Improvement of the Oxygen-depleted Layer in Dam Reservoirs and Lakes:
Supplying Oxygen to the Bottom Layer
 Configuration of gas dissolving device |
 Configuration of gas dissolving device   Low -> High: Oxygen concentration high |
Many cases have been observed in which the surface layer water in relatively deep lakes and reservoirs is warmed in summer, causing a significant temperature difference from that of the bottom layer. In this condition, blending between the surface and bottom layers of the lake water rarely occurs and the bottom layer becomes oxygen-deficient. This is known to have various adverse effects on the lake environment, such as accelerating release of nutrient salts from the bottom sediment and impacting the habitats of various organisms.
For this reason, various approaches have been taken to mitigate the oxygen-deficiency, such as forced circulation of the entire lake water and aeration of the bottom layer water. These methods, however, have problems such as the requirement of large amounts of energy for forced and large-scale circulation of lake water, as well as the possibility of supplying nutrient salts from the bottom to top layers and the difficulty in increasing the dissolved oxygen concentration because of insufficient oxygen supply.
This research has focused on efficient supply of oxygen to the bottom layer only, and a system has been developed to supply oxygenated water with the concentration increased by pure oxygen to the bottom layer only while monitoring changes in water temperature and oxygen deficiency with a water temperature gauge, dissolved oxygen sensor, or other such equipment.
The dam reservoir demonstration test has shown that oxygen can be supplied
to the bottom layer alone without destroying the thermocline in summertime
and that the dissolved oxygen concentration in the bottom layer can be
restored to approximately 10 mg/L. It has also been demonstrated that oxygen
supply can curb the release of phosphorus from the bottom sediment.
At present, this system is being applied to dam reservoirs that problems of oxygen depletion in the bottom layer to verify the effect in the field and study the method of operation.
(Contact: Water Quality Research Team)
Seismic Design Techniques for Road Retaining Walls during Large-scale Earthquakes
 Example of minor damage  Example of major damage  Model test |
A retaining wall is an artificial structure provided to prevent sediment failure and ensuring road space at artificial slopes made by embankments and cuttings, and natural slopes. There are various methods of constructing retaining walls, and seismic design is based on the Technical Guideline for Road Construction - Technical Guideline for Retaining Wall according to the characteristics of a given structure.
Meanwhile, large-scale earthquakes occurred one after another in recent years, and some cases have been identified in which damage to retaining walls has affected road traffic. To take the damage caused by Mid-Niigata Prefecture Earthquake in 2004 as an example, damage to retaining walls due to a large-scale earthquake includes minor damage such as cracks, settlement or rising of walls as well as swelling and inclination of retaining walls. Major damage includes sliding of retaining walls leading to collapse and tumbling of the walls. The existing retaining wall design does not consider the impact of large-scale earthquakes in most cases, and it is critical to develop an appropriate design method to address large-scale earthquakes.
Given this situation, past cases of earthquake damage to retaining walls have been put in order, model tests have been conducted to study deformation and collapse of retaining walls caused by earthquakes, and numerical analysis has been undertaken to find the displacement of retaining walls and faulting generated in roads to establish rational anti- large scale earthquake seismic design.
The achievements of this research, which will be reflected in the soon
to be revised Technical Guideline for Road Construction, include the following points:
(1) Identification of seismic performance and the critical state of roads and retaining walls capable of satisfying seismic performance criteria;
(2) Confirmation that existing seismic design of retaining walls is excessive; and
(3) General establishment of design methods that to some extent tolerate minor deformation and damage caused by large-scale earthquakes
This research is expected to improve the seismic performance of new retaining
walls to be constructed and to facilitate implementation of appropriate
seismic performance evaluation and reinforcement for existing retaining
walls as required.
(Contact: Soil Mechanics and Dynamics Research Team)