5:15 PM - 6:45 PM
[MIS15-P02] Effects of grain size scale on deposition of debris flow
Keywords:Debris Flow, Deposition, grain size
Introduction
Debris flow theory is built on the assumption of a single grain size and steady, isostatic flow conditions. In non-equilibrium sections such as sedimentation, both theory and experiment have not been sufficiently verified. In general, the basic equations of debris flow use the shallow water equations and have a deposition rate equation (Takahashi equation in this case) in the upwelling term of the continuous equation and the equation of motion.
The problem with this equation is that the deposition rate foctor is adjusted at the order level to match the results for the site of interest. The reason for this is that it does not sufficiently take into account differences in grain size. When determining the deposition rate for each grain size class in consideration of the current mixed grain size, the deposition rate of the entire sediment calculated by equation (1) is multiplied by the existence ratio of concentration in each grain size class. In other words, the sedimentation rate for each grain size class is the total sedimentation rate divided by each grain size class. Therefore, the interaction between different grain sizes during deposition is not considered. In particular, the deposition velocities of sediment and flood inundation may differ from the deposition velocity calculated by Equation (1).
In this study, we conducted a hydrographic experiment focusing on the deposition process. First, the relationship between deposition rate and grain size was clarified ("grain size experiment"). Next, we investigated the effects of different grain sizes, mixing ratios, and flow gradients of fine-grained sediment and coarse-grained sediment with widely different grain size ratios on the deposition rate and the interaction between different grain sizes ("mixing experiment").
Experiment
The variable gradient rectangular section channel is 8 m long and 19.5 cm wide. The channel is lined with 6 mm particles on the bed. In order to allow for sediment deposition, a slope change point was established in the middle of the channel, where the slope changes from steep (upstream) to gentle (downstream). The gradual gradient is 1 degree to allow all sediment deposition. Both upstream and downstream experiments were conducted with fixed beds because the bed material is considered large enough compared to the grain size of the flowing and depositing debris flow, so sediment exchange with the bed is not significant. Sediment was placed 1 m upstream of the gradient change point, and water was supplied from the upstream side, eroding the sediment and generating a debris flow. Then, at the point of gradient change, the sediment enters the slow gradient section and starts sedimentation. This was filmed by a digital camera and a high-speed camera (500 fps). The flow depth and deposition rate were measured from the video footage.
In the grain size experiment, sediments with d = 5.0, 3.5, 2.5, 1.5, 1.0, 0.5 and 0.35 mm were used. In the mixing experiment, coarse-grained sediment of d =6.0 mm and fine-grained sediment of d =0.7, 0.33 and 0.18 mm were used. The coarse and fine-grained sediments were mixed one at a time, and the mixing ratio was varied by 25% in the mixing experiments.
Conclusion
The effects of deposition rate and grain size were found to be proportional. It is expected that the variation of the coefficients can be limited by removing the grain size component from the deposition rate factor. This result needs to be confirmed in the future by reproduction of the realized phenomenon. In the mixing experiment, the deposition rate changed after about 60% of fine-grained sediment. The relationship between the relative flow depth (h/d) and the deposition rate shows that this boundary corresponds to the boundary between laminar and turbulent flow. The dominant particles also switch at this boundary.
Debris flow theory is built on the assumption of a single grain size and steady, isostatic flow conditions. In non-equilibrium sections such as sedimentation, both theory and experiment have not been sufficiently verified. In general, the basic equations of debris flow use the shallow water equations and have a deposition rate equation (Takahashi equation in this case) in the upwelling term of the continuous equation and the equation of motion.
The problem with this equation is that the deposition rate foctor is adjusted at the order level to match the results for the site of interest. The reason for this is that it does not sufficiently take into account differences in grain size. When determining the deposition rate for each grain size class in consideration of the current mixed grain size, the deposition rate of the entire sediment calculated by equation (1) is multiplied by the existence ratio of concentration in each grain size class. In other words, the sedimentation rate for each grain size class is the total sedimentation rate divided by each grain size class. Therefore, the interaction between different grain sizes during deposition is not considered. In particular, the deposition velocities of sediment and flood inundation may differ from the deposition velocity calculated by Equation (1).
In this study, we conducted a hydrographic experiment focusing on the deposition process. First, the relationship between deposition rate and grain size was clarified ("grain size experiment"). Next, we investigated the effects of different grain sizes, mixing ratios, and flow gradients of fine-grained sediment and coarse-grained sediment with widely different grain size ratios on the deposition rate and the interaction between different grain sizes ("mixing experiment").
Experiment
The variable gradient rectangular section channel is 8 m long and 19.5 cm wide. The channel is lined with 6 mm particles on the bed. In order to allow for sediment deposition, a slope change point was established in the middle of the channel, where the slope changes from steep (upstream) to gentle (downstream). The gradual gradient is 1 degree to allow all sediment deposition. Both upstream and downstream experiments were conducted with fixed beds because the bed material is considered large enough compared to the grain size of the flowing and depositing debris flow, so sediment exchange with the bed is not significant. Sediment was placed 1 m upstream of the gradient change point, and water was supplied from the upstream side, eroding the sediment and generating a debris flow. Then, at the point of gradient change, the sediment enters the slow gradient section and starts sedimentation. This was filmed by a digital camera and a high-speed camera (500 fps). The flow depth and deposition rate were measured from the video footage.
In the grain size experiment, sediments with d = 5.0, 3.5, 2.5, 1.5, 1.0, 0.5 and 0.35 mm were used. In the mixing experiment, coarse-grained sediment of d =6.0 mm and fine-grained sediment of d =0.7, 0.33 and 0.18 mm were used. The coarse and fine-grained sediments were mixed one at a time, and the mixing ratio was varied by 25% in the mixing experiments.
Conclusion
The effects of deposition rate and grain size were found to be proportional. It is expected that the variation of the coefficients can be limited by removing the grain size component from the deposition rate factor. This result needs to be confirmed in the future by reproduction of the realized phenomenon. In the mixing experiment, the deposition rate changed after about 60% of fine-grained sediment. The relationship between the relative flow depth (h/d) and the deposition rate shows that this boundary corresponds to the boundary between laminar and turbulent flow. The dominant particles also switch at this boundary.