مدل‌سازی تجربی تأثیر شرایط غیردائمی جریان بر رفتار بستر فرسایش‌پذیر

The streams can be divided into three groups; 1) Perennial streams, 2) Seasonal streams and 3) Ephemeral streams. Seasonal streams are the kinds of stream that have flowed in certain seasons and they often occur in the watershed due to snow melt or sudden rain. The flow in this type of streams, acts as a flash flood, consequently its flow hydrograph has a sharp peak and a relatively short base time and it can cause erosion and sediment transport in the watershed, which altogether create some problems and constraints to human. The purpose of this research is measuring the temporal changes of sediment transported (bed-load) by flash flood (unsteady flow condition) in a laboratory flume. The experiment canal that uses in this research has 10 m length, 30 cm width, and 50 cm height. Using an electrical device which is connected to the channel pump, input flow turns into an unsteady flow and then enters the channel. Some of the characteristics of this flow are imperceptible increase and decrease while passing through the laboratory flume. In this research, a new method is also presented for sampling and measuring the bed load sediment which is transported by the flow in the laboratory. In this method form of the canal is changed. We created five sections: 1) fixed roughness, 2) mobile bed load, 3) section of sampling, 4) fixed roughness and 5) glass. The sampler of sediment includes 20 net box with length, width and height 25 cm, 5 cm and 3 cm, respectively. For inscription date, use two camera system. One of them for record of bed change that set in front of mobile bed and another one for record boxes of sampler movement under the section of sampling. Time duration of sampling for each box of the sampler is between 3 to 6 second. A grading of sediment with D50=2.5 mm is used. This research includes a series of stream symmetric triangular hydrographs in different shapes and contains different base times (60 sec, 90 sec, and 120 sec) and peak flows (between 20 and 55 liters per second), which were crossed from the bed of sediment. Then the transported bed load, affected by them, was removed by using the produced sampler. Also, as base flow, two steady streams 0.95 and 1.5 liters per second is determined. The effect of some factors such as existence or lack of the base flow on amount of transported bed load were also checked and experimented by input stream hydrograph. The results show that maximum bed load transport decrease with stream of base flow and it is minimum at experiment without base flow. Also, a total of sediment transport is higher in the experiment with base flow. Therefore, the existence of the base flow can increase sediment transport and upstream erosion. In this experiment, the hydrograph had greater base flow, increase upstream erosion to 76.5 % and for another base flow they increase 26.5 %. For hydrograph with the same pick flow, when its base time increase 1.5 and 2 times, total sediment transport increase 45 and 92 percent, respectively. Also peak of sedigraph increase with base time and therefore base time is an important parameter for upstream erosion and sediment transport. In another part of this research, the effects of fixed bed roughness on characteristics of flow hydrographic in a channel is investigated. In this series of experiments, the fixed bed was made using three different roughness, including one roughness of glass and two roughness of different granulations. The results of movable bed show that bed slope has a great effect on upstream erosion and transportation of sediment to downstream. As increasing the bed slope from 0.005 to 0.01 lead to increase 50% of transported bed load and with two times the slope, the transported bed load increase to 400%. The results show that a temporal lag was found between the flow hydrographs peak and the sediment hydrograph peak. The temporal lag was found to be about 5 - 11% of flow hydrograph duration. For hydrograph with the same pick flow, average sediment transport decreases with increase the base time. Also elevation of bed load showed that the maximum erosion occurred at the start of bed load transmission