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Showing 9 results for Scour

A. Ardeshir,
Volume 2, Issue 1 (3-2004)

In this research a mathematical model was developed to study bed elevation variation of alluvial rivers. It utilizes two principal modules of hydraulics and sediment transport for simulation purposes. SDAR (Scour and Deposition model of Alluvial Rivers) is a new model with both one and semi-two dimensional (S-2D) computational schemes. It is regarded S-2D in a sence that lateral variation of velocity, hydraulic stresses, and geometrical specifications are achieved by dividing the main channel into serveral stream tubes. In order to overcome the existing limitations, a new idea of reachwise stream tube concept was also introduced. This allows to include branch connections and withdrawal points across the tube barriers. Sediment routing and bed variation calculations are accomplished along each river strip desigated by virtual interfaces of the tubes. Presently, quasi-steady gradually varied flows are processed by the model. It should also be emphasised that this version is only valid for alluvial rivers composed of noncohesive bed material. To assess the model, several river cases and laboratory data base were used. During calibration runs, the ability of model in longitudinal and transversal bed profile simulation and armor layer development predection were especially detected. Results of simulation are also compared with the results of well-known models, e.g. HEC-6, GSTARS-2, and FLUVIAL-I2. It was found that the ability of model in simulating bed variation is noticeably increased when S-2D concept is introduced. Indeed, the comparative validity tests confirm SDAR"s promising functioning in facing with complex real engineering cases. Obviously more article discussions would bring oppurtunities to demonestrate it"s technical cappabilities profoundaly.
Golparvar Fard M., Yeganeh Bakhtiary A., Cheng L.,
Volume 3, Issue 1 (3-2005)

This paper presents a k- turbulence model for simulation of steady current and itsinduced vortex shedding caused by the presence of an offshore pipeline. Performance of the modelaround a circular cylinder above a wall with gap to diameter ratios of 0.1, 0.35 and 0.5 underdifferent flow regimes with Reynolds numbers of 1500, 2500 and 7000 is studied. The flow field iscomputed with solving the Reynolds Averaged Navier-Stokes equations (RANS) the seabed underpipeline is treated as a plane boundary with no-slip boundary condition on pipe surface. Thegoverning equations are solved using Finite Volume Method in a Cartesian coordinate system.Based on the numerical solutions, the flow field, vortex shedding and distribution of shear stressdue to the presence of the pipeline near seabed are studied. In addition the mechanism of vortexshedding with different gap to diameter ratios is examined with focusing on the effect of vortexshedding on bed shear stress. It is found that the k- turbulence model can well predict the flowfield and its induced vortex shedding around a pipeline hence it can be easily applied forsimulation of scour below an offshore pipeline.
M. Salamatian, A. R. Zarrati, S. A. Zokaei, M. Karimaee,
Volume 11, Issue 3 (9-2013)

The efficiency of a collar in reducing the scour depth around circular and rectangular piers is studied at different flow intensities (ratio of upstream shear stress to sediment critical shear stress). Rectangular Piers aligned with the flow as well as skewed at 5º, 10º, 20º were examined. Previous studies had shown that with collar the equilibrium time of scouring increases considerably. To reduce the time of experiments low density sediment was used as the bed materials. Comparison between test results and available results with natural sediment showed that, though the relative equilibrium depths were approximately similar, the time to reach equilibrium condition diminished to less than 10 hours with low density sediment. Experimental results for circular and aligned rectangular pier showed that at u*/u*c=0.95 to 0.75 the collar could reduce the maximum scour hole from about 20% to 60% respectively. In rectangular pier, by increasing the skew angle and/or the flow intensity, the efficiency of collar decreased.
N. Abedimahzoon, A. Lashteh Neshaei,
Volume 11, Issue 4 (12-2013)

In this paper, a new approach is presented for estimating the vertical and horizontal distribution of undertow in the surf zone for reflective beaches. The present model is a modification of the original model presented by Okayasu et al., (1990) for natural, non-reflective beaches to include the effect of partially reflected waves. The nonlinearity of waves, wave-current interaction and nonlinear mass drift of the incident wave are also included in the present model. The results of experimental investigation and model development show that existence of reflective conditions on beaches results in a reduction in the magnitude of undertow and modifies its distribution across the beach profile. Comparison of the results by those obtained from the experiments clearly indicates that by taking the nonlinearity and wave-current interaction, the predictions of undertow in the surf zone are much improved. In particular, due to the effect of turbulence induced by wave breaking for nonlinear waves, the predicted results show more consistence with the measurements.
S. Soudmand, M. Ghatee, S. M. Hashemi,
Volume 11, Issue 4 (12-2013)

This paper proposes a new hybrid method namely SA-IP including simulated annealing and interior point algorithms to find the optimal toll prices based on level of service (LOS) in order to maximize the mobility in urban network. By considering six fuzzy LOS for flows, the tolls of congested links can be derived by a bi-level fuzzy programming problem. The objective function of the upper level problem is to minimize the difference between current LOS and desired LOS of links. In this level, to find optimal toll, a simulated annealing algorithm is used. The lower level problem is a fuzzy flow estimator model with fuzzy link costs. Applying a famous defuzzification function, a real-valued multi-commodity flow problem can be obtained. Then a polynomial time interior point algorithm is proposed to find the optimal solution regarding to the estimated flows. In pricing process, by imposing cost on some links with LOS F or E, users incline to use other links with better LOS and less cost. During the iteration of SA algorithm, the LOS of a lot of links gradually closes to their desired values and so the algorithm decreases the number of links with LOS worse than desirable LOS. Sioux Falls network is considered to illustrate the performance of SA-IP method on congestion pricing based on different LOS. In this pilot, after toll pricing, the number of links with LOS D, E and F are reduced and LOS of a great number of links becomes C. Also the value of objective function improves 65.97% after toll pricing process. It is shown optimal toll for considerable network is 5 dollar and by imposing higher toll, objective function will be worse.
H. Khalili Shayan, J. Farhoudi, H. Hamidifar,
Volume 13, Issue 1 (3-2015)

