Experimental study of scouring downstream of coastal vegetation in an inundating tsunami current
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ORIGINAL PAPER
Experimental study of scouring downstream of coastal vegetation in an inundating tsunami current Arshad Ali1 · Norio Tanaka1,2 Received: 7 August 2019 / Revised: 28 March 2020 / Accepted: 2 May 2020 © International Consortium of Landscape and Ecological Engineering 2020
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. Keywords Coastal vegetation · Driftwoods · Local scour · Backwater rise
Introduction Coastlines suffer from erosion caused by waves, sea currents, storm surges, tsunami and post-tsunami processes. Coastal flooding due to extreme weather events and sea level rise is of growing global concern (IPCC Working Group II 2014), and increasing coastal resistance to these threats are a priority for many coastal countries (Barbier 2014). To deal with these natural events, and to protect or mitigate damage to coastal areas, there are two kinds of solutions. One is structural, hard infrastructures that include sea embankments or seawalls, and the other is non-structural, soft solutions * Norio Tanaka [email protected]‑u.ac.jp 1
Graduate School of Science and Engineering, Saitama University, 255 Shimo‑okubo, Sakura‑ku, Saitama, Saitama 338‑8570, Japan
International Institute for Resilient Society, Saitama University, Saitama 338‑8570, Japan
2
that mainly use coastal forests. In addition, hybrid defenses using an embankment and a natural system (forest, wetland, lago
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