Journal of Irrigation and Drainage Engineering

Sewer bend manholes are frequent elements of urban drainage systems. Any deviation of straight-lined supercritical flow, as within the manhole, generates shock waves, possibly impinging at the manhole end or imposing a hydraulic jump. Then the free-surface flow regime abruptly breaks down and backwater effects occur. Thus it is important to know the maximum discharge that safely can pass across a bend manhole, as a function of its geometry, without generating collapsing flow. This study conducted calibrated numerical simulations to assess the hydraulic features of supercritical bend manholes with variable deflection angles, curvature radii, and lengths of straight downstream extension elements. The numerical model was validated previously with data from analogous physical model tests documented in the literature. The combined data from the numerical simulation and from the physical model indicated a hydraulic capacity of the bend manholes for different geometrical setups. It was demonstrated that the hydraulic capacity of a bend manhole increases with increased curvature radii and straight extension lengths, whereas the effect of the deflection angle is less significant. A multiple regression technique provided an empirical equation indicating the normalized discharge capacity of supercritical bend manholes as a function of the governing geometrical parameters, along with the approach flow filling ratio.

At some locations along the length of open sewers and drains, sediment particles get deposited on the bottom bed, reducing hydraulic efficiency. A sediment invert trap (SIT) can be implemented on the bottom of open sewers and drains to trap sediment particles moving on the bed surface. Several investigators have conducted experimental and computational studies to understand the particle trapping behavior of invert traps of various shapes and depths under varying sediment and flow parameters. In experimental studies, previous investigators have used actual and artificial sediment particles; in computational analyses, spherical particles have been considered. The current study investigated the particle trapping behavior of an irregular hexagonal sediment invert trap (SIT) through experiments and two-dimensional (2D) numerical modeling. Seventy-two experiments were performed using actual sewage solid particles in an open rectangular channel with a bottom-fitted invert trap with multiple depths. Sixty numerical simulations were performed using ANSYS Fluent 2020 R1 computational fluid dynamics (CFD) software, considering spherical and nonspherical sewage solid particles. The velocity field was predicted using the volume of fluid (VOF) model combined with the realizable $k\text{-}ϵ$ turbulence model. Particle trap efficiency was predicted using a stochastic discrete phase model (DPM). Trap efficiency varied significantly with depth of flow, invert trap depth, slot aperture size, and particle size and shape. Analysis of the simulated flow field showed that variation of turbulent kinetic energy above the trap’s slot aperture was also responsible for variations in trap efficiency values. Fluent simulations considering particles as nonspherical (using shape factors) agreed well with experimentally measured trap efficiency, with mean absolute percentage errors (MAPE) of 6.23%, 8.79%, and 13.77% for particle size ranges ${\mathrm{SS}}_1$, ${\mathrm{SS}}_2$, and ${\mathrm{SS}}_3$, respectively, with both slot apertures at all flow depths. The 2D-CFD study consistently overpredicted trap efficiencies compared to the experimental findings. With a fixed length and width of 0.32 and 0.15 m, respectively, the optimal depth of the irregular hexagonal SIT was found to be 0.65 m with slot aperture sizes of 0.15 and 0.03 m, whereas with slot aperture size of 0.15 m only, invert trap depths equal to 0.58, 0.43, and 0.38 m was found as optimum.

This study measured the cost efficiency and estimated the optimal cost structure in operating and maintaining the irrigation systems of eight provinces in the Red River Delta (RRD), Vietnam, from 2014 to 2016 using the input-oriented model of data envelopment analysis. The inputs included in the model were the costs of labor, materials, electric energy for irrigation, irrigation structure safety, maintenance, depreciation, administrative expenses, and overhead cost. The output was the irrigation area, which was converted to paddy areas. The findings of this study showed a large inefficiency in using the total input financial cost subsidized. The optimal technical efficiency scores under constant returns to scale (CRS) and variable returns to scale (VRS) were 0.86 and 0.98, respectively. The results implied that total input costs should be reduced by 14% under CRS and 2% under VRS to avoid cost inefficiency. The results also showed that scale efficiency occurred in the operation and maintenance (O&M) management of the irrigation systems. The O&M management in the irrigation systems in the RRD should allocate the major proportion of the costs to labor to be efficient. Application of the optimal input cost structure would improve irrigation management performance for the RRD region by 48.7% for the efficient input target (EIT) under CRS and by 5.03% for the EIT under VRS. These were the significant bases for recommending policy options for reviewing the irrigation tariff subsidy according to each input factor. And then it could create the incentives for private participation partnership in the irrigation sector.

The standard curve-number method has shown limited accuracy in estimating observed rainfall-runoff relations, particularly for less extreme rainfall events. Consequently, the Natural Resources Conservation Service is considering revising the standard initial abstraction ratio and associated published curve numbers to improve the performance of the method. A form of the curve-number equation is presented here that improves the prediction accuracy of the curve-number method without the need to adjust the initial abstraction ratio or the existing guideline curve numbers, and the improved runoff estimation is at least as good as could be achieved by adjusting the initial abstraction ratio. The improved curve-number equation can also be used to analytically estimate curve numbers corresponding to any given initial abstraction ratio, and it is demonstrated that these estimates are supported by field measurements.