will not vary along the chute. This region is defined as the uniform equilibrium flow region (Fig 1). Once the skimming flow becomes fully developed, a stepped chute behaves in the same way as un-stepped one.

Figure 1. Nappe and skimming flow.

Air Entrainment on Stepped Spillways Air Entrainment Conditions The flow conditions above a stepped channel are characterised by a high level of turbulence and large quantities of air are entrained. Air entrainment is caused by turbulence velocities acting next to the air-water free surface. Through this interface, air is continuously trapped and released. Air entrainment occurs when the turbulent kinetic energy is large enough to overcome both surface tension and gravity effects. The turbulent velocity normal to the free surface V' must overcome the surface tension pressure, and be greater than the bubble rise velocity component for the bubbles to be carried away. These conditions are:

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8  W .d ab

(1)

V ' Ur.Cos

(2)

V '

where σ is the surface tension, ρw is the water density, dab is the air bubble diameter, Ur is the bubble rise velocity and α is the channel slope. Air entrainment occurs when the turbulent velocity V' satisfies both equations 1 and 2 (Chanson, 1993). Aeration on Steps Considering a stepped chute with nappe flow regime, air is entrained at each step by a plunging jet mechanism at the intersection of the over falling jet and the receiving waters, and at the toe of the hydraulic jump. With deep pooled steps, most of the air is entrained by the plunging jet mechanism. For flat steps with shallow waters, most of the air is entrained at the toe of the hydraulic jump. The air entrainment characteristics of hydraulic jumps were analysed by a number of researchers. On a stepped spillway with skimming flow, the entraining region follows a region where the flow over the spillway is smooth and glassy. Next to the boundary however turbulence is generated and the boundary layer grows until the outer edge of the boundary layer reaches the surface. When the outer edge of boundary layer reaches the free surface, the turbulence can initiate natural free surface aeration. The location of the start of air entrainment is called the point of inception. Downstream of the inception point of freesurface aeration, the flow becomes rapidly aerated and free surface appears white. Far downstream the flow will become uniform and for a given discharge any measure of flow depth, air concentration and velocity distributions will not vary along the chute. This region is defined as the uniform equilibrium flow region (Fig 2). Effects of Aeration In supercritical flows above chutes and spillways, the amount of entrained air is important design parameters. Self aeration was initially studies because of the effects of entrained in volume of the flow. Air entrainment increases the bulk of the flow which is parameter that determines the height of spillway sidewalls. Also the presence of air within the boundary layer reduces the shear stress between the flow layers and hence the shear force. The resulting increase of flow momentum must be taken into account when designing a ski jump and a stilling basin downstream of a spillway, a stepped spillway or a rockfill dam with possible overtopping (Falvey, 1980). Further presence of air within high-velocity flows may prevent or reduce the damage caused by cavitation. Cavitation erosion may occur on stepped spillways. But the risks of cavitation damage are reduced by the flow aeration. Peterka and Russell and Sheehan showed that 4 to 8 % of air concentration next to the spillway bottom may prevent cavitation damage on concrete surfaces. On stepped spillways, the high rate of energy 4

dissipation reduces the flow momentum in comparison with a smooth chute. The reduction of flow velocity and the resulting increase of flow depth reduce also the risks of cavitation as the cavitation index increases (Chanson, 1994, Peyras et al 1992). For the designer the flow bulking is estimated from the total quantity of air entrained while the prevention of cavitation damage requires the knowledge of the air concentration in the layers close to the spillway bottom. The reduction of the drag observed with air entrainment will reduce the energy dissipation above the spillway and hence its efficiency. It must be emphasised that the reduction of the friction factor observed with increasing mean air concentration is not completely understood. The presence of bubbles across the flow and the bubble size distribution is expected to affect the turbulence and the turbulent mixing mechanisms (Avery and Novak 1978).

