Shinya Ikematsu

A study of seed dispersal by flood flow in an artificially restored floodplain

Riverine floodplains play many important roles in river ecosystems. However, many floodplains have suffered degradation or loss of ecological function due to excessive river improvements or through changes in agricultural systems. As a result, many floodplain restoration projects are being conducted worldwide. One of the many methods being implemented to restore floodplain vegetation is flood water seed dispersal. In this technique, precisely estimating the effect of seed dispersal by flood water is important in order to achieve successful floodplain revegetation. Here, we focus our attention on sediment transport by flood water into the Azamenose Swamp, a restored floodplain. We attempt to estimate the function of seed deposition in the restored floodplain and explain how the seeds are deposited in the floodplain by flood water. The result suggests that the restored floodplain functions as a more appropriate deposition site for seeds than the riverbanks of the main river. It was also found that the distance from the inflow site and the weight of the sediment were related to seed deposition.

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.

Factors of Water Quality and Feeding Environment for a Freshwater mussel’s (Anodonta lauta) Survival in a Restored Wetland

Wetland plays important roles in river ecosystems. However, the area of wetland decreased considerably due to excessive construction and land use with economic development. For recovering river ecosystem, the wetland named Azame-no-se was restored in the Matsuura River, which is located in Saga Prefecture in Kyushu, Japan. Anodonta lauta, a freshwater mussel that lives in the lentic water of wetlands, played an important role in a river. However, A. lauta lost its habitat and became endangered in Japan. A. lauta was found surviving in the Azame-no-se Wetland; however, only recently researchers have begun to understand the mechanisms that allow this mussel to now prosper there. This study investigated mussel survival mechanisms in the restored Azame-no-se Wetland. Mussel motility, coverage of Trapa japonica (an aquatic plant) and water quality were investigated in three pools (Shitaike, Ueike, and Tomboike) of the Azame-no-se Wetland and in still water in the Matsuura River (May 2011–2012). A feeding behavior experiment was also conducted in Ueike Pool two times in 2012. Results showed that this pool had abundant nutrients (thanks to its high flood frequency) that supported the growth of certain phytoplankton species possibly important for A. lauta. Dissolved oxygen was another important factor. Although Shitaike Pool shared similar conditions with Ueike Pool, the mussel became extinct when the dissolved oxygen decreased because of the lush growth of T. japonica in Shitaike Pool. This study indicated that the factors essential for A. lauta’ survival in the restored wetland were dissolved oxygen (DO), nutrients, and phytoplankton species. These results could be helpful for understanding functions of restored wetland and further offer suggestions for managing river–floodplain system.

Effect of two succesive check dams on debris flow deposition

bovolin & mizuno, 2000; busnelli, stellinG & laRCHeR, 2001; mizuyama, kobasHi & mizuno, 1995; mizuyama, oda, nisHikawa, moRita & kasai, 2000; osti, itoH & eGasHiRa, 2007; wu & CHanG, 2003). It is generally believed that check dams can reduce sediment transport to downstream river reaches and stabilize river beds. Check dams must act to lower the peak sediment discharge and to decrease the total volume of sediment outflow to the downstream area. There are two-types of check dams; one is a closed type and the other is an open type. The former type is a traditional structural measure for controlling debris flow. However, this type of check dams has to be empty in order to trap large amounts of sediment during a debris flow event. The latter type can be subdivided into slit-check dams and beamcheck dams. Concrete slit-check dam is known as the slit type, and steel-pipe open dam is known as the beam type. An open type allows finer sediments to pass through at lower discharge and coarser sediments to be trapped at higher discharge such as debris flow. However designing their appropriate opening becomes a problem. The Hofu City in Yamaguchi Prefecture, Japan had heavy rain on July 21, 2009. The accumulated rainfall was 240.5mm and the largest hourly rainfall was 63.5mm/hour. This rainfall caused many shallow landslides on the mountainous areas of this city. Most of these landslides changed into debris flows and moved downstream in the mountainous rivers. ABSTRACT This paper describes the effect of two successive check dams on the debris flow event which occurred on 21 July, 2009 in Hofu City, Yamaguchi Prefecture, Japan. The debris flow event caused sediment deposition in the check dams in the Tsurugi and Hachimandani River. In the former river with two successive closedcheck dams, driftwood did not accumulate in the check dams but in the central region of the river bend. In the latter river with two successive open-check dams, driftwood accumulated in the opening of the check dams so that the accumulation obstructed the sediment transport to the downstream direction. The upstream check dams in these rivers have sediment deposition profile of around 2°, whereas the downstream ones have deposition profile of around 1.3°. The ratio of sediment deposition volume in the downstream check dams to that in the upstream ones is 0.3 : 1. The total amount of sediments trapped by the two successive check dams can be estimated at around 9,500 m3 in each river. Specific sediment runoff volume from the mountainous areas is qs = 6,800 (m 3/km2).