APPLICATION OF SATELLITE DATA FOR FLOOD MONITORING H. Bach a, U. Dierschke a, F. Appel a, K. Fellah b, P. de Fraipont b a
VISTA Geowissenschaftliche Fernerkundung GmbH, Gabelsbergerstrasse 51, 80333 München Tel. +49 89 5238 9802 ; Fax. + 49 89 5238 9804 ;
[email protected] www.vista-geo.de b
SERTIT, Parc d’Innovation Boulevard Sébastien Brant 67400 Illkirch-Graffenstaden, France http://sertit.u-strasbg.fr
ABSTRACT Flood monitoring from satellite data provides the opportunity to quickly and precisely overview flooded areas. The extent of the flooding and affected areas can be delivered to authorities, civil protection agencies or insurances. Delivered evaluations include information to better estimate risk in future and to prepare for protection measurements. As demonstration product the conducted mapping of the Elbe flood (08/2002) is presented. For the evaluation of flood areas between Dresden and Neu Darchau (approx. 450 km), satellite data of different systems (Landsat, SPOT, ERS) were used to derive a "complete" flood extent data set. For different sections, satellite data of different dates were used to perform the best fit of product to the flood peak along the river. The use of data from radar-satellites, as ENVISAT and ERS, even allows the view through clouds. The evaluation of flooded areas enable improvements for planning as e.g. the declaration of risk areas. Large area evaluations allow to verify and improve model calculations. By adding further information, various applications open up. Additional information, as land use (industrial areas, settlements, agriculture, woods) are intersected with flooded areas. This intersection results into the detection of affected areas (and economic values), which have to be protected against flood events in future. Multitemporal evaluation of flood events (using a set of images of different days in succession) document the spatial and temporal dynamic of a flood event. They allow to better rate damages (e.g. contamination of soils) as well as consequential costs (e.g. production loss, caused by impassable access roads). In the framework of the project "Plain Flood Monitoring", supported by ESA, products and services in respect to flood events are offered. Product and service definitions are developed in partnership with addressed user groups (i.e. civil protection agencies and insurance companies). As French partner in the project the company SERTIT, supplies a 24-hours service to make flood monitoring available in best time. Keywords: flood monitoring, damage assessment, Earth Observation, SAR, Elbe
1. INTRODUCTION Flooding is one of the most devastating natural hazards on the Earth's surface. It causes damage each year to cities, roads and agriculture with a very high cost in lives and to the economy as a whole. In Europe, floods are the most frequent type of natural disaster and reach 75% of all insurance payments due to natural disasters. The indirect economic and social effects, more difficult to quantify, underline the importance of this type of natural hazard and the steps taken to alleviate the effects and associated risks. The major entities concerned by floods and their consequences are therefore administration organizations, in charge of practical in-situ flood management, and the insurance sector dealing with the economic impacts and consequences of floods through their risk management portfolios. Administration organisations operate before, during and after the crises events aiming to alert, estimate and monitor the impact. At political and legislative levels, the administrative organizations have put in
place policies aimed at providing better risk-related information to citizens, improving flood prevention measures through legislation and organizing risk management through decentralized state authorities and formalizing procedures governing the different phases of crisis management. At the operational level, Civil Protection Authorities (CPA), responsible for civilian protection, need information on flooded areas urgently while a flood happens. Actually, and besides the meteorological reports that are delivered by Meteorological services, this flood related information, provided to the CPA by flood warning services, result from the case to case comparison process of in-situ punctual river stations producing data, height and flow of the river, for the equipped basins, and pre-established flood prediction map extents, model based pre-calculated maps or historical events based flood maps, which can be refined, when necessary and possible, through expensive aerial-photo campaigns. Nevertheless, this flood related information collected using conventional methods do not show the real situation but a predicted one. Due essentially to property damage implications, private insurance companies are also concerned directly by floods. Insurance companies need information on flood risk zones and economic vulnerability for calculating insurance premiums and to quantify the damage after a flood event. This information is currently gathered together using conventional data collection methods (population statistics, administrative documents -like PPR- consultation, damages claims, …). As geographical reference mostly the ZIP-codes are used. More seldom classical geo-information analysis based on river network databases associated with simplified hydraulic models.
2. APPLICATION OF SATELLITE BASED FLOOD MAPS FOR CIVIL PROTECTION AND INSURANCES Earth Observation (EO) can contribute to improve flood management. Space-derived information can be a valuable source of up-to-date geo-information for flood risk mapping (prevention), for flood forecasting (preparedness phase) and for the crisis and post-crisis phases dealing with flood extent mapping and damage assessment. EO missions like ENVISAT, ERS, Radarsat, and SPOT can play a key role in natural risk management. This is especially the case for the all-weather and space borne SAR missions which allow the provision of timely information on regions affected by natural disasters. Furthermore, the combination within a GIS of the EO based flood map extent with optical high or very high resolution EO data (SPOT5, Ikonos, …) or derived land use maps under normal hydrological conditions are used for vulnerability assessment during the prevention phase. Flood related, derived information for flood damage assessment as well as for flood risk zonal mapping are provided which are of interest to both CPA’s (and their related services in charge of the prevention phase) and insurances. During a crisis, CPA, need up-to-date overview information on flooded areas. Periodic analyses of a complete scenario, following the progress of the flooded extent with respect to the location of infrastructure and of logistical means to support operations, guarantees an improved decision making process. It supports the elaboration of emergency plans and allows the visualization of events at emergency headquarters thus anticipating affected areas and the event’s evolution. Once the emergency is over, the Civil Protection Authorities require an assessment of both the extent of the flood damage to the general landscape and particular features like, communication networks, urban infrastructure, cultivated fields, etc, to help plan recovery actions. Less stringent temporal requirements characterize the needs of Insurance Companies, interested in geoinformation for calculating insurance premiums and compensation. The analysis of EO derived flood extent maps from past events enables an improvement in the delimitation and risk assessment of flood risk zones. The identified at-risk areas are then combined with economic vulnerability information in order to establish insurance fee criteria. In the aftermath of a crisis, damage quantification is also made easier by jointly analysing the spatial trends of flood duration, the land use within the affected area and the company contracts with their customers.
