Investigate Foundation Local Scour Type-A Piano Key Weirs with Zigzag Lateral Crest

Document Type : Research Article


1 ferdowsi university of mashhad

2 water department of Ferdowsi university of mashhad

3 water and science engineering Dept. Ferdowsi university of mashhad


Introduction: According to the research background, increasing the approaching speed has been reported as the most important factor in reducing the discharge capacity and also, the falling jet from the inlet and outlet keys has been reported as the main cause of foundation erosion in piano key weirs. Researchers believe that the geometric characteristics of piano key weirs have a significant effect on hydraulic behavior, The phenomenon of immersion on the crest, as well as the downstream hole scour of this type of weirs. so; The authors of the present study, in addition to investigating the hydraulic performance of Type A piano weir in standard and zigzag lateral crest conditions, considered it necessary to investigate the dimensions of the hole scour in the downstream of this weir. therefore; The effect of zigzagging of the lateral crest profile, which was modeled with the aim of improve transmission capacity and increase the immersion threshold in high water heads, was investigated on the geometric characteristics of the hole scour.
Materials and Methods: Experiments in a channel with a rectangular cross section, metal frame and glass wall, with a length of 10 meters, a width of 0.3 meters and a height of 0.5 meters and a slope of 0.0012 in the laboratory of hydraulic models Department of Water Science and engineering was done at Ferdowsi University of Mashhad. Based on the critical depth of the flow on the crest, Range of changes in input flow rate in the range Lit/s were set to achieve a minimum relative head of water on the weir crest with values greater than 0.1. According to this criterion, the relative water depth range of all experiments in the range ( ) was considered. Free flow conditions were adjusted by considering the range ( ) and up to the value by the sleeper valve downstream of the channel. Physical modeling of weirs was modeled with two forms of standard weir crest profile profile (type A) and zigzag crest with ratios P/Wu=1.33 and Wi/Wo=1.2 in 2.5 repetition with the standard sharp crest weir model with constant and equal height, it was used for experiments. In the second model, the form of the zigzag crest weir of the piano key was designed as a sinusoid with a height of 5 mm. During the lateral crest, 9 complete sinus zigzags were modeled. The sedimentary layer of the canal floor with two non-uniform and non-stick grains (1 and 3.2 mm), according to the control test results, was considered to be 0.4 m thick and 2 m long.
Results and Discussion: In the present study; By designing a sinusoidal zigzag in the lateral crest of a standard piano key (type A), it was shown that the maximum discharge coefficient occurs in a smaller ratio H/P but with a higher numerical value than the standard mode, so that the average discharge coefficient increases by 10% Improves discharge capacity efficiency. On the other hand; Due to the importance of scouring, changes in the geometric parameters of the hole and ridge sedimentary in the downstream erosion bed were measured and analyzed by dimensional analysis using Π-Buckingham method. The interaction of the output current jet in dealing with the sedimentary bed and intersecting with the overflow flow of the overflow inlet keys is the main cause of obstruction and the emergence of two rotating vortices in the lower hole erosion downstream of the piano key weir. The results showed that the maximum depth of the hole scour decreased by approximately 31% with the piano key weir of the linear sharp crest. Also, with the zigzagging of the lateral crest of the standard piano key weir (type A), despite an increase of 15% in the length of the hole erosion, the final scour depth decreased by 12% compared to the standard model to half the corresponding value in the linear sharp crest model.
Furthermore; At maximum critical relative depth values of hc/P and hd/P water level compared to minimum values, the maximum scour depth increases by 73% and decreases by 90%, respectively. Also, it was observed that scour values occurred in fine-grained sediments 68% more than coarse-grained sediments. The process of sediment transfer in the hole and ridge scour increases with increasing The particle Froude number. In the particle Froude number range, Frd50 = 2.7-1.2 for the PKW, the average depth and length of the hole scour are estimated to decrease by 10% and increase by 22% respectively.
Conclusion: Finally, it should be noted that the proposed crest shape has reduced the maximum depth of the concrete slab in the economic design of downstream protection structures of Type-A PKW.


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