مجله هیدرولیک
سال هفدهم شماره 1 (بهار 1401)

Scouring downstream of sediment- carrying free over fall water jetEhsan Ghasemi1 and Maoud Ghodsian*21- Ex. M.Sc. Student, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, Tehran-Iran.2- Prof., Faculty of Civil and Environmental Engineering and Water Engineering Research Iistitute, Tarbiat Modares University, Tehran, Iran*ghods@modares.ac.irExtended 

Prediction of scour and characteristics of scour hole due to out flow from hydraulic structures is important in hydraulic engineering. The dimensions of scour hole is influenced by different parameters including: flow discharge, drop height of flow, tailwater depth, sediment size, sediment load and time of scouring. Almost all the previous studies have focused on the scour hole characteristics downstream of free over fall clear water jets. Since water jets are not always clear and may carry sediment, especially during flood condition, and the effect of sediment load on the scour characteristics are not well studied. Therefore in this study, scour downstream of a free over fall clear and sediment-carrying water jet are studied experimentally. The main purpose of this study are to investigate the effects of sediment load, sediment size and discharge of the free falling jet on the scour depth and the longitudinal scour length. Also temporal variation of the sour depth and longitudinal scour length was also addressed. New dimensionless equations for scour depth and longitudinal scour length were obtained.

Experiments were conducted in a rectangular channel of 0.6 m width, 12 m length. The water was pumped from a sump to the channel at the end of which a free falling jet was formed. A rectangular free-overfall jet of 0.21 m width was established at the last 1 m length of the channel. Scour was simulated in a rectangular box of 1.5 m width and 2.51 m length located downstream of channel. Measurement of discharge was done by using a calibrated sharp crested triangular weir with apex angle of 90 degree. The depths of flow and longitudinal bed profiles were measured by using a digital pint gauge with ±0.01 mm theoretical accuracy. Temporal measurement of longitudinal bed profiles were also done by using the same digital point gauge. The bottom of the rectangular box was covered by a uniform sand layer of 0.45 mm thickness. Experiments were conducted for four different discharges 4.27, 7.48, 11.78 and 17.3 L/s and two sediment sizes (d50= 0.6 mm and 1.2 mm). Experiments were conducted with clear water free falling water jet and sediment caring free falling water jet. In experiments with sediment caring free falling jet, the dry sediment with constant rate was added to the water jet. Four values of sediment load 0.25, 0.5, 1 and 2 Kg/min were used. The added sediment to the jet was of the same size as the bed material size (d50= 0.6 mm and 1.2 mm). Time variations of scour depth and scour hole length were also studied.

Based on the result, by increasing the sediment load, the values of maximum depth of scour and length of scour hole decreased. The rate of scour reduction depend on the amount of sediment load in the water jet, water discharge and scouring time By increasing the duration of experiments, the increasing effects of densimetric Froude number Frd and jet discharge in the longitudinal bed profiles reduces. The decreasing trend of sediment load on the maximum scour depth is more pronounced in experiments with lower duration. In higher discharges, the reducing effect of the sediment load on the maximum depth of scour reduces. The reducing effect of sediment load on the longitudinal scour length is reduced with higher jet discharge. The reducing effect of sediment load on longitudinal scour length is enhanced in experiments with lower duration. Effects of dimensionless parameters on the scour depth and scour length were addressed. By increasing the densimetric Froude number the relative scour depth and relative longitudinal length of scour increases. By using the dimensional analysis, dimensionless equations for estimation the longitudinal scour profile, scour depth and scour length are obtained.

In this experimental study, the scour depths and the longitudinal scour lengths were compared in the clear water and sediment caring free falling water jets. It was found that by increasing the sediment load, the values of maximum depth of scour and length of scour decreases. The rate of scour reduction depend on the amount of sediment load in the water jet, water discharge and scouring time. New equations for estimation the longitudinal scour profile, scour depth and scour length are obtained.Keywords: free over fall jet, sediment carrying jet, scouring, dimensional analysis, densimetric Froude number, and longitudinal profile.

