Experimental investigation of the angle effect of the cylindrical bridge group piers relative to the flow direction on the maximum scour depth of the piers

Bridges are one of the important structures in the field of land communication. With the construction of these structures in the river, a flow pattern with a three-dimensional structure is formed in the vicinity of its piers, and as a result of increasing the flow speed and the formation of horseshoe vortex and wake vortices, part of the sediments around the piers and foundation will be washed away, and if the sufficient depth of foundation is not taken into account, the destruction of the bridge will result especially during floods. Road or railway crossing over the rivers is limited to the particular reach of the rivers which is determined by the general direction of the road or railway. Moreover, the general direction of the road or railway determines the position of the bridge over the river. Selection of the bridge path angle relative to the river flow direction is very important.  Sometimes, due to the geographical conditions of the region and the general direction of the road or railway, the bridge crossing directly perpendicular to the flow direction is impossible. In this case, the bridge deck diagonally crosses over the river and the bridge group piers are angled relative to the flow direction. In such case, the distance between the piers, the flow direction relative to the piers and the piers submergence are very important parameters which affect the scour depth

Reservoir sedimentation modelling

The purpose of this study is to consider sediment deposition in large reservoirs with high suspended sediment inflows. A characterising feature of this kind of reservoir is the existence of turbidity currents due to density differences between inflow and ambient waters of the reservoir. Therefore, two major parts can be found in this study: firstly, experiments and analysis of the experiments on gravity currents, and secondly the development of a computer model for reservoir sedimentation modelling. Some experiments were conducted using a laboratory flume to consider different aspects of gravity currents including: the development of the head of gravity currents, the body of subcritical gravity currents, and deposition due to the head and the body of turbidity currents. The analyses of the measured velocities, concentrations and the sizes of sediment particles have been presented. By using the data collected from this study and other available data a n e w coefficient for the equation of the head of gravity currents was proposed with the help of a statistical package. Sediment transport by the head of gravity currents is discussed. Based on the calculated water entrainment and using other available water entrainment data, an equation for the water entrainment coefficient was proposed. A new equation for sediment entrainment over an erodible bed was presented by using the available data from other investigators. A new procedure for the prediction of sediment processes in reservoirs was developed recognising the fact that turbidity currents affect long-term sedimentation, particularly when suspended sediment concentration is relatively high. Based on this a new computer program, DEPO , for the prediction of sediment processes in reservoirs was developed by incorporating the effects of turbidity currents on long term sedimentation. Although the model is theoretically one-dimensional, some options exist for the distribution of sediment deposited on the bed or for sediment scoured from the bed. This makes the model a pseudo two dimensional model. To verify the proposed model, four different turbidity currents were run in the laboratory flume. The computations performed by DEPO for: the water elevation, the height of the turbidity current and the amount of the deposited material on the bed, showed excellent agreement with the measured values. The proposed model was also tested by application to a prototype situation, Dez Reservoir (a large reservoir in the south-west of Iran). Test results showed the capabilities of the model as a practical tool for the prediction of long term reservoir sedimentation. The DEPO model was tested using Dez Reservoir to consider the effects of alternative bottom gates on deposited sediment. By using the alternative bottom gates the amount and the pattern of sediment deposited in the reservoir were affected significantly. The height of the sediment deposited in the reservoir was reduced, particularly in the region from the d a m wall to 20 km upstream of the dam. The estimated volume of deposited sediment was reduced by about 55 percent and the trap efficiency was reduced to 0.46. The model was also utilised to predict the future volume and bed elevation of Dez Reservoir after 60 years of operation. The results showed the capabilities of the model for predicting long term reservoir sedimentation, for the management of reservoirs, for considering the effects of the bottom gate on reservoir life, and for controlling turbidity currents in reservoirs

Investigating the process of changing the discharge coefficient of the flow passing through the combined structure of rectangular sharp-crested weir with multiple-gates

