Effectiveness of the Nagaragawa Estuary Barrage for Flood Control
The construction of the main structure of the Nagaragawa Estuary Barrage, which began in fiscal 1971, was completed in fiscal 1994 thanks to the cooperation of the local community including the fisheries and agricultural interests. Gate operation was started in July 1995. The dredging in the mound section, which was completed in July 1997, enhanced the floodwater discharge capacity in the lower Nagara River and proved highly effective in mitigating flood damage. In floods in September 1999, September 2000, July 2002 and October 2004 with the water level far exceeding the warning level, the water level was one to two meters below the level before the start of barrage operation and the risk of levee breach was reduced.
The flood of October 21, 2004 due to Typhoon No. 23 in particular was the most serious in the history of observation. The peak discharge was about 8,000 m3 per second at Sunomata, 39.2 km away from the mouth of the river, equivalent to the peak discharge designated for the Nagara River for flood control planning.
The peak water level that would have occurred if the peak discharge mentioned above had flowed in the cross section of the channel in 1970 before the dredging of the river channel was estimated for comparison with the measured peak water level.
The water level at Sunomata was about two meters lower than before the dredging of the channel. Floodwater was thus discharged safely.
Lowering flood stage [at Sunomata (39.2 km from the river mouth)]
Lowering of water level in major floods
Lowering water level at peak discharge
Water gauge at Sunomata during the flood of September, 1999
Water level at the Nagaragawa Estuary Barrage during the flood of October 2004