One of the main factors in the collapse of the bridge piers in rivers is local scouring. By placing the piers in the direction of the streams, a complex three-dimensional flow is formed around the pier that has been the popular subject of research by many researchers. Methods of reducing the depth of local scouring are divided into two systems: 1. increasing the strength of bed materials around the piers by using more resistant materials, such as riprap, collar and gabion in the riverbed. 2. Reducing the strength of the main factors such as downward flow and horseshoe vortex by the deflector, blade and submerged Vane or changing the geometric shape. In the present study, the effect of the deflector shapes such as triangles and curved surface on the maximum scour depth around the pier under clear water conditions were investigated. General factors of bridge pier scour include down-flows, horseshoe vortex, and wake vortex. In general, the flow impact on the pier and its separation is the main factors that form scour holes around piers.

The experiments were done in the laboratory of Khuzestan water and power authority laboratory (KWPA), equipped with a flume with a length of 10 meters and a height of 500 mm and a width of 310 mm. The flume is equipped with an electromagnetic flow-meter with an accuracy of ±0.1 liters per second and a weir downstream of the flume to adjust the water level. In this study, natural river sand with uniform grain size (δg = 1.36), relative density Gs = 2.64 and the average particle diameter of 0.95 mm. In all experiments, water depth was considered 100 mm. In this research, three different models of PVC deflector surface (the deflector surface shapes such as triangles, curved and simple surface) with angle's face (θ= 15, 30 and 45-degrees). It should be noted that the angle of flow with the deflector head is calculated as α = 90-θ, which used to describe and analyze. The unprotected pier scouring investigated to represent a basis for controlling and comparing with the other scour and bed change conditions. A 12-hour control experiment was also conducted on the control pier to determine the experiment time (equilibrium time), and scour depth changes were recorded in the time unit during experiments.

The horseshoe vortex around the scour hole accelerates digging and transfers the sediments separated from the bed downstream with the main flow. The flow's separation from around the pier also creates perpendicular vortexes on the sedimentary bed known as wake vortexes. These vortexes are active behind the pier, separate the bed particles like a tornado, expose them to the flow, and help move sedimentary particles from the front and sides of piers downstream. The scour hole digging by the horseshoe vortex continues until the water volume inside the scour hole increases and exhausts the vortex energy. In this state, the scour depth changes negligibly over time and reaches equilibrium, and Figure 1 show the scour mechanism. In this study, to reduce the scouring depth pattern around the pier, three shapes of the deflector (the deflector surface shapes such as triangles, curved and simple) with three angles were used. The results showed that by reducing the head slope from 40 to 15 degrees, scouring depth decreases. For all deflectors with 15 degrees in the parameter (U/Uc=0.70), the percentage of the scouring depth reduction is close to 83 to 89 per cent. In the parameter U / Uc=0.96 near inception motion that is the most critical state and the value most comparable to the particle incipient motion, the deflector with triangle surface shows a decrease of 85%, curved surface 77%, and simple 75%. By reducing the angle of the deflector, part of the flow lines didn't deviate towards the bed, which reduced the potential of the high-pressure zone created at the pier. This reduction in compressive potential reduced the flow velocity of the back vortices and, ultimately, reduces their ability to transport sediment downstream. Based on the results, the deflector with a triangle surface shape in all flow conditions had a better and lower scour hole depth than the carved and simple shape.

This study used a deflector structure to reduce and control the scour depth around bridge piers. The flow effect was investigated by implementing these protections and their impact in various relative velocities (U/U_c=0.97,0.83,0.70). The scouring pattern and sediment point bar created around the pier with The deflectors protection compared with the control pier (without protection) have less scouring depth due to minor deviation of flow streamlines and reduced disturbances around the pier. Finally, the deflector with a triangle surface shape has a better response to reduce the scouring hole. The results state that the deflector at the 15-degree angle significantly affects the flow deflection near the bed, corrects the flow pattern around the pier, and reduces scouring depth.

Among the most important parameters in hydraulic engineering is the flow resistance coefficient, e.g. Darcy-Wiesbach, in alluvial rivers; in which is divided into two categories: grain resistance and form resistance. The grain resistance is a function of the bed sediment size; whilst the form resistance is a function of the bed form geometry. Ripple and dune are among the most common forms formed in alluvial rivers that are triangular in shape (Shafai Bajestan, 2008; Julien, 2010). The effect of bed form on flow resistance have been studied by few researchers such as: Talebbeydokhti et al. (2006), Omid et al. (2010), Nasiri Dehsorkhi et al. (2011), Chegini and Pender (2012), Kabiri et al. (2014), and Kwoll et al. (2016). However, the effect of roughened bed form, bed form covered with artificial roughness of different sizes, on the Darcy-Wiesbach friction coefficienthas received little attraction. Therefore, it is the main goal of the present study to experimentally investigate this effect. This different flow rates and different bed slopes. In this study, sediments with sizes of 0.51 and 2.18 mm were used to rough the surface of the bed forms.

