Introduction
Kawasaki, situated along Japan’s western Tokyo Bay, faces increasing flood risks driven by its geographic and climatic challenges. With a significant portion of its population and assets concentrated on vulnerable alluvial plains, even a single flood event could result in catastrophic damage.
Map: Kawasaki City Hydrology and Water Facilities, by author.
Kawasaki’s short-term challenge lies in street flooding caused by extreme rainfall and the Tama River’s backflow. The city experiences a humid subtropical climate, with seasonal rainfall ranging from 150 to 250mm[1]. Extreme weather like Typhoon Hagibis in 2019 brought 1,000 mm of rain, overwhelming aging drainage culverts and infrastructure[2]. The city is facing an increase in Habigis-induced rainfall led by the increase in tropospheric and ocean temperatures around Japan[3], urging the city to find an immediate solution to the street flooding.
In the long term, sea-level rise exacerbates inland flooding, especially in low-lying areas where compact alluvial soil limits infiltration, combined with the risk of saltwater intrusion into Combined Sewer Overflow (CSO) pipes[4].
Many water management challenges, such as upstream runoff and inland flooding, are highly related to extreme rainfall and sea level rise. The city’s aging and dysfunctional water infrastructure such as water pipes, combined sewage system, and drainage culvert complexes the problem. To build resilience towards extreme rainfall and sea level rise, Kawasaki must seize the best chance to manage the flood risks now.
Diagram: Web of water management challenges, by author.
Why Green Stormwater Infrastructure (GSI)?
Traditional flood control methods, such as drainage culverts and floodways, offer temporary solutions. While culverts can reroute stormwater, they inadvertently increase its volume, magnitude, and flow frequency, putting further strain on downstream systems. Floodways, like the city’s Gotanda River floodway[5] provide fixed capacity but require high investments and underperform during extreme rainfall events.
Green stormwater infrastructure (GSI) offers a sustainable, cost-effective alternative. It reduces total runoff, retains stormwater, and slows water flow[6], while also being adaptable across scales and urban contexts. Worth noting that GSI alone cannot combat sea-level rise in low-lying areas, thus urging its implementation upstream and selective integration downstream.
Diagram: GSI Mechanism Map, by author.
Upstream Solutions: Retain and Infiltrate Water
The goal of upstream solutions is to reduce total stormwater runoff and slow its entry into the river systems. The upstream region has potential for large-scale interventions due to its permeable soils, available open space, and patches of urban greenspaces.
1. Constructed Wetlands
One effective intervention is the construction of wetlands. Engineered wetlands mimic natural processes to filter water, manage large stormwater volumes, and prevent shoreline erosion. Free Water Surface (FWS) wetlands[7], with shallow water flowing over the soil surface, are particularly effective. Combined with underdrain systems, these wetlands can not only enhance flood provention but also potentially recollect and redistribute water for non-potable uses.
Diagram: Free Water Surface wetland, by author.
2. Reforestation and green space
Another crucial upstream intervention is reforestation and the expansion of green spaces. As of 2023, only 653 hectares, or 4.5% of Kawasaki’s total land area, is covered by natural forests[8]. Increasing green surfaces can be achieved by planting diverse trees, shrubs, and grasses at multiple heights to improve the forest’s roughness, allowing it to absorb rainfall, stabilize soil, and reduce runoff. These efforts would strengthen the watershed’s ability to manage stormwater naturally while enhancing biodiversity.
Downstream Solutions: Manage Flow and Store Water
Downstream solutions focus on managing and safely storing stormwater in urbanized, low-lying areas where space for large-scale interventions is limited.
1. Bioretention Basins
One of the most effective measures is the use of bioretention basins. These shallow, vegetated basins filter localized runoff, improve water quality, and allow gradual infiltration of stormwater. Designed for commercial and institutional areas, bioretention basins can be installed in parking lots, sports fields, and alongside streets. Depending on their design, these basins can reduce annual runoff volume by 40–80%[9], providing a reliable solution for localized flooding while enhancing the urban landscape.
Diagram: Bioretention Basin, by author.
2. Green Roofs and Permeable Pavements
Another critical downstream intervention involves green roofs and permeable pavements. Green roofs allow rainwater to be captured and stored at the building scale, reducing the volume of runoff entering the drainage system. A notable example is the Ford Rouge Factory in Michigan, USA, which retains up to 4 million gallons of rainwater annually with its 10.4-acre green roof. Similarly, permeable pavements allow water to be collected by blue infrastructures, such as bioretention basins or water storage tanks, alleviating pressure on the city’s drainage systems.
Diagram: Proposed locations for Bioretention Basins and Green Roofs within the Odasakae Commercial Complex, highlighting opportunities for stormwater management in the low lying area, by author.
