On 12 October 2019, super-typhoon Hagibis (locally referred to as Typhoon No. 19) made landfall in Japan and caused extensive damage to a wide area, especially in the Kantō Region. Considered as the strongest typhoon to hit the country in 60 years, the typhoon with wind speed exceeding 200 km/hr dumped record-breaking amount of rainfall and caused extensive flooding in many places, necessitating mass evacuation advisories, paralysing transport systems and shutting down commercial facilities. This quick report presents some preliminary findings on the flooding-related disasters that occurred in Tokyo and adjacent areas caused by the passage of the typhoon, based on the first-hand experience and subsequent reconnaissance work conducted by the author, who was in Tokyo when Hagibis pummelled Japan. Supplemented by additional information from newspapers and online resources, various torrential rain-induced damages, such as slope failures, collapse of river embankments and other sediment-related disasters, are described and their impacts to the built environment are discussed. Finally, major observations and lessons learned from the large-scale disaster are summarised.
Record-breaking rainfall, hurricane-force winds and widespread flooding devastated many areas in the central, eastern and northeastern Japan when Typhoon Hagibis (locally referred to as Typhoon No. 19) made landfall on 12 October 2019, with direct impact to Tokyo and adjacent areas. Considered as a Category 5 super-typhoon (with winds of 259 km/h) prior to landfall, Hagibis (which means “speed” in Filipino) was downgraded to a Category 2–equivalent typhoon when it hit Izu Peninsula (located southwest of the capital), but it still caused serious damage in many places, necessitating mass evacuation advisories, paralysing transport systems and shutting down commercial facilities.
While on research and study leave at Tokyo City University (TCU), the author experienced first-hand the wrath of the typhoon, including submergence of the TCU dormitory (where he was staying) under >1.3m of floodwater. With the TCU campus closed for two weeks due to severe flooding, he then took the opportunity to conduct his own field survey and desktop investigation to learn more about the impact of the typhoon on the affected areas.
This quick report summarises his personal experience and his investigation results, supplemented by information from various online sources, over a ten-day period after the typhoon. Focus is on torrential rain-induced damages, such as flooding-related disasters, collapse of river embankments and other sediment-related disasters in the Kantō Region (which covers seven prefectures: Gunma, Tochigi, Ibaraki, Saitama, Tokyo, Chiba, and Kanagawa, and home to nearly a third of Japan’s total population). In addition, the impacts of the typhoon to engineering structures and the effectiveness of soft-type countermeasures (hazard maps and alert systems) are described. Finally, lessons learned from the disaster and their relevance to New Zealand are presented.
2 TYPHOON HAGIBIS
2.1 Overview of the Typhoon
Typhoon Hagibis developed into a tropical storm on the sea near Minami-Torishima on 6 October, proceeded to the west towards the Mariana Islands and developed into a “large and violent typhoon”, the highest category on Japan Meteorological Agency (JMA) typhoon scale. It then proceeded towards Honshu, Japan’s main island, making landfall on the night of 12 October (see Figure 1a). Data from the Environmental Modeling Center (EMC) showed that on 10 October, the typhoon had maximum sustained wind speeds of 259 kph (tagged as Category 5 typhoon), and then weakened when it made landfall two days later (see Figure 1b).
Figure 1: (a) Path of typhoon Hagibis (JMA 2019); (b) development of Hagibis in terms of wind speed, pressure intensity and storm speed (EMC 2019).
Typhoon Hagibis, the 19th to enter Japan this year, has been considered as the strongest typhoon to hit Japan in 51 years. Typhoon Ida (locally, Typhoon Kanogawa) struck in September 1958, causing the Kano River on the Izu Peninsula to overflow and flood its basin, killing 1,269 people, destroying 16,743 houses and inundating 521,715 houses (National Institute of Informatics website). It should also be mentioned that in September 2019, typhoon Faxai (locally, Typhoon No. 15) caused significant damage to the Kantō Region, with at least three people killed, hundreds of thousands of households remained without power for over a week and numerous homes lost their roofs.
