Experiment investigation the effect of unsteady flow condition on mobile bed load

The streams can be divided into 3 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 laboratory flume. The experiment canal that use 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 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 laboratory. In this method form of canal is changed. We created five section. 1) Fixed roughness, 2) mobile bed load, 3) section of sampling, 4) Fixed roughness and 5) glass. The sampler of sediment include 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 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 time (60 sec , 90 sec , and 120 sec) and peak flow (between 20 and 55 liter 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 bases flow, two steady stream 0.95 and 1.5 liter 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 transmission sediment bed load decrease with stream of base flow and it is minimum at experiment without base flow. Also total of sediment transport is higher in experiment with base flow. Therefor exist of the base flow can increase sediment transport and upstream erosion. In this experiment the result show that occur the hydrograph with elder 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. Also peak of sedigraph increase with base time and therefor base time is an important parameter for upstream erosion and sediment transport. Another part of this research also checked the effects of fixed bed roughness on characteristics of flow hydrographic in a channel. In this series of experiments, the fixed bed was made using three different roughness’s including one roughness of glass and two roughness’s 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. Since increases the bed slope from 0.005 to 0.01 increased 50% bed load transported and with two times the slope it show increase 400% in bed load transported also 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 equal to 5 - 11% of flow hydrograph duration. For hydrograph with the same pick flow, sediment transport average decrease with increase base time. Also elevation of bed load shown that maximum erosion occurs at the start of bed load.