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Abstract: Labyrinth spillways as appropriate hydraulic structure to pass PMF discharge have been considered. To replace existing spillways of reservoirs which in terms of capacity they are not hydraulically suitable, these types of spillways are recommended.  In a certain width with similar head these spillways with nonlinear crest can pass greater discharges compare to spillways with frontal crest. The Labyrinth spillway is a type of hydraulic structure which can be used for the existing and new reservoirs. This structure is considered for hydraulic engineers as a good alternative to keep the water level of the reservoirs in an acceptable position when there is a limit on the maximum level of the water surface of the reservoir. The plan shape of the labyrinth spillway consists of broken lines in the form of triangular, trapezoidal, semi circular and rectangular with several repeating cycles. The main objective of using this type of spillway is to increase the flow capacity of the reservoir while keeping the crest and the water surface of the reservoirs in a reasonable level. The use of this type of reservoir goes back to the early 1920’s; however, the real studies and research in this field started form 1968. The majority of researches in this field are for those with triangular or trapezoidal in plan. The most advantages of using labyrinth spillway in the reservoirs and channels are as follows: 1- If the width of the spillway or channels cannot be chosen enough wide due to upstream or downstream site condition; 2- If the maximum level of the water in the reservoir during the flooding season is limited; 3- If it is desirable to provide larger water storage of the reservoir by making the crest level of the spillway higher without causing floodplain of upstream lands during high inflow to the reservoir; 4- If the capacity of the existing spillways are not large enough with respect to current standards of dams safety; 5- It is more economical due to the need of smaller discharge channel downstream of the spillway; and 6- If the reservoir needs to have spillways with the gates to provide larger amount of water storage, in the case of labyrinth spillway the needs of the gates can be eliminated which is very advantageous both in terms of primary costs and maintenance costs. In this study by considering various angles of walls and lengths of nose different types of trapezoidal labyrinth spillways were designed. Applying Flow-3D software which is analytical software to solve equations of flow domain passed over spillway based on Eulerian viewpoint, numerical models were constructed. In order to model turbulence, the K-ε model in RNG state was used. Also to identify position of free surface profile, the VOF method was applied. To verify the results, experimental data of Tullis and Amanian (1995) was employed which the model results showed good agreement with the experimental data. Passing various discharges over the spillway variation of discharge coefficient was studied. It was showed with increase in length and angle of the spillway wall in flow direction the discharge coefficient would increase which is due to less interaction between the flow layers. This increase in small angles due to the reduction in over lapping layers is a considerable loss. At the end based on the obtained results a procedure to design a trapezoidal labyrinth spillway with choosing the angle of wall along the length of the nose was developed.    

Keywords: Trapezoidal Labyrinth Spillway Design, Coefficient discharge, CFD method, Flow-3D

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