The secular variations of environmental characteristics of sediment in Isahaya Bay were investigated on the basis of the data for 20 years from 1989 to 2008 obtained by the Kyushu Regional Agricultural Administration Office. The sediment grain size tends to be fining and the sulfide in the sediment is increasing after the construction of the reclamation dike. The authors sampled undisturbed sediment cores at 22 observation sites in Isahaya Bay in the summer of 2008 and investigated the spatial distributions of the environmental quantities. The organic pollution spreads over the whole region of Isahaya Bay and the sediment environments deteriorate extremely behind the south side of the reclamation dike. The comparison was made between the sediment environments in Isahaya Bay and in the innner are of Ariake Sea and it revealed that the sediment environments of Isahaya Bay in the summer of 2008 were much worse than those in the inner are of Ariake Sea in the summer of 2004.

View full abstract

  The authors sampled undisturbed sediment cores at 22 observation sites in Isahaya Bay in the summer season of 2008 and carried out the laboratory experiments of sediment oxygen consumption. The oxygen consumption velocity in Isahaya Bay ranged from 3.18×10^-3 m/h to1.68×10^-2 m/h, and the averaged value was 9.06×10^-3 m/h. The spatial distribution of the oxygen consumption velocity took the high values off Konagai and it may attribute to the deposition of organic matter derived from phytoplankton. The oxygen consumption velocity was estimated by analyzing the in-situ data of water quality obtained in the centeral area of Isahaya Bay from June to October of 2008 by the Kyushu Regional Agricultural Administration Office. The consumption velocity obtained by analyzing the in-situ data tended to increase with the water temperature.

View full abstract

  In order to understand the characteristics of the tidal flat in the Ota River Floodway and the role of the river structure on the formation of tidal flat environment, the investigation of sediment, river water, groundwater, biological environment and organic matter transport were carried out from 1996 to 2009.
  It was found that the generation of groundwater flow was caused by the water level difference around the revetment. Hence, the secondary phenomenon caused by groundwater flow was important factor for the range of bivalve and the formation of tidal flat environment. It was suggested that the various tidal flats are formed by the construction of river structure, which considered groundwater environment.

View full abstract

  For evaluating the possibility of forestation in gravel bed bars, Breakage or Overturning Index (BOI) and Wash-Out Index (WOI) were derived considering the difference of tree breaking phenomena due to floods, i.e. trunk breakage or overturning, and wash-out of trees, respectively in this study. Coupling the two indices, the method for classifying the river habitat into four, not breaking and not washout (Region A), breaking and not washout (Region B), not breaking and washout (Region C), and breaking and washout (Region D), is proposed. The applicability of the two indices, BOI and WOI, were validated in the middle stream reach of the Arakawa River and the down stream from the upper stream reach of the Tamagawa River. In addition, in seven locations where relative height from ordinary water level is different, the breaking modes of trees were classified in the BOI-WOI plot considering floods with different return periods. There are two patterns for the dynamics of plots with flood return period in the BOI-WOI figure. One is changed from Region A to Region D (Pattern I), and the second is changed from Region A to Region B (Pattern II). In Pattern II, trees are not washed out even though they are damaged. Then, the location has high possibility to be a stable forest. Therefore, the countermeasure for preventing the forestation should be considered to change the habitat from Pattern II to Pattern I.

View full abstract

  Most previous studies about laboratory measurements and field observations considered the properties of Langmuir circulations on the basis of streaky structures of convergence zones of mass and momentum. Therefore, there is almost no studies which could measure the secondary circulations with longitudinal axis under wind-induced water waves. In the present study, the secondary vortices, i.e., Langumuir circulations, could be measured successfully by PIV measurements, in which a high-speed camera was placed in the water body of the flume. It was found that the number of the secondary vortices is influenced by aspect ratio and wind-speed. Furthermore, phase analysis allowed us to understand that the structure of the secondary vortices depend significantly on the phase of water waves.

