Experimental investigation of time-dependent local scour downstream of a stepped channel

In this study, temporal variation of local scour occurring at the downstream part of the stepped channel were investigated experimentally. The experimental tests were carried out in a stepped flume with a height of 2.4 m. The width of the rectangular flume was 0.10 m and the length of the stilling basin was 2.12 m. Bed material was placed in a sediment box with a height of 24 cm and length of 2.48 m, without any compaction. Experiments were carried out by using bed material of 4 different grain size distributions, 2 different sill heights and 6 different flow rates. Two empirical equations which include Shields parameter (θ) and densimetric Froude particle number (Fd) were proposed by using the experimental findings to predict the temporal variation of the scour depth. The R2 (coefficient of determination) values were computed for both proposed equations as 0.866 and 0.865. The scatter index (SI) values were also determined and computed as 8.73% and 8.25%. The fit of the equations was also determined by means of Fisher’s test.Keywords: stepped channel, local scour, temporal variatio

Ayşegül ÖZGENÇ AKSOY1, * , Mustafa DOĞAN1

In open channels broad crested weirs are widely used to measure the discharge values. In this study the effect of the approach angle on the discharge efficiency of the broad crested weir was investigated experimentally. Four different approach angles were tested under steady state conditions by using three different discharge values. Experimental results indicate that the discharge coefficient is not only a function of crest length but also a function of approach angle. A modified new non dimensional empirical relation involved approach angle has been proposed

Experimental investigation of time-dependent local scour downstream of a stepped channel

In this study, temporal variation of local scour occurring at the downstream part of the stepped channel were investigated experimentally. The experimental tests were carried out in a stepped flume with a height of 2.4 m. The width of the rectangular flume was 0.10 m and the length of the stilling basin was 2.12 m. Bed material was placed in a sediment box with a height of 24 cm and length of 2.48 m, without any compaction. Experiments were carried out by using bed material of 4 different grain size distributions, 2 different sill heights and 6 different flow rates. Two empirical equations which include Shields parameter (θ) and densimetric Froude particle number (Fd) were proposed by using the experimental findings to predict the temporal variation of the scour depth. The R2 (coefficient of determination) values were computed for both proposed equations as 0.866 and 0.865. The scatter index (SI) values were also determined and computed as 8.73% and 8.25%. The fit of the equations was also determined by means of Fisher’s test

Experimental Study of the Coarse Surface Development Effect on the Bimodal Bed-Load Transport under Unsteady Flow Conditions

WOS: 000313361400002Gravel-bed rivers display a surface layer that is coarser than the substrate lining below this armored surface. The armored layer that is developed under certain flow conditions may be attributed as water-worked sediment bed. Both the threshold of incipient motion and the bed-load transport are affected by the presence of this layer. In this study, the coarse surface development and its effect on the sediment transport were investigated experimentally in a rectangular flume. Four antecedent flow rates were combined with an input hydrograph, without sediment feeding. The reference shear stress of individual fractions and the grain-size distribution were determined after the development of the coarse surface due to the antecedent flow. Then a triangular-shaped hydrograph was generated for both the intact surface and the coarse surface cases. It was seen that the maximum bed-load transport values obtained in unsteady flow experiments were highly dependent on the coarse surface formed by the antecedent flow. The interrelation between the armor ratio and the total bed load was also sought. It was revealed that there existed a nearly linear relation between the armor ratio and the dimensionless total bed load with a correlation coefficient of 0.99. This strong interdependence implies that the knowledge of the armor ratio is of basic and utmost importance to predict accurately the bed load to be transported. The values of the Einstein bed-load transport parameter were plotted versus those of the dimensionless shear stress. When the global sediment transport was considered, a clockwise hysteresis appeared in the case of the initial intact bed surface, whereas a counterclockwise hysteresis arose when the coarse surface was developed. As for the fractional sediment transport, in the case of the intact initial surface the clockwise hysteresis was encountered more frequently, but for the armored bed experiments the coarser the bed surface, the more dominant was the counterclockwise hysteresis, meaning that the quantity transported during the falling limb of the hydrograph was higher than that transported during the rising limb. DOI:10.1061/(ASCE)HY.1943-7900.0000640. (C) 2013 American Society of Civil Engineers.TUBITAKTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [106M274, 109M637]The authors would like to acknowledege the financial support of TUBITAK through the Projects No: 106M274 and 109M637

