Journal of Irrigation and Drainage Engineering

The water flow under a high-dam gate with a small opening has rarely been studied. The discharge coefficient and head loss under sluice gates are studied experimentally, with the relative gate opening less than 0.10, including the free and submerged flow. The semiempirical formula for the discharge coefficient is proposed, which is used to estimate the overflow capacity of the free and submerged flows with a relatively small gate opening. The equation of the bed friction head loss of the submerged flow in the boundary layer is derived by using boundary-layer theory. The local head loss and the head loss caused by the flow turbulence in the eddy area are estimated, respectively. The results show that for the free flow, the inflection point of the vertical contraction coefficient, discharge coefficient, and total-head-loss coefficient varying with the relative gate opening was 0.04. The head loss caused by the flow turbulence in the eddy area played a leading role in the total head loss for submerged flow. The results can provide a reference for the design and operation of high-dam deep-hole gates.

This study used two-dimensional (2D or depth-average) numerical modeling to examine the effects of clearwater scour on the flow field at a single bendway weir (BW) set at angles of 30°, 60°, and 90° pointing into the approach flow. The flow field was characterized in terms of depth-average velocity (location and magnitude) through the contraction formed around the BW and was supported by data and observations obtained from large and small flumes. In addition, a three-dimensional (3D) model was used to assess when a 2D model is of design use and when it is not. The 2D modeling showed that, when the ratio of flow depth (about four BW lengths upstream) to BW height ($Y:H$) equaled 2.0, the differences (pre- and post-scour) in depth-average velocity magnitude and location of flow around the BW were fairly negligible for all orientations of the BW. However, when $Y:H$ equaled 1.25, the effects of the scour on the flow field were more prominent. Similar results were obtained for all three orientations of the BW. The value of $V_{\max }$ obtained with the 2D model was closer to that measured in the large flume when $Y:H$ decreased from 2.0 to 1.25. This was due to the shallower flow’s greater effect in directing the flow around the BW than occurred for the deeper flow, in which flow had a larger vertical component of velocity when passing over the top of the BW. The practical implications of using a 2D model versus a 3D model were briefly assessed, and 2D models were found to be suitable for designing BWs to manage thalweg position but inadequate for estimating near-bank velocities.

Vortex settling basins (VSBs) are efficient and economical structures that use free-surface vortex strength to remove sediment particles from flow. In this study, computational fluid dynamics was employed to predict and evaluate the effects of basin geometry on performance and flow field structure. The flow field was calculated using Reynolds-averaged Navier–Stokes modeling and large eddy simulation. Verification of the numerical simulations was carried out based on experimental data obtained using a typical VSB model. The Smagorinsky model results for abstraction ratio, velocity component, and free-surface flow were superior to those of the other turbulence models. The distinctive features of two previously proposed designs for vortex settling basins were investigated, and a new geometry to enhance these features is presented. The results indicated that the new geometry provided better performance in generating a symmetric central vortex, lengthening the particle settling path and decreasing the abstraction ratio.

Labyrinth weirs are commonly used to increase spillway discharge capacity. Stepped chutes represent a common spillway element used to help dissipate kinetic energy, thus reducing the size requirement and often the cost of the downstream energy dissipation basin. When combined, the complex, highly turbulent, aerated flow patterns generated by the labyrinth weir create different aeration inception point behaviors relative to traditional stepped chute applications (e.g., linear weir upstream). To reduce the risk of erosion and other damage near the spillway, stepped chute sidewalls are typically designed with sufficient height to contain the majority, if not all, of the bulked air-water spillway flow and surface waves. This study evaluated some of the hydraulic impacts of coupling a labyrinth weir with a relatively steep-stepped chute (08H:1V) at the laboratory scale in an effort to provide some useful guidance related to the sizing stepped chute wall heights. Test results showed that required chute wall heights for bulked flow containments were higher at the chute entrance than the traditional stepped chute (i.e., no labyrinth weir) design predictions. Placing ramped floors in the labyrinth weir downstream cycles facilitates the transition from the labyrinth outlet cycles to the steep chute and, in some cases, reduced maximum chute water levels. To better understand this phenomenon and provide guidance toward the use of ramped floors to reduce chute wall height requirements, the hydraulic behavior of various ramped floor configurations was systematically evaluated. The results for the geometries tested ranged from no effect to a 15% reduction in maximum chute flow depth.

Barrage weirs, which are trapezoidal-sectioned sharp-crested weirs having finite crest length, are primarily used in the Indian subcontinent for diverting water for irrigation. This study presents an investigative study on submerged flows over barrage weirs, as occurs during floods, and establishes correlations for determining the structure’s coefficient of discharge using a two-dimensional (2D) Reynolds averaged Navier Stokes-volume of fluid (RANS-VOF) model, coupled with the renormalization group (RNG) $k\text{-}\epsilon$ equations. The study specifically examines the piezometric gauging technique, commonly employed in Crump weirs, for estimating the discharge reduction factor of irrigation barrage weirs under submergence and compares the results with those obtained by the traditional tailwater gauging method. The results establish a unique correlation between the discharge reduction factor and the ratio of the piezometric head at a point immediately downstream of the weir crest and the total pressure head at a location on the upstream face of the weir. The method promises a more accurate estimate of the submerged discharge capacity for the barrage weirs than the conventional procedure. The study also suggests that a steeper upstream slope of a weir potentially leads to greater reduction in discharge under submergence. Further, for a given upstream weir face slope, the discharge reduction varies inversely with the relative crest length. The submergence rate at the modular limit is found to be in the range of 0.70–0.78. The modular limit, however, is found to be independent of the upstream face slope. The accuracy and reliability of the computed results are ensured by conducting relevant grid convergence analyses and validating the results of numerical simulations against experimental data.

The cutoff technique has not been adequately investigated despite being the most practiced among smallholder farmers. This study aimed at scenarios of improving water application efficiency, distribution uniformity, and productivity for the technique. Three cutoff treatments where water supply was stopped with the advance phases reaching 75%, 80%, and 90% of furrow length were investigated with eight irrigation events. Mean application efficiencies with significant difference ($\mathrm{P}<0.05$) of 70%, 66.4%, and 63% were achieved for the 75%, 80%, and 90% length cutoff treatments respectively. Corresponding uniformities and water productivities of 90, 89.9%, 89.2% and 1.54, 1.38, ${1.18\mathrm{kg}/\mathrm{m}}^3$, respectively, were obtained with no significant difference ($\mathrm{p}>0.05$). The results demonstrated the 75% cutoff has the potential of saving water of up to 26% without compromising water productivity. It is recommended therefore that the 75% cutoff position of water supply be promoted among the smallholder farmers and that research studies should now intensify for less than 75% cutoff.

Severe drought events and sparse rainfall due to climate change in recent years are resulting in costly demand for agricultural water resources, requiring sustainable management addressing water use efficiency and timing and quantity of irrigation applications. This study illustrates the advantage of using optimization considering interactions of water production function and limited available soil-water over the use of single-equation models of yield response function alone for promoting deficit irrigation. Procedures in this study maximize crop production and minimize available soil-water for deficit irrigation scheduling over a crop growth period of 3 months: June, July, and August. Deficit irrigation reduced crop water stress in July (driest month) compared to crop water stress in June and August. Applications of procedures to main-crop potato production data showed yield increases of one-half ton/hectare, and the minimized irrigation was 4 mm less on average than the quantity of soil-water actually used for the observed yield over a land area of about 6,700 ha in East Anglia, United Kingdom. A small proportionate increase of optimum soil-water increased the same proportion of main-crop potato yield.