steady flow

Local scour around bridge piers is one of the most significant factors for the bridges’ destruction. Therefore, it is necessary to investigate the scour depth around the bridge piers. The effect of the skew angle of the single-column pier group related to the flow direction in two different arrangements including 1×2 and 1×3 piers on the maximum scour depth around the pile group was investigated in this study. The experiments were carried out under steady flow conditions. The pier group was placed in the 1×2 arrangement at the skew angles of 0 to 90 degrees and in the 1×3 arrangement at the skew angles of 0 to 45 degrees. The results showed that increasing the skew angle of the pier group is almost ineffective on the maximum scour depth around the first pier. However, it has a great effect on the maximum scour depth, its temporal development, and the expansion of the scour hole around the second and third piers in different arrangements of the pier groups. The maximum scour depth of the pier group in both different arrangements occurred at a skew angle of 30 degrees, in the arrangement of 1×2 around the second pier and by 13.33% more than the first pier and in the arrangement of 1×3 around the third pier and by 21.57% more than the first pier.

Pressure distribution is of utmost importance in both 1D and 2D analysis of steady flow in coarse-grained porous media. In the present study, the pressure distribution in length and height of the gravel media has been investigated in a experimental for three gradations of small, medium, and large in three lengths of 0.5, 1, and 1.5 meters. A piezometer is used to measure the pressure on the floor and walls of the laboratory flume. Experimental data show that; in the initial parts of the gravel media, the real pressure distribution (recorded in the laboratory) has relatively a good agreement with the hydrostatic pressure, and as the end of the gravel media is approached and the increase of water level curve, the pressure difference increases. The average Mean Relative Error (MRE) between the real pressure and the hydrostatic pressure for the two endpoints of the gravel media for the three gradations mentioned in the 0.5 meter media are respectively equal to 36.34%, 32.36%, and 33.64%, for 1 meter length is equal to 33.63%, 28.17%, and 29.34% respectively and for 1.5 meter length, it is equal to 39.46%, 25.66%, and 27.62% respectively. In other words, to check the steady flow in the coarse-grained porous media, the use of real pressure will increase the accuracy of the calculation process.

Floods are one of the natural events that cause significant morphological changes in riverbeds and adjacent lands, which result in financial, human, and other losses. When a flood occurs, the interference of the flow in the main channel and the flood plain causes the transfer of movement and complexity in the flow pattern from the main channel to the flood plain. Due to the dynamic nature of these events, statistical, experimental, and semi-experimental criteria are used to determine the dimensions of stable regime channels. Nowadays, the use of two-dimensional models to study the behavior of the regime channels has a superior advantage over the empirical and theoretical approaches, which are based on one-dimensional. There haven't been many numerical studies done on the flow pattern and sediment transport in alluvial channels, but there have been a lot of experimental studies. In this study, the behavior and hydraulic geometry of the trapezoidal compound channel have been investigated using MIKE21software for type of steady and unsteady overbank flow and Finally, the results of modeling in steady flow conditions showed less difference with real values than in unsteady flow conditions.In order to study the behavior of the main channel in the flood flow, the flow has been selected in such a way that the flood plain are in the state before the movement threshold. First, the compound trapezoidal channel section with fully losses boundaries/ with fully mobile boundaries was developed under bankfull flow, and then the behavior and geometry of the developed trapezoidal channel (regime channel) were evaluated under type of steady and unsteady flows. In the first step of permanent steady flow modeling, the flow conditions in the channel at a certain point are constant with respect to time, which means that the speed along the flow and the average depth of the flow are the same. In the second step of the modeling, the unsteady flow along the channel was checked with the same conditions as the first step, with the difference that the input discharge is variable in time.The results showed that when the cross-section of the main channel is full of water, there was no significant change in the width of the main channel, but the width of the main channel in overbank flows is higher than the full section and increases rapidly. The mean relative differences obtained at upstream and downstream width trapezoidal section, and flow depth in steady flow conditions are0.96, 0.88, and0.98,respectively, and in unsteady flow conditions, the average relative difference obtained at parameters upstream and downstream width trapezoidal section, and flow depth in steady flow conditions are1.12, 1.15and1.05,respectively. According to the sediment transport equations available in the software, the England-Hansen sediment transport relationship chosen as the appropriate relationship for the parameters of the channel dimensions, which can be more predictable to laboratory values.An stable alluvial channel with a bankfull, when subjected to flooding, tilts towards a new stable channel.Under steady flow conditions, estimating the results of the depth and width of a stable alluvial channel in the MIKE21model is more accurate than the results obtained from unsteady flow. Also, the values of width, depth and sediment concentration predicted by the above software are better estimated than the sustainable channel design methods such as brownlie, vanrajin and WBP. In all models, the width of the channel increases rapidly over the first 3hours, and then approaches the balance stage and the channel erosion rate decreases. As the discharge increases up to 15liters per second, the depth and width of the channel is more severe and then these changes are almost constant.

Due to increasing negative environmental impacts of storage dams, detention basins have been considered as an alternative solution instead of construction of concrete dams for reduction of flood damage in recent decades. Literature reviews on detention basins show that there are limited numerical and experimental studies in this regard. In this research the effect of bed roughness height of main channel on the inflow discharge has been investigated by considering the effective factors on the efficiency of basins in laboratory of Gorgan University in 2018.  Also, the general ability of using steady flow equations for unsteady flow was investigated by calibrating some of the discharge coefficient equations. For simulation, a metal tank was connected to a channel through a broad side weir. Three bed roughness heights of the flume in the range of 0.06 to 15 mm as well as three lengths of side weirs in the range of 30 to 90 cm were utilized. The experiments were carried out under unsteady flow condition by three inflow hydrographs with peak discharges of 17, 25 and 33 l/s and a base time of 504s. The flow depth was measured instantaneously by transmitter and the average inflowing discharge into the detention pond was calculated. According to the results, use of lateral flow discharge coefficient relationships of the steady flow cannot be generalized for unsteady flow. So, based on dimensional analysis of the Π-Buckingham theory, some relationships have been proposed for calculation of the average inflow rate into the detention pond and also for filling time of the pond. The maximum errors for calculation of the average inflow rate as well as the filling time of the basin were less than 15 percent. Also, increasing the streambed roughness can increase the average inflow rate to the detention basin by 25 percent.

