SYNERGY OF HIGH SAR AND OPTICAL DATA FOR FLOOD MONITORING; THE CENTRAL EUROPEAN FLOODS GAINED EXPERIENCE

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1 SYNERGY OF HIGH SAR AND OPTICAL DATA FOR FLOOD MONITORING; THE CENTRAL EUROPEAN FLOODS GAINED EXPERIENCE YESOU Hervé, ALLENBACH Bernard, ANDREOLI Rémi, BATTISTON Stéphanie, BESTAULT Claude, CLANDILLON Stephen, FELLAH Kader, MEYER Colette, THOLEY Nadine, de FRAIPONT Paul Sertit, Parc d'innovation, Boulevard Sébastien Brant, BP 10413, Illkirch Graffenstaden, France sertit@sertit.u-strasbg.fr, Abstract: After two years of flood mapping activities over Central Europe within the Charter Space and Major Disaster framework, a unique set of SAR and optical data has been constituted. It s allowing the analyses of their flood mapping and monitoring potential. It has been possible to assess and validate their flood mapping potential and accuracy over agricultural and forested areas. Finally, guidelines and recommendations in terms of resolution, polarisation and revisit frequency can be proposed, as well as scenarios for the synergestical exploitation of the future Sentinel 1 and VHR SARs. Keywords: Charter, Central Europe, Flood mapping, ENVISAT, Radarsat, SPOT, SAC C, VHR, Formosat, Romania, Bulgaria, Danube, March river 1. INTRODUCTION The past two years has seen Central Europe being badly affected by flood events, with Bulgaria and Romania being hit during the months of June and July 2005 ; and again, with Germany, the Czech Republic, Hungary and Austria in March and April The International Charter "Space and Major Disasters" was triggered to cover these events, in order to provide a unified system of space data acquisition and delivery. SERTIT was asked by the CNES and by the RISK-EOS RMS Activity 1, an ESA GMES Service Element, to process and interpret the EO data and to produce maps of the affected areas. 1 Within the framework of Risk-eos, during these two years, rapid mapping actions in Central Europe were shared between ZKI-DLR and SERTIT. For example in 2006 ZKI concentrated his work over German Elbe and Eastern Danube whereas SERTIT was involved in Austria, Check republic and Hungary as well as Bulgaria. Others actions, such as Romania 2005, have be done through bilateral agreement. Various sets of medium and high resolution ASAR and optical data have been synergistically exploited during these rapid mapping actions: Radarsat and SPOT 5 over the Siret River, eastern Romania, in July 2005 ASAR ENVISAT WSM, Radarsat, SPOT 5 and Formosat 2 images over the Elbe and Morava Rivers in the Czech Republic, in April 2006 ASAR ENVISAT WSM and APP modes, Radarsat, CONAE SAC-C, SPOT 4, over the March River, Austria, in April 2006 MODIS and Formosat 2 over the Danube River on the Bulgarian and Romanian borders, in April 2006 These data constitute a unique set of SAR and optical data allowing the analyses of their flood mapping and monitoring potentials in temperate European landscapes. 2. DATA SYNERGY FOR FLOOD MAPPING 2.1. SAR and optical synergy for flood mapping: the 2005 Siret flood example In July 2005, the Charter was triggered as devastating floods were affecting the Siret river watershed in the Vrancea Galati Judet (Eastern part of Romania). These dramatic events involved the death of 23 persons, the evacuation of more than persons. The economic losses were also very important as, 696 houses and 425 bridges have been destroyed; 2000 houses within 600 villages been affected. Between the 18 and 28 July, in collaboration with the Romanian Space Agency, 34 added value products, at scales ranging from 1: to 1:12500 were generated exploiting reference and crisis data of SPOT 2 & 5 optical data and of Radarsat standard mode images. If the SPOT2 multispectral data allow a good recognition of the affected areas, thanks to the swir band and the enhanced spatial resolution of the SPOT 5 data [1] it was possible to distinguish different events signatures such as flooded area, ie still under water, eroded river beds and mud deposit and others affected areas (Fig. 1). Broken dike Proc. Envisat Symposium 2007, Montreux, Switzerland April 2007 (ESA SP-636, July 2007)

