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Space technologies assist rapid response to cyclone disaster in Pacific Islands

 

Dr Richard Teeuw, University of Portsmouth

A research team at the University of Portsmouth spent much of Easter processing space satellite images of South Pacific islands that were recently hit by Tropical Cyclone Harold, one of the most severe storms ever to hit the region.

Cyclone Harold started in the southern Solomon Islands on 2nd April 2020, becoming a Category 5 storm when it hit northern Vanuatu on 6th April and a Category 4 storm when it clipped southern Fiji on 8th April, finally heading to Tonga where it had faded to a Tropical Storm by 11th April (Figure 1).

 

The path of Tropical Cyclone Howard
Figure 1. The path of Tropical Cyclone Harold: from the Solomon Islands on April 2nd, to Vanuatu on April 6th and on to Fiji on April 8th (source: UNITAR/UNOSAT)

 

When Cyclone Harold hit the northern Vanuatu islands of Espiritu Santo and Pentecost, it was a Category 5 storm - the most powerful and destructive type of storm, with sustained winds of 215km/h and gusts of 235 km/h (1, 2). The satellite radar image shown below captures the eye of the storm's landfall on Pentecost island. The direct hit caused widespread devastation. 

 

The eye of the storm: radar imagery from the Sentinel-1 satellite captures the moment when Tropical Cyclone Harold arrived at Pentecost Island, Vanuatu, on April 6th 2020
Figure 2. The eye of the storm: radar imagery from the Sentinel-1 satellite captures the moment when Tropical Cyclone Harold arrived at Pentecost Island, Vanuatu, on April 6th 2020 (data source: the European Space Agency, ESA; processed by Dr Mathias Leidig of UoP)
Radar imagery from the Sentinel-1 satellite, showing areas flooded by Cyclone Harold (in blue), near Luganville, the second-largest town of Vanuatu
Figure 3. Radar imagery from the Sentinel-1 satellite, showing areas flooded by Cyclone Harold (in blue), near Luganville, the second-largest town of Vanuatu (data source: ESA; processed by Dr Nasos Argyriou of UoP)

 

The University of Portsmouth researchers who volunteered to assist the Cyclone Harold disaster response efforts are also working on the disaster risk resilience elements of the CommonSensing project. 

"We have been using satellite data to map hazardous terrain and vulnerable features in Vanuatu. It is a beautiful country, with very welcoming people, but Vanuatu is very susceptible to disasters. We are hoping that we can put our skills to effective use by rapidly assessing satellite imagery to map roads and bridges damaged by Cyclone Harold"

- Dr. Richard Teeuw, University of Portsmouth Team Leader

The team first used radar imagery from the Sentinel-1 satellite, which coincidentally was orbiting over Vanuatu when Cyclone Harold swept through. Satellite radar is particularly useful for detecting areas with flooding and landslides caused by major storms, due to radar being able to detect land surface features through cloud cover. The post-cyclone radar imagery shows areas of flooding on Espiritu Santo island, near Luganville, Vanuatu's second-largest town. Next, cloud-free visible and infrared imagery from the PlanetScope constellation of micro-satellites, collected a day or two after the cyclone had passed Vanuatu, was used for rapid assessments of damage. 

The University of Portsmouth cyclone damage mapping was done in conjunction with the UNOSAT team of the United Nations Institute for Training and Research (UNITAR) who specialise in rapid post-disaster damage mapping, particularly the severity and extent of building damage. UNOSAT coordinated other emergency mapping teams in the interpretation of 0.5m pixel satellite imagery for mapping damage to buildings from Cyclone Harold: thousands of buildings were mapped, as shown here. Rapid post-disaster situational information is essential for effective disaster response and the coordination of associated humanitarian relief efforts.

The focus of the research team was on rapidly mapping the impacts of a severe storm (flooding, erosion, landslides, debris flows) on critical infrastructure such as major roads and bridges. PlanetScope is able to provide daily satellite imagery of any given location, which enabled very useful and timely mapping of the damage, as illustrated by Figures 4 and 5. 

The imagery has some limitations: it is not detailed enough for mapping sub-metre features and the imagery cannot 'see' through cloud cover. The pixel size of PlanetScope imagery is typically 3.8 metres; while that is not sufficient for accurate assessment of damage to buildings, it is adequate for mapping areas of flooding, erosion and landslides, along with associated damage to roads or bridges. Then there is issue of cloud obscuring PlanetScope imagery, which is particularly problematic with storm-driven disasters. Fortunately, after a major cyclone, there are often days with cloud-free skies, and daily (sometimes twice-daily) orbital coverage of the satellites can then provide some relatively clear imagery. 

