In Detail: Flood Mapping with Radar Imagery and Digital Terrain Models

Flood mapping in urban areas poses significant challenges for Synthetic Aperture Radar (SAR) sensors due to limitations in detecting water in regions characterized by dense vegetation, urban infrastructure, or complex surface conditions. These limitations include reduced sensitivity in vegetated or built-up areas and water-like backscatter effects on smooth, dry, or snow-covered surfaces. The Global Flood Monitoring (GFM) platform employs Sentinel-1 SAR backscatter data for automated flood delineation but recognizes the constraints posed by such conditions. To address these challenges, the GFM has integrated an exclusion mask that highlights regions prone to SAR-based misclassification.  

This recommended practice introduces a novel algorithm developed by the Joint Research Centre of the European Commission that combines SAR-derived flood layers with digital terrain models and the GFM exclusion mask. By leveraging Digital Terrain Models (DTMs), water depth calculations and hydrodynamic propagation models are applied to infer flood conditions within exclusion mask areas, enhancing the reliability of flood extent delineations. 

Input Data: 

  • GFM outputs for a flooded Area of Interest: 
    • Flooded Area
    • Permanent and Seasonal Water Bodies 
    • Exclusion Mask 
  • Digital Terrain Model 

 Software: 

  • Global Flood Monitoring Database 
  • Python 
  • QGIS or other GIS software 

This practice can be used for any area with major flooding. The practice is especially relevant for large floodings, that extend into cities and vegetated areas, like forests and agricultural fields. 

Strengths: 

  • The approach is based on physics and the actual topography of the area. That means it overcomes many limitations associated with satellite imagery. 

Limitations: 

  • The greatest limitations of the practice come from the spatial resolution and accuracy of the input flood delineation and of the Digital Terrain Model. 

  • The quality of the output is dependent on the number of flooded pixels, provided with the input flood delineation. 

Hawker, Laurence; Uhe, Peter; Paulo, Luntadila; Sosa, Jeison; Savage, James; Sampson, Christopher; Neal, Jeffrey (2022): A 30 m global map of elevation with forests and buildings removed. In Environ. Res. Lett. 17 (2), p. 24016. DOI: 10.1088/1748-9326/ac4d4f. 

Betterle, Andrea; Salamon, Peter (2024): Water depth estimate and flood extent enhancement for satellite-based inundation maps. In Nat. Hazards Earth Syst. Sci. 24 (8), pp. 2817–2836. DOI: 10.5194/nhess-24-2817-2024. 

Expert Flood Monitoring Alliance, McCormick, N., Salamon, P., Global Flood Monitoring (GFM) – Product User Manual. European Commission. 2023. 

https://www.un-spider.org/sites/default/files/RP_UN-SPIDER_logo_1_3.png

The workflow can be divided into two sections:  
1) Preparedness before the flood and 2) Response after the flood. 

 

Recommended Practice: Flood Hazard Assessment
Recommended Practice: Flood Hazard Mapping
Recommended Practice: Flood Mapping and Damage Assessment Using Sentinel-1 SAR Data in Google Earth Engine
Recommended Practice: Flood Mapping and Damage Assessment using Sentinel-2 (S2) Optical Data
Recommended Practice: Radar-based Flood Mapping
Sentinel 1 - SAR Dataset (ESA)
Sentinel 2 - Imagery (ESA)
Google Earth Engine (Google)
Python
https://www.un-spider.org/node/13452
https://www.un-spider.org/advisory-support/recommended-practices/recommended-pr…

Recommended Practice: Flood Mapping with Radar Imagery and Digital Terrain Models

This recommended practice introduces a novel algorithm developed by the Joint Research Centre of the European Commission that combines SAR-derived flood layers with digital terrain models and the Global Flood Monitoring (GFM) exclusion mask. By leveraging Digital Terrain Models (DTMs), water depth calculations and hydrodynamic propagation models are applied to infer flood conditions within exclusion mask areas, enhancing the reliability of flood extent delineations. 

https://www.un-spider.org/node/13451
https://www.un-spider.org/node/13452
Recommended Practice: Flood Hazard Assessment
Recommended Practice: Flood Hazard Mapping
Recommended Practice: Flood Mapping and Damage Assessment Using Sentinel-1 SAR Data in Google Earth Engine
Recommended Practice: Flood Mapping and Damage Assessment using Sentinel-2 (S2) Optical Data
Recommended Practice: Radar-based Flood Mapping
Google Earth Engine (Google)
Python

The objective of this practice is to improve flood maps with DTMs. This is necessary, as Synthetic Aperture Radar (SAR) sensors are not sensible to flood water on every land surface / land use class equally. That is especially the case in urban and densely vegetated areas. 

