Effect of stream-wise groins on the reduction of ship-waves energy flux and sediment transportation

For preventing bank erosion due to ship waves, many stream-wise groins have been installed in the downstream region of Arakawa River, Japan. However, the bank erosion still occurs at some locations even though similar-type groins are installed. In this study, field investigation at three locations on Arakawa River and numerical simulation were conducted. At each field investigation site, wave height, particle distribution of bed material and amount of sediment movement due to ship waves were measured. Amount of sediment volume increased with decreasing cross slope. As for numerical simulation on the flow around groins, total energy flux of ship waves by a ship was decreased due to installation of groins. The observed amount of bank erosion also decreased according to the decrement of the total energy flux. In addition, the amount of bank erosion increased with decreasing cross slope. These results indicate that the location with mild slope and steep slope has tendency to be accumulated or eroded, respectively, even after the groins were installed

Effect of river bed slope and particle size distribution on washing out condition of trees in rivers

In previous studies, washing out of trees can be evaluated using Breaking or Overturning Index(BOI) and Washing-Out Index(WOI). However, the applicability of those indices were only validated some middle-stream rivers where the bed slope is not so steep. For validating the indices at different river condition from past studies, this survey carried out at 7 rivers with different bed slopes. Accordingly, a relationship between WOI and particle size distribution became clear. Bed slope is found to have no influence on the critical condition of BOI, but particle size distribution can affect the critical wash-out condition for WOI

Overflow pattern and the formation of scoured region by the Tsunami propagated in river channels in Great East Japan earthquake

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, with a magnitude of 9.0, 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. Three, 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 coast and river, and the situation, including the evacuation of people, became complex. 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

Effect of drifted houses on inundation region and the washout region of houses at the tsunami caused by the great east Japan earthquake

Breaking or washout condition of houses by a tsunami was usually analyzed using fluid force and moment by drag force. However, the condition was not validated in actual large tsunami that could cause catastrophic damage on houses, and produce a large amount of debris including a floating houses or broken houses. Therefore, the objective of this study is to develop a numerical model considering the effect of drifted house on the tsunami inundation distance from the shore line and washout situation of neighboring houses at large tsunami event. Field investigations were conducted at 18 locations in Sendai Plain between April to May after the tsunami caused by the Great East Japan Earthquake. In the field survey, tsunami water depths and situation of broken or washed out houses were investigated. In this study, to elucidate the effect of the drifted house on the damage of neighboring houses, one location was selected. The effects of the drifted house were included into numerical model as an additional resistance. In addition, the numerical model was validated by the observed water depths and washout region of houses at the 2011 Japanese tsunami. The numerical simulation demonstrates that the drag force by debris or floating houses on flow reduces the washout region of houses, but on contrary, it also increases the drag force on the neighboring house and thereby increases the washout region of houses. The washout region of houses was reduced because the effect of the drag force by debris or floating houses on flow are greater than that of the additional drag force on the neighboring house. This result indicates that evaluating the effect of the drag force acting on the debris or floating houses on flow is quite important. Moreover, the simulation could reproduce well the washout region within the reasonable limits. The model enables the analysis of large tsunami that produces large amount of debris in densely populated area

Effects of the coastal forests, sea embankment and sand dune on reducing washout region of houses at the tsunami caused by the Great East Japan Earthquake

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, broke most of the sea embankment and coastal forests, and caused dreadful damage to people and buildings in Tohoku and Kanto districts of Japan. This study hypothesized that the coastal forest had a tsunami mitigation effect even when the coastal vegetation was bent down, because most of the vegetation was not washed out, hence could acts as a dense roughness element. Therefore, the objective of this study is to evaluate the vegetation effect on reducing the washout region of houses under severe tree breaking phenomenon using numerical simulation and data from field investigation in April and May 2011. Numerical simulations estimated the effects of a 640m-coastal forest, sea embankment around 5.4m in height or sand dune (2m increase) on reducing the washout region of houses by around 100 m, 600m and 600m for 10m height tsunami at coast. It was observed/concluded that although the quantitative effect of coastal forest is smaller than sea embankment, the coastal forest and sand dune is not a negligible component of the mitigation measures when a large tsunami occurs and overflows the sea embankment

Quantitative analysis on the tsunami inundation area and erosion along the Abukumagawa river at the great east Japan earthquake