The experiments were performed in a sloping straight flume (manufactured by Armfield, UK). The length and width of the flume were 12 and 0.3 m, respectively (Figure 1). In this study, flow rates of 10, 15, 20, 25, and 30 l/s and bed slopes of 0, 0.0001, 0.0005, 0.001, and 0.0015 were examined.The present study experiments were divided into two categories: bed without form and bed with form. Each form was made by P.V.C sheet in a triangular shape. The bed form length and height were equal to 20 and 4 cm, respectively, and the angles of its upstream and downstream to the horizon were selected as 16.4 and 32 degrees, respectively. After making each form, the desired sediments were glued on their surface. In this study, two types of uniform granulation with average sizes (d50) of 0.51 and 2.18 mm were used. The total number of experiments in the present study was 100.

Ripple and dune form are usually being developed in lower flow regime, in which the Froude number is less than 1 (Shafai Bajestan, 2008; Julien, 2010). In this study, the Froude number values in all tests with bed form were from 0.435 to 0.6, indicating a lower flow regime. Figure 5 shows the changes in the total Darcy-Weisbach's coefficient (f_b) against Froude number for sediment-covered with sand sizes of 0.51 and 2.18 mm. It can be seen that as the Froude number increased, the total Darcy-Wiesbach's coefficient (f_b) decreased. In addition, increasing the longitudinal slope of the bed, causes the f_b to increase. In addition, increasing the longitudinal slope of the bed, causes the f_b to increase.Figure 6 shows the trend of changes in the total Darcy-Weisbach's coefficient (f_b) versus relative submergence (y/∆) for the slope of 0.0001. This figure shows that with increasing relative submergence rate, the total Darcy-Wiesbach's coefficient decreased due to the relative roughness reduction. In addition, Figure 6 shows that f_b increased with increasing particle size. Calculations showed that the value of f_b in beds with a sediment size of 2.18 mm for slopes of 0, 0.0001, 0.0005, 0.001, and 0.0015 on average 32.8, 28.8, 28.46, 33.8, and 35.9% are more than the bed covered with 0.5 mm sediment size, respectively.The results of Table 2 shows that the grain Darcy-Weisbach's coefficient ((f_b ) ́) for particles with sizes of 0.51 and 2.18 mm are on average 25.45 and 26.8% of the total friction coefficient (f_b), respectively. In addition, from Darcy-Weisbach's coefficient (f_b^'') for particles with sizes of 0.51 and 2.18 mm is on average 74.55 and 73.2% of the total friction coefficient (f_b), respectively. According to the results, it can be seen that the value of f_b^'' for particles with a size of 0.51 and 2.18 mm on average is 193.6 and 173.4% more than (f_b ) ́, respectively.

The results showed that with increasing the particle size of the bed, the total Darcy-Wiesbach's coefficient (f_b) and the grain Darcy-Wiesbach's coefficient ((f_b ) ́) increased. The value of f_b in sedimentary beds with a size of 2.18 mm is on average 32% higher than sedimentary beds with a size of 0.5 mm. Meanwhile, the value of the form Darcy-Wiesbach's coefficient (f_b^'') for particles with a size of 0.51 and 2.18 mm on average is 193.6 and 173.4% more than (f_b ) ́, respectively.Keywords: Bed form, Ripple, Dune, Darcy-Weisbach friction coefficient.