Experimental investigation of equilibrium and temporal scour around the round head of the rubble mound breakwater in case of breaking and non-breaking waves

In this study, local sediment movements caused by regular wave effects around the head section of rubble mound breakwater were investigated experimentally. The experiments were performed in a wave channel with a length of 33 m, a width of 3.6 m and a depth of 1.2 m. A plunger-type wave generator was located at the offshore side, generating regular waves of varying heights and periods. Uniform sandbed material of three different grain sizes was used, alongside two distinct breakwater slopes and six different wave characteristics. Experiments were performed under both breaking and non-breaking wave conditions. Time-dependent scour depth and water level measurements were determined by using measuring equipment working with an ultrasonic method. In the paper, new empirical relations were derived for temporal relative scour depths and the non-dimensional time scale parameter and the non-dimensional time scale parameter. The determination coefficient and scatter index of these derived relations were calculated and evaluated statistically. It was observed that the relations are in good agreement with those obtained from experiments. Additionally, the findings indicate that wave breaking significantly reduces the equilibrium scour depth

Study of the time-dependent clear water scour around circular bridge piers

The local scour around bridge piers influences their stabilities and plays a key role in the bridge failures. The estimation of the maximum possible scour depth around bridge piers is an important step in the design of the bridge pier foundations. In this study, the temporal evolution of local scour depths as well as the equilibrium scour depths were analyzed

Study of the time-dependent clear water scour around circular bridge piers

WOS: 000394839900003The local scour around bridge piers influences their stabilities and plays a key role in the bridge failures. The estimation of the maximum possible scour depth around bridge piers is an important step in the design of the bridge pier foundations. In this study, the temporal evolution of local scour depths as well as the equilibrium scour depths were analyzed. The experiments were carried out in a rectangular flume by using uniform sediment with median diameter of 3.5 mm and geometric standard deviation of 1.4. The diameters of the tested circular bridge piers were 40 mm, 80 mm, 150 mm and 200 mm. The flow and scour depths were determined by ultrasonic sensors. The experiments were realized in clear water conditions with various constant flow rates. The experimental findings were compared with those calculated from some empirical equations existing in the literature. A new empirical relation involving the flow intensity, the relative water depth and the dimensionless time is also introduced. The advantage of this proposed relation is that the only parameter requiring the calculation is the critical velocity, other parameters being known geometric and hydraulic parameters. The performance of this relation was tested by using experimental data available in the literature, and a satisfactory compatibility was revealed between the experimental and numerical results.TUBITAKTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [106M274, 109M637]The authors thank TUBITAK (project number 106M274 and 109M637) for the financial support

Study of the time-dependent clear water scour around circular bridge piers

WOS: 000394839900003The local scour around bridge piers influences their stabilities and plays a key role in the bridge failures. The estimation of the maximum possible scour depth around bridge piers is an important step in the design of the bridge pier foundations. In this study, the temporal evolution of local scour depths as well as the equilibrium scour depths were analyzed. The experiments were carried out in a rectangular flume by using uniform sediment with median diameter of 3.5 mm and geometric standard deviation of 1.4. The diameters of the tested circular bridge piers were 40 mm, 80 mm, 150 mm and 200 mm. The flow and scour depths were determined by ultrasonic sensors. The experiments were realized in clear water conditions with various constant flow rates. The experimental findings were compared with those calculated from some empirical equations existing in the literature. A new empirical relation involving the flow intensity, the relative water depth and the dimensionless time is also introduced. The advantage of this proposed relation is that the only parameter requiring the calculation is the critical velocity, other parameters being known geometric and hydraulic parameters. The performance of this relation was tested by using experimental data available in the literature, and a satisfactory compatibility was revealed between the experimental and numerical results.TUBITAKTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [106M274, 109M637]The authors thank TUBITAK (project number 106M274 and 109M637) for the financial support