It is worth mentioning that in the morning of 12 October, just hours before the typhoon hit the Kantō Region, a tornado was reported in Ichihara City, Chiba Prefecture. Late afternoon of the same day, a magnitude 5.7 earthquake occurred off the coast of Chiba Prefecture, registering a peak seismic intensity of 4 in Japan intensity scale of 7.
2.2 Rainfall Records
The typhoon dumped record breaking amounts of rain over a wide area. Table 1 shows the highest 24-hr and 1-hr rainfall intensities recorded during the typhoon. The rainfall intensity in Hakone is depicted in Figure 2, where it is seen that the total amount of rainfall that fell in the area during the passage of the typhoon was well in excess of 1,000 mm. Also indicated in the figure is the average monthly rainfall intensity for October in the area, which is 334.3 mm. This indicates that the amount of rain that fell in Hakone during the passage of the typhoon is about three times the monthly average for October.
Table 1: Maximum recorded rainfall intensities due to Typhoon Hagibis (data from JMA 2019).
Figure 2: Rainfall intensity recorded in Hakone, Kanagawa Prefecture (from JMA 2019).
With such record-breaking rainfall, JMA, for the first time, issued a Level 5 special warning for heavy rain (the highest in their scale) to 13 prefectures. The warning level advised millions of residents within the affected areas to evacuate to more secure buildings or move to higher floors.
3 GENERAL FEATURES OF DAMAGE
Based on data as of 21/10/2019, Hagibis left at least 70 people dead, 12 people missing and 408 people injured. In addition, 275 houses were completely destroyed, 1,711 houses partially destroyed and 2,997 suffered some form of damage. Moreover, 29,707 houses had their ground floor flooded while 23,386 houses had underfloor inundation. In terms of non-residential building damage, 190 public and 1,061 private buildings were also affected (FDMA 2019).
Following the emergency alerts issued, more than 800,000 households across 13 prefectures were urged to evacuate. In addition, more than 210,000 households suffered power outages across the Kantō region and Shizuoka Prefecture.
As of 21/10/2019, the following damages were reported (MLIT 2019):
- Damage to river networks:
– Nationally-administered rivers: 108 sections in 26 rivers (mostly river overflow, levee collapse and inland inundation)
– River management facilities: 51 sections in 24 rivers (mostly bank failure, breakage, seepage, and erosion)
- Sediment-related disasters: in total, 79 cases broken down as follows:
– Debris flows – 8cases (in 6 prefectures)
– Slope failures – 65 cases (in 23 prefectures, with 5 cases in Tokyo Metropolitan area)
– Landslides – 6 cases (all in Niigata Prefecture)
- Road networks:
– State highways: 9 sections along 8 routes
– National roads: 24 sections in 10 routes
– Prefectural roads: 74 sections of 41 routes
- Railway networks:
– 56 routes operated by 16 railway companies affected
- Airport facilities:
– While there was no reported damage to airport facilities, 2,622 flights were cancelled
– Treatment plants: inundation damage occurred at 13 plants and treatment function stopped
– Pump stations: suspended at 9 locations due to flood damage
– Manhole pumps: 12 pumps affected due to flooding and ground damage (in 6 cities, 1 town and 1 village)
In addition, based on reports from various news sources, the passage of the typhoon affected some international events. These included the cancellation of several rugby matches, the first time in Rugby World Cup history, as well as the disruption of the Suzuka Grand Prix. The typhoon also forced the first-ever all-day shutdown of Tokyo’s Disneyland and DisneySea theme parks in Chiba Prefecture.
4 FLOOD-RELATED DISASTERS AND LEVEE PERFORMANCE
Severe flooding triggered by the torrential rain submerged large areas in central Japan. Most of the affected areas were located near rivers, where in many cases, the levees and floodwalls either collapsed or were overtopped. In other cases, areas underwent inland flooding due to the intense rain that overwhelmed existing drainage infrastructure.