Natural rivers are commonly characterized by a main channel for primary flow conveyance and a floodplain to carry extra flow during floods. Floodplains are usually partially or completely covered with vegetation such as shrubs or trees. Vegetation affects the depth of flow, velocity distribution as well as sediment transport(Yang et al.,2007). Predicting the lateral velocity distribution in compound channels is important for determining the stage-discharge curve and supporting the management of fluvial processes in vegetation condition in river(Tong and Knight, 2009). Previous research in the area of vegetated floodplains has primarily focused on the adaptation of theory driven resistance equations. Since the 1990’s, several Lateral Distribution Models (LDM) have been developed for obtaining lateral velocity distribution in compound channels. Among the velocity lateral distribution models, the Shiono and Knight Model (SKM) is more popular with widespread applications(Unal et al, 2010). Three calibrating coefficients need to be estimated for applying the SKM, namely eddy viscosity coefficient (λ), friction factor (f) and secondary flow coefficient (k). Determining these coefficients in natural channels is not always feasible and requires some experiences (Knight et al. 2010). Utilizing soft computing (SC) methods to solve different problems is another progressing concept. One of the newest and most powerful SC methods is gene expression programming (GEP), which is an extension of GP and GA, and was first introduced by Ferreira(2001). The GEP method mitigates the majority of problems of principal SC methods related to the absence of equations for practical engineering by presenting explicit equations. The aim of this study was to use GEP modeling to predict the depth averaged velocity distribution in compound channels with vegetated floodplains. The results of the best GEP model are presented as an equation and compared with the result of SKM model.

In this study, by aid of nearly 508 depth-averaged velocity data reported in a study by Tavakoli(2019) and using gene expression programming (GEP), the depth-averaged velocity in compound channels with vegetated floodplains was modeled. 9 dimensionless input variables including, Relative flow depth 〖(D〗_r), Relative distance (χr), vegetation density (λ), shading factors , Dsa and Dsr, either aligned or randomly arranged, respectively, Dfp (the vegetation diameter over the width of the floodplain), y_n1,y_n2(the distance from the channel centerline to the measurement location in main channel and flooplain) and one output variable (depth averaged velocity) have been used in GEP. 70% of the experimental results are used for the training process and the remaining 30% for testing. After selecting the training set, the GEP learning environment should be defined. The five main steps in GEP training are as follows: First step: selecting fitness function. Second step: determining the function set(F) and terminal set (T) for chromosome generation. Third step: specifying the number of genes and the head length. Fourth step: defining the linking function for linking different sub-ETs. Fifth step: setting the values of different genetic operators, such as inversion, transposition and recombination(Fuladipanah, 2020). The values of these operators and other parameters used in GeneXpro program are presented in Table 2. Various statistical error analyses were performed to verify the reliability of the developed GEP model. An equation was derived from the best GEP model and its results were compared with the analytical method of Shiono and Knight. To obtain analytical solutions by SKM with an accepted accuracy, the drag coefficient, the shading factor, local friction factor, eddy viscosity and secondary flow term need to be determined, these parameters were discussed in the paper. Importance of the predictor variables for GEP models were also presented by using sensitivity analysis.

A number of expressions have been generated to predict depth averaged velocity distribution within a compound channel with vegetated floodplains using gene expression programming (GEP). For an initial attempt, the gene expression programming was run with all data in the non- dimensional form. The best produced formula was as given in Eq. 27. The amount of CC, RMSE, and MAE for GEP at the first scenario during training and testing phases were calculated as (0.919,0.13,0.093) and (0.874,0.156,0.096) respectively. This expression shows high positive correlation, however, this value may be misleading as correlation should only be used as a measure for normally distributed variables. Analysis of the Experimental data showed two distinct normalised distributions, for the lower velocities on the floodplain and the higher velocities on the main channel, respectively(Harris et al.,2003). So the data sets separated in to two data sets and the GeneXpro program was then applied to the two data sets separately, thus giving separate expressions for the two zones. Evaluation of model performance showed that the model presented for main channel, with CC of 0.902 and RMSE of 0.083 outperformed than the model presented for floodplain with CC of 0.843 and RMSE of 0.092. The velocity prediction on the main channel shows good correlation with R2=0.8536, see Fig. 2 but the floodplain results show a degree of scattering with R2=0.78 , this is due, in part, to data collection experimental error and the complexity of the flow around the vegetation. The sensitivity analysis results demonstrate that dimensionless shading factor of vegetation (Dsr), is the most influential parameter with regard to the depth averaged velocity distribution. As it is presented in Fig. 4. Dsr are the most important variable for GEP model, this conclusion is supported by the work of Naot et al. (1996). The results showed that the Shiono and Knight method (SKM) has shown satisfactory results for the prediction of depth-averaged velocity distribution in the lateral direction. The GEP model, with RMSE of 0.15, exhibits superior performance over the SKM model with RMSE of 0.24 for all data.

