Typhoon No. 19 Emergency dam 6 dams Pre-release implementation system is not in place November 12th 17:04
“Emergency discharge” that discharges the same amount of water as the inflowing water when the dam's water storage capacity approaches the limit, and the possibility of flooding in the downstream area. Last month's typhoon No.19 was held in six dams. However, all of these dams were found to have no pre-discharge implementation system in place, for example, the rules for discharging water in advance were not decided in consultation with water users.
In the heavy rain caused by Typhoon No.19, 146 dams, mainly in the Kanto and Tohoku areas, conducted “flood control” to reduce the amount of water discharged downstream to prevent flooding.
Of these, the Takashiba Dam in Fukushima Prefecture, the Mizunuma / Ryujin Dam in Ibaraki Prefecture, the Shiobara Dam in Tochigi Prefecture, the Shiroyama Dam in Kanagawa Prefecture, and the Miwa Dam in Nagano Prefecture, which are managed by the country, all have water storage. Because it was expected to exceed the limit, an “emergency discharge” was conducted to discharge the same amount of incoming water.
Since “emergency discharge” may result in inundation downstream, the country states that “preliminary discharge” that releases water in advance and lowers the level of the dam to secure capacity is effective as one workaround.
However, if there is little rain, it will affect the users who use the water of the dam for water supply, power generation, agriculture, etc. Therefore, it is necessary to prepare an implementation system in advance to perform “pre-release”.
However, when NHK interviewed, all six of these dams did not have a pre-release implementation system in place, such as having decided in advance with water users and deciding rules for discharging water in advance. I understand.
Among them, there were dams that were “pre-released” suddenly with the approval of the typhoon, such as the Takashiba Dam and Miwa Dam, but they were concerned about the case where the amount of water did not recover. The water level could not be lowered.
According to the Ministry of Land, Infrastructure, Transport and Tourism, there are only about 10% of dams in the country that have an implementation system in place. It is a policy.
Professor Tetsuya Kakuno of Kyoto University, who is familiar with the dam, said, “During a disaster, it is very difficult to make a pre-release decision in a situation where it is not possible to coordinate with water users. The number of dams that set rules in West Japan is increasing little by little based on lessons learned from the heavy rain in West Japan last year, but there are still few in East Japan, and I hope that the typhoon No. 19 will be used to expand the movement. ”
"Pre-release" two issues
“Pre-release” is an operation that lowers the dam's water level in advance by discharging part of the water capacity originally used for water supply or power generation when a flood risk is expected downstream of the dam due to a typhoon or heavy rain. Say that.
In order to conduct “pre-release”, it is necessary to discuss in advance with water users and to establish rules and systems, but when the Ministry of Land, Infrastructure, Transport and Tourism investigated, the system was in place as of last month. It means that there were only 54 dams out of 562 dams.
Why is the system not ready? Issues to be addressed are "risk of drought" and "dam structure".
[Issue 1: Risk of drought]
If the water used for water supply or power generation is discharged in advance to control flooding, if the discharged water does not return without heavy rain, the water users will be greatly affected. .
Typhoon 19 resulted in heavy rains and a lot of water flowing into many dams, but there is little rainfall during the autumn and winter seasons, and there is a particular concern about the risk of drought.
For this reason, in order to introduce “pre-release”, the minimum amount of water storage required for water users, how to compensate for drought as a result of discharge, dam managers and water users in advance However, it is necessary to proceed with difficult consultations, which is an issue.
Furthermore, to prevent drought, it is also important to understand the amount of rainwater inflows a few days ahead, and how to improve the accuracy of weather forecasting is also an issue.
[Problem 2: Impossible due to dam structure]
Depending on the dam, there is not enough room for pre-discharge, or a large amount of water cannot be discharged unless the position of the “discharge pipe” installed in the dam is lowered in the first place. Some dams cannot be released.
For this, it is necessary to increase the water storage capacity by raising the body of the dam and to install new discharge pipes.
Takashiba Dam “No rule” is also released immediately
One of the six dams that performed “emergency discharge”, the Takashiba dam in Fukushima Prefecture, had not established a system for implementing “preliminary discharge”.
According to the Fukushima Prefecture Yodogawa Water System Dam Management Office, the Takashiba Dam water is provided as industrial water to businesses in Iwaki City.
At that time, heavy rain was expected due to the approach of the typhoon, so after obtaining approval from the prefectural corporate bureau, about 40% of the capacity for users of spilled water was released in advance. Did.
After that, however, “emergency discharge” was carried out because there was a high possibility that the dam would exceed the limit where it was able to save water at the upstream of the dam.
Akihito Ohtake, Director of the Fukushima Prefecture Yodogawa Water System Dam Management Office, said, “If there is no prior discharge, the time for emergency discharge will be accelerated, or the amount of water discharged will increase further, causing a large inundation in the downstream area of the dam. There was also. "
On the other hand, there were concerns. This is a risk when the course of the typhoon has changed even after “pre-release” and heavy rain has not fallen and the amount of water released has not returned.
Otake said, “In preparation for heavy rain and flooding, it is safe to lower the water level in advance, but if there is not enough water for industrial use, there is a risk of having a major impact on businesses.” I was talking.
In the future, we will discuss with the water users how much we should lower the water level in advance when heavy rain is expected, and how to fill in the water if necessary water can not be secured in advance discharge. I wanted to do it. "
Driftwood dams… Effects and challenges
In Typhoon No. 19, many dams also demonstrated the effect of blocking driftwood from upstream. On the other hand, there is still a problem of how to remove a large amount of driftwood accumulated in the dam.
At the Takashiba Dam in Fukushima Prefecture, a large amount of driftwood was generated upstream of the dam due to the rain of Typhoon No. 19 and No. 15, and the dam was dammed up.
Driftwood is approximately 2200 cubic meters, which is equivalent to approximately 11,000 bottles when converted to a 200-liter drum.
Regarding the effect of stopping driftwood, Professor Tsuno of Kyoto University said, “In a recent heavy rain disaster such as the heavy rain in northern Kyushu two years ago, the damage of driftwood is conspicuous along with flooding. If it flows into the river and gets caught on a bridge, it may cause flooding of the river or breakage of the embankment, not only lowering the downstream water level but also blocking the driftwood is a big role for the dam. " It is.
On the other hand, how to remove a large amount of driftwood accumulated in the dam is a big challenge for the dam manager.
Although the Takashiba Dam is being removed, it is difficult to put heavy machinery into the dam lake, and workers on the boat collect driftwood, carry it close to the main body of the dam, and lift it using heavy machinery. Is doing.
It means that it will take about 5 months to remove all driftwood.
If a large amount of driftwood continues to accumulate, it will lead to deterioration of water quality, and if heavy rain falls during the next flood season etc., it can be removed as soon as driftwood may affect the operation of the dam. Is a challenge.