Rokkaku River Research Articles

Disaster Response Activities of Geospatial Information Authority of Japan to the Heavy Rain Event in August 2019

A heavy rain accompanied by a front hit Saga prefecture in Kyushu, Japan in August 2019, flooded the Rokkaku river and others, and brought an inundation disaster nearby. Geospatial Information Authority of Japan (GSI), which is one of the designated government oraganizations enacted by the Disaster Countermeasures Basic Act, took aerial photographs and produced inundation-depth estimation maps to support activities of relevant disaster management organizations. These products were also published on GSI's website for general use. This article shows major GSI's activities, especially for providing the geospatial products that are helpful for flood disaster management. In addition, outreach activities to government and public organizations that conduct disaster response activities are also reported.

Cyt b gene and D-loop sequence analyses of Coilia nasus from the Rokkaku River of Japan

The estuarine tapertail anchovy (Coilia nasus) is a small anadromous fish mainly distributed in the northwest and western Pacific, extending southward toward Guangdong in southeastern China and northward to the Ariake Sound of southwestern Japan. This important commercial anchovy has suffered from population decline in both Japan and China, and the taxonomic status of the populations from Japan and China has been disputed. To ascertain the taxonomic status of the populations from Japan (C. nasus) and China (once named C. ectenes), the genetic diversity was comparatively analyzed between Cyt b gene and D-loop sequences of C. nasus from Rokkaku River in Ariake Sound, Japan, and those of C. nasus in waters of China downloaded from GenBank. The results showed that the nucleotide diversity of Cyt b haplotypes of C. nasus from Rokkaku River was 0.0033, and the nucleotide diversity based on D-loop analysis was 0.0061. The level of fixation index (Cyt b: 0.2861/D-loop: 0.2890) between C. nasus from Japan and China were relatively high, and the level of genetic distance (Cyt b: 0.0052 ± 0.0014/D-loop: 0.0168 ± 0.0038) was lower than that between C. nasus and C. mystus. The phylogenetic trees and haplotype network constructed based on Cyt b gene and D-loop sequences indicated that C. nasus populations from Japan and China failed to form monophyletic clades respectively, suggesting that the anchovies from Japan and China should be the same species but from populations that had separated from each other.

Hypoxic water mass in the highly turbid well-mixed macrotidal Rokkaku River Estuary, Ariake Sea, Japan

Hypoxic water masses are usually formed in the bottom layer of stratified water columns. However, we found that hypoxia was formed throughout the water column in a well-mixed macrotidal estuary, the Rokkaku River Estuary, Japan. There was a spring-neap variation in the dissolved oxygen (DO) concentration where hypoxia developed during spring tides. Minimum DO concentration was observed 1 or a few days after the spring tide. When hypoxia developed, all the water column from bottom to surface became hypoxic (<2 mg l−1 or < 30% saturation). The hypoxic water mass was associated with the estuarine turbidity maximum (ETM). Oxygen demand of water in this area was very large (>2 mg l−1 day−1). Potential oxygen consumption by microorganisms was positively correlated with turbidity. Concentration of suspended solid (SS) at the ETM exceeded 1000 mg l−1. Furthermore, particulate organic carbon (POC) concentration in the estuary increased in spring tides. C/N ratio and δ13C of organic matter suggested that POC was transported from the sea into the estuary. These results suggest that the active resuspension of organic matter caused by the strong tidal current during spring tides enhanced the respiratory activity of microorganisms and the main source of the organic matter was detrital material from the sea. Such a large amount of organic matter should fuel microbial respiration effectively. Since microbial oxygen consumption was larger than the oxygen supply from the water surface even in the well mixed condition, a hypoxic water mass was formed in this estuary.

Feeding habits, age, growth and reproductive biology of Coilia nasus (Engraulidae) in the Rokkaku River estuary, innermost area of Ariake Bay, Japan

Feeding habits, age, growth and reproductive biology of<i> Coilia nasus</i> (Engraulidae) in the Rokkaku River estuary, innermost area of Ariake Bay, Japan

Unravelling habitat use of Coilia nasus from the Rokkaku River and Chikugo River estuaries of Japan by otolith strontium and calcium