In this paper, two algorithms namely SKM and GEP have been applied to predict depth averaged velocity distribution in compound channels with vegetated floodplains. The results of these two mentioned algorithms were compared with experimental modeling. The paper highlights the advantages of using intelligent algorithm rather than the traditional approach to predict and extract the complicated and hidden relationship among dependent and independents variables.

A compound channel consists of one main channel with a deeper flow in the middle and one or two floodplains around the main channel by looser flow depth. The difference between velocity in the main channel and floodplains in compound channels creates a strong shear layer at interface between the main channel and floodplains. Also, because of the three-dimensional (3D) structure of flow, the Investigation of flow characteristics in compound channels is completely complicated. In non-prismatic compound channels, due to the mass exchange between subsections, the study of flow is more complex. Therefore, the prediction of flow behavior in the non-prismatic compound channel is an important subject for river and hydraulic engineers. The skewed compound channel is one kind of non-prismatic compound channels. In compound channel with skewed floodplains, one of the floodplains is divergent and the other is convergent. The flow patterns in skewed compound channels have been studied experimentally by many researchers (James and Brown, 1977; Jasem, 1990; Elliott, 1990; Ervine and Jasem, 1995; Chlebek, 2009; Bousmar et al., 2012). However, numerical studies on flow characteristics in skewed compound channels were rarely performed. In this research, by using the computational fluid dynamics (CFD) and two turbulence models of the RNG and LES, the velocity, boundary shear stress distributions, secondary current circulation, and water surface profile in a compound channel with skewed floodplains has been numerically investigated.

In this research, modeled compound channel is similar to the experimental channel using by Chlebek (2009) at the hydraulic laboratory of Birmingham University, Department of Civil Engineering. The experimental studies were performed in a straight flume of 17 m long, 1.198 m wide, 0.4 m depth, and with an average bed slope of 2.003×10-3 (Fig. 1). By using the PVC material, the cross-section of this flume was made compound shape, a rectangular main channel of 0.398 m wide and 0.05 m deep in middle, and two floodplains with 0.4 m wide around the main channel (Fig. 2). The skewed compound channel was made by isolated floodplains using L-shaped aluminum profiles. Experiments were conducted at the skewed angle of 3.81° and four relative depths of 0.205, 0.313, 0.415, and 0.514. The lateral distributions of depth-averaged velocity and boundary shear stress were measured at six sections along the skewed compound channel (see Fig. 3), using a Novar Nixon miniature propeller current meter and Preston tube of 4.77 mm diameter, respectively.For numerical simulation of the flow field in the skewed compound channel, the FLOW-3D computational software was used. Also, the renormalization group (RNG) and large eddy simulation (LES) turbulence models were selected. Two mesh blocks were utilized for gridding, mesh block 1 by coarser mesh size at the upstream of the skewed portion of the channel, and mesh block 2 by smaller mesh size for skewed part (Fig. 5). The flow field is numerically simulated by three computational meshes (fine, medium, and coarse mesh size). Details of griding for different computational meshes are summarized in Table 2. Finally, the medium mesh by 1653498 cells was selected. For boundary conditions, using volume flow rate condition for inlet, outflow condition for the outlet, symmetry condition for water surface area and the interface of two mesh blocks, and wall condition for lateral boundaries and floor (see Fig. 8 and Table 3).

The results of numerical simulation show that the RNG turbulence model, can predict the depth-averaged velocity and boundary shear stress distributions in the skewed compound channel fairly well (Figs. 9 and 10). In addition, in the skewed compound channel, the mean velocity and boundary shear stress on the diverging floodplain is more than these values on the converging floodplain at the same section. The longitudinal discharge distribution on floodplains of the skewed compound channel is linear, and the numerical modeling can compute these values very well (Figs. 11 and 12). By moving along the skewed part of the channel, areas with more flow velocity move toward the diverging floodplain. Also, the position of the maximum velocity, instead of the main channel centerline, move to the interface between the main channel and diverging floodplain, too (see Figs. 13 and 14). The lateral flow that leaves the converging floodplain, caused by changing the geometry of the channel along the skewed portion, created a secondary flow circulation in the main channel near the converging floodplain. Also, by going to the end of the skewed compound channel, this secondary flow becomes stronger (Figs. 15 and 16). For prediction of water surface profile in the skewed compound channel, two turbulence models of RNG and LES can compute the water depth along the channel fairly well, especially the RNG turbulence model (Fig. 17). In addition, the error analysis by using experimental data and numerical results are investigated. For error analysis, mean absolute error (MAE), mean absolute percentage error (MAPE), root mean square error (RMSE), and the coefficient of determination (R2) were calculated by using the equations of (12) to (15), respectively. These computation errors between numerical simulation results and experimental studies data are presented in Table 5 and are showed in Figs. 18 and 19.