To investigate the effect of the number and dimensions of the gates-weir structure, 117 experimental tests were carried out at different flow rates, numbers and dimensions of square gates. The experiments were carried out in a laboratory flume with a length of 12 m. The results showed that with the increase in the number and dimensions of the gates in combined weir-gate structure, the combined discharge coefficient was increased. These changes are such that the combined discharge coefficient is increased by 3.8% with the increase of each gate. The results showed that the lowest and the highest increase in the flow coefficient was observed in the comparison between the single gate and the 5-gate condition compared to the no-gate condition by 16% and 18%, respectively. In the following, a relationship was presented to determine the combined discharge coefficient with RMSE = 0.001

Experimental investigation on density currents propagating over smooth and rough beds

Density currents occur when fluid of one density propagates along a horizontal boundary into fluid of a different density. They are also called turbidity currents when the main driving mechanism is from suspended sediments. Reservoir sedimentation is often related to sediment transport by turbidity currents. The leading edge of a density current is deeper than the following current and is called head or front. In this paper, the effects of bed roughness on density currents propagation were studied. Experiments were carried out over a smooth bed as well as three artificially roughened beds by cylindrical roughness elements. Temporal and spatial evolutions of the current front were analyzed. In experiments performed over rough beds, the measured head velocities were smaller than that of smooth bed. The observed trend is that as the surface roughness increases the front velocity decreases

Temporal Scour Variations at Permeable and Angled Spur Dikes under Steady and Unsteady Flows

Spur dikes are river protection structures typically used for flow diversion from erodible banks. However, scouring might be a severe problem that compromises their stability and, consequently, their hydraulic function. This paper aims to study the maximum scour depth at permeable and angled spur dikes under hydrographs of different duration. Experiments were carried out in a rectangular channel 10 m long, 0.76 m wide, and 0.6 m deep. The mobile bed was made of nearly uniform sand with a median grain size of 0.8 mm. A total of 36 new experiments were performed with a detailed data collection over the time (i.e., 216 datasets), which could provide a useful contribution to the topic. The impact of the spur dike orientation angle, θ, and the degree of permeability, φ, on the temporal scour evolution were explored. Results were found physically consistent and revealed that the spur dike permeability implies a significant attenuation of the scouring processes in comparison to the impermeable spur dikes and generally its effect is more beneficial than that from a favorable orientation angle. The differences in percentage between the maximum scour depth for impermeable spur dikes and the maximum scour depths for various degrees of spur dike permeability were found ranging from 44% (at φ = 33% and θ = 60°) up to 88% (at φ = 66% and θ = 120°). Other results include the effect of the hydrograph base-time on the scour depth and the comparison between scouring processes under steady and unsteady flow conditions. By quadrupling the hydrograph base-time, keeping constant the peak and base flood discharges, the maximum scour depths increased by about 29%, 42%, and 25% in case of impermeable spur dike, spur dike with 33% degree of permeability, and spur dike with 66% degree of permeability. Furthermore, starting from dimensional analysis a new empirical model (with coefficient of determination R2 equal to 0.94) is introduced to predict the time-dependent scour depth due to the passage of a flood wave. The model suggests that the main independent dimensionless variables which control local scour processes are: the densimetric Froude number, the time t normalized to the hydrograph base-time, the degree of permeability, and the orientation angle. These dimensionless variables would generalize the laboratory results to the real-world scenarios, although caution should always be taken because of possible scale effects

Effects of cylindrical and cubic piles on motion of density currents

Gradijentno strujanje je gibanje fluida kroz neki drugi fluid drugačije gustoće, uz često negativan utjecaj na okoliš. Zbog problema taloženja uzrokovanih djelovanjem gradijentnih struja u blizini tijela brana, često se poduzimaju odgovarajuće mjere kako bi se ublažio ili eliminirao utjecaj takvih struja u srednjem dijelu akumulacija. U tom se smislu u sredini akumulacije postavljaju odgovarajuće prepreke. U ovom se radu eksperimentalno istražuje utjecaj valjkastih i kockastih prepreka na kretanje prednje fronte (glave) gradijentnih struja. Rezultati istraživanja pokazuju da je utjecaj kockastih prepreka na parametre strujanja veći od utjecaja valjkastih prepreka.Density current is the motion of a fluid in another fluid of a different density, with frequent negative environmental impacts. According to the deposition problems caused by density currents in the vicinity of dam bodies, attempts are usually made to weaken or eliminate these types of currents in the middle of the reservoir. Appropriate barriers are placed in the middle of the reservoir for this purpose. The effects of cylindrical and cubic obstacles on the motion of the head of the saline density current are experimentally investigated in this study. The results show that the effect of cubic obstacles on current parameters is greater compared to cylindrical obstacles

