Document Type : Original Article

Authors

• Alireza Afzalian ¹
• seyed mohsen sajjadi ²
• Mahmood shafai bajestan ³
• javad Ahadiyan ⁴

¹ Ph.D. student in Water Structures, Faculty of Water and Environment, Shahid Chamran University of Ahvaz

² Assistant Professor faculty of water and environmental Engineering shahid chamran university of Ahvaz

³ professor, faculty of water and environmental shahid chamran university of Ahvaz

⁴ Associate Professor, Faculty of Water and Environment, Shahid Chamran University of Ahvaz

Abstract

Extended Abstract
The side weirs are widely used in sewage networks to aerate streams, irrigation and drainage networks to control water levels for dewatering, rivers for coastal management, and flood management of dams. One of the newest types of weirs researchers have considered in recent years is the piano key weir. Piano key weirs have a higher efficiency than other weirs due to their special geometry, especially the presence of upstream and downstream overhangs, as well as inlet and outlet keys. The use of piano key weirs as side weirs has received less attention from researchers, and because these weirs show better performance in discharge, more and more research is needed. The main objectives of the present study are first to investigate the effect of trapezoidal piano key weir height as one of the geometric parameters affecting the discharge coefficient and second to investigate the water surface profiles at the upstream and downstream ends and within the location of the weir in the main channel. In parallel with the main objectives, the study of the Dimarchi hypothesis in estimating the discharge coefficient and the study of discharge efficiency of the trapezoidal piano key is also followed. In this regard, four models of trapezoidal piano key overflow with a height of 10, 15, 20, and 25 cm (TPKSWp10, TPKSWp15, TPKSWp20, and TPKSWp25), a labyrinth trapezoidal weir model with a height of 20 cm (TNRSW) and also rectangular sharp crest weir as The control model (Lisw) was tested under the subcritical flow with the Froude numbers in the range of 0/10 to 0/74. De Marchi, in 1934, assuming that the specific energy was constant at the upstream and downstream ends, calculated an equation for estimating the discharge coefficient of the side weirs that were related to the hydraulic parameters of the flow at both the upstream and downstream ends of the weir. In this study, the main hypothesis for estimating the piano key weirs' discharge coefficient is the Di Marchi hypothesis. The results show that, firstly, due to the specific energy changes at the upstream and downstream ends of the trapezoidal piano key models (ΔE/E1), especially in the TPKSWp10 and TPKSWp15 models, and the occurrence of hydraulic jump that affects the essence of the flow, use the Dimarchi hypothesis And comparing the discharge coefficients of trapezoidal piano key side weirs should be done with caution. The discharge capacity of the side weirs is defined as the ratio of flow spill from them to the inflow to the main channel. The results of this study show that the discharge capacity of trapezoidal piano key weirs increases with increasing height due to the more uniform water surface profile, reducing the interference of the outlet blades of the inlet and side crest and reducing the vortex in the inlet openings and faster exiting than the outlet keys. In the TPKSWp25 model, the discharge capacity is 2/60 times higher than the Lisw, and for the TPKSWp20, TPKSWp15, and TNRSW models, it is 1/92, 1/59, and 1/38 times higher than the Lisw, respectively. Also, the water level decreases in the longitudinal section Z*=1 (on the crest weirs) and at the upstream end of the weir due to the increase in the longitudinal acceleration of the flow and being affected by the suction of the flow by the weir in this range in all experimental models. This water level reduction for TPKSWp15,20,25 models, and TNRSW models is almost the same and equal to 25%.
Introduction
In irrigation and drainage networks that are designed with a certain capacity, for various reasons, including improper operation by water collectors and in rivers and other natural channels, by imposing excessive capacity currently on them, excess current is created which can cause serious damage to them. And the economy of the project will be challenged and of course, it can lead to life-threatening risks. In the design of dams, according to the location of the dam and in order to use more of the volume of dams for flood control purposes or just more reservoir volume, sometimes the dam is located in such a way that the possibility of constructing a main weir perpendicular to the mainstream is technically and economically justified. Therefore, in such cases, the side weirs are introduced and designed as the main weirs of the dam. Also, in some dams that have been constructed with old hydrological information and meteorological statistics and information indicate that the weir will be potentially dangerous for future floods, the option of overflow correction or using side overflows as auxiliary overflows are suggested. Emin Emiroglu et al. (2010) performed about 2,900 experiments in the subcritical flow mode to analyze the water surface profile on the weir and the flow velocities along with the weir. Their results showed that the weir discharge coefficient in labyrinth weir mode is about 1.5 to 4 times higher than in rectangular weirs. Bagheri and Haidarpour (2012), by measuring the three-dimensional components of the flow velocity in the main channel near the rectangular side weir, concluded that the horizontal flow velocity component at the lower end of the stream decreases. Also, by examining the transverse and deep components of the flow, it was concluded that most of the flow is discharged from the lower end of the weir. Using physical models, Michelazzo (2015) proposed a new approach to solving the Dimarchi equation for zero-height side weirs in an open canal. To solve this model, the flow conditions were considered subcritical and the substrate constant. Solving them without using numerical methods makes it possible to estimate the weir outflow according to upstream and downstream hydraulic conditions. Aydin (2015), by placing the sill on the bed of the canal in three positions of the upstream end, the downstream end and in the middle of the weir, concluded that the presence of an obstacle at the lower end of the weir increases the lateral rectangular weir coefficient. Maranzoni et al. (Maranzoni, 2017) performed numerical and laboratory analyses on a side weir in a rectangular convergent channel. Their experiments, which were performed under subcritical and sustained flow conditions, show that the number of downstream landings and the dimensionless height of the weir has the greatest effect on the flow rate through the lateral weir in a converging channel. The approach of studies in recent years has tended to increase the efficiency and innovation in the use of new geometries of lateral weirs as well as new strategies and methods for estimating the discharge coefficient of these weirs. These findings include Ghaderi et al.'s (2020) studies to use numerical models to estimate the flow rate of trapezoidal zigzag weirs, Karimi et al. (2018) on the use of piano key weirs with a rectangular plan As side weirs, Saghari et al.'s (2019) studies and Seyed Javad et al. (2019) studies on the use of trapezoidal piano key weirs and Dibaco & Scorzini (2019) studies to estimate The lateral weir flow coefficient using neural network methods was pointed out.
 