3. THE ELBE EVENT IN 2002 Flood monitoring from satellite data proved to provide the opportunity to quickly and precisely overview flooded areas. The Elbe flood in August 2002 was mapped using a set of satellite data. For the evaluation of flood areas between Dresden and Neu Darchau (450 km), satellite data of different systems (Landsat, SPOT, ERS) were used to derive a "complete" flood extent data set. For different sections, satellite data of different dates, from 17.08. – 25.08. 2002, were used to perform the best fit to the flood peak along the river. The use of data from radar-satellites, as ENVISAT and ERS, even allows the view through clouds. Some technical information regarding the resulting flood extent products for the Elbe event in August 2002: •
The flooding areas have been produced from 10 satellite images.
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The spatial resolution of the data set is 30m.
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The total flooded area mapped amounts to almost 1000 km².
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The length of the mapped river section is approximately 450 km.
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The number of affected ZIP-areas are approx.: 100
Fig. 1 shows a section of flooding along river Elbe at Torgau (centre of image) with a Landsat TM7 image from as background. Impressive is the effect of broken dams, that can be seen in the lower right part of the image, where flooding broadens up to approx. 13 km.
Fig 1: Flooding at river Elbe at Torgau, with broken dams in the lower right part of image; background image: Landsat TM7 from 20.08.2002
Support of damage assessment can be conducted using flood extent overlayed with a city map in high resolution. Fig. 2 shows an example for the city of Dresden. Dresden was one of the Cities in Germany most seriously affected by the Elbe flood. During the maximum flood extent a SPOT acquisition was programmed and evaluated. The maximum flood extent was classified and superimposed within a Dresden GIS system. A German insurance company was interested in obtaining a street map where the flooded area was clearly highlighted. These maps were handed over to the local insurance advisors in order to optimise their damage assessment. The hatched area in Fig. 2 is the flooded area; which has been expanded by 50 m with a dashed buffer-line in order to show the spatial accuracy.
Fig. 2: Map showing the flood extent overlayed with a map of a subset of the city of Dresden (flood area is displayed as hatched signature).
The evaluation of flooded areas enables improvements for planning as e.g. the declaration of risk areas. Large area evaluations allow to verify and improve model calculations. By adding further information, various applications open up. Additional information, as landuse (industrial areas, settlements, agriculture, forest) are intersected with flooded areas. This intersection results into the detection of affected areas (and economic values), which have to be protected against flood events in future. E.g. the evaluation of affected settled areas per ZIP-code area allows an insurance to estimate losses for a certain flood event. Examples for such a GIS application are given in Fig. 3 for the Elbe event. For each ZIP-code the percentage of flooded areas related to the area of the ZIP-code region and the area of the settlements in the ZIP-code regions are calculated and illustrated. Obviously the settlements are statistically less affected by floods except of the Dresden region. Multitemporal evaluation of flood events (using a set of images of different days in succession) document the spatial and temporal dynamic of a flood event. They allow to better rate damages (e.g. contamination of soils) as well as con-sequential costs (e.g. production loss, caused by impassable access roads).
Fig. 3: Evaluation of flooding per ZIP-code areas (left) and in percentage of settled areas (right) along river Elbe for the 2002 event
4. CONCLUSIONS Flood monitoring from satellite data proved to provide the opportunity to quickly and precisely overview flooded areas. Through multitemporal analyses also the duration of a flooding can be determined. The extent of the flooding and affected areas is delivered to authorities, civil protection agencies or insurances. Combination of the flood layer with other GIS information e.g. land use and flood risk allows the generation of value added products. Delivered evaluations include information to better estimate risk in future and to prepare for protection measurements. The demonstrated service is supported by the “Operational Centre for Plain-Flood Monitoring” set up by SERTIT. The centre intends to provide a 24-hour service delivering products. Civil protection authorities can obtain rapid flood dynamic maps - within 6 hours of data reception - and flood impact on specific landscape features – within 12 hours of data reception.
ACKNOWLEDGMENTS This work is part of the project "Plain Flood Monitoring", supported by the European Space Agency ESA within the framework of Earth Observation Market Development. The projects targets to develop and offer products and services in respect to flood events. We are grateful that our work is strongly supported by the users i.e. the civil protection agency EMZ-COZ of Metz, and the reinsurance Swiss Re.