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, 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. Field surveys were conducted to elucidate the volume of embankment erosion and their hinterlands (residential area) by tsunami propagation in river channels and overtopping of embankments. Ten rivers were selected for the field investigation. This study focuses on the situation around the Abukumagawa River where severe erosion due to overtopping of embankments was occurred. To elucidate the relationship between the scoured area and the tsunami overtopping time, numerical simulation was conducted around the river mouth area of Abukumagawa River including the hinterland. For reproducing the Japanese tsunami, non-linear long-wave equations and a fault movement model were used in this study. For calculating tsunami from large region where fault motion occurred to small region where tsunami inundation and overtopping from river embankment occurred, five different regions with different grid size were set and flux and tsunami height in boundary grid were interpolated from large size grid to small size grid by nesting method. This simulation validated in comparison with the observed data and simulated one as to maximum water level near Abukumagawa River. The simulation reproduced well the tsunami water depth and inundation area within a reasonable limit, and showed a clear positive tendency between the time while tsunami overflow occurred and the size of scoured region on and around embankments measured by the post tsunami survey in April 2011. However, the volume of embankment erosion was greatly affected by the locality along the river, i.e. meandering, sand bar at river mouth, obstruction by a bridge, even when the tsunami-overtopping time was similar. This indicates that erosion volume due to overtopping from embankments is affected not only by overtopping time but also by the locality, and the mechanism should be included in future

Effect of physical tree characterisitcs and substrate Condition on maximum overturning moment

Effects of physical tree characteristics and soil shear strength on overturning moment due to flooding were investigated using Salix babylonica and Juglans ailanthifolia, exotic and invasive plants in Japanese rivers. Tree pulling experiments were conducted, and the resulting damage was examined in order to assess the effects of physical tree characteristics on the maximum overturning moment (Mmax). In situ soil shear strength tests were conducted in order to measure soil strength parameters. The effects of species differences on the Mmax were examined by analysis of the root architecture. S. babylonica has a heart-root system that produces a greater overturning moment due to the strong root anchorage and the large amount of substrate that must be mobilized during overturning. J. ailanthifolia has a plate-root system that produces a smaller overturning moment. However, trees with the plate-root system may withstand overturning better due to an increased root:shoot ratio. Considering the strategy of J. ailanthifolia to increase the root:shoot ratio for anchoring in the substrate, the trunk volume index (height*Dbh 2) is a better parameter than Dbh 2 because it indirectly involves the difference in belowground volume and surface area. Different soil cohesion values were found at different experimental sites, and the average Mmax for overturning each species decreased linearly with increasing soil cohesion

Analysis of drag force characteristics of real trees with three different types of vegetation for bioshield in coast

This paper presents the experimental investigations on drag force characteristics of vegetation in mitigating the impact of tsunami and other surge effects by the resistance offered to the flow. The experiment was conducted in a laboratory towing tank of 50m x 2m x 2m. Three types of vegetation species used were the trees with small thin broad leaves (Wetakeyya), large broad leaves (Kottamba) and stick type leaves (Kasa). The drag force characteristics of the vegetations mainly depend on the differences in the distribution of foliation, different streamlining mechanism of the leaves against flow, the roughness and the shape of the tree trunk. Drag coefficient of vegetation varies with the flow velocity; the lower flow velocities show higher drag coefficients because of the maximum frontal projected area of the plant. The drag coefficients for the canopies show higher values for the Reynolds numbers less than 106. For canopies with large broad leaves (Kottamba), it ranges from 0.02 to 0.2. The drag coefficients for small thin broad leaves (Wetakeyya) and stick type leaves (Kasa) range from 0.1 to 1.7 and 0.18 to 0.7. Comparatively the drag coefficient of Wetakeyya is greater than Kottamba and Kasa at larger Reynolds numbers (Re > 106). Previous studies on vegetal drag are mainly focused on the single rigid cylinders and colony of rigid cylinders. The studies with single rigid cylinders show an almost linear relationship between drag force and square of the mean velocity of flow. However, the limited studies with natural flexible vegetation show a linear relationship between drag force and mean velocity. Drag coefficient for the trunks of above three types of trees were found less than the smooth cylinder for the region of Re > 60000. For this region the drag coefficient for Kasa trunk ranged in between 0.9 to 1.0 while for the smooth PVC pipe it ranged in between 1.2 – 1.4. For Kottamba it was in between 0.8 – 0.9 and for Wetakeyya it was around 0.6

Estimation of drag coefficient of trees considering the tree bending or overturning situations

Drag coefficients of a real tree trunk and the sheltering effects of an upstream trunk on a downstream one in a linear arrangement with different spacings and inclinations were investigated in detail. In addition, for elucidating the change of drag coefficient for an overturned tree, drag force acting on a real tree with roots was also measured in this study. For the measurement of drag force with different inclinations, Terminalia Cattapa and Albizia sp., vegetated in Sri Lanka, were selected in this study. Drag coefficient of inclined tree trunk has the similar tendency in relation to the Reynolds number with that of vertical standing tree investigated in Tanaka et al.(2011). For the vertical tree trunk with rough surface, drag coefficient of rear-side tree trunk was decreased with decreasing L/d (where, L is spacing and d is the diameter of trunk). In addition, as a result of mutual interference experiment of two inclined tree trunk, the drag coefficient of rear-side trunk decreased with the increase of the inclination. Under the influence of the increment of projected area due to existence of roots and shear force acting on tree trunk surface, the drag coefficient of a tree with roots became similar value (1.0-1.2) comparing with that of a vertical standing tree