Methodology
Considering several geometric parameters that affect the discharge coefficient of trapezoidal piano key side weirs, this study was designed to achieve the two main objectives of investigating the effect of trapezoidal piano key side weir height on its discharge coefficient and investigating the longitudinal profiles of flow depth in the main channel. In parallel with the main objectives, this study follows the study of the Dimarchi hypothesis in estimating the discharge coefficient of the piano key weir as well as its discharge capacity of it. In this regard, four trapezoidal piano key weir models differ only in height and therefore the inclination of the inlet and outlet keys, one trapezoidal labyrinth weir model, and a rectangular linear model as a control model will be tested in different Froude numbers.
In line with the purpose of the present study, six models were designed and built. These models included a sharp rectangular linear model (control model), a trapezoidal labyrinth model, and four trapezoidal piano key weir models with heights of 10, 15, 20, and 25 (Figure 1). Figure 1 shows all the models in three perspective views, plan and cross-section.
Results and Discussion
Adjusting the water level in irrigation and drainage networks in order not to disrupt the operation of reservoirs b creating a water level profile at the upstream end of the reservoir and also the pressure on the body of piano s, especially the side crown and inlet and outlet gutters according to the thickness and materials used. They are important topics for design engineers in irrigation and drainage networks and spillway design. In the present study, two types of diagrams have been used to investigate the effect of the height of trapezoidal piano weirs on the water surface profile due to the complexity of the flow pattern and the simultaneous effect of geometric and hydraulic parameters of the weir and better analysis of these parameters on the water surface profile.
Figure 3-a shows the Head-discharge curve of experimental models and 3-b shows the average flow rate of experimental models compared to the control model. As shown in this figure, for all dimensionless Y₁/P ratios, the discharge through the TNRSW is higher than the control model (Lisw). Also, the flow rate through TPKSWP20 is higher than the TNRSW model. It is worth noting that in all dimensionless dimensions Y₁/P, the flow through the piano key weir with a height of 10cm is less than all models. This rate is even 30% lower compared to the control model. The high velocity of the flow in the main channel and the impossibility of discharging the current even from the downstream cycle, which plays the greatest role in discharging the side weir, in the TPKSWp10 model reduces the effective length of the overflow crest in discharging the flow and is the main reason for this difference. Also, as shown in this figure, the flow rate of the TPKSWp25 model is higher than other models. A comparison of average flow rate overflows in Y₁/P ratios and the Froude range of the present study shows that the average flow rates of TPKSWp25, TPKSWp20, TNRSW, and TPKSWp15 models are 2/60, 1/92, 1/59 and 1/38 times higher than the control model, respectively.
 

Keywords

• Irrigation and drainage networks
• Dimarchi hypothesis
• the discharge capacity of side weir

Main Subjects

• Irrigation network management