This is particularly important in the region between the crest and the point of inception, because this region does not contain air to mitigate cavitations damage. Far below, in the region of uniform flow, air has reached the bottom layer hence; this reach is well protected against cavitations damage .
Figures 14to16 show minimum piezometric head distribution for various discharges over horizontal face for stepped spillway. It was found that, the piezometric head on the crest of spillway will increase under the lower heads and decrease until accrue negative readings under the higher heads. Also, the negative readings observed on the horizontal face of step number four under the lower heads and increases under the higher heads for model No.
Figures 17 to 19 show minimum piezometric head distribution for various discharges over vertical face of stepped spillway. It was found that, the vertical face of all the steps of spillway mostly was subjected to negative pressure in two cases firstly at low discharge, resultingjetadherencecausesthestreamlinestobecomemore curvedandtheflowvelocitytobecomehigher and secondly at higher number of steps; this resulting to converted jet flow to partially nappe flow and then to nappe flow generating triangular cells, contains maximum turbulence.
The pressure field in these cells is generating exhibit intense pressure fluctuations, these negative pressure converted to positive pressure when increase the discharge.
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Matos et al. This could be attributed to the reason that; as the head above crest of high spillway increases the overflowing process becomes easier and developing suctionpressureatthecrest and first stepresultingnappeadherencecausesthestreamlinestobecomemore curvedandtheflowvelocitytobecomehigher, this lead to developing jet flow over semicircular stepped spillway after hit it at first step, trying to speed the jet and consequently increase the flow rate passing over it and increasing the energy dissipation. These results agree very well with previously published results by Chanson .
As well as these figures show that spillway model No. In order to show the effect of height of step size on energy dissipation Figure 27 shows a comparison between three configurations of model, model No. Also, it is observed from the same figure that stepped spillway dissipated energy at low discharge higher than at high discharges. Three models are stepped in series No. In general view this figure showed a decending trend in energy dissipation with increasing the flow rate for all types of spillway, this can be attributed to the fact that as flow increases skimming flow dominates over nappe flow.
It is clear that the energy dissipation over stepped spillway is more than over traditional spillway with fully smooth.
Also; for stepped spillway when the number of stepped increases, the energy dissipation over the spillway decreases. This means that the small steps have little significant effect and they are like a smooth surface, this conclusion is agreed with Amanj . Also, minimums piezometric head distribution measured over vertical face of stepped spillway, mostly the vertical face of all steps of spillway was subjected to negative pressure in two cases firstly at low dischargeand secondly at higher number of steps.
As well as the results show that spillway model No.
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Also, observed that stepped spillway dissipated energy at low discharge higher than at higher discharges. Novak, A. Maffat, R. Narayanan, Hydraulic structures.
Hydraulics of Spillways and Energy Dissipators
Published by the academic division of uniwin Ltd. Chanson, Air bubble entrainment in free surface turbulent shears flows. Academic Press, London, UK, pp: Chanson, H. Dam Energy, Vol. Rajaratnam, Skimming flow in stepped spillway. Hydraulic Eng. Chamani and N.
Rajaratnam Characteristic of skimming flow over stepped spillways. Barani, M. Rahnama, N. Sohrabipoor, Investigation of flow energy dissipation over different stepped spillways. American Journal of Applied Sciences, 2 6 , Al-Talib, Laboratory study of flow energy dissipation using stepped weirs. Hussein, I. Juma and S. Shareef Flow characteristics and energy dissipation over single step broad crested weirs. Irzooki, and S. Boesand and W. Hager, Hydraulic design of stepped spillway. Ohtsu, Y. Yasuda, and M. Takahashi, Flow characteristics of skimming flow in stepped channels.
Pinheiro, and C. Fael, Nappe flow in stepped channels, Occurrence and energy dissipation. Amador, B. Valenzano, M. Sanchez-Juny, J. Pomares, , and J. Chanson, Hydraulics of skimming flows over stepped channels and spillways, Journal of Hydraulic Research, Vol.
Hydraulics of spillways and energy dissipators
Essery, and M. Horner, the hydraulic design of stepped spillway. Ciria Report 33, London, 45 pp. Chow Open-Channel Hydraulics. Khatsuria, Hydraulics of spillways and energy dissipaters. Marcel Dekker, U. Matos, M. Sanchez, A. Quintela, and J. Dolz, Air entrainment and safety against cavitation damage in spillway over RCC dams. Chao-Yang, p.
Rui-Wan, and H. Xue-Min, Stepped dissipater on spillway face, Proc. Related Papers. By iaeme iaeme. By Najm Alghazali. It is noteworthy that no book on spillways published until the late eighties included the topic of aeration, and reference sources remained scattered in journals and proceedings. The book has been organized into four sections dealing with spillways, energy dissipators, cavitation and air entrainment, and hydraulic modeling.
Emphasis has been on discussing first the hydraulics of different types of spillways and energy dissipators and to illustrate its application top ractical design problems. To this end, illustrative examples have been included at appropriate places. The reason is the electronic devices divert your attention and also cause strains while reading eBooks.
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