During this practice, flood delineations from GFM are used and improved. Furthermore, water depths are calculated for the flooded areas, which is valuable information for rapid response teams as well as for estimating economic loss. Water depth cannot be estimated by satellite-based flood mapping (Betterle and Salamon 2024). 

Relief & Response
Flood
Sri Lanka

In October 2024, many provinces of Sri Lanka have been experiencing heavy rainfall and strong winds, causing floods and severe weather-related incidents that resulted in casualties and damage (Sri Lanka: Floods - Oct 2024 | ReliefWeb, accessed: 22.11.2024).  

Nepal, Sri Lanka and Bangladesh experience an increased flood risk and extend of the monsoon season from June to October, with heavier rainfall potentially disrupting agricultural activities and affecting infrastructure (OCHA, 19.11.2024). 

This practice can be applied to flooding events globally. It requires little to no knowledge of GIS and Remote Sensing. A basic understanding of Python code and virtual environments in Python is helpful to the analysis but is no requirement. 

Sentinel-1C

ESA
No

Sentinel-1 is a two satellite constellation with the prime objectives of land and ocean monitoring. The goal of the mission is to provide C-Band Synthetic Aperture Radar (SAR) data continuity following the retirement of ERS-2 and the end of the Envisat mission.
To accomplish this the satellites carry a C-SAR sensor, which offers medium and high resolution imaging in all weather conditiions. The C-SAR is capable of obtaining night imagery and detecting small movement on the ground, which makes it useful for land and sea monitoring.

SAR
7years

World Space Forum 2024 in Bonn kicks off with a Vision for a Sustainable Space Future

Group Picture of World Space Forum

The World Space Forum 2024, organized by the United Nations Office for Outer Space Affairs (UNOOSA) in collaboration with Germany, Peru and the United Arab Emirates, has officially opened its doors in Bonn, Germany, setting the stage for three days of transformative discussions on international collaboration and space sustainability.

UN-SPIDER ZFL Expert Meeting: Confronting the challenges of natural hazards and climate change - solutions from the space community

UN-SPIDER ZFL Expert Meeting

UN-SPIDER and the Centre for Remote Sensing of Land Surfaces of the University of Bonn (ZFL) joined forces to organise the UN-Spider/ZFL Expert Meeting ‘Confronting the Challenges of Natural Hazards and Climate Change: Solutions from the Space Community’ on 02 December 2024. The Expert Meeting was organised as a pre-event to the United Nations World Space Forum 2024, also held in the UN Campus in Bonn, Germany.

WMO and UNDRR launch Status report on Multi-Hazard Early Warning Systems

Cyclone Fany in the coast of India, Image courtesy of ESA

Across the globe, natural hazards have triggered widespread death and destruction, with the human and economic impact of disasters continuing to rise. Despite ongoing advancements in disaster risk governance and comprehensive risk management, Least Developed Countries (LDCs), Landlocked Developing Countries (LLDCs), and Small Island Developing States (SIDS) continue to experience devastating impacts, eroding hard-won development gains.

https://library.wmo.int/idurl/4/69085

Handbook on the use of risk knowledge for multi-hazard early warning systems 2024

Handbook Multi-Hazard Early Warning Systems

Since several decades ago, developed and developing countries began to implement early warning systems to reduce the impacts of natural hazards such as severe weather, floods, and tsunamis. In March 2015, 187 Members States of the United Nations committed to substantially increase the availability of, and access to multi-hazard early warning system as one of the targets included in the Sendai Framework for Disaster Risk Reduction 2015-2030.

Handbook on the use of risk knowledge for multi-hazard early warning systems 20…

New book chapter about the Global Flood Awareness System (GloFAS)

Global Flood Awareness System (GloFAS)

Floods are impacting many communities around the world, triggering devastating disasters as in the case of the recent floods in Southern Brazil in May 2024, in the South Eastern United States in September 2024, and more recently in Valencia, Spain at the end of October 2024.  Since several decades ago, early warning systems have been implemented to minimize impacts in case of floods and other hazards.  Flood forecasting is one of the key essential elements of any flood early warning systems.

Global Flood Awareness System - GloFAS News

Atlas of the Human Planet 2024 launch event

This is event is available for participation on an ongoing basis
Atlas of the Human Planet

The launch event of the Atlas of the Human Planet 2024 will be an occasion to explore how information on population and human settlements derived from Copernicus satellites can help tackle a broad spectrum of societal challenges.

The geography of humans on the planet defines many issues of security, environment, economics and equality. The diversity of cities and rural areas also offers models for the change necessary to adapt to a changing climate and geopolitical landscape.

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Sustainable Development
Urban Development
disaster risk reduction
Researchers and academics; data providers; national and international public authorities addressing population, regional and urban policies; NGOs and anyone interested in the analysis of human settlements and its population.
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10/30/2024, 12:00am
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European Comission, Joint Research Centre

Atlas of the Human Planet - JRC
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