An experimental study to determine the obstacle height required for the control of subcritical and supercritical gravity currents

<p>Turbidity currents are important phenomena involved in sediment processes in long and deep reservoirs. Deviation or dissipation of the turbidity currents in reservoirs can be achieved by using obstacles. In this paper, we study the effects of obstacle height on complete control of both subcritical and supercritical gravity currents (.7 ≤ <i>Fr</i>_<i>d</i> ≤ 1.37) with various inflow characteristics by means of a physical model. First, a gravity current with different concentrations and discharges without using an obstacle is measured along a flume with three different slopes. Then, the height required to block the gravity current is determined by taking into account inflow characteristics and other results reflected in the literature. Finally, using the obstacle same as embankment barrier with various heights, the effects of obstacle height on the gravity current are studied in both subcritical and supercritical flows. The results specify what obstacle height is required for complete blockage of subcritical and supercritical gravity currents.</p

Experimental Observation of Saline Underflows and Turbidity Currents, Flowing over Rough Beds

Density currents are formed when gravity acts upon a density difference between two different fluids, and the driving force is the buoyancy force. These currents are the most important transport mechanisms and deposition of noncohesive sediments in narrow and deep reservoirs. In this research, 126 experiments were performed to investigate the effects of artificial bed roughness on saline and sediment-laden density currents. Conic and cylindrical shapes of roughness were used with three different heights. Velocity and concentration profiles were measured in 4 and 3 cross-sections, respectively. Presence of roughness causes increasing density current body thickness, decreasing maximum value of velocity and increasing distance of peak value of velocity point from the bed in the normal velocity profile. Coefficient of entrainment in the rough beds was more than smooth bed and boosted for greater roughness heights. A special behavior, named as “Lifting phenomenon”, was appeared in some of the tests which effects on the velocity profiles.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Spatial and temporal evaluation of global rainfall products in a data-scarce region: The Dez Basin, Iran

The limitation in approachability to rainfall data sources with an appropriate spatial-temporal distribution is a significant challenge in different parts of the world. The development of general circulation models and mathematical algorithms has led to the generation of various rainfall products as new sources with the potential to overcome the shortage in datascarce basins. In this study, the performance of the PERSIANN-CCS and CMORPH satellite-based rainfall product, as well as the ERA5 and ERA-Interim reanalysis, was evaluated based on detection skill and quantitative metrics in a daily, monthly and seasonal time scales in the Dez basin located in the southwest of Iran. The basin has a wide topographic variation and scattered rain gauge stations. Overall results denote that the ERA5 dataset has the best performance in all statistic verification than other rainfall products. Based on the daily evaluation of all rainfall products, the false alarm rate (FAR) is higher than 0.5, so none of the datasets could capture the temporal variability of rainfall occurrence. This study has covered the western parts of the Zagros steep slopes in which the topographic conditions have a significant effect on the activity of rainfall systems. On a monthly scale, the mean value of the correlation coefficient (CC) for ERA5, ERA-Interim, PER-SIANN-CCS, and CMORPH was equal to 0.86, 0.85, 0.51, 0.39, respectively. The results of seasonal evaluation suggested that all datasets have better rainfall estimation in autumn and winter, and the capability of all datasets dramatically decreased in the spring. The current paper argues that the ERA5 reanalysis typically outperforms ERA-Interim and can be considered as a reliable rainfall source in the future hydrological investigation in the southwest of Iran