4.1 Rate of Floodwater Increase
One of the surprises this typhoon showed was how fast floodwater rose and how quickly it disappeared. Figure 3 shows how the water level in Tama River, a major river that separates Tokyo Metropolitan Area and Kanagawa Prefecture, changed during the passage of the typhoon. The water level shown in the figure was monitored near Futako Bridge in Setagaya Ward, Tokyo by the Keihin Office of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). It can be observed from the graph that at this monitoring station, the water level rose at a rate of approximately 0.4~0/5 m/hr, peaking in excess of 9 m at about 10 pm on the night of the typhoon. When the rainfall intensity finally subsided, the water level decreased at almost similar rate.
Figure 3: Variation of water level in Tama River during the passage of typhoon Hagibis (from https://dot.asahi.com/print_image/index.html?photo=2019102100087_2).
A visual description of such rapid change in water level is depicted in Figure 4, which illustrates the flooding that occurred at the author’s 4-storey accommodation (photos taken from the second floor) near Tokyo City University (see location on Figure 5). The white car and white container at the middle of the photos can be taken as reference. The sequence of photos showed the floodwater rose by about 0.5 m in 6 hrs, half-submerging the car, which was at an elevated position with respect to the road. At sunrise the next day, all the water was gone. Subsequent measurements showed the road in front of the accommodation building was inundated by at least 1.7 m high floodwater.
Figure 4: Photos showing the changes in floodwater level at various times due to inland flooding.
4.2 Damage along Tama River
Major cities in Japan built near river systems are located in areas below the design high water levels. Hence, when levees break or floodwalls are overtopped, serious inundation can occur. Levee management is one of the most important flood control activities in Japan.
The author’s accommodation was adjacent to Tama River, which extends over 138 km between Tokyo Metropolitan Region and Kanagawa Prefecture (see Figure 5). Levees all along this river were designed for a 1-in-200 year rainfall event. At the Ishihara Monitoring Station located in Chofu City (see figure), water levels reached their peak at 6.24 m by 11 pm on the night the typhoon struck, in excess of the target 5.9 m level the levees were designed for. Fortunately, an additional 1.5 m safety margin is provided beyond this threshold level. Thus, the total levee height at this monitoring station was 7.4 m and the levee survived the typhoon with just a little above a metre to spare. Note that any levee failure along the Tama River could have caused devastating flooding to many areas downstream of the river, severely affecting millions of residents in Tokyo and Kanagawa. For the first time ever, the Kawasaki City Office (located in Kanagawa Prefecture, downstream of the Tama River)) issued an emergency warning to all its one million+ residents to evacuate by 7 pm that night. Similarly, the author received similar urgent warnings.
Figure 5: Tama River and some of the observation points of interest (base map from Google Maps).
In order to accommodate the extra floodwater, the levees in Japan are constructed some distance away from the river itself. Typically, the open area between the levees and the river are converted into useful belt of parks, greenery and playing fields. In both banks of the Tama River, sports fields, playgrounds, parks and golf driving ranges have been constructed. However, a visit to the area the day after the typhoon indicated that most of these have been obliterated by the raging waters of the Tama River, as seen in Figure 6a. What used to be tennis courts and soccer fields were covered by mud and debris, with many of the sporting facilities washed away. Several portaloos were also damaged, an indication of the force the raging floodwater had as it flowed downstream towards the sea. Debris and other materials carried by the flood were also observed along the river levees, indicating the maximum possible heights by which the floodwater rose. In Figure 6b, the floodwater was just < 2 m away from overtopping the levee at this location.
Figure 6: (a) Washed out playgrounds and sports field adjacent to Tama River; and (b) trace of floodwater on the levee.
