Investigation of channel capability for sediment transport is an important factor in sanitary sewer and urban runoff drainage system design. This is because of the undesirable effects arising from the reduced channel hydraulic capacity due to the deposition of sediment particles. Self-cleansing channel designs minimize sediment deposition and، thereby، prevent reduced channel hydraulic capacity. Our literature review revealed no previous report on self-cleansing models yet proposed for channels with trapezoidal sections. An experimental study was conducted using a trapezoidal channel and a non-deposited bed to investigate the self-cleansing process with four different sediment particle sizes under various slopes and discharge rates. The experimental data were exploited to develop a bed load self-cleansing model for the trapezoidal channel considering the specific flow، fluid، sediment، channel، and sediment characteristics. The proposed model was then compared with those reported in the literature for circular and rectangular channels. It was found that channel section geometry is a decisive factor in channel self-cleansing rate. Comparisons revealed that circular and rectangular self-cleansing models produce an error of approximately 15% when employed for designing channels with trapezoidal sections. Hence، the model developed in this study is recommended for designing channels with trapezoidal sections

Trapezoidal open channels are among the most common and optimized sections that used in water conveyance system. Thus investigation of flow conditions in them is very essential. One of the most important related subjects is lateral velocity component that produced cells in secondary flow. Several parameters such as channel roughness can intervene on how cells were formed. In the present study applying CFD methods using ANSYS CFX package flow condition in trapezoidal open channels withroughed boundary were simulated and variation of cells in secondary flow and distribution of hydraulic parameters such as mean depth velocity and boundary shear stress under uniform increase of boundary roughness were investigated. The results showed changes in sizes as well as the cells location due to uniform roughness increase. Also this increase was caused decrease in mean depth velocity and increase in boundary shear stress.

The bed and sidewall average shear stresses in smooth trapezoidal open-channels are determined by solving the continuity and momentum equations. The isovels and orthogonal for a trapezoidal channel section with 1: 1 sidewall aspect ratio were obtained by using conformal mapping technique. New equations were drived by determining a portion of bed shear stress from total shear stress. As a first approximation, the boundary shear stress equations were obtained after neglecting the secondary currents and assuming a constant eddy viscosity. In comparison with laboratory measurements, the first assumption overestimated the average bed stress by 20% but it underestimated the average sidewall shear stress by -24%. The second approximation was then presented by introducing two lumped empirical correction factors for the effects of secondary currents, variable eddy viscosity and other unexpected effects. Comparison between laboratory measurements and those from the second approximation, showed an excellent agreement (R2>0.993 and average relative error less than 1.2%) between them with experimental measurements over a wide range of width-depth ratios.

In the present research, height and velocity of shock waves in contractions of open-channel with trapezoidal and rectangular sections was investigated using experimental models. For this purpose, length of transition (0. 5m), convergence ratio (1/2, 1/3 and 1/4) and side slope angle (45º , 60º , 70º and 90º ) were considered as geometric variables of the experiments. Also 4 Froude number was assumed as a hydraulic variable of the experiments in the range of 3. 25-9. 23. Analysis of free surface and velocity profiles of shock waves showed that in general, the increasing of side slope angle of the transition wall, reduction of convergence ratio, and increasing of Froude number have a direct relationship with the increasing height and velocity of shock waves. The results showed that in the contraction with trapezoidal section with the convergence ratio (1/2) and side slope angle (45º , 60º and 70º ) compared with rectangular cross-section, on average maximum height of shock waves respectively decreased to: 59. 34, 44. 27 and 24. 04. In the same condition, the maximum velocity of shock waves respectively decreased to: 37. 51, 25. 23 and 14. 67. As Executive channels mainly with a trapezoidal cross-section are building and operating, so the findings of this study can be very useful for design engineers.

Aim of this study is to measure shear stress in smooth rectangular and trapezoidal open channels. A new approach is presented to measure direct shear on wet perimeter of open channels. A water flume with a section on a frictionless support (called knife edge flume) along with a system of load cell was used to measure total force on the wet perimeter. The local boundary shear stress in rectangular and trapezoidal cross sections was measured using a 4mm diameter Preston tube connected to a differential pressure transducer and a data acquisition system. The calibration curves proposed by Patel were used to convert the pressure readings to shear stress. Local and total shear measurements were used to differentiate the channel wall and bed shear forces. This method allows minimizing of the error as result of the Preston tube diameter. Comparison of the results confirmed advantage of the momentum method over the energy method in term of accuracy. Results are presented for smooth rectangular and trapezoidal channel cross sections with side slopes of 1, 1.5, and 2.

