Inspection report of Tsukubagawa Tsukigata Town

The first place I visited was Susubo River in the tributary Pon and three low dams about 60 cm in height were installed at the curved places of the river. In this place, in addition to the blooming flowers of Ezomisohagi planted by Dr. Eastern, shrubs such as willows and many grasses are growing and it seems that the river bank is stable.
According to my teacher, I wanted to be a place where children can play.
The amount of water is very low, the water temperature is high, aquatic insects such as tobikela and Ugii were observed.

Next, I visited the low dam construction area of ​​Susabu River mainstream. I got off the car, walked down the road along the river, walked in the river from the most downstream to the upstream of the construction site and observed the state of the river.

The place we walked on the way towards the downstream was a forest like Yanagi, Alder, Harnil. This place said that before the construction of the low dam group 25 years ago, there were no forests, it was a wasteland on one side. The land was stabilized by the establishment of the low dam group, and the forest grew up to this state.
In the explanation of Mr. Higashi, it is that many people misunderstood, but instead of "Because there are forests the land is stable", it was "to be a stable land so that forests can be formed."
In the lowest dam of the low dam group, due to the upstream low dam group, because the supply of soil and sand was lost, severe river bed decline occurred, so we established a new low dam group in 1998, The group is devised not to hinder the movement of fish by rubbing against the natural rock, by preventing the river bed decline and keeping the fall low. Beneath the low dam group, there is a depth of about 1 m, which is deeper than the other parts. Sediment accumulates immediately downstream of it and it has a depth of about 20 cm. By the low dam, you can see that the flow of water is controlled to suppress the movement of earth and sand. In addition, this work constitutes a continuous structure of ice, deep in the linear direction.
Immediately under the dam, there is sufficient depth of water and the head is low, so it seems that there is little influence on the movement of low dam damaged fish. In addition, what was installed in 1998 is cut diagonally on the downstream side, so that fish are considered to be easy to move.

Between low dams, riparian forests are formed on the sediments deposited on both banks of the river, and the width of the river narrows to the minimum necessary width with respect to the amount of water by the action of nature, and the state close to the environment of a natural river It is getting.
Also, due to fallen trees falling from the river to the river, a deep pool is formed or under the roots of the riparian forest are gouged, a cover of about 30 cm in depth is formed, and a space suitable for fish life is formed It was.
Indeed, as I was walking along the river, I saw many fish and could catch a fish and a loach.

However, aquatic insects are very few, and it was said that there are too few aquatic insects to feed for live fish. As a cause, the quality of the riverbed is not coarse gravel, but it is thought that there are many fine sand.

According to Dr. Higashi, it is useless to have a lot of fine sand on the geology of the mountain. Because the low dam group is for depositing sediment, the possibility that it accumulates more sand than usual due to its work is also considered, so when applying it to other rivers, the geology of the mountain and the upstream part It is necessary to pay close attention such as considering the construction law and the place, comprehensively looking at the situation of the entire river basin such as the characteristics of sediment movement of.

At the end of the tour, we planted a commemorative tree planting using Kamineckon and dissolved it.

Although some questions were issued from participants, it was demonstrated in SUBe river that low dams group can control the movement of sediment well, prevent the collapse of river banks and regenerate the forests around the river So its effectiveness is very high. Also, since ecosystems are constantly changing in the time axis, the fact that there were no plants at all prior to construction took about 25 years of time to look like this and that in the future It is necessary to think about continuing the change.

Also, as pointed out by Dr. Higashi, the low dam group said that it is not necessary to use heavy concrete like the present situation, and that simple construction using stones and logs will function adequately. With this method, in small rivers, residents can repair themselves, and there is also the possibility of implementing river improvement projects and river refurbishment work involving residents.
In the future, I think that it is necessary to introduce such river refurbishment with cheap expenses, less burden on the environment, and the residents to act on their own.