Because of flooding in the past, levees have been constructed all along the Tama River to protect neighbouring residential areas, except for approximately 540 m-stretch section southwest of Futako-Tamagawa Train Station (see Figure 7a). This area has undergone extensive redevelopment in recent years and has been transformed into upscale residential and shopping neighbourhood, with large hotels, high-rise residential buildings and shopping complexes. Interview with local residents indicated that over a century ago when it was still a sparsely populated fishing area, local authorities planned of building levees as flood protection measure. However, restaurant owners who have been occupying the area for a very long time refused to re-locate, prioritising river view over the possibility of flooding. Hence, the authorities then yielded to their demand and instead built a levee behind the restaurants. More recently, some residents in the area still opposed the plan of increasing the height of the riverbank, again prioritising the river view. Accordingly, the 540 m section where the levee should be was not erected, while levees were constructed in other sections along the river. Then, on the night of 12 October, the Tama River overflowed from that low-lying bank and water gushed through the opening, flooding several residential houses facing the river (see Figures 7b and 7c).
Figure 7: (a) Map showing location of missing levee in Futako-Tamagawa (modified from MLIT document); and (b) condition during inundation (view towards the river) (from https://mainichi.jp); and (c) condition after the typhoon (view towards inland).
Just south of the Futako-Tamagawa Station, on the Kawasaki City side of the river, another localised flooding occurred (see Figure 8a). Here, Hirase River, a tributary of the Tama River, flows towards the east to join the main stream. The affected zone, which was about 50 m away from the confluence of the two rivers, was sandwiched between the floodwalls of the Hirase River and the levee, which was apparently built in the mid-Edo period (i.e. 1700s). All the residential houses and apartment complexes were built lower than the ground. Interviews with local people indicated that the water level of Hirase River was already very high by the late afternoon of 12 October. From this time, emergency warnings to evacuate were frequent. By 8 pm of that day, the river overflowed and the adjacent road was flooded. The ground floor of many residential houses were then inundated in less than 15 min. An elderly resident died, apparently from drowning. Figures 8b and 8c depict the conditions of the area the morning following the typhoon.
It is unclear why residential houses were allowed to be built in the area on the north side of the levee. Inspection done on-site showed that houses located on the south side of the levee escaped flooding, because of the protection provided by the levee. Detailed investigation through residents’ accounts showed that in the past, there have been frequent inland flooding in the area because rainwater could not be drained. The city authorities has set up pumping facilities as a measure to direct rainwater to the Hirase River. Although the pump was operated again this time, the water level of the Hirase River, which was the destination of the pumped water, rose very rapidly.
The fact that only the region at the confluence between the two rivers was affected suggests the possibility that the flow of Hirase River was blocked by the higher water level in the Tama River. Known as “backwater phenomenon”, the flow of the tributary (Hirase River) rose too high and overtopped its banks because it was blocked by its main stream (Tama River), which was filled with far more water than usual from the record amount of rainfall that fell in the mountainous area of the upper Tama River.
Figure 8: (a) Map showing the location of affected zone in Takatsu-ward, Kawasaki City (base map from https://www.mapion.co.jp); (b) flooded condominiums; (c) inundated residential houses.
4.3 Condition in other parts of Kantō Region
As in most cases in Japan, many rivers in the Kantō region are characterised by their moderately short lengths and substantially steep gradients because of the narrow and rugged topography of the region. Following the passage of the typhoon, many low-lying areas adjacent to rivers were also inundated in, more or less, the same fashion as those observed near the Tama River. Breaching of levees and floodwalls as well as inland flooding were the main culprits, as the enormous amounts of rainfall raised up water levels of many rivers in Tokyo, Saitama and Tochigi prefectures (Figure 9).
Figure 9: Reported river inundation condition during the typhoon, with data from JMA, MLIT and prefectural offices. Fonts in blue are the prefectures comprising the Kantō region (modified from https://mainichi.jp/articles/20191012/k00/00m/040/266000c)
An example of a collapsed section of levee occurred in Akiruno City, located on the outskirts of Tokyo (see Figure 5) adjacent to the swollen Aki River, a tributary of the Tama River. The collapsed portion of the levee, about 70-80 m long, allowed floodwater to inundate the community (see Figure 10a). In Ushinuma, 13 houses located along the Aki River suffered various amounts of tilt when the foundation of the 5 m high revetment wall in the area was eroded; one of the houses eventually collapsed into the river (see Figure 10b). As a result, the city’s fire department placed entry restrictions to the remaining houses.
