Modeling boulder transport by coastal waves on cliff topography: Case study at Hachijo Island,Japan

Clifftop coastal boulders transported by storm waves or tsunamis have been reported around the world. Although numerical calculation of boulder transport is a strong tool for the identification of tsunami or storm boulders, and for estimation of the wave size emplacing boulders, models which can reasonably solve boulder transport from below a cliff or from a cliff-edge onto a cliff-top do not yet exist. In this study, we developed a new numerical formulation for cliff-top deposition of boulders from the cliff edge or below the cliff, with validation from laboratory tests. We then applied the model using storm and tsunami wave forcing to simulate the observed boulder deposits at the northwest coast of Hachijo Island, Japan. Using the model, the actual distribution of boulders was explained well using a reasonable storm wave height without assumption of anomalously high-water level by storm surge. Results show that boulder transport from the cliff edge or under the cliff onto the cliff-top was possible from a tsunami with periods of 5~10 min or storm waves with no storm surge. However, the actual distribution of boulders on the cliff was explained only by storm waves, but not by tsunami. Therefore, the boulders distributed at this site are likely of storm wave origin. Our developed model for the boulder transport calculation can be useful for identifying a boulder's origin and can reasonably calculate cliff-top deposition of boulders by tsunami and storm waves. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Simulations of measures to control lava flows

We evaluated the quantitative effects of artificial barriers, water-cooling and guiding channels on lava flow using the lava simulation program LavaSIM. Lava flow is basically subject to the topography around the path, effusive rate and viscosity. To prevent damage due to lava flow, we conducted experiments in controlling the flow direction, velocity and temperature. The simulation demonstrated that artificial barriers can successfully change the direction of a lava flow and is more effective when placed nearly parallel to the flow direction at a point where the topography is not very steep, while a barrier placed perpendicular to the flow direction can only stop the flux temporarily, ultimately allowing the solidified crust to accumulate and causing the following mass to go over the barrier. The water-cooling trial was also effective in controlling the direction and temperature, although the amount of water was as much order as 10⁵ m³. The guiding channels successfully control the direction and inundated area but only in local areas.     

Effects of the 2011 Tohoku-oki tsunami and human activities on long-term coastal geomorphic development in northeastern Tohoku,Japan

The 2011 Tohoku-oki tsunami caused large-scale topographic changes along the Pacific coast of northeastern Japan. More than 10 years have passed since the tsunami waves struck the area. Today, because of reconstruction work, very few places exist where natural post-tsunami topographic changes can be monitored continuously. For this study, the authors investigated topographic changes caused not only by the 2011 tsunami but also by natural and artificial activities during the 50 years before and after the tsunami based on aerial photographs, excavations and subsurface explorations using ground-penetrating radar at the Osuka coast in Aomori prefecture, Japan. The site is rare because it is a protected area with few and superficial engineering activities, making it suitable for continuous observation of pre-tsunami, syn-tsunami and post-tsunami topographic changes. The findings indicate that the 2011 tsunami waves generated large topographic changes: depositional and erosional features produced by the tsunami can be recognized, respectively, as tsunami deposits and erosional channels across the sand dunes. During the post-tsunami phase, the sand volume at the coast quickly recovered naturally. Tsunami deposits and the erosional channels were well preserved underground even at 10 years after the event. However, dynamic movement of the dunes started after the tsunami. The shifting was attributable to the artificial clearing of coastal forests rather than the tsunami effects on the coast. Our results first indicate not only that the sedimentary features of paleo-tsunamis but also the erosional features have some probability of being preserved in the subsurface of the beach and sand dunes at tsunami-affected areas. Also, artificial activities such as deforestation are much more crucially undermining of the stability of the coastal geomorphology than the tsunami effects: the coast is now reaching a different status from its pre-tsunami situation.     

Variations in the thermal conductivities of surface sediments in the Nankai subduction zone off Tokai, central Japan

We investigated the relationship between variations in the thermal conductivity of surface sediments and the topography in the Nankai subduction zone off Tokai, central Japan, the easternmost part of the Nankai subduction zone, which has an accretionary prism with varied topography. We analyzed sediment thermal conductivity data obtained from the trough floor and accretionary prism. Variations in the thermal conductivity of sediments were related to the topographic features formed by accretionary prism development. Thermal conductivities of 1.1?W/m?K were measured on the trough floor where thick terrigenous turbidites have been deposited. The thermal conductivity of Nankai Trough floor sediments decreases from northeast to southwest along the trough, probably because of the decreased grain size and/or changes in sediment mineral composition. High thermal conductivities (??1.0?W/m?K) were measured in fault scarps on the accretionary prism. A landward increase in these values on the prism may be explained by decreased porosity of the sediments attributable to tectonic deformation during accretionary prism development. At the base of the fault scarp of the frontal thrust, low thermal conductivities (<0.9?W/m?K) were measured, likely reflecting the high porosity of the talus deposits. Low thermal conductivity (0.9?W/m?K) was also measured in slope basins on the accretionary prism, likely also related to the high porosity of the sediments. Our results demonstrate that, for accurate heat flow measurement in an area of varied topography, the geothermal gradient and the thermal conductivity of the sediments must be measured within regions with similar topographic features.     

Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island,Japan

Tsunami boulders deposited along the coast constitute important geological evidence for paleotsunami activity. However, boulders can also be deposited by large storm waves. Although several sedimentological and theoretical methods have been proposed to differentiate tsunami and storm wave affected boulders, no appropriate numerical method exists for their differentiation. Therefore, we developed a new numerical scheme to differentiate tsunami and storm wave boulders for coastal boulders on Ishigaki Island, Japan. In this area, tsunami and storm waves have emplaced numerous boulders on the reef and the coast. By conducting numerical calculations of storm waves in this region, we estimated the size of a storm wave that can explain the maximum clast size distribution of boulders on the reef. Consequently, we showed that a wave with a combination of 8 m in initial wave height and 10 s period can satisfy the above conditions when we assume mean sea level. In contrast to the boulders on the reef, all boulders deposited along the shore are heavier than the calculated possible maximum clast size distribution by the storm wave. Therefore, we confirmed these boulders as being of tsunami origin. Results of previous studies showed that they were most likely deposited or reworked by the 1771 Meiwa tsunami. Then, using the tsunami boulders, we numerically estimated the wave period and amplitude of the 1771 Meiwa tsunami, which should have had a 4–5 min period and 5.6–5.9, 6.3–7.0 m amplitude, respectively. Using the proposed scheme, it is possible to differentiate tsunami and storm wave boulders and estimate the size of past storm waves and tsunami waves, although it is noteworthy that there are exceptions for which the scheme cannot be applied.     

Relative importance of multiple environmental variables in structuring benthic macroinfaunal assemblages in chronically metal-polluted salt marshes

In this study, we assessed importance of sediment-associated trace metals in structuring benthic macroinfaunal assemblages along multiple environmental gradients in chronically polluted salt marshes of the Arthur Kill - AK (New York, USA). More than 90% of benthic macroinfaunal communities at the northern AK sites consisted of a considerably large number of only a few polychaete and oligochaete species. Approximately 70% of among-site variances in abundance and biomass of benthic macroinfaunal communities was strongly associated with a few environmental variables; only sediment-associated mercury consistently contributed to a significant proportion of the explained variances in species composition along natural environmental gradients (e.g., salinity). Although sediment-associated copper, lead, and zinc were substantially elevated at some of the AK sites, their ecological impacts on benthic macroinfaunal communities appeared to be negligible. These findings suggest that cumulative metal-specific impacts may have played an important role in structuring benthic macroinfaunal communities in chronically polluted AK ecosystems.     

The reduction effects of mangrove forest on a tsunami based on field surveys at Pakarang Cape,Thailand and numerical analysis

Using an integrated approach including satellite imagery analysis, field measurements, and numerical modeling, we investigated the damage to mangroves caused by the 2004 Indian Ocean tsunami at Pakarang Cape in Pang Nga Province, Thailand. Comparing pre- and post-tsunami satellite imagery of the study area, we found that approximately 70% of the mangrove forest was destroyed by the tsunami. Based on field observations, we found that the survival rate of mangroves increased with increasing stem diameter. Specifically, we found that 72% of Rhizophora trees with a 25–30 cm stem diameter survived the tsunami impact, whereas only 19% with a 15–20 cm stem diameter survived. We simulated the 2004 Indian Ocean tsunami using the nonlinear shallow-water wave theory to reproduce the tsunami inundation flow and investigated the bending moment acting on the mangrove trees. Results of the numerical model showed that the tsunami inundated areas along the mangrove creeks, and its current velocity reached 5.0 m s⁻¹. Based on the field measurements and numerical results, we proposed a fragility function for mangroves, which is the relationship between the probability of damage and the bending stress caused by the maximum bending moment. We refined the numerical model to include the damage probability of mangrove forests using the obtained fragility function to investigate the tsunami reduction effect of mangrove forest. Under simple numerical conditions related to the mangrove forest, ground level, and incident wave, the model showed that a mangrove forest of Rhizophora sp. with a density of 0.2 trees m⁻² and a stem diameter of 15 cm in a 400 m wide area can reduce the tsunami inundation depth by 30% when the incident wave is assumed to have a 3.0 m inundation depth and a wave period of 30 min at the shoreline. However, 50% of the mangrove forest is destroyed by a 4.5 m tsunami inundation depth, and most of the mangrove forest is destroyed by a tsunami inundation depth greater than 6 m. The reduction effect of tsunami inundation depth decreased when the tsunami inundation depth exceeded 3 m, and was mostly lost when the tsunami inundation depth exceeded 6 m.     

Seasonal variation of the ITCZ and its characteristics over central Africa

We investigated the seasonal march of the Intertropical Convergence Zone (ITCZ) shown by the 22 coupled general circulation models of the 20th Century Climate in Coupled Models experiment in seven regions (Africa, Indian Ocean, western Pacific, central Pacific, eastern Pacific, South America, and Atlantic Ocean). Inter-model differences in the seasonal march of the ITCZ over Africa (10?C40°E) were significantly smaller than those over other regions. This finding indicates that the seasonal march of the ITCZ over Africa is insensitive to differences in model physics and resolution and suggests that the seasonal march of the African ITCZ is controlled by robust and simple mechanisms. Motivated by this result, we tried to understand the process of the seasonal march of the ITCZ over central Africa (15?C30°E) based on an analysis of ERA-40 data. The analysis results revealed the following features of the ITCZ in this region: (1) The ITCZ itself produces large convective available potential energy that generates deep convection. (2) The abundant water vapor within the ITCZ is maintained by horizontal moisture flux. (3) Outside but near the ITCZ, shallow convection exists and may act to pre-moisten deep convection in spring and autumn. (4) The seasonal change of the ITCZ is preceded by that of the vertical moist instability in the lower free atmosphere caused by the seasonal change in insolation.