The Sube Miyako River that we visited this time is the same size as the Rubeshibe River flowing through Niseko Town, but there are no farmland or residential land in the surroundings, and there are many fish that live in Niseko. There is a difference that it is not a fish, and it is necessary to discuss whether it is better to introduce the low dam group of Sube Miyakogawa as it is in the future. However, judging from the case of Sube Miyako River, the combination of low dams, water retarding basin, and sand basin can reduce flood damage even in rivers adjacent to agricultural land.
In any case, it is very difficult to meet all the conditions required of a river 100%. It is important to think about methods that can satisfy each request as much as possible and to discuss and decide on which part to put emphasis on at the same time.
In addition, the flood control measures are not merely to construct the inside of a river to ensure the cross-sectional area of ​​the river, but to visit the river, the ecological system of the entire river basin, and the lifestyle of the local residents, the familiarity , The idea of ​​the person who uses it, and so on.
In order to leave valuable assets to the next generation, sufficient discussion is required.

On behalf of the participants, Mr. Miyata, Mr. Ikeda, Mr. Ito asked about impressions, doubts, etc. about the observation of the low dam group of SUBE CITY River.

• Almost no aquatic insects were found. If it is not a matter of water quality, it can be cited that the bottom sediment is sand. There seems to be no problem in water quality.
• A story that Iwana was inhabited in the past.
• I can not feel a sense of life.
• Even though the water in the incoming river is clean, why is the water of the dam badly cloudy?
• In the case of the Rubeshve River, too, the sediment accumulation situation of the present erosion control dam may show that sand accumulates on the river bed due to the installation of a low dam group.
• Due to the construction of the low dam group, the flow of water spreads and the flow becomes too flat. There is a problem with the movement of fish etc.
• In the low dam group, the flow may be shallow and wider, so that the water temperature may rise.
• It was good that I was not able to see and listen quite often in my normal life.
• "The teacher's words that I do not know even if I ask the people in the town about the low dam group construction method" was impressive. I realized that the local people did not know the projects to be done in the country or the road. In Niseko if you do river construction etc., I would like the opportunity to think about in the area and townspeople, even if it is not a town business.
• I visited places that my child made to play, but in the situation where the townspeople really do not know, I was wondering if the children really are playing.
• Although it looks beautiful, I thought that there is no change in the flow of the river and it is not a natural-centered river.
• Even if I saw the same river, there were various viewpoints depending on the people, so it was good that I could hear various stories.
• I would like to ask how children feel.
• The gradient of SUBE CITY River is 25%, so when applying to Niseko river, I was concerned about how much river can be used. There are very few gentle rivers in Niseko.
• In the explanation of the teacher, I did not need concrete, so it was to work well using trees such as logs, so I wanted to see a case using wood instead of concrete construction.
• I think that it is unavoidable how to construct using materials other than local ones like concrete.
• It is much better than the high sabo dam like the conventional one.
• Vegetation is almost the same as Niseko. Of the approximately 150 kinds of plants observed, there were only three types (Dokutsugi, Ohba Giboshi, Himedokusa) not seen in Niseko.
• It is hardly conceivable that the low dam group construction method has an adverse effect on vegetation.
• There is a problem with invasive species entering due to construction or post - plantation forest. In construction, it is necessary to devise measures to minimize the transfer of alien species, such as bringing soil as far away as possible.
• I was concerned about the accumulation of sand on the river bed.
• The valley is narrow, the slope is gentle, unusual terrain that can not be seen in Niseko.
• Many willows were seen in the forest of the construction site, but things that do not get used at first. It seems that Harnilé etc will increase due to the land becoming dry in the future.
• The sediment deposition method seemed a little unnatural.

Mr. Hirakawa who participated in the inspection of SUBE CITY River, as well as the visit this time, as the impression including the things heard and heard so far, we receive the following opinion.

Although I am feeling listening etc., I do not think that the current situation is necessarily good. However, I think that there is a fear that it will be worse if you add your hands naturally like river refurbishment etc.

One is because the construction work so far was so, and the other one is that we can not wait for recovery after construction.

Even with the low dam group construction method, if you make it from concrete (it seems that a fallen tree or the like may be OK), you will once put hands in the river bed, and there are people who remember resistance even after several artificial structures appear I thought it was strange.

