سمیرا رضاپوریان قهفرخی

Bank erosion consists of two processes: basal erosion due to hydraulic force and bank failure under the influence of gravity. Because riverbank resistance force varies with the degree of saturation of bank material, the probability of bank failure is the probability of the driving force of bank failure being greater than the bank resistance force. . The degree of saturation of bank material increases with river stage; therefore, the frequency of bank failure is correlated to the frequency of flooding. Consequently, the rate of bank is a probabilistic phenomenon. In this research, a mathematical model was developed to predict riverbank erosion rate. This model is based on Duan (2005) approach. The model was used to compute riverbank erosion rate of a meander reach in the Zayande Rud river. The model result indicates that the rate of bank erosion at the ZayandeRud river is approximately 0.4 m/year, and bank surfaces are not stable

The geomorphological changes of the bed and bank of rivers can affect the migration and expansion rate of the meandering rivers. A knickpoint in the bed slope is one of the most common geomorphologic changes in the river. Measuring the migration and expansion rate of meandering rivers, in addition to taking the necessary actions in order to protect the structures located along and over the river are considered as engineering measures in the field of river engineering. In this regard, to study and measure the rate of migration and expansion of meandering rivers with knickpoint in the flow at downstream, experiments were carried out in a laboratory flume with an erodible bed. The channel was carved on the sedimentary bed, and was studied and evaluated the migration and the expansion rate in to two experimental groups in the presence and absence of the knickpoint (control tests) for three different widths of the channel (different wavelengths of the initial bend), three discharges, and three different bed slopes. Results showed that the flow began to create meandering bends at a very high rate in order to modify the knickpoint at downstream, and as a result, the migration rate of meander has increased compared to the control experiments and decreased the expansion rate. An increase in the rate of migration up to 32% and the decrease in the rate of expansion up to 36% are achieved by comparing the results of experiments with presence of the knickpoint at downstream and its absence.

Scouring is one of the destructive factors in the hydraulic structures such as bridges. It occurs mainly during flood events. To find the most effective countermeasure to protect water structures against scouring phenomena, extensive investigations have been performed. One of such methods is to use submerged series of vanes which is not fully investigated yet. In this study, two series of submerged vanes were used either separately or attached to the upstream face of bridge pier and arranged symmetrically on different positions and angles of attack to control scour at cylindrical pier under clear water condition. The results showed that the highest reduction in scour depth occurs as the vanes series stands closer to the pier. Under the best condition, scour depth was reduced by 80.9 percent.