The construction of "green hills" in the lowland areas of the damaged Tohoku region are underway as a symbol of the reconstruction due to the mitigating effects that some hills contributed to during the 2011 huge tsunami event. In this study, after comparing the individual mitigation effects of a vegetation, dune and seawall against a tsunami, the combined mitigation effects of these elements were experimentally studied. It is shown that the countermeasure comprising of vegetation, dune and seawall can effectively reduce flood depth, flow rate, and specific energy, as well as further delay the tsunami arrival time. An examination of the amount of water transported by a tsunami to the area behind the coupled countermeasure shows the effectiveness of the comprehensive countermeasures.
The catch of Manila clam Ruditapes philippinarum in Japan has drastically decreased in weight (ton) recently. To increase the weight of clams that inhabit the tidal flats, it is necessary to promote the growth of benthic microalgae, which are the primary food source for clams. In this study, we applied dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and an Fe-eluting fertilizer to tidal flats surrounded by a sealing plate to prevent fertilizer outflow. We further investigated the effects of this application on the growth of benthic microalgae and, consequently, the clam population due to the elution of nutrient components from the fertilizer. We found DIN and DIP in pore waters and chlorophyll a in the surface sediments almost increased in concentration at fertilizing sites. However, periods of nutrient elution from the fertilizer were considered to be less than half a year. Consequently, total weights of clams collected from the control site and ferti-lizing sites increased to approximately 7.2 and 10.4-12.6 times greater than initial weights, respectively, 4 months after the experiment start. In addition, the weight of clams collected from the 1600g fertilizing site was greater significantly (t-test, p<0.05) than that from the control area. In conclusion, using a combination of fertilizers and sealing plates is effective for increasing the weight of clams inhabiting the tidal flats.
The Nankai Trough Earthquake Tsunami is predicted to occur in the near future in Japan. One of the measures to be undertaken against a tsunami attack is to predict the duration for which the water intakes of a nearby water purification plant in the estuary region are to be shutdown to prevent machinery damage due to salinization. In the tidal area of the Yodo River in Osaka, western Japan, there is an estuary barrage and many intakes upstream. The taken water is purified and supplied for drinking and industrial usage over a large urban area. When a tsunami overtops the barrage, it is necessary to prevent a high level of salinity from entering into the purification plant, as salt causes a breakdown of the machinery. Therefore, prediction of the maximum value and duration of salinity is important. In this study, we developed a new model that could simulate the propagation of a tsunami from the wave source in horizontal two-dimension and the flow in the estuary area in three-dimension, where the density current was considerable, and the interaction between these domains was also considered. This model enabled us to conduct a tsunami simulation from the time when the tsunami occurs to the time when the overtopped salinity is washed away by the river flow; this was impossible to achieve in extant studies. We made predictions for five conditions of the river flow rate. It was determined that the water intakes of the purification plant must remain shut for at least five hours under the condition of normal river flow rate. We also conducted a few simulations with operational modifications made to the barrage to develop methods to diminish the salinity influence. Concepts that could shorten the intake shutdown periods by more than an hour have also been proposed.