At the time of writing, other levee breaches reported in the Kantō Region include:
- one location in Higashi-Matsuyama City, Saitama Prefecture, on the Toki River;
- two locations in the cities of Higashi-Matsuyama and Kawagoe, both in Saitama Prefecture, on the Oppe River;
- three locations in the cities of Naka and Hitachiomiya, both in Ibaraki Prefecture, on the Naka River;
- three locations in Hitachiomiya City in Ibaraki Prefecture on the Kuji River.
In addition, news reports mentioned that the operators of several dams in mountainous areas, such as the Shiroyama Dam in Sagamihara (Kanagawa Prefecture), released water during the typhoon to prevent the collapse of the dams. This added to the volume of water that flooded many downstream areas.
Interestingly, the newly constructed Yamba Dam, a multi-purpose dam constructed in the middle of the Azuma River, the main tributary of the Tone River, and located in Gunma Prefecture (see Figure 11a), was put to a test by the typhoon. The 116 m-high dam, with storage capacity of 107.5 million cubic metres, just started “initial impoundment” (test flooding) on 01/10/2019 to confirm safety before going into full-scale operation. However, before it could begin actual operations, rainfall from Hagibis filled it nearly to the capacity level it was designed for (see Figure 11b). According to the Maebashi Local Meteorological Observatory, 442 mm of rain, one-third of the annual precipitation, fell upstream of the dam within 48 hours from 6 pm on 11/10/2019. There was no confirmed damage in the basin where it is located. Had Yamba Dam been in full operation, the space reserved for floodwater storage likely would have been smaller, increasing the risk for the dam and the downstream areas. The construction of the dam started in 1952, but was stopped several times due to protests from local residents.
Figure 11: (a) Photo of the just completed Yamba Dam taken in 4/08/2019 (by Akira Sakurai); and (b) the dam last 13/10/2019, filled to near capacity due to heavy rains brought by Hagibis (from http://www.asahi.com/ajw/articles/AJ201910150058.html)
4.4 Conditions outside the Kantō Region
Other prefectures in the central and northeastern part of Honshu Island were devastated by the typhoon as well. Among the worst affected areas were Nagano, Miyagi and Fukushima Prefectures.
Some of the spectacular flood-induced failures were reported when a 70 m-stretch of the levees on the Chikuma River in Nagano Prefecture were breached, and water gushed into the residential area and inundated many houses (Figure 12a). News and social media footages reported of people standing on the second floors of their houses, waving to grab the attention of the Self-Defence Forces in helicopters flying above (Figure 12b). A railway bridge along the Bessho Line of the Ueda Dentetsu Railway Company, which crossed the Chikuma River, collapsed into the water below (Figure 12c). Figure 12d depicts half-submerged Shinkansen (bullet) trains in the rail yard in the aftermath of the levee collapse. As a result, the inundated trains may be scrapped due to serious damage to their electrical systems.
Figure 12: (a) Photo showing the collapsed portion of the levee along the Chikuma River that resulted in inundation of a large area near the river banks (from http://japan-forward.com); (b) residents stranded by the flood and waiting to be rescued by the Self-Defence Forces (from https://www.sankei.com); (c) a collapsed railway bridge crossing the Chinuma River (from https://www.theatlantic.com); and (d) flooded Shinkansen train rail yard near the bank of the river (from https://www.japantimes.co.jp).