In this research, the flow field in compound channel with skewed floodplains was numerically simulated. The FLOW-3D software and two turbulence models of the RNG and the LES were used to model the depth-averaged velocity, boundary shear stress distributions, and discharge distribution at different sections of skewed compound channel. The results of simulations indicate that comparing with the LES turbulence model, the RNG turbulence model are able to predict the velocity and bed shear stress distributions quite well especially in the first half of the skew portion. Also, by increasing the flow relative depth, the accuracy of numerical modeling is increased to compute different flow characteristics, but in water surface profile, by increasing the relative depth, the precision of simulation decreases (see Fig. 18).

Bridges are one of the most important structures built on rivers and are considered as a structure connecting the two parts of the road. One of the most important reasons for the destruction of bridges is the scouring of its piers. New bridge design challenges, due to climate change and human intervention, as well as uncertainties associated with maximum events, may not adequately lead to accurate hydraulically design of bridges and may therefore as a result, in some floods, the bridge deck submerged. Under these conditions, the flow can be converted to a pressurized. This pressurized flow passes at high velocity in the region of bridge piers. As a result, it can increase the erosion potential of bed materials near bridge piers. Up to now, many studies have been performed to determine the relationship between estimating the rate of scouring of bridge piers in laboratory conditions with clear water and living bed, Such as: CSU equation.Under pressurized flow condition, researchers such as Umbrel et al., Richardson and Davis, Zehi, and Karankina et al. Have developed relationships to determine the amount of scouring of bridge piers in simple channels. Due to the difference in flow velocity in the main channel and floodplains in the compound open channels, the important changes occur in the kinetic structure of the flow near the connection line between the main channel and floodplains. These changes also cause vortices as a result of excess energy loss in the flow. In addition, the presence of vegetation on floodplains complicates the hydraulic analysis of the flow in such sections. Up to now, many studies have been performed to explain the hydraulic conditions of the flow in compound channels with and without vegetation, including Shiono knight (1991), Rameshwaran and Shiono (2007), Zarati et al. (2008), Yu-qi Shan et al. (2016), Tanino et al. (2008). and Sonnenwald et al. (2018).In previous studies, the amount of scouring of bridge piers in the conditions of pressurized flow under the deck in compound channels with vegetation has not been investigated. The aim of this study was to investigate the effects of vegetation density, pressurized flow under the bridge deck with different geometric and hydraulic conditions on the scour depth of bridge piers in a compound channel.

Experiments of this research was performed in a laboratory channel with a width of 1.5 meters and a length of 10 meters. The experiments in this study were performed with 3 geometric ratios of cross section (=B/b), 3 relative depths (Dr) and 3 vegetation densities (). It should be noted that the experiments are designed in such a way that in all of relative depths, the bridge deck is submerged and the flow pressurized.The maximum depth of scouring under the flow pressurized passing under the bridge can be expressed as a simple and dimensionless equation (1):( (1 Considering the control volume from the upstream of the bridge deck to the downstream of it, the momentum equation can be written to calculate the apparent shear stress as follows:(2)

A: Depth averaged velocityIn vegetation densities used in this study, the average velocity on floodplains with vegetation is relatively constant in most cases. This shows that except in the interface of the main channel and floodplains, the flow distribution on floodplains can be considered two-dimensional. As the vegetation density increases, the depth averaged velocity difference between the main channel and the floodplain increases between 50%-80%.B: Shear stressDue to the presence of vegetation, the reduction of the average flow velocity on the floodplain occurred as a result of shear stress has also decreased. The transverse changes of shear stress downstream of the bridge, due to the behavior of the pressurized flow passing in the deck, have more fluctuations and are on average about 25% more than the average values upstream of the bridge.C: Local friction factorThe Darcy–Weisbach friction factor in the floodplain area increases significantly due to the presence of vegetation elements. The pattern of variability of Darcy–Weisbach friction factor on the floodplain also causes a sinusoidal pattern due to the reduction of flow velocity and the presence of skin friction on the surface of the rods.D: Apparent shear stressDue to the resistance due to increasing vegetation density, the amount of apparent shear stress at higher densities increases. On the other hand, with increasing relative depth and decreasing of secondary current, the amount of apparent shear stress decreases. As the width of the floodplain increases and the secondary currents become stronger, it shows an average of 40% apparent shear stress.E: Equation for predicting maximum scour depthBased on determining the effective parameters in the amount of scour rate and using the data of this study, the following equation is presented to estimate the amount of scour of the bridge pier under pressurizes flow conditions.(3)

