Yuji Kishi: The Era of Basin Thinking and River Basin Disaster Resilience Begins

It has been nearly half a century since I became involved in the theory and practice of nature conservation and water/sediment disaster prevention based on basin thinking. It has been a long road, but I have been able to play a part in achieving model results in the small basin of Koajiro on the Miura Peninsula and the basin of the Tsurumi River, an urban river.

And finally, in response to the intensifying large-scale flooding and sediment disasters, the government announced a long-awaited policy this July to shift future flood control from the conventional approach centered on the River Act and Sewerage Act to "River Basin Disaster Resilience." This was a courageous decision made after long deliberation by the Water and Disaster Management Bureau (formerly the River Bureau), which has prepared for the hardships of the era of global warming and torrential rains. Although I retired last year, looking back on the nearly 20 years I spent participating in related deliberations, I am filled with deep emotion.

The Earth's land area can be broadly divided into three categories: glaciers, deserts, and rain-fed lands. Since rain falling on the ground flows downward due to gravity, a nested structure of depressions that inevitably collect rainwater is formed on the surface of rain-fed lands. These large and small depressions spread across the land—the water-collecting topography that receives rainwater and creates water systems—are ecosystems called "basins." Floods are phenomena generated by this topography, not by administrative districts.

If we adopt the standard method of defining a "basin" as the water collection area upstream from a river mouth as the reference point, there are as many river basins as there are rivers. If we generalize and define the water collection area corresponding to any position in a water system as the basin for that point, a nested structure of virtually countless basins divides the land where we live. The claim of "basin thinking" is that we should respect this inevitable arrangement of water-circulating topography in the biosphere, preserve the environment using the basin structure as a framework regardless of scale, and respond to water and sediment disasters accordingly.

Until today, the mainstream of both nature conservation and flood control in Japan has been outside of basin thinking. It has been 20 years since "Satoyama" rather than "basin" was established as the framework for nature conservation. As for flood control, the basic method has been one where two laws—the River Act, which manages rivers as natural public property, and the Sewerage Act, which manages sewerage as urban facilities—are burdened with the heavy responsibility of avoiding inundation, without emphasizing comprehensive consideration of the water cycle circumstances of the basin. In the near future, where "extreme heavy rains" exceeding expectations may strike, budgets and construction projects will no longer keep up with this method. The shift to River Basin Disaster Resilience was inevitable.

In the first place, in Japan, there is no education regarding basins in elementary, middle, or high school education. With the declaration of River Basin Disaster Resilience, there will likely be major changes in the field of education as well.

Even without assuming global warming, heavy rains that occur once every 50 or 100 years will come. To respond to this systematically, it remains important to improve and maintain rivers, build dams, and develop sewerage systems. To support this from the side, it is good to comprehensively re-evaluate green areas in basins as flood control infrastructure, re-examine disaster prevention responses for towns where large-scale flooding is unavoidable starting from the urban structure itself, and, if regional and agricultural sectors agree, appropriately devise basin measures for "flood control by overflowing" where vast rice paddies receive torrential rain floods.

Such basin-thinking-based River Basin Disaster Resilience is, in fact, not just starting now. Although the media mostly ignores it, a preceding example called "Comprehensive Flood Control," which serves as a model for River Basin Disaster Resilience, is celebrating its 40th anniversary this year in the Tsurumi River basin, a Class A river system where Keio University Hiyoshi Campus is also located.

The Tsurumi River system, with the 43km-long Tsurumi River as its main stream, has Machida City in Tokyo as its headwater city, stretches across the hills of Kawasaki and Yokohama, and spreads across vast lowlands in Kohoku Ward and Tsurumi Ward of Yokohama City, and Saiwai Ward of Kawasaki City. During heavy rains, floods rushing down from the hills that make up 70% of the basin have habitually stagnated and overflowed in the downstream lowlands since the Edo period. However, with the rapid development of bedroom communities after the war, the basin, which saw the sudden disappearance of fields and groves in the hills, suffered a sharp decline in water retention and retarding capacity. Starting with the massive flooding caused by the Kanogawa Typhoon in 1958, major water disasters submerging thousands or tens of thousands of homes were repeated. In 1976, as development accelerated, the Keihin Construction Office of the Ministry of Construction (now the Keihin River Office), which manages the Tsurumi River, judged that it was no longer possible to avoid major water disasters through normal flood control methods. They called upon Yokohama City and other local governments in the basin to embark on "River Basin Disaster Resilience" under the name of "Comprehensive Flood Control," centered on green conservation and the installation of rainwater regulation ponds accompanying development, alongside river and sewerage improvements. This was in 1980.

The collaboration proceeded successfully. Through basin measures such as large-scale green conservation in the headwater city of Machida and the hills of Yokohama City in the southern part of the basin, and the installation of various large and small rainwater regulation ponds at 4,900 locations throughout the basin, a water retention capacity of probably several million tons has been secured. During the East Japan Typhoon on October 12 and 13, 2019, the news that a rugby match between Japan and Scotland was successfully held the day after the heavy rain at the Yokohama International Stadium in the middle reaches of the Tsurumi River was broadcast overseas and reported as a brilliant result of Japan's flood control efforts. What drew attention was that Shin-Yokohama Park, where the pilotis-style stadium is located, is actually a massive flood control facility—a multi-purpose retarding basin with a capacity of 3.9 million tons. A heroic tale spread at home and abroad that the retarding basin, which took in the massive rise of the main stream due to the heavy rain, stored 940,000 tons of floodwater and protected the downstream town from water damage. However, this is the Tsurumi River basin of River Basin Disaster Resilience. It could be said that the true heroes were not the multi-purpose retarding basin, but the groves of Machida and Yokohama and the thousands of rainwater regulation ponds in the upstream area that retained probably well over 3 million tons of rainwater. If this water retention capacity accumulated through 40 years of basin measures had not supported it, the multi-purpose retarding basin could have become full during that rain, and the flood might have struck the downstream town.