5 SEDIMENT DISASTERS
Sediment disasters (土砂災害) is the general term used in Japan for phenomena involving large-scale movement of soil and rock that threaten human life and property, typically due to heavy rain and earthquakes. According to IDI (2004), sediment disasters can be classified as debris flows, slope failures, and landslides. In the context of this report: (1) debris flow involves soil and rock mix with water (rainwater and groundwater) flowing down rivers and mountain streams; (2) slope failure (i.e. rock/soil fall) comprises abrupt collapse of slope when the soil becomes unstable due to heavy rain; and (3) landslide has soil mass moving downward slowly at the boundary of the discontinuous surface under the influence of groundwater and gravity.
As indicated in the previous section, majority of the sediment disasters that occurred due to the passage of Hagibis were in the form of slope failures (65 cases) while some were debris flows (8 cases), which in turn affected many residential houses. This is not surprising because the Japanese archipelago has predominantly rugged and mountainous terrain and therefore the lack of habitable area has driven people to modify the hillside for residential purposes over many centuries. When the typhoon dumped significant amount of rainfall in such mountainous areas, both slope failures and debris flows were mobilised.
Slope failures occurred in at least eight sections along National Highway 413 in Sagamihara, Kanagawa Prefecture. The failures were triggered when the typhoon dumped more than 700 mm of rain in the area. Figure 13a shows one of the failed sections, which is part of the Olympic cycling road racecourse scheduled on 25/07/2020 for men and on the following day for women. About 30 km of the Olympic road racecourse runs through Sagamihara. On the other hand, Figure 13b shows a 23 m section of the track ballast between Miyanoshita and Kowakidani stations, located in the tourist town of Hakone, which disappeared because of slope failure. The intense rainfall, topping 1,000 mm in Hakone, caused major disruptions in at least 20 locations of the tracks, operated by Hakone Tozan Railway Co.
Figure 13: Typical sediment disasters: (a) slope failure along National Highway 41 in Sagamihara, Kanagawa Prefecture (from https://www.japantimes.co.jp); (b) slope failure affecting a railway track in Hakone (from https://mainichi.jp).
6 EFFECTIVENESS OF SOFT-TYPE COUNTERMEASURES
Because of its location, which is characterised by intense rainfall, rugged topography and criss-crossing river networks, Japan has experienced many flood events since ancient times. Learning from experience, the national government and administrative authorities have developed mitigation measures to minimise the impact of flood-induced disasters. Aside from hard-type (or structural) countermeasures (e.g. flood protection works, embankment and levee constructions, drainage system provisions, etc.), soft-type (or non-structural) countermeasures (e.g. rainfall and flood forecasting, flood/rain warning systems, hazard mapping, etc.) have been developed and implemented. The latter has been tested during the passage of typhoon Hagibis.
6.1 Emergency Warning System
In addition to warnings, advisories and other bulletins associated with possible catastrophes that may be caused by extraordinary natural phenomena, the Japan Meteorological Agency (JMA) has begun issuing emergency warnings since 30 August 2013. This is to notify people if the scale of a particular natural phenomenon would be of extraordinary magnitude such that it will far exceed the warning criteria (JMA 2013). Figure 14a illustrates the overview of the emergency warning in case of heavy rain.
Figure 14: (a) Overview of the emergency warning system for heavy rain (from JMA 2013); and (b) the simplified 5-level warning system for floods and landslides (from https://www3.nhk.or.jp).
In early 2019, JMA rolled out a new five-level disaster warning scale to be used for floods and landslides. Designed to simplify the existing system and to reduce casualties by speeding up evacuations, the warning system includes clear instructions tied to the scales. Figure 14b shows the 5-level warning system. For example, Level 4 means all residents must evacuate, while the elderly and physically challenged must evacuate at Level 3. The emergency message is sent to smartphones of residents (causing the devices to ring so loudly to catch attention) and is also delivered via loudspeakers.