Increasing the density of vegetation increases the longitudinal velocity in the main canal and decreases it in the floodplain.-Bridge pier scouring develops faster in pressurized flow than in free surface flow.-With the exception of the height of the dune in the pressurized flow, the depth of scour hole on a small laboratory scale is less than 50% of the depth of the upstream of the bridge deck.-The position of the maximum scouring depth quickly reaches its equilibrium position near the downstream edge of the bridge deck.

Estimating hydraulic characteristics in the channels and floodplains, in the case of vegetation is very difficult. Individual factors and their effects can be determined with acceptable accuracy, such as for a meandering channel with the presence of vegetation. In view of the need for further research on overbank flow in a meandering channel with the presence of vegetation, work will be carried out to fill the gaps to provide the required information. The key purposes of the present research are to enhance our knowledge of the flow resistance caused by vegetation and to report the results of laboratory investigations into the physical processes involved in the flow structure as well as to understand the flow characteristics and flow mechanisms in compound meandering channel with different arrangements of non-vegetated and vegetated floodplains. This study is focused on the influence of vegetation on overbank flow characteristics. In this research, the effect of submerged flexible artificial vegetation in the floodplain on two relative depths of 0.35 and 0.55 has been studied in the laboratory.

All the experiments reported here were conducted in one flume at the Ferdowsi University of Mashhad. The flume is built on a number of rigid steel structures to support its weight, achieve maximum stability and maintain its longitudinal gradient. It was constructed include to tanks, sumps and pipeworks. Both sidewalls of the flume built were using glass to ease visibility during the setting-up of the instruments used. The experimental research was carried out in a non-mobile bed meandering channel constructed in a 10 m long and 0.78 m wide flume which included the main channel and two floodplains on its sides. The channel wavelength and meander belt width were one meter and 0.58 m, respectively with the sinuosity of 1.3. The geometrical parameters for the main channel were: width, Bmc=0.2 m and depth, Hmc= 0.1 m. Artificial grass with an average height of 2.5 cm are used to simulate the emergent floodplain vegetation. A movable weir located at downstream of flume controlled water level. Velocity data were extracted and analyzed using Acoustic Doppler Velocimetry. The minimum recording time for each point velocity was 60s. ADV measures the 3D velocities of water particles located 5 cm below its probe. The measurement sections located 6 m downstream of channel inlet, with the names of S1 to S5.

The results showed that the presence of flexible vegetation in the floodplain for a constant relative depth has reduced the flow capacity. The pattern of contour lines of the longitudinal velocity in the main channel in the presence of vegetation changes at both relative flow depths relative to the uncovered state. By examining the velocity contour lines in the presence of vegetation, the core of the maximum velocity in the main channel is increased. Also, the longitudinal velocity above the submerged vegetation has been significantly increased. The values in the transverse and vertical velocity components in floodplains with vegetation are much higher than in uncovered conditions. The directional secondary vectors of the flow in section S1 indicate a counter-clockwise flow and in section S3 indicate a round clock flow in the main channel. The presence of vegetation disturbed the secondary flow pattern, and larger vectors were observed at the junction of the two channels in the presence of vegetation. It seems that the presence of vegetation, as observed during the experiments, has changed the patterns and directions of vectors on the floodplain. These changes are also observed at the relative depth of 0.35. Although the capacity of covered flow is less than the uncovered one, flow velocities in and around the main channel seem to be close to those measured in uncovered channel. This indicates the high impact of floodplain vegetation on the hydraulics of the flow in the compound meandering channels. So that, the presence of vegetation has increased the transmission flow of the main canal compared to the simple state. So that if you calculate the average of sections, the rate of flow through the main channel compared to the total flow of compound meandering channel for relative depth of 0.35 and 0.55 is equal to 54 and 36%, which shows a 19% and 6% increase compared to the control mode of transmission through the main channel, respectively.