The migratory history of the engraulid fish Coilia nasus in the Rokkaku and Chikugo River estuaries of the Ariake Sea, Japan was assessed using otolith strontium (Sr) X-ray intensity maps and strontium:calcium (Sr:Ca) ratio life history transects by an electron probe microanalyzer (EPMA). The results showed that seven of the ten specimens from the Rokkaku River Estuary (LJC) and all 15 specimens collected in the Chikugo River Estuary (ZHC) had low Sr:Ca ratios (⩽3) at the central otolith area, indicating their riverine origin and initial freshwater residence. After the first regime shift adjacent to natal regions, the Sr level mapping displayed a wide variety of color patterns, and the Sr:Ca ratios obtained by the line transect analysis could be divided into one to six significantly different phases indicative of gradual life history transition. The other three specimens from the Rokkaku River Estuary had high Sr:Ca ratios (3–6.7) at the central otolith area but showed alternating changes between low and high values outside the natal region, suggesting that estuarine-origin individuals occurred in the Rokkaku River Estuary. The two-dimensional maps of the Sr level and average of the otolith Sr:Ca ratios along the life-history transects could be used as effective tools for reconstruction of past habitat use of the tapertail anchovy in estuaries of the Ariake Sea, Japan.

Distribution of Coilia nasus (Engraulidae) larvae and juveniles in the Rokkaku River, Ariake Bay, Japan

Distribution of <I>Coilia nasus</I> (Engraulidae) larvae and juveniles in the Rokkaku River, Ariake Bay, Japan

数値解析を用いた遊水地の洪水調節効果の評価-牟田辺遊水地を対象として-

A retarding basin, that temporarily stores stormwater to reduce downstream flow rate, is one of the most effective countermeasures to mitigate existing downstream flooding problems. Effectiveness of the basin is strongly dependent on design of an overflow levee. Experimental works have been commonly employed for the overflow levee design, because the flows around the levee are very difficult to be handled by theoretical and/or numerical methods. The objective of this study is to demonstrate that a numerical model, which comprises the finite volume method based on unstructured grid using FDS technique along with appropriate boundary conditions, is effective to calculate the stage-discharge relationship of the Mutabe retarding basin in the Rokkaku River.

Clay fabric of fluid-mud deposits from laboratory and field observations: Potential application to the stratigraphic record

Abstract The clay fabric of fluid-mud deposits was investigated with the aim of identifying characteristic features that potentially apply to the recognition of fluid-mud deposits in the stratigraphic record. We examined both experimentally formed and natural fluid-mud deposits. The clay fabric of experimentally formed fluid-mud deposits is characterized by aggregates of clay particles in face-to-face contact with each other (herein termed ‘FF-aggregates’), with long-axis lengths of up to 20 μm. Some flocs that formed in fluid with a high initial suspended sediment concentration (ISSC), such as fluid mud, would have a high preservation potential as FF-aggregates; once they have settled to the sea floor, to distinguish them from flocs that form in fluids with a lower ISSC. Aggregates that are similar to FF-aggregates are also observed in natural fluid-mud deposits that formed in a modern tide-dominated estuary at the mouth of the Rokkaku River in Kyushu, Japan. Thus, in conjunction with formally proposed lithofacies and ichnofacies features of fluid-mud deposits, the observation of FF-aggregates is potentially useful in identifying fluid-mud deposits when examining limited volumes of muddy samples or thin- to very thin-bedded muddy deposits in the stratigraphic record.

Field Evaluation on the Strength Increase of Marine Clay under Staged Construction of Embankment

This article presents a case history of an embankment constructed on the soft Ariake clay on the bank of Rokkaku River, Saga, Japan. The staged method was used in the construction of embankment. To shorten the time of construction, the soft subsoil was treated by a prefabricated vertical drain (PVD). The physical and mechanical properties of soft subsoil, such as water content, unit weight, and unconfined compressive strength, were investigated before and during embankment construction. Based on the field data, the strength increase of Ariake clay during construction was revealed. A nonlinear relationship between strength increment and the degree of consolidation was found. Finite element method (FEM) was employed to investigate the strength increase behavior. The results showed that it is difficult to predict the filed behavior of strength increase using Cam-clay model. Based on the concept of structure loss of structured clay, the strength variation of Ariake clay during consolidation is discussed.

Late-stage metamorphosing Conger myriaster leptocephali collected in a river estuary of Ariake Bay, Japan

Little is known about where anguilliform fishes complete metamorphosis from the leptocephalus to juvenile stage. Conger myriaster leptocephali in the final stages of metamorphosing were collected from the estuarine portions of the lower Rokkaku River and in the northwest region of Ariake Bay. Three metamorphosing leptocephali with PAM/TM ratios of 0.27–0.34 were collected in the lower river, and five (PAM/TM, 0.30–0.46) were collected at sea stations in the bay. The collection of metamorphosing larvae of this species in a river estuary with lower salinity and high turbidity raises the question of how much these habitats are used by the metamorphosing larvae of this marine eel species.

