Laboratory study of motion threshold on rough bed and the effect of temperature on Shields diagram

The transfer of solid particles in fluid flow has been a common phenomenon in nature. The threshold of movement of sediments from the physical dimension is mixed with both deterministic and random characteristics. Certainty in the movement threshold is caused by the average flow rate, while its randomness is caused by the turbulence of the flow. The initial movement of sediment is the beginning of scouring process in natural streams. The sedimentation threshold is the condition where the destabilizing forces equal the stabilizing forces and the sediment particle is on the verge of experiencing primary motion. Two common sedimentation threshold parameters, Shields parameter and moving number, have been studied by different researchers. The initial movement of bed sediments is an important component for various researches such as sustainable waterway design, river management, sediment transport and sedimentation prevention. The beginning of the movement of sedimentary particles is called the threshold of movement and the conditions in which the particles are on the threshold of movement are called threshold or critical conditions. In low-speed flows, the substrate particles do not have any movement and are fixed in their place, but with the increase in the flow speed, the substrate particles start to move. This movement is initially in the form of up and down of the particle without transfer, but gradually the movement The particles start downstream.

In the current research, the criterion of the threshold of particle movement is the beginning of non-stop movement of particles that are set in motion in the model and move along the flume to the downstream side of the flume as the threshold of particle movement. During the test, the flow rate of the model and the upstream water level are controlled several times so that there may be no change. From dividing the flow rate, leading to the movement threshold by the flow section, the critical speed or the movement threshold speed is determined. To find the threshold speed of movement of a sediment sample in a specific slope of the channel, each experiment is repeated several times and the results are averaged. After finishing the test, the remaining sediment on the metal plate is removed. Some of the sediments have also entered the tank, and after collecting the sediments, they are dried and prepared for the next stage of tests. After the completion of each test, the slope of the channel is changed so that the main section is placed in another slope. Then all the steps mentioned above are repeated again. It seems that the Shields diagram is the reference point for all the research done in the field of sediment transport. Due to the fact that no research has been done on the effect of temperature on the Shields diagram, the present study aims to investigate the sediment movement threshold using the effect of temperature on the Shields parameter and shear stress in a laboratory model with three slopes of 0.005, 00.1 and 0.05 at four temperatures of 8, 12, 20, and 40 degrees Celsius and three flow rates. In the current research, the criterion of the threshold of particle movement is the beginning of non-stop movement of particles that are set in motion in the model and move along the flume to the downstream side of the flume as the threshold of particle movement.

In this research, using the physical model and dimensional analysis, the condition of movement threshold in open channels with rectangular section was investigated. The results showed that with the increase of relative roughness (ds/y) regardless of the temperature variable, the stability number of the particle at the movement threshold (SN) decreases and the influence of the slope on these two parameters is not noticeable. Also, in the condition of constant slope and relative roughness (ds/y), with increasing temperature, the value of particle stability at the movement threshold (SN) decreases. Changes in particle stability number at the threshold of movement (SN) have an upward trend with respect to temperature, and with increasing temperature, the number of particle stability at the threshold of movement (SN) increases. In addition, the changes of the relative roughness parameter (ds/y) with respect to temperature are associated with a downward trend, and with the increase in temperature, the relative roughness parameter (ds/y) decreases. In addition, as the slope increases, the particle stability parameter decreases. This result can be extended to different seasons of the year so that the threshold of sediment movement and sedimentation in seasonal and permanent rivers is lower than in cold seasons in hot seasons. Comparing the results of the current research with the results of other researches, there is a very small difference between the results of this research and the results of other researchers. The main reasons for these differences are the different definitions of movement threshold among researchers, the different flow conditions and test conditions.