In a certain river, residents have a strong resistance to headgear, so I thought that it would not be acceptable even if it was effective. I feel that the shape of the river which the river good for the ecosystem and the residents are good is different.

Mr. Hirakawa who participated in the inspection of SUBE CITY River, as well as the visit this time, as the impression including the things heard and heard so far, we receive the following opinion.

Although I am feeling listening etc., I do not think that the current situation is necessarily good. However, I think that there is a fear that it will be worse if you add your hands naturally like river refurbishment etc.

One is because the construction work so far was so, and the other one is that we can not wait for recovery after construction.

Even with the low dam group construction method, if you make it from concrete (it seems that a fallen tree or the like may be OK), you will once put hands in the river bed, and there are people who remember resistance even after several artificial structures appear I thought it was strange.

In a certain river, residents have a strong resistance to headgear, so I thought that it would not be acceptable even if it was effective. I feel that the shape of the river which the river good for the ecosystem and the residents are good is different.

The following comments have been received from Mr. Jun Sakamoto who lives in Sapporo city who heard the contents of Susabe River inspection.

It is a very interesting story.
The ecosystem is not only a matter of balance, but also the problem of how much span and area to consider, I think that it is a theme that can not easily draw a conclusion.
Perhaps 50 years may be as short as the experiment period ... · · · It is necessary for citizens and administrative officials to persist for a long period of time with trial and error. It is a fatally lacking part in the current administrative system, and even if the citizen side also rises up against the opposition campaign it is not good at long-term efforts ....

Mr. Umeda of the Community Research Institute, who helped formulate the Niseko Town Environmental Basic Plan, also commented as he participated in this inspection.

Several doubts were pointed out in the inspection of low dam group construction method in the upper part of the SUBE CITY ROOM, which was held in July 30. I would like to try to sort out some of these doubts.

"Problem pointed out"
1) In the riverbed part of the low dam group, gravel and the like were spreading, and few aquatic insects were observed. There are few kinds of fish inhabiting, and I feel some doubt as a habitat for aquatic life.
2) Although it was understood that it is an effective construction method in the forest in the upstream part, in view of the necessity of a wide riverbed, etc., it is important to know whether it is suitable for river with river basin of middle- I have anxiety.
3) Even though I heard the explanation of the construction method at the work site, there were not a few people who did not come to the pins. The principle of the construction method is still hard to understand. I am worried whether many townspeople can understand.

"My feeling about the problem"
About 1
A major characteristic of the low dam group construction method is the compatibility between the flood control function and the ecological recovery function. Sabo function of the rampant river is the main purpose, but as a by-product of its function, sediments are deposited stably on both sides of the river, the riverside forests are advanced there, and stable species such as Mizunara are grown to flood plains I will. Also, the ecological system that the sediment is distributed to both sides, the flow muscle is determined, and insects falling from the riparian forests are added to induce the inhabitation of fish, and the ecosystem recovers in the course of time. Although the problem that the aquatic insect was not much in the riverbed of the observation point seems to be accepted to the present condition, it seems that the current situation is that sand and gravel, mud and so on will be deposited and it will be decided whether future rich habitat will be obtained or not I think that it is an issue. However, because the observation point is the upstream part of the lunar dam, migratory fish from the sea etc. can not go up (According to Mr. Seno's report below, 4 species of 4 families of snake clover, elephant goby, Fukudodajo, Hana kajika inhabit. I also think that structural constraints such as Himemasu (?) Was also observed on that day) may be one of the causes. Either way, the ecosystem changes over time. I think that it is necessary to imagine how it will change rather than capturing fixedly the current state of the ecosystem as not changing.
An important point of view is that, in some functions, if each of the functions can not be fully satisfied by 100%, I think that what function and importance is important. In the case of Rampakawa, the flood control function can not be ignored. In other words, if there is a social requirement that can not be left alone as a natural river, I think that it will be how to preserve and restore the natural ecosystem to what extent.
(For the actual state of the aquatic ecosystem at the low dams in Subu Miyakogawa, refer to the research report by Mr. Yuji Senoo, "Low dams construction method and fish habitation-Ishikari river area private forest management project". According to this report, although some problems remain, the formation of the pool is evaluated as "a suitable habitat for fry" (page 4).)

