Flash floods in ephemeral streams are rare, short and difficult to forecast and thus to monitor. During these events, bedload transport reaches very high rates and most sediment transport occurs within a limited number of hours during the course of a year. Because monitoring of bedload in ephemeral rivers is challenging, here we present the results of a series of flume experiments designed to simulate short, flashy floods. Since most flume experiments usually involve single events, here we add to existing evidence by testing the effects of sequences of multiple floods in rapid succession. The flume is 10 m long, 0.3 m wide and 0.5 m deep. Two bed sediment mixtures (well sorted and poorly sorted) with similar median grain size but a different standard deviation were used. Bedload was monitored continuously during each hydrograph, but no sediment was fed. The flume experiments used six triangular hydrographs with peak flows ranging from 0.0147 to 0.02 m3s−1 and durations ranging from 150 to 400 s. Results indicate that the sediment transport rate decreases progressively from the first to the third hydrograph, and that this pattern is consistent for all permutations of peak discharge and flood duration. In all of the runs, the sediment transport rate at a specified flow was higher during the rising limb than the falling limb of the hydrograph, indicating clockwise hysteresis. Furthermore, in the subsequent repetitions of the same hydrograph, the degree of hysteresis generally diminishes in magnitude from the first to the last repetition for all the experiments, irrespective of their magnitude and duration.
Rapid growth of population and human activities has caused a perceptible change in the climate in some parts of Zayandeh-Roud Basin (Z.R.B.).High temperatures and aridity in some parts are a growing environmental problem.Assessment and monitoring of aridity is essential to protect desertification and mitigate drought.A Geographical Information System (GIS) was used in this paper for the assessment of (Z.R.B) using climatic data collected from 11 stations situated in the basin.In order to assess aridity, three well-known aridity models, viz De Martonne Aridity Index, Thornthwait Precipitation Effectiveness index and UNESCO Aridity Index, were used.Result showed that climate of Z.R.B is mostly becoming drier.There exists a small climatically wet zone in the area.
Heavy metal pollution has become one of the most cucial environmental obstacle today.The treatment of heavy metals is of special concern due to their refractoriness and resistence in the environment.In recent years, different procedures for heavy metal removal from wastewater have been widely studied.This paper revises the common procedures that have been applied to treat heavy metal wastewater and considers these techniques.Presently kinetic adsorption equations are widely used in adsorption processes.The main objective of the present research is to evaluate the linear and non-linear equations of the Lagergren and Ho et al. models in the kinetics of nickel adsorption by activated carbon.For this purpose, powdered activated carbon, the raw material of which is wood, was purchased from the "Merck" company (Germany).The optimum PH of the adsorption was obtained to be 6.As well, the kinetics of adsorption showed that for the initial nickel concentration of 2.5, 5, 10, 50 and 125 mg/L, equilibrium time was 40, 75, 120, 150, 50 minutes respectively.The maximum removal efficiency of nickel in the initial concentration of 2.5 mg/L was obtained to be 89.6%.The fitting of the four linear kinetic models of Ho et al. (1995), and its non-linear model, as well as the linear model ofkinetic data of adsorption at different concentrations showed that the non-linear model of Ho et al. (1995) better describes the kinetic data of nickel adsorption by activated carbon thanhis linear models.Comparison of the non-linear models of Ho et al. and Lagergren (1893) showed that in low concentrations the non-linear model of Ho et al., and in high concentrations non-linear model of Lagergren better describes the kinetic data of adsorption.First and second-order rate equations were pragmatic to etude adsorption kinetics.The most significant advantage of the offered model is its capability to predicate the balance time of adsorption, which is very essential to optimize the expense of the adsorption system design.The received consequences depict that the new model can analyze the experiential information very well.The equation is received clearly by changing the mass balance equation of a single step batch-type system into integrated first order rate equation.The proposed new formulation depends on maximum sorption (or removal) rather than equilibrium sorption, as given by Lagergren equation.The precise value of equilibrium sorption is not easy to describe in many cases, and it is exhibit that the modified formulation prepares better correlation with sorption data when the procedure is far from completion.A corrected formulation of the popular Lagergren pseudo first order rate equation is proposed.The equation is created clearly by changing the mass balance equation of a single step batch-type system into unified first order rate equation.The proposed new formulation depends on maximum sorption (or removal) rather than equilibrium sorption, as given by Lagergren equation.The precise value of equilibrium sorption is not simple to describe in many cases, and it is exhibited that the modified formulation provides better correlation with sorption data when the process is far from completion.
