Even in the feudal period, the Seta River was dredged to reduce the risk of flooding around Biwako. If the flow of the Seta River is improved by dredging, the risk of flooding downstream along the Yodo River increases. On the other hand, if the water flows away easily, during times of low rainfall, water shortages may occur. A weir is a means of resolving these two issues. In 1905, as part of the Yodo River Improvement Plan implemented from 1900 to 1908, the first Seta River weir, now known as the Old Nango Weir, was built. It went out service when it was replaced by the new weir in 1971. It remains an important historical relic and part of the structure is kept in the same state as when it was operational. On November 18, 2002, the Japan Society of Civil Engineers listed Old Nango Weir as a Civil Engineering Heritage Site.
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| former Nango Weir | Seta River Weir |
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| Monument in front of former Nango Weir | Certificate |
In accordance with an ordinance dated March 1992, during discharge, the water in Biwako is maintained at a level within 300 mm above B.S.L. (standard Biwa surface level). When there is a perceived risk of flooding, to allow for the anticipated peak influx, the water level may be decreased to -200 mm or -300 mm B.S.L. In times of drought, to supply, as necessary, the household, industrial, and agricultural needs of downstream users, the water level may be decreased by as much as -1,500 mm B.S.L. to maintain the flow of the Yodo River.
The Seta River Weir itself was completed in March 1961 and, linked to the Biwako Development Project, bypass channels were completed in March 1992.
The current weir, known as the Seta River Weir, was completed in 1961. Later, in 1992, construction was completed of bypass channels adjacent to the weir. The previous Old Nango Weir was operated by manhandling large pieces of timber in and out of the sluice gates in the weir. It used to take all of a day to completely open the gates and two whole days to fully close them. By contrast, it takes only 30 minutes to fully open all the gates in the new Seta River Weir. Moreover, using the two-leaf gates of the new weir, and the three-leaf gates of the bypass channels, throughput control is more precise.
Flow adjustment Seta River Weir is normally carried out by controlling how much water flows over the gates when they are configured as conventional deversoir (overflow) weir gates. Maximum overflow occurs when the upper and lower leaves of the gate are lowered to the bottom of the weir. In the fully open configuration, the upper and lower leaves are completely raised out of the water. In this state, the downflow of the Seta River is natural and unrestricted.
The gates of the Seta River Weir were designed for overflow operation. When the level of Biwako falls to 1,000 mm below the standard level, precise control of the flow through the weir is not possible.
Consequently, bypass channels were constructed next to the weir to enable precise control of throughput during times of drought, when the lake water level falls below the crest of the lowered weir gates. This has enabled both precise flow control down to a lake water level of -1,500 mm, and maintenance of the downstream supply of water.
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In the north and south of the Biwako catchment, the climate is distinctly different. The south has the wet summers of the Seto Inland Sea climate zone; the north has the heavy snowfall typical of highlands bordering the Japan Sea. Because the lake occupies one sixth (16.7%) of the catchment, it is essential to closely observe and monitor the weather conditions in the north and south of the lake. At three locations - offshore from Ado River and Okinoshima (Oki Island) in the northern lake, and offshore from Ogoto in the south - general automatic monitoring stations have been set up to monitor the weather, hydrological conditions, and water quality.
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