About 2)
A suitable point of the low dam group is a place where a riverbed can be wide. This will be the same whether in the upstream mountain area or in the middle agricultural area. Taking the Rubesivet river as an example, places where we can afford riverbeds with surrounding agricultural lands will be candidate destinations for the low dam group. It can be said that the actual effects need to be considered at the local workshop.

About 3)
I think the challenge is what kind of approach will be tried in the workshop to be done in the future. Even if I listen to Mr. Eastern's story, even if I listen to Mr. Masao's or Ms. Senoo's explanation, the confusibility will certainly remain. That may be the cause of the low penetration rate of the low dam group. However, it may be convinced by entering the river and experiencing the flow pressure and changes in the flow path. I wonder what kind of workshop it will be, while talking with you.

Yuji Senoo is an expert on river ecosystems including fish and consultants for river improvement. We also conduct ecological surveys of SUBe cheon in the past. We accompanied this visit and received comments from experts' viewpoints on the impressions of the participants.

About sediment deposition

• The low dam group originally meant to save earth and sand. As a result, the sand which has flowed from the upstream has been deposited.
• It is a flat river due to the low dam group. As a result, it seems that the fact that the flow velocity becomes loose and constant makes it possible for only sand to accumulate. By changing the flow, it is possible to separate sand and stone well.
• A bigger cobblestone was plucked five years ago. Perhaps sand has flowed in due to construction in the upper part and accumulated.
• Due to the geology of the mountains, the quality of water is not so good, it seems that Iwana and others did not inhabit.
• As the water surface becomes wide and shallow like the low dam group, the water temperature rises.
• Because the falling tree was removed, the flow became uniform.

On the future creation of a new river

• It is necessary to plan a river channel, considering the flow of water between the time of the normal water and the flood.
• How to change the flow
• A river channel is formed when returning from the flood to the common water. At this time, give an opportunity to become a varied flow.
• On the Rubeshve River, the left bank almost sticks to the mountain. Can you make plans for the river by using this well?
• To protect existing plants, do not rectify river channels. Yanagi and others come to the floodplain soon.
• During flat water, water may collide with plenty of water. It is advisable to protect the river bank with large fallen trees, woodworking sediments and the like. However, when flooding, it is necessary to design a river channel that creates a flow of water that does not wash that part.
• Partly, by applying the idea of ​​the low dam group, it prevents the decline of the river bed as a whole.
• In the current renovation work, the cross section of the river way is uniform, as the flow rate increases, the flow velocity also becomes faster, and the river bed is washed uniformly, resulting in a drop in the river bed. It is necessary to design to control the flow velocity according to the situation.
• There may be places to wash in the river. Because water energy is absorbed in those places, it is deposited on the downstream side.
• In fact, it was constructed with this idea at the Kobairashibe river in the Rumoi tube, although we got a good result, because we upgraded its upstream part in the conventional way, it is now a terrible sediment deposition It has come to be seen.
• It is necessary to design the river way with a model of pure natural river as a model.
• Natural rivers are flooded but never broken. Various problems come up by artificially changing hands.

Dr. Higashi is a creator of the Low Dam Group Construction Method and also designs the Low Dam group of the SUBe cherished River that we visited this time. I got a comment on the opinions from the participants.

"Read the impressions of Susaburokawa's visit etc."