View full abstract

  Vegetation such as trees and shrubs are often observed at the floodplain edge in natural rivers. Spanwise profiles of streamwise velocity component are influenced significantly by the drag force of trees. It is thus very important to investigate these hydrodynamic properties and turbulence structure considering the emergent vegetation effects in river engineering and eco hydraulics. So, in the present study, turbulence measurements by 3-D acoustic Doppler anemometer (ADV) were conducted in wide laboratory flume, in which several cylinders are placed as tree models. It is, furthermore, 3-D LES was conducted to simulate concentration transport properties in these complex flows and it is found that there is a maxim exchange rate of concentration between the main-channel and floodplains.

View full abstract

  The wind response of hypoxic water mass in Isahaya Bay was investigated by using the data of wind and water quality measured in July and August of 2008 by the Kyushu Regional Agricultural Administration Office. In Isahaya Bay, the south-southwest wind and north-northeast wind were prevailing in summer season. When the southerly wind is blowing for a long period, the hypoxic water mass formed between the mouth and the central part of Isahaya Bay was transported shorewards and was upwelled at the inner area of the bay. On the other hand, the northerly wind improved the water environments in the inner area of the bay by transporting onshorewards the surface layer water with high DO-concentration and by pushing away offshorewards the hypoxic water mass. The front of the hypoxic water mass moves at the order of several hundreds meters per hour along the on-offshore direction.

View full abstract

  Ability of the gas dissolving device to improve the DO condition of water is evaluated by using the oxygen transfer rate. Usually the oxygen transfer rate is measured in the mixed type treatment where air bubbles are released in the water tank. This method, however, has disadvantaeges that it can not be used for the compressed type gas dissolving devices and that it requires the evaluation of the overall gas transfer coefficient, etc. The present study proposes a convenient method of evaluating the oxygen transfer rate which is applicable to gas dissolving devices of both compressed and pressure free types, applicable to the water with arbitrary DO concentration, and does not require the evaluation of the overall gas transfer coefficient. Evaluation of the DO improvement is done by using the equivalent DO increment devised to eliminate the influence of the DO concentration of the water before treatement. The value of the oxygen transfer rate is represented as the value at a temperature of 20 degree celsius. Laboratory experiments to check the method have shown that the proposed method of measurement is valid.

View full abstract

  This paper describes the function of the self-clarification of water quality by tidal flat in the Tokyo Port Wild Bird Park. In this study field, the authors carried out the field observations of water quality over 2 tildal cycles and estimated nutrient fluxes between tidal flat and adjacent sea in the summers in 2006, 2007 and 2008. And the sampling of benthos and bottom sediments and counting of number of water birds were carried out in this field. From this study, the authors found following results. (1) Function of this flat is net sink for nitrogen and chlorophyll-α and net source for phosphorus in summer. (2) Nutrient fluxes between sediment and water column are large in budget of tidal flat area. (3) Amount of TN and TP removal by water birds is not so large compared to TN and TP fluxes between tidal flat and adjacent sea.

View full abstract

  Quantitative effects of the coastal forest destruction by tsunami on the decrement of tsunami force behind the forest were evaluated for the different coastal vegetation width of Pandanus odoratissimus, and with or without including the destruction mode into the model. The analysis satisfies the previous field investigation knowledge that the critical breaking tsunami water depth is around 80% of the tree height when tree height is larger than 2m. It also shows that the critical tsunami water depth for breaking the frontal tree of forest increases with increasing forest width because of the reflection by trees. When the tree height is 2 and 4m, if the tsunami force exceeds the critical value most of the trees in forest has a possibility to be broken. However, the destruction is stopped inside forest for the case of 6 and 8-m trees, and high tsunami mitigation effects are maintained. Even when the trees are destructed at just above the aerial roots, tsunami force reduction rate are decreased by 20%, 10% for 2-4m trees, 6-8 m trees, respectively, because of the existence of dense aerial roots in case of a P. odoratissimus.

View full abstract