Experimental Study of the Rough Bed Effect on Discharge Coefficient of Combined Flow Trapezoidal Labyrinth Weir-Gate

Background and objectives: Labyrinth weirs are often a desirable design option to regulate upstream water elevations and increase flow capacity and the gate structures have some advantages including passing the floating substances and the sediments in using combined weir-gate structure. But, it can be difficult to design due to the complex flow characteristics of a labyrinth weir-gate. A labyrinth weir could be described as a continuous and broken weir plan in a trapezoidal or triangular form. Thus, for a fixed width, labyrinth weirs have a longer crest distance when compared to linear one. Most weirs create a relatively static water zone in their upstream, which can be the site for sedimentation and waste materials, which is a disadvantage of these structures. Because of the sediments deposition in upstream of the weirs the flow conditions change and the accuracy of the presented relationships is reduced. Although numerous methods of design have been published for labyrinth weirs, there is insufficient design information available about the combined models of Labyrinth weir-gate.
Materials and Methods: This study was conducted to improve labyrinth weir -gate design and analyses techniques using physical-model-based data sets. The experiments were conducted in a metal and glass flume with a rectangular cross-section. The flume was 0.25 m wide, 0.5 m deep, and 10 m long. In each test the upstream subcritical depth was measured using point gauges of 0.1 mm accuracy The location for measuring the total head of the water upstream of the weir is a horizontal distance of three to four times the maximum water head on the crest of the weir. The discharge was measured with a triangle sharp weir placed at the end of the flume. The discharge-head relationship (Q-h) for triangular weir in experiments is as Q=0.6918 h2.5. The trapezoidal labyrinth weir-gate models was installed at the distance of 3 m of the beginning of flume. The base material roughness was made of natural sand with a mean diameter of 3 mm. In this research, experimental study of combined flow trapezoidal labyrinth weir-gate with one cycle has done for three sidewall angles of 15, 20 and 25 degrees, three gate openings 2, 4 and 6 cm and the weir height of 14 cm in a rectangular channel. According to the effective parameters of the combined models including sidewall angel (), gate opening (a) and the hydraulic head (Ht), the discharge coefficient has evaluated. By applying the Buckingham π theorem an equation was obtained. The discharge coefficient of trapezoidal labyrinth weir-gate can be expressed as a function of the variables of Fr,Re,We,H_t/P,L/H_t ,H_t/a,ϕ,α. In this study, the depth of water measured on the weir crest is at least 3 cm, so the effect of surface tension on the weir (We) is negligible. The effect of dynamic viscosity on the hydraulic behavior of the flow can be ignored. Therefore, the Reynolds number (Re) can be removed.
Results: The results show that the discharge coefficient decreases with increasing the ratio of Ht/P for both smooth and rough beds and it reaches a constant discharge coefficient for Ht/P >0.6. According to the effective parameters of the combined models, the discharge coefficient has obtained averagely in the range of 0.61-0.75. The discharge coefficient of the combined flow increases with increasing angle of the weir. The increase in discharge coefficient is due to the decrease in the length of weir which decreases the flow mixing. Also for a specified angle, the discharge coefficient increases with increasing of L/ Ht then gets a constant value. The effects of artificial roughness on discharge capacity are also presented. It can be shown that the discharge coefficient increases in rough bed condition compared to the smooth bed condition. The present test data and those of Crookston (2010) were compared and it can be seen that the discharge coefficient in combined flow trapezoidal labyrinth weir-gate is more than the discharge coefficient of the trapezoidal labyrinth weir (without gate) in Crookston (2010) investigation.
Conclusion: The discharge coefficient of trapezoidal labyrinth weir-gate has the highest value for weirs with a sidewall angle of 25° and gate opening of 6 cm in rough bed condition about 17% more than the smooth bed. Among the different experimental models with a sidewall angle of 25, the labyrinth weir-gate in the rough bed condition has the highest discharge coefficient (approximately 0.93) compared to the smooth bed (approximately 0.61).