Junji Yagisawa

Effects of tree characteristics and substrate condition on critical breaking moment of trees due to heavy flooding

To elucidate tree breakage conditions with different breaking mechanisms, i.e., moment by drag force, local scour, and degradation of the substrate around trees, field surveys were conducted after a flood event (September 2007 flood due to Typhoon 9) in the Tamagawa River, Japan. Trees in a river have two main breaking mechanisms during a flood event, moment by fluid force and erosion of the substrate. Moment by fluid force causes two breaking phenomena, trunk damage (bending, breakage) and overturning. Trunk bending or breakage can be expressed as a function of dc, where d is the trunk diameter at breast height and the power c equals 3 for trunk bending or breakage, and approximately 2 for overturning. Smaller diameter trees experienced trunk breakage, but larger trees were overturned. The range for these two breaking patterns changes with the substrate condition. If severe scouring has occurred, the threshold for overturning moment can be quite small. Tree overturning occurred mostly on the bank side of the gravel bar; however, some trees, especially Robiniapseudoacacia and Morusbombycis, were overturned if the substrate was a thin deposited soil or silt layer on gravel. The roots were anchored in the small-particle deposited layer in that case. As for the erosion of the substrate, the tree-breaking patterns can be classified into three types depending on the relationship between the nondimensionalized bed shear stress of d50 and d84, the representative grain diameters at which 50 and 84% of the volume of the material, respectively, is finer. The nondimensionalized shear stress of d84 is an important parameter for discussing the rehabilitation of the gravel bed bar. The boundary region for tree overturning can be changed by the effects of plant cover and debris attachment.

Characteristics of damage due to tsunami propagation in river channels and overflow of their embankments in Great East Japan Earthquake

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, with a magnitude of 9.0, broke most of the sea embankment and coastal vegetation belt and caused catastrophic damage to people and buildings in the Tohoku and Kanto regions of Japan. A field survey was conducted to elucidate the damage to river embankments and their hinterlands (residential area) by tsunami propagation in river channels and overtopping of embankments. Two, three, and four rivers in Iwate Pref., Miyagi Pref., and the Kanto region, respectively, were selected for the field investigation. In the hinterlands, the tsunami came from two directions, coast and river, and the situation, including the evacuation of people, became complex. Therefore, it is necessary to identify locations of river embankments that can be easily overtopped by a tsunami in different tsunami conditions. Tsunami inundation patterns were classified by the river capacity and whether a river or sea embankment was breached or not. This will provide useful information for making new hazard maps and planning new cities.

Estimation of drag coefficient of a real tree considering the vertical stand structure of trunk, branches, and leaves

Drag coefficients of a real tree trunk and branch and the sheltering effects of an upstream trunk or branch on a downstream one in a linear arrangement with different spacings were investigated in detail. Willow and Robinia pseudoacacia, representative and invasive trees in Japanese rivers, respectively, were selected for the study. The drag coefficient of the real tree trunk started to decrease from a relatively low Reynolds number and was smaller than that of a smooth circular cylinder in the entire Reynolds number range investigated. Leaves increased the drag coefficient by around 40–100% in comparison to that of a branch alone and increased with increasing the area ratio of leaves and branches. The sheltering effects of an upstream branch increased with the existence of leaves. The drag coefficient of a downstream branch approached around 95% of that of a single branch with increasing Reynolds number, regardless of the presence of leaves. The moment for drag force of a whole tree was calculated for a strong wind event that overturned trees in the Arakawa River, Japan. The calculated value was compared with the experimental threshold values and was validated within a reasonable limit.

Flood wash-out conditions of an exotic and invasive plant, Eragrostis curvula, in Arakawa River, Japan

The effect of an exotic and invasive plant, Eragrostis curvula, on the threshold of gravel movement and wash-out conditions of the plant due to flood was investigated in the midstream Arakawa River, Japan. Under various hydraulic conditions (grain diameter, bed slope, and water depth) and plant growth characteristics (clump diameter and plant density), the Shields parameter of the gravel in the plant-vegetated area was estimated using the drag characteristics of the plant measured directly in field and wind tunnel experiments, and the plants effect on friction velocity was evaluated. The removal threshold of E. curvula could be defined when the friction velocity around the plants was equal to the critical friction velocity of d 84 grain diameter at which 84% volume passed through the sieve. The threshold condition was found to be well expressed by the relationship between the bed slope direction component of the water weight and the drag characteristics due to the plant MD c 1.5, where M is the plant density and D c the clump diameter of E. curvula.

Index of medium-class flood disturbance for increasing diversity of vegetation area at gravel bars or islands in middle of rivers

To elucidate the relationship between biodiversity on gravel islands in a river and flood disturbance characteristics, wash-out conditions of trees and perennial grasses and breaking conditions of trees were analysed and the applicability of these indices was investigated. Two indexes are defined to express breaking and wash-out conditions of trees, breaking or overturning index (BOI) and wash-out index (WOI), respectively, and one index, WOI50, is used to express the removal condition of annual grasses. Using WOI, WOI50, and BOI, this study classified the habitats on gravel islands into five regions. The relationship between the diversity of vegetation area calculated by the vegetation species map in this study, and the flood disturbance index, a kind of probability expectation value of area for each region integrated from 2- to 40-year return periods of floods disturbance, was analysed on six gravel islands in the Arakawa and Tamagawa rivers. Within the five regions (Regions A–E), important trends were found for three regions. The diversity of vegetated areas in the gravel river habitat increased with increasing I d, the flood disturbance index in Region D, which expresses a flood disturbance that can break trees and move medium-size gravel. However, the diversity index has peak values for I a and I e, the flood disturbance indices in Regions A and E, respectively, where I a can express the immobility of gravels and lack of damage to trees and I e can express the mobility of large-size gravel and the wash-out condition of vegetation. The indices I a and I e may describe a medium-class disturbance to the habitat on gravel islands in the middle of a river.