Addressing Challenges: Low-Lying Areas and Sea-Level Rise
While GSI excels at managing stormwater, Kawasaki’s compact, impermeable alluvial soils and the impacts of sea-level rise create significant challenges. The inevitable flooding in low-lying areas, exacerbated by saltwater intrusion into aging CSO pipes, requires integrated solutions. Investing in upper watershed green infrastructure, such as wetlands and reforestation, can reduce the volume of water flowing downstream. This reduction would alleviate stress on Kawasaki’s vulnerable zones while enhancing the efficiency of downstream systems.
To complement GSI, critical downstream infrastructure should also be upgraded. Reinforcing embankments and constructing super levees can provide essential flood protection. An integrated approach should be adopt to address the limitations of GSI and build resilience to future challenges.
GSI at Planning Level
One of the key challenges for implementing green infrastructure in Kawasaki is the absence of dedicated green space zoning. Unlike many other countries, Japan does not designate “green space” as a specific zoning category, embedding green infrastructure within urban development projects instead. This unique situation provides Kawasaki with an opportunity to incorporate green infrastructure into new and ongoing development.
To encourage GSI adoption, Kawasaki can introduce development regulations requiring developers to integrate GSI solutions in upstream areas and along river corridors. For instance, redevelopment projects could mandate a specific percentage of floodable spaces or green infrastructure components. Public-private partnerships can also play a significant role. Under the 1999 Private Finance Initiatives Act, the city can collaborate with corporations to implement GSI as part of their corporate responsibility programs. Additionally, a fee structure based on impervious surfaces can incentivize property owners to adopt green infrastructure systems, with rebates offered for successful installations that manage upstream stormwater.
Conclusions
To address Kawasaki’s growing flood risks, a multi-scale approach combining Green Stormwater Infrastructure (GSI) and critical infrastructure upgrades has been proposed. Upstream, methods like constructed wetlands and reforestation will reduce stormwater runoff, stabilize soils, and strengthen the watershed’s ability to manage heavy rainfall. Downstream, solutions such as bioretention basins, green roofs, and permeable pavements will effectively manage and store water in compact urban areas, alleviating pressure on aging drainage systems.
While GSI alone cannot fully mitigate the challenges posed by sea-level rise, its integration with reinforced embankments, super levees, and blue infrastructure offers a comprehensive strategy for flood protection. By embedding green infrastructure into development regulations, encouraging public-private partnerships, and incentivizing property-level solutions, Kawasaki can implement cost-effective and sustainable flood resilience measures.
[1] “Climate and Average Weather Year Round in Kawasaki,” Weather Spark, accessed December 11, 2024, https://weatherspark.com/y/142921/Average-Weather-in-Kawasaki-Japan-Year-Round.
[2] 上下水道局, “令和元年東⽇本台⾵による排⽔樋管周辺地域の浸⽔に関する検証報告書 【概要版】,” n.d., https://t-shigetomi.com/wp-content/uploads/2020/04/83118d8c572650ba723f901d70c62ab8.pdf.
[3] Keita Fujiwara, Tetsuya Takemi, and Nobuhito Mori, “Impact of Historical and Future Warming on Heavy Rainfall over Eastern Japan Induced by Typhoon Hagibis (2019) in Dynamical Downscaling Simulations,” Natural Hazards, August 22, 2024, https://doi.org/10.1007/s11069-024-06875-9.
[4] “川崎市水道百年史 川崎市上下水道局,” accessed October 14, 2024, https://www.city.kawasaki.jp/800/cmsfiles/contents/0000139/139012/html5.html#page=265.
[5] 川崎市水道百年史 川崎市上下水道局“五反田川放水路の施設概要,” 川崎市, October 12, 2012, https://www.city.kawasaki.jp/530/page/0000018230.html.
[6] Matthew J. Burns et al., “Hydrological Shortcomings of Conventional Water Management and Opportunities for Reform,” Landscape and Urban Planning, accessed December 11, 2024, https://canvas.upenn.edu/courses/1809205/files/136037552?wrap=1.
[7] Izharul Haq Farooqi, Farrukh Basheer, and Rahat Jahan Chaudhari, “Constructed Wetland System for Wastewater Treatment,” n.d.
[8] Vizzuality, “Kawasaki, Kanagawa, Japan Deforestation Rates & Statistics | GFW,” accessed December 11, 2024, https://www.globalforestwatch.org/dashboards/country/JPN/19/14?category=land-cover.
[9] Virginia Stormwater BMP Clearinghouse, “Bioretention: Virginia DCR Stormwater Design Specification,” n.d., https://www.deq.virginia.gov/our-programs/water/stormwater/stormwater-construction/bmp-clearinghouse., depending on the design levels, providing a reliable solution for localized flooding while enhancing the urban landscape.
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