In this study, using a laboratory model, the effect of submerged flexible artificial vegetation on the floodplain of a compound meandering channel was investigated. The following is a summary of the results of this study. The presence of vegetation reduced the water transfer capacity, due to the increased resistance to flow. The average longitudinal velocity of the flow in the floodplains of the uncovered state is higher than in the case with the cover. Although, flow velocities in and around the main channel seem to be close to those measured in uncovered channel which indicates the diversion of flow to the main channel.

The importance of providing fresh water as one of the principles of achieving sustainable development is not hidden from anyone. Unfortunately, in many developing countries such as Iran, access to fresh water is associated with many problems. The water in aquifers, which supply water to one-third of the world's population, is being consumed more than nature can recover them. On the other hand, almost all large surface water storage dams were built in gorgeous sites. Despite spending a lot of money and innovations in the construction of surface reservoirs, according to UN surveys, more than one billion people currently lack adequate and clean drinking water. Therefore, water shortage will become one of the main constraints on economic development in the near future. In recent years, population growth and rapid economic development have exacerbated the problem of water shortage, especially in coastal areas, to the point that meeting freshwater demand has become a serious challenge for coastal communities. A coastal reservoir is defined as a water storage structure constructed at river estuary or other coastal area to store fresh water and control water resources. One of the obvious advantages of coastal reservoirs is providing additional fresh water storage capacity for water supply networks. In areas under water stress, coastal reservoirs, which are often the basis of local economic development, can help reduce water scarcity. Many coastal reservoirs have been developed in China, South Korea, Hong Kong and Singapore. Despite the importance of coastal reservoirs, there is a little scientific research on these structure in the authorities and many issues in this regard have not yet been resolved. In the present review study, the most important issues and problems related to these structures are presented. Finally, different new topics for future studies are presented.

Accordingly, while introducing coastal reservoirs, their advantages and disadvantages and the appropriate potential that Iran has for the construction of these reservoirs were discussed. In the following, the studies performed on the reservoir dynamics and the mechanism of salinity transfer and diffusion, were mentioned. The most important concern about the performance of this method of water supply is the control of salinity entry into the reservoir, which requires the adoption of special measures in reservoir operation policies. In the study of different authorities, while introducing this important challenge in the two periods of washing and operation parts, 6 boundary conditions controlling the performance of the reservoir were presented. These boundaries are water level, side, Intertidal zone, inlet rivers, reservoir bed and dam. Also, the factors affecting hydrodynamics and salinity transfer in coastal reservoirs such as meteorological conditions, sediment and intertidal zone, upstream rivers, saltwater-freshwater mixing rate, seawater leakage, exchange between coastal reservoir and groundwater, coastal reservoir and lateral side groundwater, coastal reservoir and underlying groundwater, coastal reservoir and submarine groundwater and finally subsidiary functions of coastal reservoirs were investigated. Then, special attention was paid to salinity as the main factor affecting water quality in reservoirs and studies on salinity transfer in coastal reservoirs or wetlands in three parts: desalination, salinization or seawater infiltration and numerical simulation. Finally, the titles of new projects that could be done by researchers in the future were introduced.

Coastal reservoirs enable the storage of excess river flood waters near the coast for future use in areas known to be drought-prone or at those times of the year when water supplies become scarce. Construction of a coastal reservoir does not involve many risk factors and disadvantages like relocation which would be there in inland dam construction. The construction of coastal reservoirs in Iran, especially the southern coasts can solve the problem of water shortage in in these areas. On the other hand, it seems that due to the problems of building inland reservoirs such as suitable topography in some southern part of the country, the use of these reservoirs is the only solution.

Coastal reservoirs need to be rationally utilized. In the present study, with considering different findings from previous studies, several avenues for future research on coastal reservoirs were presented. Hydrodynamics of reservoir water and adjacent groundwater, especially the hydrodynamics near the six boundaries as discussed in this paper. Methods of washing the salty water and sediment, Morphological changes and coastal sedimentation at the vicinity of coastal reservoirs, Sediment transport in the reservoir, Operation of sluices and the influence of dams, Integrated three-dimensional models of water in the reservoir, access channel, groundwater and river and Transfer of contaminants such as heavy metals into the reservoir are some important topics which should be considered in future researches.