Figure 2. Aeration on stepped spillway

Stream Reaeration One of the most important water quality parameters in rivers and streams is the dissolved oxygen content (DO). The oxygen concentration is a prime indicator of the quality of the water. Dams and weirs across a stream affect the air-water gas transfer dynamics. Deep and slow pools of water upstream of a dam reduce the gas transfer process and the natural reaeration as compared to an open river. But some hydraulic structures (e.g. spillway, stilling basin) can enhance the oxygen transfer during water releases. Organic waste from municipal, agricultural and industrial sources may overload the natural system, causing a serious depletion of the oxygen supply in the water. Water rich in nutrients produces algae in quantities which upon decomposition deplete the oxygen supply. Fish kills are often associated with this process of eutrophication. Habitats for warm water fish populations should contain DO concentrations of not less than 4.0 mg/l. habitats for cold water fish populations should not be less than 5.0 mg/l (Berkun and Onal, 2004).

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On stepped spillways, both the aeration of the flow and the strong turbulent mixing enhance the air-water transfer of chemicals. The chemicals can be atmospheric gases or polluted matters. The strong turbulent mixing increases the coefficient of transfer in comparison with a quiescent fluid. And the large amount of entrained air bubbles increases the air-water interface area due to the cumulative bubble surface areas. Overall gas transfer at cascades and overflow weirs can be measured by the deficit ratio (r) defined as:

r

C S  CUS C S  C DS

(3)

where Cus is the upstream dissolved gas concentration and CDS is the dissolved gas concentration at the downstream end of the channel. Upstream DO deficit of greater than 2.5 mg/l is normally required for accuracy in oxygen transfer efficiency measurement. The primary source of measurement uncertainty was found to be uncertainty in the oxygen saturation concentration. In the summer time, when the average saturation concentration is about 7 mg/l in most areas, this specification results in an upstream DO of less than 4.5 mg/l (Tebutt, 1972).

Experimental Study All experiments were conducted in Water Researches Laboratory of Karadeniz Technical University. A stepped spillway model made of wood was tested. The spillway crest is 13.5 cm above the toe and the length of spillway is 12 cm. There were 4 steps on the whole stepped spillway. The spillway was 10 cm wide and an about 3m-long channel was provided downstream of the crest. Both the approach channel and the tail channel had the same width of 10 cm as the spillway. Dissolved oxygen concentration measured at upstream of the spillway (A) and four different point (B, C, D, E) of downstream of the spillway (Fig 3). The discharge of the spillway was 0.10, 0.15, 0.20, 0.32, 0.40 lt/sn respectively and channel slope 0°, 0.5°, 1.0°, 1.5° respectively. Dissolved oxygen concentration has measured by oxygen meter (WTW Oxi 39).

Figure 3. Laboratory channel model

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Two distinct flow regimes occur on stepped spillways, so-called nappe and skimming flow. Whereas in nappe flow the steps act as a series of overfalls with the water plunging from one step to another, the water flows as a coherent stream over the pseubottom formed by the outer step edges in skimming flow, without air pockets under the jets. Nappe flow is found for low discharges (0.10, 0.15, 0.20 lt/sn) and skimming flow is found high discharges (0.32 and 0.40 lt/sn) (Aras, 2003). The results of our experiments are presented in figure 4. Usage of Spillway Aeration Recently air entrainment on spillways and chutes has been recognised also for its contribution to the air-water transfer of atmospheric gases such as oxygen and nitrogen. This process must be taken into account for the re-oxygenation of polluted stream and rivers, but also to explain the high fish mortality downstream of large hydraulic structures. In rivers, artificial stepped cascades and weirs can be introduced to enhance the dissolved oxygen content of polluted or eutrophic streams. With large dams, the downstream nitrogen content is another important parameter. Nitrogen supersaturation may increase the mortality of some fish species. In the Columbia and Snake rivers, gas bubble disease caused by water supersaturated with nitrogen gas was a significant cause of fish mortality for salmonids and steelheads (Orlins and Gulliver 1997). In the treatment of drinking water, cascade aeration can be used to remove gases and to eliminate or reduce offensive taste and odour. Inclined corrugated channels and drop structures are effective means for water treatment. The combination of flow aeration and high flow turbulence enhances greatly the mass transfer of VOC’s such as chlorine. When power is not required, this process can be successful, since the operation requires little maintenance (Toombes and Chanson, 2000).