Comprehensive Flood Control began to yield full-scale results in the mid-1980s. In addition to river channel improvement, large-scale dredging, revetment work, and large-scale underground storage pipes, the Tsurumi River basin—where basin measures such as the conservation of headwater retention forests and the development of rainwater regulation ponds throughout the area progressed—has reached the present day without a single instance of external water flooding (flooding from the river) since 1982. The start of the Shin-Yokohama Multi-purpose Retarding Basin's functions in 2003 further improved flood control safety, and the downstream area of the Tsurumi River, where thousands of homes once submerged even in rains of a scale occurring once every 10 years, is thought to have become an area that can withstand heavy rains occurring once every 40 to 50 years.

Nevertheless, challenges remain piled high. If heavy rain of a scale occurring once every 100 or 150 years strikes, the downstream part of the Tsurumi River basin will be hit by a major water disaster with no way to stop it. We must also prepare for even larger-scale rains expected due to climate change. In the downstream area of the Tsurumi River, not only inundation assumptions corresponding to planned heavy rains of a scale occurring once every 150 years, but also maximum assumed scale inundation hazard maps corresponding to heavy rains of a scale occurring once every 1,000 years have been published.

However, what should citizens do when they see hazard maps predicting submersion depths of 5m or 10m depending on the area? Both the administration and citizens have no choice but to enter an era where they fundamentally re-examine flood control and disaster prevention responses from the perspective of the basin, based firmly on the characteristics of the basin's water cycle, starting from urban planning and the basics of daily life.

Water and sediment disasters caused by heavy rains are not limited to large-scale flooding. Water and sediment disasters caused by cliffs and slopes also cause significant human casualties. While sediment hazard maps have been published for landslides, the reality is that responses to complex water and sediment disasters in small-scale basins on slopes have not progressed at all. In 2014, in the hills of Yagi, Hiroshima, a small-scale basin of several dozen hectares with dense greenery was struck by a linear precipitation zone of 80mm per hour for several hours, and the resulting debris flow destroyed the town below, resulting in dozens of victims. Even in the Tsurumi River basin, where 70% of the basin is hilly, responding to Hiroshima-Yagi type small-scale basin water and sediment disasters is becoming a major challenge for River Basin Disaster Resilience.

If the administration cannot respond, the community and landowners have no choice but to start with exploratory basin thinking. In the Tsurumi River basin, now in its 40th year of Comprehensive Flood Control, there is a model of voluntary small-scale basin management by landowners in this field as well. The one who has been promoting it is Keio University. The location is Mamushi-dani, a typical small-scale basin on the Hiyoshi Campus.

Mamushi-dani is a small-scale basin with an area of about 12 hectares. Most of the slopes are groves, but there are various sports facilities on the valley floor and the plateau surface. During my more than 40 years at Hiyoshi, I witnessed several landslides in this area. Centered on faculty collaboration, we have been conducting surveys of the vegetation and topography of the entire valley since the 1990s. In the 2000s, with the support of the administration office, students, and the government, we attempted handmade disaster prevention and green management based on the topographical and vegetation classifications of the small basin. Thanks to a courageous decision by the Jukukan-kyoku (Keio Corporate Administration), two rainwater regulation ponds have now been installed in the Mamushi-dani small basin, measures to eliminate seepage water in dangerous micro-basins have been taken, and attempts to prevent sediment disasters in cooperation with the government should also be progressing.

In Ichino-tani (common name), one of the micro-basins in Mamushi-dani that experienced a major collapse in the 1980s, handmade multi-nature micro-basin management work has been continued since 2000 by faculty (including active and retired), graduates, and local citizens (Hiyoshimaru-no-kai). This work aims to mitigate sediment disasters and restore biodiversity in coordination with biodiversity plans for the entire Tsurumi River basin. This series of activities by the school corporation and campus organizations in Mamushi-dani has drawn attention as a model for multi-nature and disaster prevention activities in the Tsurumi River basin and is used as a site for various types of learning and training. In 2018, Keio University Hiyoshi Campus received the "Tsurumi River Basin Water Cycle System Soundness Contributor Award" from the Tsurumi River Basin Water Council. The awarding body is an administrative collaboration organization that works on the "Tsurumi River Basin Water Master Plan," a next-generation basin vision to make Comprehensive Flood Control multi-natural and multi-functional, with the Kanto Regional Development Bureau of the Ministry of Land, Infrastructure, Transport and Tourism serving as the secretariat.

Basin thinking is a new way of measuring the Earth. Measuring the Earth's surface is basically based on Cartesian coordinates, and various urban policies have been promoted under the framework of administrative districts that divide the expanse, and even water and sediment disasters have been forced into this framework in the conventional method. However, if we look honestly at the uneven patterns of the land, there exists a magnificent water cycle pattern created by basin topography (ecosystems) without gaps, large or small, before being divided by Cartesian coordinates. Since water and sediment disasters are disasters caused by that water cycle pattern, a disaster prevention adaptation culture for the era of global warming and torrential rains should arise from the rediscovery of that pattern—namely, the nested pattern of basin topography—not only by the administration but by all citizens, companies, and corporations.

The Tsurumi River basin has a 40-year history as the birthplace of the adaptation culture of basin thinking in the Japanese archipelago. Mamushi-dani at Keio University Hiyoshi Campus, located in one corner of it, might be, to put it grandly, a literal small laboratory for Japan as it gropes for a new way of interacting with the biosphere in anticipation of the era of global warming and torrential rains.