Yuji Kishi: The Era of Watershed Thinking and Watershed-Based Flood Control Begins

2020/11/18

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

And now, perhaps I should say at last, in response to intensifying large-scale floods and sediment disasters, the national government in July of this year announced the long-awaited policy of shifting future flood control from the conventional approach centered on the River Act and the Sewerage Act to "watershed-based flood control." This was a bold decision made by the Water and Disaster Management Bureau (formerly the River Bureau), which has braced itself for the hardships of an era of heavy rains due to global warming, after long deliberation. Although I retired last year, looking back on the nearly 20 years I participated in related deliberations, I am filled with deep emotion.

The Earth's land area can be broadly divided into three types: glaciers, deserts, and rain-fed lands. Since rain that falls on the surface flows downward due to gravity, a nested structure of depressions that collect rainwater inevitably forms on the surface of rain-fed lands. The large and small depressions that spread out there, the water-collecting topography that catches rainwater and creates a water system, is the ecosystem called a "watershed." Floods are a phenomenon caused by this topography, not by administrative districts.

If we adopt the usual method of defining a "watershed" as the upstream catchment area with the river mouth as the reference point, then there are as many river watersheds as there are rivers. If we generalize and define the catchment area corresponding to any point in a water system as the watershed for that point, then a virtually infinite nested structure of watersheds divides the land where we live. The argument of "watershed thinking" is that we should respect this inevitable arrangement of water cycle topography in the biosphere and, regardless of scale, use the watershed structure as a framework to conserve the environment and respond to water and sediment disasters.

Until today, in Japan, both nature conservation and flood control have largely been outside the mainstream of watershed thinking. For 20 years, the framework for nature conservation has been "satoyama" (community-based forests and landscapes), not "watershed." As for flood control, two laws—the River Act, which manages the natural public property of rivers, and the Sewerage Act, which manages the urban facilities of sewerage systems—have been assigned the grave responsibility of flood prevention, and the basic approach has been one that does not emphasize comprehensive consideration of the watershed's water cycle conditions. In the near future, where there is a possibility of "super heavy rains" exceeding expectations, this approach can no longer keep up in terms of budget or development projects. The shift to watershed-based flood control was inevitable.

To begin with, in Japan, there is no education about watersheds at the elementary, middle, or high school levels. The declaration of watershed-based flood control will likely bring significant changes to 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, river improvement and maintenance, dam construction, and sewerage system maintenance remain important. To supplement these efforts, it is also advisable to comprehensively review the green areas of the watershed as flood control infrastructure; for towns where large-scale inundation is unavoidable, to re-examine disaster prevention measures starting from the urban structure itself; and if the local community and agricultural sector agree, to appropriately devise watershed measures such as "flood control that allows flooding," which uses vast paddy fields to absorb heavy rainfall and floods.

In fact, this kind of watershed-based flood control based on watershed thinking is not starting for the first time now. Although largely ignored by the media, a preceding example called "comprehensive flood control," which serves as a model for watershed-based flood control, is celebrating its 40th anniversary this year in the watershed of the Tsurumi River, a Class A water system, where the Keio University Hiyoshi Campus is also located.

The Tsurumi River system, with the 43-km-long Tsurumi River as its main stream, has its headwaters in Machida City, Tokyo, with its watershed extending into the hilly areas of Kawasaki and Yokohama, and a vast lowland area spreading across Kohoku and Tsurumi Wards in Yokohama City and Saiwai Ward in Kawasaki City. During heavy rains, floods rushing down from the hilly areas, which account for 70% of the watershed, would pool and flood the downstream lowlands, a common occurrence since the Edo period. However, with the rapid post-war development of bedroom communities, the watershed experienced a sharp decrease in its water retention and storage capacity due to the rapid loss of fields and woodlands in the hilly areas. This led to repeated major floods that submerged tens of thousands or thousands of homes, beginning with the great flood caused by the Kanogawa Typhoon in 1958. In 1976, as development accelerated, the Keihin Construction Office of the Ministry of Construction (now the Keihin River Office), which manages the Tsurumi River, determined that it was no longer possible to prevent major floods with conventional flood control methods. It called on Yokohama City and other watershed municipalities to embark on "watershed-based flood control" under the name "comprehensive flood control," with pillars including green space conservation and the installation of rainwater regulating ponds alongside river and sewerage system improvements. This was in 1980.

The collaboration proceeded successfully, and it is estimated that a water retention capacity of several million tons has been secured through watershed measures separate from various river and sewerage improvements, such as large-scale green space conservation in the headwater city of Machida and the hilly areas of southern Yokohama, and the installation of 4,900 large and small rainwater regulating ponds throughout the watershed. On October 12 and 13, 2019, during Typhoon Hagibis, news that the Japan vs. Scotland rugby match was held safely at the International Stadium Yokohama in the middle reaches of the Tsurumi River the day after the heavy rains was broadcast overseas and reported as a splendid achievement of Japan's flood control efforts. The focus was on the fact that Shin-Yokohama Park, where the pilotis-style stadium is located, is also a huge 3.9-million-ton capacity flood control facility—a multi-purpose retarding basin. A heroic tale spread both domestically and internationally that the retarding basin, which took in the massive surge of the main stream caused by the heavy rain, stored 940,000 tons of floodwater and protected the downstream towns from flooding. However, this is the Tsurumi River watershed, a place of watershed-based flood control. It could be said that the real heroes were not the multi-purpose retarding basin, but the woodlands and thousands of rainwater regulating ponds in Machida and Yokohama in the upstream area, which likely retained well over 3 million tons of rainwater. Without the support of this water retention capacity accumulated over 40 years of watershed measures, the multi-purpose retarding basin would have filled to capacity in that rain, and the floodwaters could have struck the downstream towns.

