Document Type : Research Article
Marine Physics Department, Tarbiat Modares University, Mazandaran, Iran.
Rip current is one of the most important phenomena in coastal areas. Due to the importance of rip currents, which are directly related to human lives, they have been studied and evaluated from different approaches. This research aimed to determine the effect of rip currents on the granulometry of the sediments through sampling of the sediments in the rip channel and its surroundings area in the Caspian Sea coast. Therefore, one station was selected in Noor city, Mazandaran province. Various factors such as lack of private property, easy access, lack of human manipulation and knowledge of drowning rescuers about the location of the drowned were considered in the selection of the station. Then, in two seasons, winter (December and March) and spring (May and June), with the help of lifeguards and existing signs, the location of the rip canal was identified in the station and at least three sediment samples were taken from the channel and three samples from the environment around the channel using Grab. In addition, the water depths were estimated in the sediment sampling points using Echo sounder. The sediment samples were transferred to the laboratory and sediment particles were separated based on grain size using a shaker and multiple sieves (with a network mesh of 30, 50, 60, 80, 100, 140, 200 and 230). The data obtained from the shaker were entered into the excel page of GRADISTAT software and the characteristics of sediment samples including mean diameter (D50), mean, degree of sorting, skewness and kurtosis calculated by Folk and Ward method were obtained. All sediment particle characteristics were compared between the rip channels and surrounding areas using unpaired t-test. The results showed that among the sedimentation characteristics, no significant difference was observed between the channel surrounding areas in the degree of sorting and kurtosis neither in the winter nor in the spring. In addition, the results of granulometry showed that D50, mean particle size and skewness of grain distribution of sediment particles were significantly different between the rip channel and the surrounding areas in the spring. The highest amount of D50 (with an average of 185.8 mm), mean (with an average of 202.7 mm) and skewness (0.48) of sediment particles were observed in the rip channel. These significant differences of particle characteristics were not observed in the winter samples probably due to the turbulence of the weather and sea currents. In addition, the average water depth of the rip channel was obtained of ca. 120 cm, while the average water depth of surrounding areas was estimated of ca. 85 cm. Therefore, we can state that some of the channel sediments have been removed by water flow in the channel. The higher energy and velocity of the flow in the channel than the surrounding environment has caused the transfer of sediments. On the other hand, sediment transport is directly related to other parameters such as sediment grain size. The present study showed that rip channels have sediments with significantly different characteristics than the surrounding sediments. The higher flow rate in the rip channel probably causes the removal of some fine-grained sediments and is not able to remove coarse-grained sediments. Removal of fine-grained sediments causes a change in the texture of the remaining sediments towards larger grains and these changes have caused a significant granulometric change between the rip channel and the surrounding environment, especially in the spring.