A study on technologies to prevent/reduce tsunami damage to rivers in cold regions
The tsunami caused by the 2011 off the Pacific coast of Tohoku Earthquake and Tsunami on March 11, brought extensive destruction to the Pacific coast of the Tohoku region. It even reached coastal areas of Hokkaido, severely damaging port facilities and propagating into Hokkaido rivers flowing into the Pacific Ocean, the Sea of Okhotsk and the Sea of Japan. Rivers in Hokkaido freeze in winter from around late December to early April; when the tsunami propagated into these ice-covered rivers, it broke the ice and caused a variety of related phenomena.
One such phenomenon is ice jamming (upper-left photo), in which broken ice becomes stuck in the lower reaches of the river and blocks the flow, resulting in a sudden increase in the water level. A photo of the same location under normal conditions (upper right) is shown for comparison. During periods when river-patrol observation of actual conditions is difficult due to snowfall or other influences, sudden rises in the river water level represent a serious river management problem. Another phenomenon stemming from this tsunami involved masses of broken river ice flowing down river channels and accumulating in sluice outlet channels (lower-left and center photos), and floating ice also blocked a sluice gate (lower right). Such blockages are likely to cause problems with sluice gate operations.
Although tsunami action on ice-covered rivers is relatively rare, there is a need to establish technologies to deal with and predict locations where the above phenomena may occur. In this context, CERIs River Engineering Research Team conducts studies to support the establishment of technologies for the prevention/reduction of damage caused by tsunami propagation into rivers in cold regions. Specifically, the actual conditions of rivers are monitored through observation of their ice-cover conditions (in particular, the relationship between atmospheric temperature and ice thickness) to support the prediction of how much floating ice will be generated if a tsunami runs up an ice-covered river. Hydraulic experiments and calculation model development are also conducted to predict ice movement. The results of this research are combined to enable identification of locations where ice jamming, blockage of sluice gates and other facilities and flooding damage are likely in ice-covered rivers hit by tsunami, and to develop countermeasure technologies including both tangible and intangible approaches.
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(Contact: River Engineering Research Team, CERI)
New Type of Dam That Reduces the effects on Natural Environment and the Costs
Fig. 1: Characteristics of a trapezoidal CSG dam
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Photo 1: Construction of a trapezoidal CSG dam
Photo 2: Completed trapezoidal CSG dam
Dams are roughly classified into concrete dams and embankment dams consisting of rocks and/or soil, and the appropriate type is selected by considering economic and construction efficiency based on topography and geological conditions. A dam construction project generally requires a large volume of materials; a new type of dam, trapezoidal CSG (Cemented Sand and Gravel) dam, however, makes it possible to use surplus materials obtained near the construction site effectively without extensive quarry excavation to obtain materials and thus is economically efficient and environmentally friendly.
The trapezoidal CSG dam can simultaneously rationalize materials, designs, and construction by combining the highly stabilized trapezoidal dam with the CSG construction method, which facilitates construction using materials easily obtained at site and simple facilities (Fig. 1). A dam with a trapezoidal cross section has a larger cross-sectional area than a general concrete gravity dam with a right-triangular cross section to improve the stability of dam body and mitigates the strength required for dam materials. In the CSG construction method, the CSG is produced by mixing CSG material, cement and water without classification and washing of CSG material as is done in the case of concrete dams.
PWRI has engaged in developing the design and construction technology of the trapezoidal CSG dam in collaboration with the National Institute for Land and Infrastructure Management, the Japan Dam Engineering Center, and other organizations. The construction of two ttrapezoidal CSG dams have already been completed.
The main characteristics of the design and construction of the trapezoidal CSG dam is to permit variation in the grain size of material and unit water content (the amount of water to be included per unit volume), and to design the dam body or quality control during construction such that necessary strength will be ensured in the permitted variation range. It is much different from the case of concrete dams where the material contents and strength of hardened concrete must be controlled strictly. Recent studies by PWRI seek a further rationalized design and construction of the trapezoidal CSG dams, such as improving the analytical method to evaluate the safety of this type of dams more reasonable by reflecting the variation in material properties .
(Contact: Dam and Appurtenant Structures Research Team)