During the passage of Hagibis, the author received seven emergency alerts in his mobile phone, starting as early as 3:42 pm of 12 October (Figure 15a). The warning was Level 4, advising residents in the ward (where I was staying) to evacuate because the water level in Tama River has reached dangerous level. The same warning advised to seek information from various media (television, ward homepages) and listed possible evacuation centres. If going out is dangerous, it advised residents to move to second floor or higher. Later at around 10:34 pm, the warning issued was the highest at Level 5 (Figure 15b), and residents were advised to take measures to protect their lives. A couple of minutes after, another emergency alert was received (Figure 15c), warning residents in the ward that water has started to gush out though the portion of the Tama River without levee (as discussed in Section 3).
Figure 15: Screenshots of emergency alerts received at various times during the passage of typhoon Hagibis.
Having just arrived in the ward 10 days earlier and with his NZ mobile phone not registered anywhere, the author was amazed at how efficient this emergency alert system worked. Such constant alerts and updates provided to residents are indispensable, especially during the times when the power was cut-off and there was no other source of information available. (Note: It was fortunate that the author can “decipher” the emergency message, written in Japanese; many foreign residents complained in social media because they could not understand what the message was all about, delivered in an emergency situation such as during this very strong typhoon.)
After the typhoon passed, the media reported that at least six million people were told to evacuate their homes. If all those people heeded the emergency alert, flood-related casualties would have been significantly minimised. Obviously, not everybody believed the emergency messages. Possible reasons are: (1) people tend to underestimate the likelihood of a disaster and its possible impact, i.e. there is a common human tendency to believe things will be fine under any circumstances; and (2) during the passage of previous typhoons, relatively high warnings at levels 3 or 4 may have been issued quite frequently and the “expected disasters” did not occur in those cases; hence, people may have ended up ignoring the warnings during this particular typhoon. In any case, no matter how many warnings JMA and other agencies issue, actually evacuating is up to each individual. Nevertheless, the warning system gives opportunities for people to take action to save themselves.
6.2 Hazard maps
Because large urban areas, such as the seaside cities of Tokyo and Yokohama, have several rivers traversing them, they are susceptible to flooding after intense rains. For this purpose, local government websites have available hazard maps that delineate the area most vulnerable to flood, particularly when overflowing of major rivers occur. As an example, the hazard map of Setagaya Ward (Tama River version) is shown in Figure 16, which was published in 2016 by the MLIT’s Keihin River Office based on an estimated total 2-day rainfall of 588 mm in the Tama River basin. The map shows the expected inundation area, inundation depth, shelter, etc. when the Tama River levees break down during a heavy rain and flooding occurs. The map is available to residents in order to provide them with general information, including the location of evacuation shelters in case of emergency, so that they can discuss possible actions during evacuation with their family members.
Figure 16: Hazard map of Setagaya Ward (from https://www.city.setagaya.lg.jp)
Figure 17, on the other hand, compares the estimated flooded area near Futako-Tamagawa Station and the flood hazard map in the area. Reports indicate that the flood during the typhoon was several centimetres deep in the affected area, which correspond to the region expected to undergo 5~10 m of floodwater in the event of design overflow of the Tama River. While the actual rainfall intensity did not reach the design rainfall used in developing the map and Tama River did not actually breach the levee, as the water found its way through the section with missing levee, it can be surmised that the spread of the floodwater coincided well with the map projection.
Figure 17: Side-by-side comparison between: (a) estimated flooded area in Futako-Tamagawa (base map from Google Maps); and (b) the flood hazard map in the area, enlarged from the full-scale hazard map (from https://www.city.setagaya.lg.jp)
Similar observations have been reported in some news bulletins, notably in areas adjacent to Chikuma River in Nagano City. According to Mainichi Shimbun (www.mainichi.jp), the predicted flood areas in the hazard map compiled and circulated by the Nagano Municipal Government corresponded almost exactly with the locations that actually flooded.
It appeared that many residents in areas that were flooded failed to use local hazard maps to consider evacuating because they did not know the maps existed. Hence, there is a need for local administrative bodies to consider more effective ways to distribute information about hazard maps and for residents to make good use of them to devise evacuation plans in anticipation of flooding.
