Search

A rectangular slit weir is used to measure small flows, namely, those flows less than 5 L/s and contraction ratios (b/B) less than 0.25. In the present study, flow tests were conducted in a laboratory setup with various contraction ratios (b/B) under models A (b/B = 1/12), B (b/B = 1/8), C (b/B = 1/6), D (b/B = 5/24), and E (b/B = 1/4). The dimensional analysis revealed that b/B, the height of the flow on the crest of the weir to the height of the weir crest (h/P), the Reynolds number (Re), and the Weber number (We) are effective parameters for predicting the discharge coefficient of the slit weir. The experimental results indicated that the discharge coefficient is variable for different contraction ratios; the maximum average discharge coefficient was achieved at the lowest b/B (Model A), with a value of 0.72. The minimum average discharge coefficient obtained in Model B equals 0.639. In addition, former prediction equations were compared with the obtained experimental data, and the minimum, average, and maximum errors were calculated. Furthermore, a relationship was established for deriving the discharge coefficient in slit weirs with R2 = 0.9212, RMSE = 0.02244, and MAE = 0.0134 based on the obtained experimental data with four effective parameters, b/B, h/P, Re, and We. [ABSTRACT FROM AUTHOR]

Grade control structure (GCS) can be used to reduce erosion in river beds. They decrease the river bed scour by changing the flow regime and reducing its velocity. It is required to maintain the stability of these structures against the scour hole that generates downstream of them, so investigating the scour hole characteristics downstream of the GCS is inevitable. In the present research, the equilibrium scour holes downstream of GCS were investigated under three discharges (q=0.00467, 0.00583, 0.00700 m3/s.m) and two different sedimentation conditions (d50=0.082, 1.6 mm). The results show that in the constant GCS geometry and finer sediment, the maximum scour depth (ds) and maximum scour hole length (ls) increase. In the same sedimentary conditions and different geometry, when the slope of the GCS downstream face varied from 90° to 60°, ds increases, but ls decreases. Also, the hydraulic jump forms on the scour hole at high flow intensity, which has an essential influence on the geometry of the equilibrium scour hole. Comparison of ds/Hc between the present study and Ben Meftah and Mossa (2020) has R2=0.891 and RE=10.66%, which reveals acceptable compatibility between the present study results and previous ones. [ABSTRACT FROM AUTHOR]

Central Asia (CA) is among the world's most vulnerable regions to climate change. Increasing anthropogenic greenhouse gas concentrations (GHGs) are the primary forcing of the current and future climate system for the time scale of a century. By analysing observation datasets, we show that a warming of 1.2°C led to a decrease of 20% in snow‐depth CA during the last 70 years, especially over the mountains. In recent decades, longer summer times and fewer icing days (more than 20 days·year−1) have exposed unprecedented shock to CA's climate system's components. Furthermore, we analyse 442 model simulations from Coupled Model Inter‐comparison Project Phase 5 and 6 (CMIP5, CMIP6) and show that CMIP6 simulations are generally warmer and wetter than the CMIP5 ones in CA. For instance, under the highest emission scenarios (RCP8.5 and SSP5‐8.5), CMIP6 projects a 6.1°C increase, while CMIP5 projects a 5.3°C increase, suggesting CMIP6 anticipates greater warming with high emissions. In contrast to CMIP6, the CMIP5 precipitation trends suggest a potential nonlinear relationship between increased greenhouse gas emissions and changes in precipitation, though the impact is much less pronounced than the temperature changes. Our analysis shows that CMIP6 models are more sensitive to temperature rise than CMIP5 ones. Both simulation sets' ensemble means capture well the observed warming trend. The imposed snow‐melting leads to an increase in the run‐off in the vicinity of glaciers. Such climatic shifts lead to more flooding events in CA. Given the projected warming range of 2–6°C in CA at the end of the century in various scenarios and models, such warming trends might be catastrophic in this region. The seasonal cycle of the temperature change indicates an extension of the glacier's melting period under future scenarios with fossil‐fueled development. The models' uncertainty increases for the far‐future time‐slice, and warming larger than 4°C in CA is very likely among all the models and during all the seasons if no sustainable action is taken. This study also incorporates a detailed Köppen climate classification analysis, revealing significant shifts towards warmer climate categories in Central Asia, which may have profound implications for regional hydrological cycles and water resource management, particularly in the Amu Darya and Syr Darya river basins under warmer scenario by the end of the century. The Tundra and ice cap climate categories will lose more than 60% of their coverage at the end of the century compared to the historical period in the Amu Darya and Syr Darya river basins. [ABSTRACT FROM AUTHOR]

