10:00 AM - 10:15 AM
[ACG45-05] Experimental study of sediment binding effect by vegetation cover at the front edge of the Futtsu coastal forest floor in Chiba
Keywords:Vegetation coverage, Hydraulic experiments, Tsunami
Coastal forests are not only effective in preventing sand and wind during normal times. In the event of a tsunami, they are effective in attenuating wave force and trapping drifting debris. However, it is feared that driftwood caused by sediment runoff from the coastal forest floor will lead to increased damage in coastal areas. Therefore, there is a need for sediment runoff prevention measures in and around the forest floor, but there are no clear maintenance and management methods. For the development of maintenance and management methods, it is necessary to clarify the effects of vegetation cover and type on the sediment control effect. In this study, a hydraulic experiment was conducted using actual coastal vegetation to clarify the sediment control effects of cover and vegetation type.
A two-dimensional wave-making tank capable of generating tsunami-simulated bore waves from upstream was used for the hydraulic experiment. In the center of the channel, a slope and a flat section were installed to allow waves to run up. The flat section can be switched between a fixed bed and a mobile bed, and sand and vegetation were installed in the moveable bed experiment. The vegetation samples were picked from each dune at the front edge of the coastal forest at Futtsu Coast in Chiba. The vegetation type and coverage were selected so that they would be varied, and 11 types of samples were picked up. To clarify the amount of sediment transport in the absence of vegetation cover, a case was conducted in which only beach sand was set up. Sediment runoff from the sample was captured and its dry weight was measured. Two types of bore waves with different initial water levels were conducted to examine the effect of wave size. Water levels and velocities at each point in the channel were measured in a fixed-bed experiment without vegetation. The maximum values of water level and velocity of large waves at the center of the section where the vegetation samples were placed were 0.12 m and 1.72 m/s, respectively. On the other hand, the maximum values of water level and velocity of small waves were 0.04m and 1.45m/s, respectively. As a result, it was confirmed that the amount of sediment transport decreased with the increase of the coverage even though the magnitude of the waves was different. In the case of small magnitude waves, the rate of decrease in the amount of sediment transport due to the increase in the degree of coverage from no coverage is small. On the other hand, in the case of large magnitude waves, the amount of sediment transported decreased by about 60% when the degree of coverage was about 40%, indicating that the rate of decrease in sediment transport due to an increase in the degree of coverage from no coverage was large. The effect of coverage was more significant than differences in various types of vegetation and length.
A two-dimensional wave-making tank capable of generating tsunami-simulated bore waves from upstream was used for the hydraulic experiment. In the center of the channel, a slope and a flat section were installed to allow waves to run up. The flat section can be switched between a fixed bed and a mobile bed, and sand and vegetation were installed in the moveable bed experiment. The vegetation samples were picked from each dune at the front edge of the coastal forest at Futtsu Coast in Chiba. The vegetation type and coverage were selected so that they would be varied, and 11 types of samples were picked up. To clarify the amount of sediment transport in the absence of vegetation cover, a case was conducted in which only beach sand was set up. Sediment runoff from the sample was captured and its dry weight was measured. Two types of bore waves with different initial water levels were conducted to examine the effect of wave size. Water levels and velocities at each point in the channel were measured in a fixed-bed experiment without vegetation. The maximum values of water level and velocity of large waves at the center of the section where the vegetation samples were placed were 0.12 m and 1.72 m/s, respectively. On the other hand, the maximum values of water level and velocity of small waves were 0.04m and 1.45m/s, respectively. As a result, it was confirmed that the amount of sediment transport decreased with the increase of the coverage even though the magnitude of the waves was different. In the case of small magnitude waves, the rate of decrease in the amount of sediment transport due to the increase in the degree of coverage from no coverage is small. On the other hand, in the case of large magnitude waves, the amount of sediment transported decreased by about 60% when the degree of coverage was about 40%, indicating that the rate of decrease in sediment transport due to an increase in the degree of coverage from no coverage was large. The effect of coverage was more significant than differences in various types of vegetation and length.