Rei Itsukushima

Evaluation of Infiltration Capacity and Water Retention Potential of Amended Soil Using Bamboo Charcoal and Humus for Urban Flood Prevention

In Japan, floods occur frequently in urban areas because non-infiltrating areas are seeing increased urbanization. To prevent floods, urban basins must improve the infiltration capacity and water retention of the whole basin. There are several basic technologies for river basin management, such as infiltration trenches or rainwater storage. However, a method of soil amendment that prevents flood disasters has not been established. This study aims to evaluate the infiltration capacity and water retention potential of soil amendments using bamboo charcoal and humus. A constant-head infiltration test and rainfall simulation were conducted to evaluate the properties of the soil amendments. The combination ratios of the improved soils used in the experiment were (1) 100% decomposed granite, (2) mixed with 10% bamboo charcoal, (3) mixed with 20% bamboo charcoal, (4) mixed with 30% bamboo charcoal, (5) mixed with 10% humus, (6) mixed with 20% humus, and (7) mixed with 30% humus. The constanthead infiltration tests results showed that soils mixed with 30% humus had the greatest potential for influencing initial and final infiltration rates, and the more the mixing rates of bamboo charcoal and humus were increased, the higher the water retention capacity. The results of the rainfall simulation showed that soils mixed with 30% humus had the highest final infiltration rates and lowest multiplication spillage. To reduce the runoff volume using soil amendment technology, it is important to delay overland flow, and the hydraulic properties of the soils mixed with bamboo charcoal and humus were as effective as those of granite soils.

The effectiveness of delineating ecoregions in the Kyushu region of Japan to establish environmental indicators

Throughout Japan, numerous restoration projects and efforts to conserve the river environment are underway. However, in spite of such efforts, effective measures to conserve the river ecosystem or post-evaluation of restoration projects remain insufficient due to a lack of environmental indicators. In many European countries and the United States, a number of biological indicators have been developed and adapted. However, it appears to be difficult to directly apply these indicators to Japan, because its biota is finely classified according to its many islands and peninsulas. The ultimate goal of this study is to comprehensively evaluate indicators of aquatic biodiversity from both physical and biological aspects. We divided the Kyushu region in Japan into several ecoregions as a preliminary step to establish an indicator. We delineated the ecoregions of the Kyushu region using the fish fauna data of 21 rivers within the Kyushu region. Presence–absence (0/1) data for each fish species were used to run a two-way indicator species analysis (TWINSPAN analysis). As a result, the Kyushu region was divided into four ecoregions (A: northwest Kyushu, B1: northeast Kyushu, B2: south Kyushu, and C: Amami-Oshima Island). Each ecoregion was characterized by the following fish species: (A) Cyprinidae, in particular Acheilognathinae, is abundant; (B1) Gobiidae is abundant, while Cyprinidae such as Tanakia limbata (not confirmed in B2) are also present; (B2) Gobiidae is abundant, while Rhingobius sp. DA and Rhingobius sp. CO (not confirmed in B1) are present. These results indicate that the fish fauna of the Kyushu region is finely delineated, and that an assessment standard for biological integrity should be established, based on differences in fish fauna.

Optimal structure of grated bottom intakes designed for small hydroelectric power generation

Water intake structure is an important element technology for small hydroelectric generation. Currently, intake structures with bar screens have been broadly introduced; however, these require constant maintenance to avoid the clogging of bars by dust or gravel. This study considers the optimal structure of bottom intakes by focusing on two criteria: efficient water intake and prevention of clogs by trapping trash. Grating was selected as the intake structure because it is convenient to construct, widely available, and cheaper than other materials. A flume experiment was conducted to examine the relation between the grating structure and the intake efficiency and trash-trapping rate. Results indicate a clear linear relation between the installation angle and water intake capacity. Furthermore, the trash-trapping rate is low for gratings that have high opening area ratios because their surface areas are small and friction resistance is low.