Debris upstream of the bridge pier and through the flow variations, changes scour hole and river morphology. Dune forms downstream of the pier as a result of sediment movement in the pier scour phenomenon. The present study investigates the dune characteristics and geometrical parameters in experimental models. Experimental models are categorized into four cases including the pier, pier with buried debris, pier with free debris, and pier with free debris which was protected by bed sill at four various distances from the pier downstream face. It was concluded that debris submergence, densimetric particle Froude number, flow intensity, and bed sill affect dune geometrical parameters such as dune height (hd), dune crest position (xd), and dune length (ld). Results show dune height increases with debris accumulation up to almost 150% and its crest distance from the pier reduces up to approximately 100%. Also, in higher Fd, the dune height and its crest distance reduce with bed sill up to 50%, and in lower Fd, hd, and xd increase by up to 80% by using bed sill. Based on these effective parameters, three separate equations were proposed for hd, xd, and ld in pier scour with debris accumulation protected by bed sill. [ABSTRACT FROM AUTHOR]

Sluice gates are utilized in dams and irrigation channels as regulating structures in the channel have the role of regulating the water level for proper water intake of upstream intakes. As high velocity and energy of the flow issuing from these hydraulic structures scour occurred in erodible downstream beds. Occurrence of the hydraulic jump is very possible, and when it forms, therefore the necessity of investigating the flow condition increases. The scouring phenomenon is a time evolution and a long-time procedure. This research focused on time effects on experimental data received by a sediment bed profiler at different times from the experimental setup conducted in a laboratory. The maximum scour depth (dse), the maximum dune height (hd), the horizontal distance of the maximum scour depth from the beginning of the sedimentary bed (xse), and the horizontal distance of the maximum dune height from the beginning of the sedimentary bed (xd) were investigated. Laboratory observations show that the extension of the location of the maximum scour depth and the maximum height of the dune formed with time is evident. The laboratory data for the dimensionless conditions of 4 scour profiles characteristics including dse/b0, hd/b0, xse/b0, and xd/b0 were analyzed by the nonlinear regression method, and separate equations were proposed for each characteristic. RMSE=0.045, 0.0677, 0.358, and 2.146 respectively, and the R² =0.996, 0.975, 0.974, and 0.977 respectively. [ABSTRACT FROM AUTHOR]

Dust weather has an impact on human health and climate change in the Middle East. In this study, we examined the influence of the Arctic Oscillation (AO) on spring dust weather frequency over the Middle East at an interannual timescale during 1974–2019. The results show that there was an interdecadal shift of the influence of AO on dust weather frequency in the Middle East, with a correlation coefficient between the AO index and dust weather frequency changing from −0.52 during 1974–1994 period (P1) to 0.49 during 1995–2013 period (P2). During P1, negative correlations between AO index and dust weather frequency were found over the northern Arabian Peninsula, and during P2, positive ones were mostly over the central and southwestern Arabian Peninsula. In the lower and middle troposphere, negative geopotential height anomalies are associated with the negative phase of the AO over South Europe during P1, whereas negative geopotential height anomalies are associated with the positive phase of the AO over Northeast Africa during P2. The negative height anomalies resulted in the occurrence of more low‐pressure systems and, thus, dust weather in the Middle East. It is found that the interdecadal shift of the influence of the AO on dust weather was related to an AO‐related northward displacement of wave train propagation during P2. Specifically, during P1 period, the northern branch of AO‐related wave train dominated over the middle latitudes of the North Atlantic across South Europe to the Arabian Peninsula, which resulted in the negative height anomaly over South Europe in the lower and middle troposphere. However, during P2 period, the AO‐related wave train propagation moved northward, and then, the southern branch of the AO‐related wave train dominated over the low latitudes of the North Atlantic across northern Africa to the Arabian Peninsula, which played an important role in the formation of the negative height anomalies over Northeast Africa. [ABSTRACT FROM AUTHOR]

Investigating the effect of the geometrical characteristics of the piers, i.e. increase in the length of the pier end, could reduce the transverse wave height and help to achieve the optimal design for the converging chute walls height and reducing the construction costs. In this paper, FLOW-3D is used for three-dimensional simulations of the Khair-Abad dam spillway and investigating the spillway piers geometry effects on forming the transverse waves. The simulation results show that the heights of the second and third waves increase by increasing the length of the pier end which is located adjacent to the chute walls due to the superiority of the effect of the chute convergence on the piers end length changes. It is shown that the suitable way to reduce the transverse wave height is increasing the length of the middle pier end. The simulation results demonstrated that the third wave height does not significantly change by increasing the length of the middle pier end more than a specific value and increasing the end length to more than this value to just increases the construction costs. Consequently, a specific length could be recommended for the spillway piers end optimum length to reduce the possibility of generation of the second and third waves and also to reduce the chute walls height. [ABSTRACT FROM AUTHOR]