Because of the complexity of the physical processes in the vicinity of the hydraulic structures due to the separation of the flow, traditional methods for for prediction of maximum scour depth downstream of hydraulic structures are mostly based on empirical approaches. Hence, only a few theoretical works have been reported to study this phenomenon. The present paper describes a new approach based on the momentum principles to estimate the maximum local scour depth downstream of a submerged sluice gate flowing over horizontal or adverse stilling basin. A control volume of the fluid in the equilibrium state of the scour hole was considered and based on momentum principles, some equations are derived to estimate the scour depth at equilibrium state. To verify the proposed equations, large numbers of experiments were planned and conducted under wide range of characteristic parameters such as, incoming Froude number, sediment size, tailwater depth, length and slope of the apron. It was found that the proposed equations fall in a good agreement with experimental results. It was also observed that, in the case of horizontal apron, a specific tailwater depth exists with which the local scour depth attains a minimum value. However, in the case of adverse basins when the tailwater depth takes a specific value, the maximum depth of the scour hole reaches to its maximum and then decreases to a constant value as the tailwater depth increases. This critical tailwater depth was formulated using a semi-theoretical equation.
Duc Do Minh, Mr. Hieu Nguyen Minh,
Volume 15, Issue 3 (5-2017)

Climate change and global warming have led to severe typhoons and sea level rise (SLR) which may threat the stability of coastal structures. However, countermeasures to enhance coastal protection against SLR and severe typhoons have not appropriately considered. The Hai Hau coast with 33.3 km of sea dikes is located in the Red River delta of Vietnam. Herein, coastal dikes have collapsed twice over the last 30 years, which lead to about 1.5 km of coastline retreat. This paper aims to assess quantitative impacts of SLR on sea dike stability. Change in pore water pressure (PWP) in the dikes was monitored by piezometers. Distribution of PWP at different tide levels was then used to calculate factors of safety (Fs) of inner and outer slopes. Projected SLR until 2100 will reduce Fs of the outer slopes, but sea dikes have no problem with sliding stability. The main threats of SLR to sea dike stability are indirect impacts such as accelerated erosion, scouring, and wave overtopping-induced soil erosion on the inner slopes. Troughs of sea dikes in Hai Hau coast could fail in 6–10 years due to accelerated coastal erosion. A solution of multiple protections to adapt to SLR in Hai Hau coast was proposed which includes conventional structures (i.e. dike, revetment, groins, and mangrove) together with geotubes as submerged breakwaters and vetiver grass.

Younes Aminpour, Javad Farhoudi,
Volume 15, Issue 5 (7-2017)

Local scour downstream of hydraulic structures is one of the critical phenomena which has absorbed a vast amount of interests by researchers. The designers of hydraulic structures, particularly, spillways try to utilize proper means to minimize the consequences of excess energies downstream of such structures which usually tend the erosion at their immediate downstream reaches. The stepped spillway is designed to create a large amount of energy dissipation by means of steps and would decrease the amount of scour evolution at its downstream. This article presents the results of 67 experiments conducted at two different scales of stepped spillways, to study the local scour downstream the structure. The experiments were planned to consider a wide range of geometrical factors, flow characteristics, and sediment properties. The time length of experiments was ranged from 6 to 24 hours which produced more than 80000 data points for analytical considerations. The results were used to render a regression equation to define the similarity among the scour hole profiles. It was observed that, a long term observation would be needed to reach the equilibrium state. However, semi-equilibrium conditions will be achieved after 24 hours. It was also noted that the depth of scour hole adjacent to channel walls was bigger than that at centerline. 

Volume 15, Issue 7 (10-2017)

In this paper, the scour hole dimensions around submerged and emerged spur dike in a 90o bend along with the mean and turbulent flowfield were investigated experimentally. Two types of re-circulating flow at the downstream of the spur dike and around the spur dike wing were observed. A direct relation between the estimated bed shear stress using TKE and the scour process prevails. More attentions is needed in estimating the bed shear stress using vertical velocity fluctuations. The scour hole dimensions increase by increasing the ratio of radius of channel bend to channel width , the Froude number of the spur dike, ratio of the length of spur dike to channel width and ratio of the approach mean flow velocity to the approach flow velocity at threshold condition. However, vice versa trends were observed by increasing ratio of the spur dike length to the median sediment size, ratio of the wing length of spur dike to the length of spur dike and the submergence ratio. A particular location of the spur dike in the sharp bend was specified beyond which the scour hole dimensions increase. The ratio of the spur dike length to the median sediment size has a secondary effect on the scour hole dimensions. New equations are proposed for prediction of the scour hole dimensions considering the submergence ratio along with other effective parameters.

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