General equations for predicting depth-averaged velocity in open channels are not well suited for trapezoidal open channels. In this study, an innovative method is developed for predicting the depth-averaged velocity distributions in smooth trapezoidal open channels with the best hydraulic section. Depth-averaged velocity was obtained using the boundary shear stress obtained in the present study. To calibrate the present analytical results, an experimental investigation was also performed. Analytical results compared well with the experimental data.

The non-uniform flow in a prismatic channel with gradual changes in the free water surface level is called the gradually varied flow (GVF). Calculation of the GVF profiles over the last century has become a significant topic for the researchers in the relevant fields.  To obtain this profile, the nonlinear ordinary differential equation of the GVF needs to be solved. This can be carried out either numerically or analytically. Although several studies have been conducted on the GVF in open channels in various forms (Jan & Chen, 2013; Vatankhah, 2010, 2015; Homayoon & Abedini, 2019), the number of semi-analytical studies in the field of gradual variable flow in trapezoidal and triangular channels is limited, which requires further investigation. In this research, the Adomian Decomposition Method (ADM) is used to find a semi-analytical solution for solving the GVF equation in the triangular and trapezoidal prismatic channels. In this method, the Manning equation is used as the resistance equation. Moreover, for the aim of verifying the semi-analytical solutions, the ADM results are compared with the finite difference method (FDM). The presented semi-analytical solutions in this paper can be used to validate other numerical methods in similar studies.

Background and Objectives: Contractions have many uses in supercritical flows, such as flow conveyance from intake channels of dams to tunnel spillways, reduction of chutes width and reduction of flow conveyance time in the flood conduits. In supercritical flows studies, the formation of the shock waves has an important role. Technically, production and development of the mentioned waves are undesirable due to water depth increase because of several times increasing of inflow water depth, its spread at a wide range in downstream of channel and water surface roughness. Any weak design of channels under supercritical condition can cause to scour channel’s bed and walls, damage to equipment in the flow direction, raising maintenance costs and reduce water conveyance efficiency. In the present research, the formation of shock waves in converged transitions of open channel with rectangular and trapezoidal sections was investigated using laboratory and physical models.Materials and Methods: In order to investigate hydraulic parameters of shock waves in the converged transitions, twelve models with different geometries were used. In the present research, the studied geometric variables were the diagonal length of transition walls (0.5, 0.75 and 1 m) and side wall angle (33.69o, 45o, 60o and 90o). In all used models, the convergence ratio was 0.5. The height and instantaneous velocity were measured in different points of formed shock waves in the mentioned models for four different Froude number in the range of 3.25 to 9.23.Results: The measured values in the converged transitions showed that the velocity distribution was not uniform in the vertical direction of shock waves. Also, the results showed that by traveling wave front toward downstream cause to reduce wave velocity and increase wave height so that for various geometries, the changes trend was different. The results showed that on average, and for side slopes angels of 33.69o, 45o and 60o, the maximum height of shock waves was reduced 64.8%, 54.3% and 39.6% respectively in the comparison of trapezoidal and rectangular sections. Also, in the converged transitions and for the mentioned side slope angles, maximum shock wave velocity was reduced 39.1%, 31.6% and 16.5% respectively in the comparison of trapezoidal and rectangular sections.Increasing of side slope angle was accompanied with energy dissipation increment of shock waves for a constant Froude number and transition wall length. Also, maximum value of energy dissipation was seen for 0.5 m of wall length. The values of energy dissipation for the mentioned length, Fr1=7.26 and side slopes angels of 33.69º, 45º, 60º and 90º were achieved 14.69%, 15.43%, 16.34% and 18.72%, respectively.Conclusion: The analysis of the velocity profiles and free surface of shock waves showed that in general the reduction of side slope angle (increasing side slopes) of the transition wall, increase of diagonal wall length of the transition and reduction of Froude number have a direct relationship with the reduction of waves velocity and height. Since channels are constructed in the form of trapezoidal, the obtained results of the present research can be very useful for designer engineers.

Measurement of irrigation water in agricultural areas is necessary, especially, in arid and semi-arid regions. Long-throated flumes have been used to measure irrigation water and are known to be inexpensive devices with high performances. In these structures, head-discharge tables are predicted theoretically with no need to calibrate. In this study, the model of LOTF (LOng Throated Flumes) was developed in VB. net programming language, in which, friction head loss and h-Q (h is head at measuring station and Q is discharge) tables were predicted by using the Manning's equation. The predicted tables of h-Q by the LOTF model were perfectly similar to the results of Win Flume and HEC-RAS model which are universally used for these flumes and open channel flow, respectively. Calculated h-Q tables in trapezoidal flumes with two different side slopes of 0 and 1, 0 and 2, and 0 and 4 were computed by the LOTF and HEC-RAS models and were the same as those predicted by the WinFlume model with equal average of these side slopes of channel. At a water depth, in trapezoidal sections with different and equal average side slopes, the resulted h-Q tables in the three models were the same.