 

Figure 4. Before and after Cyclone Harold: PlanetScope satellite images of Pentecost Island. Note the rainforest hillsides stripped of tree cover, the river erosion, debris and sediment plume along the coast (data source: Planet Inc.; processed by Dr Richard Teeuw of UoP)
Figure 4. Before and after Cyclone Harold: PlanetScope satellite images of Pentecost Island. Note the rainforest hillsides stripped of tree cover, the river erosion, debris and sediment plume along the coast (data source: Planet Inc.; processed by Dr Richard Teeuw of UoP)
Figure 5. PlanetScope imagery of Pentecost Island, before and after Cyclone Harold, with the red arrow indicating a major road closed by a landslide (data source: Planet Inc.; processed by Dr Richard Teeuw of UoP)
Figure 5. PlanetScope imagery of Pentecost Island, before and after Cyclone Harold, with the red arrow indicating a major road closed by a landslide (data source: Planet Inc.; processed by Dr Richard Teeuw of UoP)
Figure 6. PlanetScope imagery of Espiritu Santos Island, before and after Cyclone Harold, with the red arrow indicating a major road bridge destroyed by flooding and debris flows (data source: Planet Inc.; processed by Dr Nasos Argyriou of UoP)
Figure 6. PlanetScope imagery of Espiritu Santos Island, before and after Cyclone Harold, with the red arrow indicating a major road bridge destroyed by flooding and debris flows (data source: Planet Inc.; processed by Dr Nasos Argyriou of UoP)
Figure 7. Part of a webmap produced by UNOSAT, showing damage from Cyclone Harold on Espiritu Santos Island. Locations of flooding, landslides and road or bridge damage are marked with the blue icons. The detailed mapping of building damage, shown in red, was carried out by UNOSAT (map source: https://unosat.maps.arcgis.com/apps/webappviewer/index.html?id=33f0d22c718745079fdc7 a635565223a)
Figure 7. Part of a webmap produced by UNOSAT, showing damage from Cyclone Harold on Espiritu Santos Island. Locations of flooding, landslides and road or bridge damage are marked with the blue icons. The detailed mapping of building damage, shown in red, was carried out by UNOSAT (map source: https://unosat.maps.arcgis.com/apps/webappviewer/index.html?id=33f0d22c718745079fdc7 a635565223a)

 

In Figure 7, the locations of flooding, erosion, landslides and road or bridge damage are marked with the blue icons; these were rapidly mapped by the University of Portsmouth team using PlanetScope imagery. The more detailed mapping of building damage, shown in red, was carried out by UNOSAT mappers using Maxar sub-metre imagery.

An estimated 160,000 people - about half the population of Vanuatu - were affected by Cyclone Harold, with at least 4 fatalities (3). Up to 20% of the population of Pentecost Islands, which was directly hit by the cyclone (Figure 2), are thought to be injured (4). The satellite-based damage assessments show that 90% of buildings on Pentecost Island were damaged, while 70% of the buildings in Luganville city were damaged (5).

This disaster is made further complicated by the current Covid-19 pandemic. As a result of this, there are significant emergency measurement issues, with multiple coinciding hazards and many potential increases in vulnerability. Cyclone Harold resulted in thousands of displaced people, many having to relocate to cramped storm shelters, at a time of Covid-19 containment and social distancing measures, with lots of increased public anxiety about health and safety. Covid-19 quarantine measures might also impact the activities of incoming international disaster response teams and slow the distribution of overseas aid cargo (4, 6).

References

  1. RNZ: Cyclone Harold updates 06/04/2020
    https://www.rnz.co.nz/news/pacific/413549/cyclone-harold-updates-storm-makes-landfall-on-vanuatu-s-santo
  2. GDACS 13/04/2020
    https://www.gdacs.org/report.aspx?eventtype=TC&eventid=1000663
  3. OCHA 08/04/2020
    https://reliefweb.int/report/vanuatu/vanuatu-tropical-cyclone-harold-humanitarian-snapshot-8- april-2020
  4. NewsHub 13/04/2020
    https://www.newshub.co.nz/home/world/2020/04/cyclone-harold-s-damage-to-vanuatu-exceptionally-bad.html
  5. The Guardian 10/04/2020
    https://www.theguardian.com/world/2020/apr/10/its-all-gone-cyclone-harold-cuts-a-deadly-path-through-vanuatu
  6. The Guardian 14/04/2020
    https://www.theguardian.com/world/2020/apr/14/cyclone-harold-relief-for-vanuatu-delayed-by-coronavirus-contamination-fears 

 

2020-04-24

Richard Teeuw

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