2 where also identified exploiting merged THR Xj SPOT5 data. Comparison of the SPOT 5 derived product and Radarsat diachronic colour composite is very interesting. Firstly, even if less thematic differentiation are possible exploiting the Radarsat set, areas identified as flood affected are very similar on SAR and optical derived products. The areas under water 2.2. Medium and high resolution optical synergy for flood mapping: the March flood example As already mentioned, during the March river medium and high resolution optical data and Sar images were available. Figure 1: Siret flood event: flood map derived from SPOT 5 image acquired the 19 th of July 2005 In addition, if the major features on the Radarsat diachronic colour composite correspond to the inundated areas, yellow aureoles surrounding these are noticeable. These aureoles correspond to water drawn off, allowing the delimitation of former larger water extension [2]. This is well confirmed by the analysis of the SPOT 5 derived products. Figure 3: CONAE SAC SAC-C 175m data acquired on the 09 April 2006 Figure 2: July 2004 Siret flood event mapping based on Radarsat set exploitation. Dark blue corresponds to areas still under water whereas areas where water drawn off has been done drawn yellow aureoles surrounding blue patches Figure 4: Flood space-map dressed over the Charter Call 119 area of interest in Austria, realised using SPOT 4 (20 m) data acquired on the 09 April 2006

3 The first set consisted in SPOT 4, 20 m multipsectral data, and the SAC-C from CONAE the Argentina Space Agency. SAC C has a quasipolar sun-syncronous orbit at an altitude of 705 km. It is a 5 bands from blue to Swir sensors with a pixel size of 175 m with a observed swath of 360 km allowing a revisit time of 9/7 days. band data, less for C band, it have been possible to distinguish inundated forest from none affected one. On diachronical product, flooded forest characterised by highest backscattered signal appears in bright-yellow tone whereas the none affected forest are in a blue grey colour allowing recognition of inundated forest (Fig. 5). The Charter Call 119 was one of the first time that CONAE data were exploited in a crisis context in Europe. Comparison of products derived from SAC and SPOT 2 data is interesting. It shows that for a global overview of the flood situation, ie production of spatial map at scale of 1: , the CONAE derived product are very satisfaying, allowing a good general overview, without real omission and no flagrant commission. For derived product with a finer scale, SPOT 2 data were exploited and their spatial and thematic accuracy assessed Flood extent validation in a crisis context The Charter Call 119, over the March River, was also very interesting as offering an opportunity of in NRT validation of the Spot 4, 10m, derived flood extent exploiting aerial photos. A stripe along the march river, at a scale of 1: and oblique air photos were few days after the first product generation done available thanks to the Regional Alarm Centre Lower Austria and to the Austrian federal office of Meteorology and Survey.. This comparison was done over the city of the Durnkrut town, a badly affected village surrounded by agricultural field. The area recognized as flooded both on aerial stripe and SPOT 4 represent near 99%. The level of omission on the SPOT4 derived flood extent is very low as well as the commission error. These results are very similar to those obtained over the city of Dresden exploiting also SPOT data [3] Multi SAR synergy for flood mapping: the case of the Dyje and March rivers flood The sites of interest of the Charter calls 117, 118, 119, ie Elbe river (East of Pragua, Czech Republic), March river (Austria- Slovakia borders) have been covered by a single image acquired the 4 th of April 2006, an ENVISAT WSM. From this first available crisis data covering more a swath of 400 km, despite the lack of reference data, it has been possible to produce a first crisis map presenting a global overview of the flood situation over three countries. In addition to this WSM image, high resolution SAR data have been acquired, Radarsat standard mode and APP ENVISAT data, allowing combination and comparison. Thanks to double bounce effect, a phenomenon often described for L Figure 5: Multitemporal Radarsat colour composite with flooded forest (1), none affected forest (2). Blue patches correspond to open water of flooded areas Comparison between HH from Radarsat and from Envisat APP, HH, highlights the better discrimination potential of the single polarized data due to its better image contrast associated with a lesser level of speckle. Figure 6: Radarsat, gama map filtered, image, acquired over perennial water body the 12 of April 2006 with 46,50 of incidence angle