Each person's view of nature can not be standardized, and there is no need to impose a personal view. However, logic that will reach an agreement is required if it becomes public investment. Trying to join the logic of each specialty field in a division of labor society has different time orders and spatial scales, so there is something frustrating without discussion.
River flood control and water use are the places where river administrators argue and are designed and constructed based on the heavy rain of 30, 50 or 100 years probability. Geology discusses the structure of deep subsurface in the order of tens of thousands of years, millions of years. Sabo has dealt with technical problems within 100 years at best. What time order and how much spatial scale are the objects to be discussed by ecology?
Because the trace that the erosion control was carried out "when", "where" and "what" remains, it makes it easier to judge exactly and conflicting results externally, and also to people who are not specialized. It is the severe criticism that "Sabo dam damages the inhabitation of fish" as its most important event. Sabo action is a so-called outdoor large-scale experiment. Since river construction is being conducted as a means to prevent disasters accompanying land use throughout the basin, if we want to maintain the ideal form of fish ecology, we will focus on the way of land use sooner or later, historically We should consider what the residents have done and what the expectations of the residents were.
Since Niseko is a volcanic rock zone and the lunar shape is the Tertiary zone (sedimentary rock), the geological conditions are different, and the bottom sediments of rivers and streams are different from each other. The water of the reservoir dam built in the mud and mud river is always cloudy because the movement of fine particles is intense (example: Lake Katsurazawa, Hobetsudamu). However, from the viewpoint of sabo technique concerning suppression of sediment discharge, in principle it is possible to sufficiently cope with the low dam group construction method (for this point, please see the construction example inside the road which was fried in the pamphlet). In addition, please refer to the related system diagram of the back cover and the purpose of technical intervention.
28 years have passed since the construction of the Low Dam group in 1974 at the construction site of the mountainous area of ​​SUBE CITY River. The section which was once a wasteland of river width 80 m × extension 300 m became a forest and mountain stream like the present situation. In retrospect, due to the recordable heavy rain in 1975 right after the construction and the whole-road torrential rain in 1981, the sediment accumulated in the construction section formed splendidly the river terraces, on top of which the pioneering plant Yanagi A community is born, and it is moving to the alaskan forest along with the flow of time. From now on it seems that it will shift to tall trees such as Harnile or Onigurumi, but as described below, artificial erosion control has been effective and it has stabilized so that vegetation can be established in the changing space until then Become.
An artificial rock (erosion control dam) is provided at about 50 m pitch on the ridge of SUBE CITY River, and longitudinal erosion is suppressed. Needless to say, although it is "unnatural existence", this artificial rock is characterized by not being seen in a high river workpiece, which does not extremely hinder the fish's run-up. In this case, however, low dams are floating on a fragile gravel layer under an organic relationship with each other, water overflows without resistance, megaliths sleep under the dam top end (Temba) -ing Despite the fact that 28 years have passed since the construction of this low dam group, functional defects are not found. This is so-called maintenance-free.
Sabo action is extremely unnatural due to human-oriented natural intervention, but it can be said that it is an example of human knowledge (human power) that suppresses exercise without countering nature by making use of natural power. Whether this is "possible or impossible" is a difference in opinion on the standpoint and if you have a better way to respond to irreversibility of nature, please provide it.
Development prioritizes road opening, aiming at expansion of production area and living area, and has struggled to increase economic efficiency. It forced the emergence of an antisocial space of cutting natural forests and killing ecosystems. It is customary that it will eventually become a moment to encounter disasters, to notice river disaster prevention, and to entrust safety to huge structures. And when the disaster prevention act walks alone, the purpose of the means becomes everyday, unnecessary urgent structures are born, and the original natural appearance is lost. That is why among adults enjoying substance civilization, the adult society, which is immersed in economic supremacy in particular, is asked "what can be left to the next generation" now.
The things that do not lead to profit were other people 's appointment, leaving the office, leaving the agency, leaving the country. If there is no burden act, information will not come back. On the other hand, hierarchical type systems are revealing various defects due to the lack of on-site recognition, and remind me of the way of civilized society which has become too divided. It seems that if a net type system that at least recognizes individual possibilities is developed on the scene, it will lead to the creation of a new regional culture. In addition, if there is interchange between different generations during the river setting, concrete solutions can be found.
It is impossible for children to go out alone to Susubo River in that mountain. However, if it is a public event, you can prepare a variety of outdoor learning menu such as water play and midsummer tree planting. Facts In the summer of 1999, related memorial events of the forest and river, green and fish were carried out at the ceremonial occasion of the national forest direct control project, and public and private sectors became unified, boasting skinship of adults and children. It seems that it was a time that felt nature with the skin and was touched by the activities of people. I would like to expect the day to enjoy total field thinking thinking.