Tropical Cyclone Maria. Inventory of landslides and flooded areas

Name: Tropical Cyclone Maria. Inventory of landslides and flooded areas
Creator: United Nations Satellite Centre (UNOSAT)
License: http://www.opendefinition.org/licenses/cc-by

✦ DISCOVERED BY ORIONAIUnited Nations Satellite Centre (UNOSAT)EnvironmentDominican Republic

Dominican Republic

About this dataset

.A large scale landslide inventory was carried out by a team from the University of Twente, use of 5 scenes of Pléiades satellite imageries with resolution of 0.5m, which were obtained in September 23 and October 5 after the hurricane, made available through UNITAR-UNOSAT. Apart from these also a series of Digital Globe Images were used that were collected for the Google Crisis Response through a KML layer. The images were visually interpreted by image interpretation experts, and landslides were mapped as polygons, separating scarp, transport and accumulation areas, and classifying the landslides in types. A total of 9,960 landslides were identified, which include 8,576 debris slides, 1,010 debris flows and 374 rock falls, with area of 7.30km2, 2.50km2, and 0.50 km2 respectively. The whole area of landslide is 10.30 km2, which covers 1.37 percent of the island. The source of landslides is 3.30km2, and the other 7.0 km2 is transportation and deposition area. Almost all of the rivers flooded due to intensive precipitation. The flooded area is 13.03km2, which covers 1.74% of the island. Dominica will face some new problems for mountain hazards in the coming years, as many of the fresh scarps may produce more debris, and many tree trunks are still on the slopes or in the river channels. With so many fresh landslides in the upper catchments, it is likely that debris flows will be triggered with rainfall thresholds that are substantially lower than before the hurricane. Hurricane Maria damaged the forest cover dramatically, which changed the conditions for hazard initiation. Without the protection of vegetation, more new shallow landslides could happen in the near future. A series of cascading hazards may happen, for example landslides or debris flow blocking rivers and resulting in outburst floods. Therefore more detailed evaluation of the post-Maria hazard and risk situation is very important.

Documentation

UNOSAT datasets and maps are produced using a variety of methods. In general, analysts closely review satellite imagery, often comparing two or more images together, and determine notable changes between the images. For damage assessments, refugee or IDP assessments, and similar analyses, these changes are then manually documented in the vector data by the analyst. For flood extractions, landcover mapping and similar analyses, a variety of automated remote sensing techniques are used to extract the relevant information which is then reviewed and revised as necessary by the analyst. In all cases, resulting data is then loaded into a standardized UNOSAT geodatabase and exported asshapefiles for dissemination. This is a preliminary assessment and has not yet been validated in the field. It is important to consider the characteristics of the source imagery used in the analyses when interpreting results. For damage assessments it should be noted that only significant damage to the structural integrity of the buildings analyzed can be seen in imagery, while minor damage such as cracks or holes may not be visible at all. For flood extractions using radar data it is important to note that urban areas and highly vegetated areas may mask the flood signature and result in underestimation of flood waters. Users with specific questions or concerns should contact unosat@unitar.org to seek clarification.

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Tropical Cyclone Maria. Damage Density in the North-western Part of Dominica

This map illustrates potentially damaged structures and buildings in the north-western part of Dominica (St. John, St. Andrew, St. Peter & St. Joseph Parishes) as detected by satellite images acquired after landfall of the Tropical Cyclone Maria-17 on 19 September 2017. UNITAR-UNOSAT analysis used a Pleiades images acquired on 20 and 23 September 2017 as post imagery. Within the extent of this map UNITAR-UNOSAT identified in the cloud free zones about 3723 potentially damaged structures. Taking into account the pre-building footprints provided by Humanitarian OpenStreetMap, this represents about 70 % of the total number of structures within the analysed cloud free areas. This is a preliminary analysis and has not yet been validated in the field. Please send ground feedback to UNITAR-UNOSAT.