Yugo AOKI

A Study of Hydraulic Countermeasures for Shinanogawa Diversion Bridge of JR Echigo-Tsumari Line during Rising Water

Minjiao LU,Hongxuan YANG

In recent years, heavy rains have caused rivers to rise and flood rapidly, making it imperative that measures be taken to reduce damage. In Niigata Prefecture, Typhoon No. 19 in October 2019 caused record-breaking rainfall, with the largest water level ever observed at several river stations; near the Shinanogawa Diversion Bridge on the JR Echigo Line, the water rose so high that the bridge piers could no longer be seen and driftwood hit the bridge, causing damage. In response, the government took countermeasures by strengthening the levees on both sides of the river, but these measures could prevent overtopping, but could not lower the water level and thus prevent the destruction of the bridge.
Therefore, this study was conducted to examine measures to reduce the water level hydraulically by using river simulation to analyze the situation near the Shinano River diversion bridge on the JR Echigo Line under multiple conditions when the water level rises.
There are two types of water flow in an open channel: subcritical flow and supercritical flow. The flow near the Shinanogawa Diversion Bridge on the JR Echigo Line during a rise in water level has been confirmed by calculations to be subcritical flow. There are three possible measures to lower the water level in a subcritical flowing river: 1) creating a projection on the riverbed at the bridge location, 2) shrinking the width of the river at the bridge location, and 3) lowering the roughness coefficient.
In the verification of the countermeasure by creating protrusions on the riverbed at the bridge location  in 1), the water level dropped by approximately 0.6 m at the bridge location by creating the protrusions. In addition, we thought that it would be better to make projections locally rather than across the entire width of the river due to cost and time issues, so we made projections only on both banks, only at the center, and on both banks and at the center, and calculated the results. As a result, it was found that the effect of each method was only 0.1 m less than the result of making protrusions along the entire width of the river, indicating that even if protrusions are made locally, they are still effective.
Verification of the countermeasure by narrow the river width at the bridge location in 2), did not result in a large decrease in water level, but a slight rise in water level upstream of the bridge location. Therefore, this method was considered to be less effective, and the countermeasure that causes the water level to rise in front of the bridge was not effective.
In the verification of the countermeasure by lower the roughness coefficient in 3), it was found that the water level began to drop before the coarseness coefficient was changed, the water level dropped the most at the point where the coarseness coefficient started to change, and the minimum water level value dropped as the section where the coarseness coefficient was changed was widened.
Based on the above results, considering the cost performance and feasibility, we thought that measures 1) by creating protrusions on the riverbed and 3) by lowering the roughness coefficient should be considered in a real topography in the future.