Erratum to: Comparison of the early life histories of two Cynoglossus species in the inner estuary of Ariake Bay, Japan

The larval distribution and feeding habits of Cynoglossus abbreviatus and Cynoglossus lighti were investigated around the Rokkaku estuary in Ariake Bay during October 2004 and March and April 2005. Cynoglossus abbreviatus started and completed metamorphosis at larger sizes than C. lighti. Developmental phases consisted primarily of individuals at metamorphosis and postmetamorphosis, which were mainly distributed inside and just outside the Rokkaku River in March–April for C. abbreviatus and October for C. lighti. Tidal changes in vertical distribution just outside the river mouth differed between the closely related species; C. abbreviatus was distributed in the surface and middle layers at flood tide, and aggregated near the bottom (where the current speed was lowest) at ebb tide. Cynoglossus lighti stayed mainly near the bottom during all tidal phases. Larvae of both Cynoglossus species selectively fed on copepods (Pseudobradya sp.), which were scarce in the waters. These facts suggest that the nursery grounds of the two Cynoglossus species were different in terms of their tidal vertical distribution patterns.

電気泳動法的手法による干潟底泥に存在するプロテアーゼの分析

We present here an electrophoretic method to analyze proteases involved in tideland sediments based on a modified method reported by Heussen and Dowdle. The method revealed at least five proteases with different molecular masses in the tideland sediments from the Rokkaku River in Saga City. These proteases are selectively distributed in the soil fraction, but not in the tideland seawater. The electrophoretic analysis showed that there were marked differences in the protease patterns obtained from the sediments of various estuary tidelands; the Kamo River in Takehara City, the Enko River in Hiroshima City, and the Rokkaku River in Saga City. These results suggested that the proteases secreted from microorganisms into the tideland sediments are significantly influenced by the microorganism species and also environmental conditions that surround the tidal flats.Moreover, we have found that the activities of the proteases revealed in an electrophoretic gel were quantitatively analyzed using image editing software.

One-Dimensional Water Quality Model for Water Management in Tidal Rivers.

Water systems in lowlands consist of tidal rivers, open channel networks and several water structures. The water management of such systems requires problem analysis on water quantity and water quality. In this paper the water problems of tidal rivers in Indonesia are analyzed and the problems to be solved for the future and present time are proposed. As major subjects, it is pointed out that the integrated water management for the water systems from the aspect of water quantity and water quality is necessary. Additionally, the concept for the development of tools (computer models) is proposed. The major water quality parameters for the water management of tidal rivers in Indonesia are chloride and suspended solids. Through the water quality problem analysis, one-dimensional water quality model, so called branch-node model, is developed and the applicability of the model is examined in the Rokkaku River in Japan. The water movement is simulated using existing computer model for water management systems. It shows that those calculated results of water movements, water level and discharge, agree with the observed data. The calibration of chloride concentration is carried out to estimate dispersion coefficient and the calculated results agree with the observed data. The calculations for the suspended solid concentration match with observed data.

On the erosion and transportation in a channel formed with cohesive soil (gata soil)

There are serious problems of the deposition of gata soil along the bank in rivers inflowing to the Ariake Sea. The erosion and transportation of the gata soil is investigated by the experiment using an open channel excavated on the flood channel of the Rokkaku river. It is found that while naturally-deposited gata soil on the channel bed is eroded by the tractive force of the flow as much as 1/40 of gata soil cohesive resistance, the gata soil forming the flood channel and deposited along the river bank has very strong erosive resistance. The removal methods of the gata soil which are deposited on the bank are proposed by the field experiment, considering characteristics of gata soil.

Evacuation in Flood Plains with Different Characteristic

Evacuation is one of the most important flood disaster mitigations. It is necessary to know the actual condition of evacuation for its simulation and planning. However, evacuation has different actual conditions according to flood plain conditions which are flood plain gradient, inundation depth, and so on. We have questionnaires with relation to actual evacuation in the two river basins, namely, the Mifune and the Rokkaku river. People there suffered considerable losses from flood in 1988 and 1990 respectively. We analyze the answers, then we get evacuation velocity, evacuation ratio, etc.

六角川流域における水循環システムの変遷に関わる安全度

In the Basin of the Rokkaku River, paddy fields have spread over the extensively reclaimed land area and various kinds of equipment for water utilization such as irrigation pond and deep well has been constracting because of lack of water flow in the river. Since the amount of water available for irrigation is not sufficient for whole paddy fields, drought has visited often since 1500. Water volume necessary for irrigation in a certain age was estimated, and the total area of paddy fields was compared with the area which is supplied water from the irrigation ponds. As results of this estimation, the present state at which irrigation with deep wells has been conducted is shown to be very dangerous. Furthermore, ground subsidence has been caused by the pumping subteranean water from the wells, and it is necessary for solving this trouble to develop new sources of water supply