7 KEY OBSERVATIONS AND LESSONS LEARNED
From the ground survey and information collected from various sources, the following are the main observations from this large-scale disaster.
- The amount of rain that fell during the passage of typhoon Hagibis was unprecedented. More than 1,000 mm of rain fell in some areas, three times more than the monthly average rainfall intensity. This caused extensive flooding in many places due to overflowing of rivers through floodwalls and levees, destruction of some river embankments, and inland flooding when the rainwater was not able to drain out. The intense rain and flooding necessitated the issuance of mass evacuation advisories. It also paralysed many of the region’s transport systems and shut down commercial facilities.
- Honed up by many last-scale disasters in the past, various disaster mitigation systems that have been developed appeared to work. Many of the flood control measures, such as the levees along the Tama River, were able to withstand the large amount of water collected in rivers. National and prefectural authorities issued warnings and advisories for floods and sediment disasters before and during the passage of the typhoon. In the days and hours prior to the typhoon’s arrival, television/radio stations provided important information and advised residents to take precaution. Although there were still casualties and some hiccups in local responses, it was clear that the lessons of past disasters have been gradually put to use. However, the intensity of the typhoon and the amount of rainfall it brought were unprecedented, almost beyond what the systems were designed.
- Many commercial establishments, railway networks and other public transportation operators announced very early the cancellation of their operations with extraordinary speed, i.e. 2-3 days before the typhoon’s arrival. This action minimised public exposure to the extreme weather event, although the fact that the typhoon struck over a weekend when the number of commuters was fewer also helped.
Based on the experience, both national and local authorities need to consider the following issues.
- As a country frequently experiencing natural disasters, Japan has developed various disaster mitigation measures. These include “hard” measures, such as strengthening river dikes, and “soft” measures, such as promoting evacuations of hard-hit areas. However, community planning and urban structures designed on earlier assumptions may be inadequate to deal with intensifying natural disasters.
- While many river levees withstood the typhoon well, there is likelihood that, considering the progression of climate change, more intense typhoons would strike Tokyo and other metropolitan areas in the future and those levees are not designed to handle larger volume of water. There is a need to for a thorough review of the nation’s flood control countermeasures in order to prepare for such increasingly abnormal weather.
- Businesses and schools must put plans in place for strong typhoons and other disasters. At the same time, homeowners need to realistically assess the geographical risks and other features of their homes and agree on measures to take to protect lives when disasters occur.
While the national government and other administrative authorities have implemented measures that were designed to minimise lost in life and property, there are limitations to what they can do. Many of the casualties in this typhoon died while trying to escape the rising flood; however, they have been adequately advised, with emergency alerts issued hours (or even days) before the typhoon struck. Each individual citizen need to be aware of what would happen in the face of such massive disaster and should adjust their behaviour accordingly.
8 RELEVANCE TO NEW ZEALAND
Widespread flooding induced by intense rainfall and levee breach is not new to New Zealand. For example, in April 2017, water from the Rangitāiki River breached a stopbank protecting Edgecumbe, and caused extensive flooding across the whole town. In March 2019, a local state of emergency was declared in Westland District when record rainfall hit the area and the flood destroyed a bridge and several roads. As extreme weather is becoming more frequent and catastrophic due to global warming, NZ’s coastal regions are becoming more prone to storms and flooding, while inland regions are facing heatwaves and flash flooding.
Hence, all the lessons learned from the impact of typhoon Hagibis discussed above are equally applicable to New Zealand setting. Various flood protection and sediment disaster measures designed from past experiences need to be re-evaluated. While structural (hard-type) approaches, including building stronger flood defence systems like stop banks, drainage systems, etc. would work, there appears to be a limit to their effectiveness when viewed from the perspective that intense rainfall is becoming the new norm. Non-structural (soft-type) approaches, such as improved flood warning systems, can significantly reduce the impact of flooding and potentially save more lives. Efforts should also be focused on increasing public awareness by educating people about these disasters and their potential risks so that when they happen, people could act appropriately.
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