In recent decades, several factors such as population growth, limited surface water resources due to reduced rainfall, and over-withdrawal of aquifers have caused a sharp drop in groundwater levels (GWLs) in many plains of Iran, including Salmas plain located in the province of West Azerbaijan. In this study, the numerical model of the Salmas plain aquifer was prepared using MODFLOW model in GMS software in both states of steady flow (September 2015) and non-steady flow (from September 2016 to August 2017). The input parameters of the model were calibrated for the measured values of GWL at the location of observation wells and validated from August 2016 to July 2018. Finally, the GWL was investigated applying 6 scenarios including a 10% to 30% decrease in pumping wells and a 5% to 15% increase in rainfall. The results of studied scenarios showed that reducing the pumping rate of the wells significantly improves the condition of groundwater resources. By reducing the pumping rate of the wells up to 30%, the volume of groundwater will increase by 13.66 mcm and shortage of the aquifer reservoir will be compensated. However, 15% increase in annual rainfall will improve the condition of the aquifer by only 0.5 mcm. Results show that reducing the rate of withdrawal from wells reduced the amount of groundwater flow entering the aquifer from 41.98 mcm in the calibration state of the model to 38.7 mcm in terms of 30% reduction of discharge rate in the wells. However, the increase in rainfall did not cause significant changes in the amount of groundwater inflow and outflow. The results of this study indicate that management scenarios such as preventing over-withdrawal from authorized wells and blocking unauthorized wells have more significant effects on improving the GWL than possible climatic scenarios.

The analysis of steady-state flow within rockfill materials is performed using two methods of gradually varied flow theory that analyzes the flow as one-dimensional and the Parkin equation that analyzes the flow as two-dimensional. The calculation of the hydraulic gradient (i) is of great importance in both methods. Most of the research in this area has been done in homogeneous rockfill materials and limited studies have been carried out on the flow in heterogeneous rockfill materials. In this study, we used the results of experiments in homogeneous and horizontal heterogeneous rockfill materials with three layers of aggregates with large, medium and small size. In the experimental data used in the present study, changes in hydraulic gradient relative to the flow velocity in each of the three homogeneous layers were available separately and also the changes in the horizontal heterogeneous media consisting of three layers were available separately. Due to the fact that in order to form a horizontal heterogeneous rockfill media, large, medium, and small homogeneous layers are placed on top of each other, respectively, in the approach presented in the present study, the data of the steady flow of homogeneous media are placed in the same order in the Particle Swarm Optimization (PSO) algorithm to optimize the coefficients of the binomial equation (a, b) and consequently the calculation of hydraulic gradient. In other words, in the present study, the values of the coefficients a, b, equivalent to the horizontal heterogeneous media, depending on how the homogeneous layers are placed on top of each other and only using the data of steady flow in the homogeneous media (large, medium and small size) Optimized. The results show that the difference of mean relative error (MRE) the approach presented in the present study than conditions where flow data in a horizontal heterogeneous media are directly used to calculate the coefficients a and b and consequently the hydraulic gradient, is 1.38 percent.

Accelerated soil erosion is the most important degradation factor of soil and water resources. Typically, soil erosion involves the detachment and transport of soil particles by rainfall, shallow surface flow or the interaction of these two factors. Therefore, understanding the motion threshold of sediment particles and temporal variation of sediment concentration in flow-induced can provide a detailed cognition of the processes inducing soil erosion and sediment transport and their eventual interactions. It is also important for increasing the accuracy of soil erosion models. In this study, the particle motion threshold and temporal variation of sediment were studied for a sandy sample at three slopes; 3.1, 5.9, 8.9% and-dunder three flow discharges of 4.78, 7.12 and 9.05 (×10-5 m2 s-1). This study was carried out in the laboratory conditions using a flume with 240 cm long by 40 cm width. The results showed that the Shields curve is not suitable for this study to determine the motion threshold. The threshold stream power of particle motion was determined 0.035 W m-2. Also, with increasing slope and consequently increasing stream power up to 0.05 W m-2, the erosion intensity increased and soil erosion changed from sheet erosion to rill erosion. The results indicate that the formation and development of rill erosion would be the main factor for soil loss and sediment production in hillslopes. Therefore, prevention of rill formation by strip croping, terracing and terrace farming is an effective strategy for soil conservation.

Local scour is one of the main drivers of instability and failure of bridge piers. Therefore, in addition to the stability issue, the scour depth reduction is also important. Numerous solutions have been presented for scour depth reduction. In this study, a new idea environmentally-friendly and non-structural approach has been introduced for bridge pier scour depth reduction. Nano-structured materials are waterproof, they can be used for scouring reduction in alluvial beds. For achievement of research aims, the bed sediment around the bridge pier was mixed with some nano-structured material called nanoclay. The experiments were carried out in a laboratory flume under steady conditions on a cylindrical bridge pier. The obtained results showed that, the highest scour depth reduction percentage (58.3%) occurred for the largest flow discharge (16 L s-1) by a large difference compared to other values of flow discharge. In this case, the scour depth was decreased from 60mm to 25mm due to presence of nano-structured material. The lowest percentage of scour depth reduction was nearly 41.6 for the minimum flow discharge (4 L s-1).