Conclusions In both nappe and skimming flow regimes, the air-water gas transfer taking place above a stepped chute is enhanced by the strong turbulent mixing and by the aeration of the flow. The flow aeration must be taken into account for the re-oxygenation of polluted streams and rivers, but also to explain the high fish mortality downstream of large hydraulic structures. As the flow patterns of stepped channel flows are very different from smooth chute flow situations, designers must analyse carefully stepped channel flows. Firstly, engineers must select the appropriate flow regime. Then, the hydraulic calculations are developed for that flow situation. Designers must take into account the hydraulic characteristics, the flow aeration and the hydrodynamic interactions between the flow and the steps.

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

a)=0



b)  = 0.5

8



c)  = 1.0



d)  = 1.5

Figure 4. Dissolved oxygen concentration at different channel slope 9

The results of our experimental study indicate that the aeration efficiency of nappe flows is primarily a function of the discharge and channel length. The oxygen transfer increases with increasing chute length. Also the oxygen transfer increases with the discharge for a constant ratio Hdam/dc. In skimming flow situations, aeration efficiency is nearly zero as long as no free-surface aeration occurs. When free-surface aeration takes place on the stepped channel, the oxygen transfer increases sharply with the dam height. The largest aeration efficiencies are obtained with small discharges. The analysis shows also that the channel slope and number of steps have little effect on the aeration efficiency of skimming flows.

References Aras, E. (2003). Stepped Spillway Aeration On Dams and Effects in Stream Reaeration (In Turkish), M.Sc. Thesis, Karadeniz Technical University Institute of Natural Sciences, Trabzon, Turkey. Avery, S.T. and Novak, P. (1978). Oxygen Transfer at Hydraulics Structures, JI of Hyd. Div., ASCE, Vol.104, No.HY11, 1521-1540. Berkun, M. and Onal, A. (2004). Effects of İnorganic Metals on Respirometric Oxygen Uptake and Related Sag Curve Formations in Streams, Water SA, Vol.30, No.2, 273-278. Chanson, H. (1993). Stepped Spillways Flows and Air Entrainment, Can JI of Civil Eng., Vol.20, No.3, 422-435. Chanson, H. (1994) Hydraulic Design of Stepped Cascade, Channels, Weirs and Spillways, Elsevier Science Ltd., Oxford, England. Falvey, H.T. (1980). Air Water Flow in Hydraulic Structure, USBR Eng. Monograph, No. 41, Denver, USA. Frizell, K.H. (1992) Hydraulics of Stepped Spillways for RCC Dams and Dam Rehabilition Proc. 3nd. Specialty Conf. on Roller Compacted Concrete, ASCE, 423-439, USA. Orlins, J.J. and Gulliver, J.S. (1997) Prediction of Dissolved Gas Concentrations Downstream of A Spillway, Proceedings, XXVII. IAHR Congress, 524-529, New York. Peyras, L., Royet, P. and Degoutte, G. (1992) Flow and Energy Dissipation Over Stepped Gabion Weirs, JI of Hyd. Eng., ASCE Vol.118, No.5, 707-717. Tebutt, T.H.Y. (1972) Some Studies on Rearation in Cascades, Water Research, Pergamon Press, Vol.6, 297-304, England. Toombes, L. and Chanson, H. (2000). Air Water Flow and Gas Transfer at Aeration Cascades: A Comparative Study of Smooth and Stepped Chutes, Hydraulics of stepped Spillways, Balkema, Rotterdam.

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