Effects of underscour depth and horizontal spacing between two bed protection blocks on stability of frontal block

The stability of a river-bed protection block in a flood event is affected by underscouring of the block. The effects of block spacing on block stability were investigated in different underscour conditions. Three blocks were placed at the bottom in a flume, and the underscour depth of the frontal block and distance between the first and second blocks were changed. Drag and lift forces were measured directly in each experimental condition. Particle Image Velocimetry (PIV) was used to visualize the flow and evaluate the streamline pattern. Pressure measurements around the block to understand the pressure distribution show that the pressure distribution on the rear and bottom surfaces of the block is significantly affected by changes in block spacing and underscour depth. The experimental results show that for the same underscour gap, lift, and drag coefficients decreased with increasing space between the two blocks. In addition, if the underscour gap is increased with the same horizontal spacing, the lift coefficient increases and drag coefficient decreases. These results demonstrate that wider spacing enhance stability of the frontal block in the underscour condition. Moreover, analysis of block stability with changing underscouring and horizontal spacing shows that bed protection blocks seem to have more stability when the gap between two bed protection blocks was equal to height of the blocks.

Tsunami mitigation by combination of coastal vegetation and a backward-facing step

ABSTRACT Since the 2011 Great East Japan tsunami, many improvements have been made in both hard and soft solutions for tsunami mitigation. After the 2011 Great East Japan tsunami, a post-tsunami survey was conducted along the coast in Miyagi Prefecture, which was one of the most tsunami-affected sites because rapid acceleration of the tsunami currents broke and washed away the trees, resulting in extensive damage to inland houses. In contrast, some of the houses located inland and away from vegetation with a dropping step survived. This shows a possibility that a step combined with the vegetation offers greater tsunami energy reduction by providing additional resistance. Laboratory experiments were conducted to investigate the energy reduction through a compound defense system (vegetation and a backward-facing step). Vegetation with a step (VS) showed greater energy reduction compared to that of only vegetation without a step (OV) due to additional loss by collision with the bed surface. However, the relative energy reduction in OV remained almost constant with the increase in the initial Froude number (F0, where the Froude number is obtained from a model without vegetation in a flume), whereas the relative energy reduction in VS showed a decreasing trend with increasing F0 because the energy reduction due to collision decreases with increase in water depth or F0.

Effect of Submergence Condition and Overtopping Depth on Gully Scour Dimensions: Observations from the 2011 Great East Japan Tsunami

When a tsunami reaches a shore, it propagates along river channels with tremendous energy and overtops river levees. It causes severe damage to the levee structures through scouring along levee slope. In the present study, gully scours created along the levee slopes of two rivers (Omotogawa and Sakarigawa Rivers) in Iwate Prefecture and three rivers (Nanakitagawa, New Kitakamigawa, and Abukumagawa Rivers) in Miyagi Prefecture were investigated after the 2011 Great East Japan tsunami. The energy head during levee overtopping for each location was calculated from post-tsunami surveys. The relationship between the energy head and gully scour dimensions (length, width, and depth) was analyzed. The analyses clarified that gully scour dimensions were greatly dominated by two important phenomena: (i) overflow type (free overflow or submerged overflow), and (ii) soil texture at the scoured region. The present study results can be utilized to obtain the overtopping flow height for post-tsunami analysis.

Flow Structures and Sedimentation Characteristics Around Colony-Type Vegetation at Flood Events

Colony models, comprising seven equally spaced cylinders with staggered arrangement are mounted on a water flume bed. The characteristics of flow structures around the colony-type roughness model in uniform flow were investigated. For elucidating the effects of the vegetation density and emergent or submerged condition of the colony-type roughness according to the flood water depth, drag force is measured with changing both of the relative space L/d and the relative height h/H,where L is the space between neighbored cylinders, d is the cylinder diameter, h is the model height and H is the water depth and the drag coefficient (C d ) is calculated. The flow structure around the colony models and the drag coefficients values are changed depending on L/d and h/H. Two types of flow structures, a large-scale Karman vortex street behind the colony models and a primitive Karman vortex street behind the individual cylinders are generated. The C d in emergent condition becomes larger than that in submerged condition. The C d increases sharply when h/H is ranged from 0.8 to 1.0 and then approaches the C d value in unsubmerged condition. In field observation, sedimentation around the two roughness type was investigated. When the vegetation density is large, like willow, i.e. Salix subfragilis or Eragrostis curvula (colony-type vegetation), the sedimentation is occurred behind the vegetatation region When vegetation denesity is small, it occurres inside the vegetation region, i.e., Robinia pseudo-acacia or Phragmites japonica. Similar flow structure are supposed to be occurred in water flume experiment and in field. This study elucidated the effect of vegetation density on flow structure.