[A-CG42] Water and sediment dynamics from land to coastal zones

[ACG42-11] Experimental study on the effect of vegetation cover and location by a forest floor model on reducing sediment transport

*Ako YAMAMOTO¹, Akira Kojima², Ryusei Shiotsuki², Keita Kawaharada³, Makjoto Sotoyama³, Tsuyoshi Tada², Yoshinori Shigihara², Yo Fukutani³ (1.Forest of Research and Management Organization Forestry and Forest Products Research Institute, 2.National Defense Academy, 3.Kanto Gakuin University)

Keywords:Vegetation coverage, Hydraulic experiments, Tsunami

In coastal areas hit by the 2011 Tohoku Tsunami, coastal forests were reported to have mitigated the disaster and prevented some damage, including reducing water levels and flow velocities as well as trapping sediment. Coastal forests are being restored on embankments in the affected coastal areas. The embankments were created using soft engineering to maintain the deep root system of the trees. There is a concern that another tsunami may cause erosion and sediment runoff. To prevent the trees from being washed away, it is necessary to control sediment runoff at the ground surface. However, there is no method to evaluate the effect of vegetation cover on sediment transport at present. In this study, hydraulic experiments were carried out to clarify the effects of vegetation trapping and location on sediment transport. A two-dimensional wave-making tank was used for the experiments, consisting of a water storage tank, a 30-m long channel, and a drainage tank from the upstream side. The upstream gate was opened abruptly to generate a tsunami-like stepped wave. A forest floor model with different vegetation coverage and locations was set up in the sand bed section of the channel. We also conducted experiments using models with more complex vegetation patterns to simulate native coastal vegetation. The results confirmed that there was a decrease in the amount of sediment transported with an increase in the degree of cover when the location was the same. However, it was found that the amount of sediment transport was affected not only by the amount of vegetation cover, but also by the direction and density of the vegetation in relation to the flow. It was also confirmed from the image analysis that the effect of the waveform of the wave on the vegetation was very different. It is not clear whether these causes are due to the effect of vegetation trapping or location.