Building Resilience After the Disaster: One Concern in Kumamoto City, Japan
Part Two of our Digital Anthropology Showcase
By Trigg Hutchinson, Digital Anthropologist at One Concern
At 9:26 PM on 14 April, 2016, a magnitude 6.5 strike-slip earthquake at shallow depth occurred below Mount Kinpo, just to the northwest of the city of Kumamoto in Western Japan. This earthquake was powerful enough to be felt in Honshu, the largest island in Japan, and was followed by more than 140 aftershocks in 48 hours. The Japanese Self Defense Forces immediately leapt into action, evacuating more than 40,000 people, but the earthquake proved to be a foreshock. On the 16th, a magnitude 7.0 earthquake occurred just below the eastern section of Kumamoto, this time strong enough to be felt in Korea.
In addition to the massive, widespread structural damage throughout the city — over 30,000 buildings were destroyed or seriously damaged, including the Kumamoto castle and nearby Aso shrine — more than 60 people lost their lives and nearly 2,000 were injured. Fascinatingly, these earthquakes occurred on two different faults — the tectonic line that runs under Japan forks to the east of Kumamoto, creating a complex and volatile seismic environment.
Since then, the prefecture has experienced numerous floods, leading to catastrophic mudslides and further loss of life. Another section of Kumamoto castle collapsed following heavy rains in 2018, and the numerous rivers that weave through the city have burst their banks multiple times. Other threats lie temporarily dormant — Kumamoto lies just west of Mount Aso, the largest active volcano in Japan, and across the Ariake Bay from Mount Unzen, whose 1991 eruption killed 43 volcanologists and journalists caught off guard by pyroclastic flows.
When I lived in Tokyo, I was quick to forget the very real risk of natural disasters in Japan — it’s easy to ignore while in the center of a megacity — but viewing the smoke emerging from Mount Aso’s caldera on our approach into Kumamoto Airport snapped things back into focus. Despite all the technical know-how of Japanese structural engineers and urban planners, natural hazards remain a constant threat. And at first glance, it doesn’t feel that way — Kumamoto is a typical Japanese city, modern and convenient in every regard.
Our task in Kumamoto was to map the hydrological environment in and around the city, to ultimately provide high-resolution data, which can then power comprehensive flood models for the entire watershed. This was a particularly challenging problem, as the rivers that run through the city are fed by three separate drainage areas to the north, east, and southeast — meaning that one river may burst its banks after a storm that happens miles away, while the others remain comfortably below the tops of their levies, or they could all overflow at the same time, depending on the distribution of rainfall. And the city is located on the coast, so the storm surge created by seasonal typhoons introduces yet another level of complexity. For our data science team to successfully model the city, it was critical for us to understand the drivers behind these dynamics.
Bathymetric and topographic data is generally readily available when we aggregate data for a new city — unlike other types of data we rely on, such as structural information or demographic data, the natural environment does not typically change, and a strong understanding of the natural environment is necessary for any sort of construction. And as we deploy our platform exclusively in developed areas (at least for now), we can usually be confident that this data exists and can be acquired. However, locating the data outside of the US can be a lengthy process, and at times our development timelines require access to complete data sets well before we can even identify who to ask for them. Should our timelines be excessively concise, we are fully capable of quickly and efficiently creating the data ourselves.
There are a number of techniques that can be employed to digitize the physical space and create a bathymetric map. The most technically sophisticated is water-penetrating LiDAR, or light detection and ranging, which is typically mounted to a drone or small aircraft. This is commonly used for topographic mapping, as the standard LiDAR wavelength is capable of penetrating vegetation but attenuates — loses intensity — too rapidly in water.
The second and perhaps most straightforward approach is photogrammetric analysis, in which a single location is imaged from multiple aspects and measurements extracted from the results. Unfortunately, photogrammetry is an option only with very shallow, relatively clear waterways, and the rivers in Kumamoto prefecture run too deep and turbid for this method to be employed.
With LiDAR and photogrammetry ruled out, we chose a third option — sonar. A typical commercial sonar can suffer significant accuracy issues in shallow or turbulent water. In these conditions, shallow water can lead to the sound wave bouncing between the river bottom and surface multiple times before being detected by the transducer, and bubbles created by fast currents can create noise and cause the sonar to ‘fuzz’ up and generate inconsistent returns. To get ahead of these concerns, our team modified a sonar for extreme shallow depths and adjusted the transducer’s sounding arc to project straight down — giving us an accurate view of river geometry in as shallow as one foot of water.
Once we started surveying the waterways, it became immediately apparent just how much effort the government of Kumamoto puts into risk mitigation. The major river running through the city — the Shirakawa — and the two larger rivers to the south — the Midorigawa and Kasegawa — are constrained by comprehensive flood defence architecture, including levies, locks, numerous dams, and curious concrete blocks that look like old tank traps, which dissipate the kinetic energy of water during periods of increased water flow. The tank traps in particular are broadly deployed along river banks across the prefecture and proved very difficult to scamper up and down as our survey proceeded.
Nevertheless, after just a week in the city our team completed our efforts to digitize the geometry of every major river in and around Kumamoto city, allowing our data scientists to finalize their models. Our approach proved rapid, accurate, and replicable in a range of environments, so we could come home happy. But the standing question is always, how can our work translate to improved outcomes for vulnerable populations on the ground? In a city like Kumamoto, with all of its mitigative architecture, it’s clear that the threat of natural disasters is constant — and just within the last decade this threat has been demonstrated multiple times.
Our purpose, then, is simply to move the needle. By identifying and quantifying risk at a hyperlocal level, we can allow community stakeholders to take tangible steps to better prepare themselves for when a disaster ultimately strikes. Kumamoto city, and Japan is a whole, is a community that fully understands the potential risk of living atop multiple fault lines and alongside an extensive coastline — and there’s simply no need to raise awareness of this risk.
What our team can do, however, is point to exactly where that risk is most significant, to identify the cascading impacts of a natural hazard on critical infrastructure and the complex interdependencies of an urban environment, and showcase exactly which steps can be taken to mitigate the impact. At the conclusion of our project our team returned home armed with the data to generate these insights, and despite being exhausted and sunburned we left satisfied, knowing that the next earthquake or flood may impact a community incrementally better prepared.
If you enjoyed this Q&A, keep an eye out for the rest of our Digital Anthropology Showcase in the following weeks!
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