Packed columns are an important part of the broad selection of mass and heat transfer equipment. Nowadays, the use of packed columns is increasing, which is because of its lower pressure drop, higher capacity and higher mass transfer in comparison to tray columns. The experiential tests and the hypothetical analysis display that the chemical dehumidification of air by hygroscopic salt solutions confirms the stable reduction in humidity ratio, which is appropriate for uses to air conditioning or drying processes The mass transfer factors in the pulse were found to correspond nearly to the factors that would be achieved in the distributed bubble flow regime In the present study, parameters that affect column performance, such as, fluid retention and gas-phase mass transfer coefficient in a humidification column using random packing in towers with 0.1 m and 0.2 m diameters and 1m height, were measured Air velocity was 1.32 to 3.92 m3 per hour and liquid velocity was 10 to 70 m3 per hour. In this research, the Nakajima model was used to calculate the effective area. Thereafter, experimental values for gas-phase mass transfer coefficients were compared to Zech, Shi, Grouff, Shulman, Billet and Ondamodels The mean relative errors of these models with the present study’s experimental findings were 7%, 15%, 29%, 21%, 45% and 195%. Findings showed that by decreasing the column diameter, the gas-phase mass transfer coefficient (Kg.ae) also increases Further, the obtained retention values showed that retention in the column with a 0.1 m diameter was higher than the column with a 0.2 m column.
Compound channels are hydraulic sections that consist of two parts including the main channel and the floodplain.The main channel conveys the usual runoffs and the basic discharge that often flows in the river.Predicting the flow characteristics in the prismatic compound channels includes the effect of the interaction between a fast flow in the main channel and a slower one in the floodplains.This speed difference creates a shear layer at the interface between the main channel and the floodplain, which results in the exchange of the size of the movement between the main channel and the floodplains.The result is a reduction in the flow-conveyance capacity of the main channel and its increase in the floodplains, which should be considered in flow modeling.On the other hand, Flow3D software is a great software for Computational Fluid Dynamics that is used to solve complex geometry problems.The main objective of this study is to evaluate the Flow3D model in numerical simulation of behavior of hydraulic flow characteristics such as velocity distribution, shear stress distribution and in prismatic compound channels.In this study, Volume of Fluid Method (VOF) was used to simulate water surface profiles and the dimensional Buckingham π theorem was used to extract parameters without effective dimension; besides, the model's precision was tested by an experimental analysis in a rectangular flume.The results showed that the Flow3D numerical model has high accuracy in computing the flow passing the prismatic compound channel and has been able to model the hydraulic parameters of the flow and provide appropriate results.
Velocity at the toe of structures, such as spillways, drops, and chutes, plays an important role in designing protective and control hydraulic structures. In this study, laboratory experiments were conducted to investigate velocity at an inclined surface with a height of 40 cm made up of galvanized steel in a flume 20 m long and 0.6 m wide. Using three different slopes, five different discharges and five kinds of roughness, 75 experiments were done. Comparison of measured and theoretical (measured by energy equation without considering losses) flow velocities showed that over the inclined bed, the correction coefficient of theoretical velocity for different Froude numbers varied from 0.87 to 0.88, reducing with roughness size. Roughness size, Froude number and the ratio of water head to the length of bed play a dominant role in the prediction of velocity.
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