Experimental study of continues and discontinues parapet walls on piano key weir flow

A non-linear spillway is appropriate and acceptable as an economic structure with high efficiency. This hydraulic structure is applied in the dams and irrigation networks as an appropriate option. Effective discharge of the non-linear spillways such as labyrinth weirs increases up to 3 or 4 times of linear spillways in the constant width channels. The Piano keys weirs (PKW) are special forms of labyrinth weirs which were conducted by the Institute Hydrocoop in France. There are several types of piano key weirs that their difference is about upstream and downstream sloped faces. In this research, type “A” of PKW has been studied. In addition, in accordance with previous studies, the inlet pier shape is one of the effective factors on piano key weirs performance. Because the triangular pier has the best hydraulically performance at the many physical models, in this research we have chosen the triangular shape of piers. As has been shown in the previous studies, piano key weirs have relatively high discharge capacity at low head. So that in high head, spillway efficiency is decreased due to submergence in the start of outlet key. According to the literature review there are no studies in field of discontinues parapet wall. In this research, this object has been investigated and the results are compared to continue case. In this research, parapet wall with various slopes have been used that B́ = (0.25, 0.5, 0.75 and 1.0) ×B where B is the entire key length. This object was done to reduce submergence of spillway entrance. To achieve these purposes in addition to sloped parapet walls, the un-sloped parapet walls with discontinues conditions in part of inlet and outlet keys were used. Experiments of this research were done in physical and hydraulically laboratory of Shahid Chamran University (SCU) on a flume with 10 m length, 0.8 m width and 0.6 m height. The walls of the flume are made of glass and its floor is made of stainless steel. However, the bottom slope of the flume was zero. On the base of hydraulic requirement a type A of piano key weirs was designed and built with 3.5 cycles and geometrical ratio, P/Wu, equal to 1.33, relative length, Bi/Bo, 1 and relative width, Wi/Wo, 1.5. The discharges and its head of water were measured after the weir installation and the models of parapet walls on the weir crest. The upstream water level was measured by point gage and flow discharge was measured by calibrated rectangular weir where it was placed end of the flume. However, 22 various models of parapet walls were tested that are described on three categories: 1-piano key weir without any parapet wall as control experiment, 2-unsloped parapet walls (2.5, 3.5 and 4.5 cm height) with discontinues on (a): side crest (b): side crest and inlet key crest (c) entire weir crest and 3-sloped parapet walls in three heights ,2.5, 3.5 and 4.5 cm, with length B́ where B́ = (0.25, 0.5, 0.75 and 1.0) ×B. After accomplishing the tests and gathering data, in order to analyze discharge coefficient, the general equation of weirs was used. According to discontinues of parapet walls, the modified equations were used to calculate the theoretical discharge. The results show that sloped parapet walls on side crest have a little effect on spillway efficiency and could not prevent turbulence and interference flow jets. However, parapet walls with constant height have more effect. Un-sloped parapet walls with discontinues on the inlet and outlet key crest improved the weir efficiency in the high head water. So in the parapet wall with 4.5 cm height, discharge coefficient increased up to 34 percent (rather than control test) with discontinues in outlet key part and high head water. Due to increasing flow volume in outlet key and decreasing of inlet friction (due to discontinue parapet wall), the discharge efficiency increases. Finally, a statistical relationship with suitable regression coefficient using SPSS software was derived for prediction of discharge coefficient. This relationship with upstream Weber Number less and more than 50 were extracted. Moreover, the results of this research were compared to previous studies and it was found that the sloped parapet wall with discontinues in inlet and outlet keys improve the discharge coefficient