Comprehensive flood control began to produce tangible results in the mid-1980s. With the advancement of channel maintenance, large-scale dredging, revetment work, and large-scale underground storage pipes, in addition to watershed measures such as the conservation of headwater retention forests and the development of rainwater regulating ponds throughout the area, the Tsurumi River watershed has not experienced a single case of external flooding (flooding from the river) since 1982. The start of operations of the Shin-Yokohama multi-purpose retarding basin in 2003 further improved the level of flood safety, and the lower Tsurumi River watershed, where thousands of homes once submerged even with rain on a 10-year scale, is now thought to be an area that can withstand heavy rains on a 40- to 50-year scale.

Nevertheless, many challenges remain. If a heavy rain on a 100- or 150-year scale were to strike, the lower Tsurumi River watershed would be helplessly hit by a major flood. We must also prepare for even larger-scale rains anticipated due to climate change. In the lower Tsurumi River watershed, not only has an inundation forecast for a planned-scale heavy rain of a 150-year frequency been published, but also an assumed maximum scale inundation hazard map for a heavy rain of a 1,000-year frequency.

However, looking at a hazard map that predicts inundation depths of 5 or 10 meters in some areas, what are citizens to do? Both the government and citizens have no choice but to enter an era of fundamentally re-examining flood control and disaster prevention measures from a watershed perspective, starting from the basics of urban planning and daily life, based on a solid understanding of the characteristics of the watershed's water cycle.

Water and sediment disasters caused by heavy rain are not limited to large-scale floods. Water and sediment disasters caused by cliffs and slopes also lead to significant human casualties. While sediment disaster hazard maps for cliff collapses have been published, the reality is that measures for complex water and sediment disasters in small watersheds on sloped land have not progressed at all. In 2014, in the hills of Yagi, Hiroshima, a small, lushly green watershed of several tens of hectares was hit by a linear precipitation band with an intensity of 80 mm per hour for several hours, and the resulting debris flow destroyed the town below, causing dozens of casualties. In the Tsurumi River watershed, where 70% of the area is hilly, responding to Hiroshima-Yagi-type small-watershed water and sediment disasters is also becoming a major challenge for watershed-based flood control.

If the government cannot respond, the community and landowners have no choice but to start with an exploratory approach based on watershed thinking. In the Tsurumi River watershed, now in its 40th year of comprehensive flood control, there is a model in this field as well for voluntary small-watershed management by landowners. The one promoting it is Keio University. The location is Mamushi-dani Valley, a typical small watershed on the Hiyoshi Campus.

Mamushi-dani Valley is a small watershed of about 12 hectares. Most of the slopes are woodlands, but there are various athletic facilities at the bottom of the valley and on the plateau. During my more than 40 years working at Hiyoshi, I witnessed several landslides in this area. Centered on faculty collaboration, I promoted surveys of the entire valley's vegetation and topography starting in the 1990s. In the 2000s, with the support of administrative offices, students, and the government, I attempted handmade disaster prevention and green space management based on the small watershed's topographical and vegetation classifications. Thanks in part to a bold decision by the Jukukan-kyoku (Keio Corporate Administration), two rainwater regulating ponds have now been installed in the Mamushi-dani small watershed, measures for seepage water drainage in dangerous micro-watersheds have been taken, and efforts to prevent sediment disasters in collaboration with the government should also be underway.

In one of Mamushi-dani's micro-watersheds, Ichi-no-tani (common name), which also experienced a major collapse in the 1980s, handmade, nature-rich micro-watershed management work to mitigate sediment disasters and restore biodiversity has been continuously carried out since 2000 by faculty (including current and former members), alumni, and local citizens (the Hiyoshimaru-no-kai), in conjunction with the biodiversity plan for the entire Tsurumi River watershed. This series of activities by the educational corporation and on-campus groups in Mamushi-dani has attracted attention as a model for nature-rich disaster prevention activities in the Tsurumi River watershed. It is used as a venue for various learning and training sessions, and in 2018, Keio University Hiyoshi Campus received the "Award for Contribution to the Soundness of the Tsurumi River Basin Water Cycle System" from the Tsurumi River Basin Water Council. The presenter of the award is an administrative collaborative organization, with the Kanto Regional Development Bureau of the MLIT as its secretariat, that is working on the "Tsurumi River Basin Water Master Plan," a next-generation watershed vision to make comprehensive flood control more nature-rich and multifunctional.

Watershed thinking is a new way of measuring the Earth. The conventional method has been to measure the Earth's surface based on Cartesian coordinates, promoting various urban policies within the framework of administrative districts that divide up the land, and even forcing water and sediment disasters into this framework. However, if we look honestly at the uneven topography of the land, we see that before it was divided by Cartesian coordinates, there existed a magnificent water cycle pattern created by a gapless mosaic of large and small watershed topographies (ecosystems). Since water and sediment disasters are caused by this water cycle pattern, the culture of disaster prevention and adaptation for the era of heavy rains due to global warming should emerge from the rediscovery of this pattern—that is, the nested pattern of watershed topography—by not only the government but also all citizens, companies, and corporations.

The Tsurumi River watershed has a 40-year history as the birthplace of an adaptation culture based on watershed thinking in the Japanese archipelago. Mamushi-dani Valley on the Keio University Hiyoshi Campus, located in a corner of this watershed, may be, to put it grandly, a literal small experimental site for Japan as it seeks a new way of interacting with the biosphere in anticipation of the era of heavy rains due to global warming.