Experimental study of scouring downstream of coastal vegetation in an inundating tsunami current

Abstract

Local scour by inundating tsunami currents on the land side of coastal forests can greatly decrease the tsunami mitigation advantage of the forest by producing driftwood. The purpose of this study was to analyze the scour profiles inside and behind a forest model (FM) due to the flow passing through it, and to compare the scour phenomenon around fixed and moveable bed cases within the FM. Two series of flume experiments were conducted by changing initial water depths (water depth without FM in the flume, ho = 3 cm to 7 cm), densities of FM (dense G/d = 0.25, intermediate G/d = 1.09, and sparse G/d = 2.13, where G = clear spacing between each cylinder in a cross-stream direction, d = diameter of a cylinder) and vegetation thicknesses (dn) of 180, 380, and 580 No.cm. In series 1 and 2, experiments were performed with a fixed bed inside FM for the three vegetation densities and for moveable bed case only for the intermediate density, respectively. Downstream scour profiles behind FM, backwater rise, and water surface gradient inside FM were investigated. For the fixed bed case, minimum scour was measured in the intermediate FM case for the three densities of 180, 380, and 580 No.cm. In the fixed bed inside FM, the location of maximum scouring moved away from the downstream end of FM while in cases of a moveable bed inside FM, the location of maximum scour moved towards the downstream end inside FM. Scour inside coastal forest damages the trees; therefore, it is recommended to stabilize the bed inside the forest by planting grasses, etc., if there is no constraint on the light environment inside the forest and the space behind the forest where local scour occurs.