4 The the mono polarized Radarsat image has an ENL of 4 whereas the one for ENVISAT HH from APP data is only 1.8. On the HH single polarized data acquired with a high incidence angle, landscape elements within the flooded area (trees, banks, roads) are more visible than on the HH derived from a dual polarized data (Fig. 6-7). Figure 8: Formosat raw data acquired the , presenting an impressive EW compression. Figure 7: Envisat APP image, with HH, HH, HV respectively in RGB, acquired with an incidence angle of 23 over perennial water bodies 4. FLOOD MAPPING EXPLOITING FORMOSAT: PREFIGURATION OF THE FUTURE AGILE OPTICAL SATELLITE CONSTELLATIONS For the calls 117 and 121 2, in addition to Charter images, SPOT Image provided to SERTIT sets of Formosat II images. These data by their geometric characteristics, by their spectral bands and swath, 24 km, present lot of similarities with the future agile satellites such as the Pleiades constellation. These future satellites, with a nominal image swath of 20 km will be able to acquired data over much larger areas, delivering mosaic of images covering more than one hundred km [5]. Therefore, these mosaics will be constituted by images acquired with various across and along angles. Figure 9: Orthorectified Formasat data Formosat sensors, as the future Pleiades, get, in addition to a high resolution panchromatic, four multispectral bands ranging from the Blue to Near Infrared domain [5, 7]. Comparing with the SPOT system, the addition of a blue band allow firstly to produced real natural colour images and secondly to get more information on water characteristics. Associated with the high resolution, 2m, the three first bands allow generating highly document natural colour products (Fig 10). With its acquisition characteristics, ie huge satellite incidence angle: 46.50, very important viewing angle across track, and also variations along the along track 2.39, the Czech set of Formosat data sets allowed investigating this geometrical aspect as well as it incidences on thematic issues. It highlights the need of specialised tools in order to produce real ortho image, an absolutely necessary processing step during the generation of added value products [6]. 2 The charter images acquired within the framework of the 2006 Danube flood, call 121, were processed by DLR, the Romanian Space Agency, ROSA, and the Romanian Meteorological Institute [4]. Figure 10: Formosat fused image, 2m of spatial resolution, acquired the 25 of April 2006 over the inundated lowland of Lom town and affected harbour infrastructure (warehouses, docks and cranes).

5 Blue band brings lot information on water characteristics such as colour linked with suspended material content. Exploiting natural colour composite, analysis of water bodies depending of their colour allows distinguishing, stream, captive water, pluming which can be related with the break of levees (Fig. 11). Figure 13: Formosat data acquired the 18 of April over the Danube. Figure 11: Plume indicating a broken dike using natural colours Formosat image With their daily capability of revisit, future constellations, as well as the actual would allow monitoring of the flood events such as the flooding of agricultural areas, or of villages (Fig 12). During the Danube flood is was possible to see from the 17 of April to the 24 of April, the progression of the flow within large agricultural fields (fig.12-14). It also has been possible to monitor the arrival of the water inside the streets of the Rast village (Romania) and the damage over the harbour of Lom (Bulgaria). Figure 12: Formosat data acquired the 17 of April 2006 over the Danube. Black patches correspond to inundated parcels after the break of dikes Figure 14: Formosat data acquired the 25 of April over the Danube presenting an important extent of the inundated parcels comparing to the 18 of April images. (image size: 15 *8 km 2 ) DISCUSSSION AND CONCLUSION Along these Charter actions carried over temperate European landscapes, in 2005 and 2006, a unique set of SAR and optical data has been constituted allowing the analysis of their synergy for flood mapping and monitoring. Finally after two years of flood mapping activities over Central Europe, it is possible to propose guidelines and recommendations in terms of resolution, polarisation and revisit frequency. Concerning SAR data few remarks and recommendation can be done. The best polarisation for water recognition and extraction is HH. Plus, single polarized data are superior to dual one due to their better radiometric quality. For future actions, it can be advice to acquire systematically more images in WSM mode and to acquire it on a regularly basis in order to set up an exploitable archive of up dated reference data. These WSM images from ENVISAT provide a good idea of the potential of the future Sentinel 1 images when acquired in the back ground mission mode. With the coming launch of VHR SAR, Cosmo Skymed and Terra SAR, WSM image either from ENVISAT than Sentinel 1, would provide a unique source of

6 continuous information at regional scale. Whereas the VHR SAR resource would be fully concentrated over smaller sensitive areas, such towns, industrial areas which are major sources of pollution in case of flooding, as well as nuclear plant. Same remarks, concerning future scenarios of synergistical exploitation, can be done for the medium, high and very high resolution optical images, such as the SAC C from COANE and SPOT. One key issue of these Charter actions was the possibility of assessing the accuracy of the crisis products. It is as been shown that there is a great similarity between inundated surfaces recognized from high resolution satellite optical imagery and those derived from more conventional airborne cameras. And other key issue was the possibility of demonstrating the daily revisit potential from a thematic point of view of flood mapping over agricultural and forested areas. Flood Rapid Mapping. Envisat Symposium, Montreux, April Arnaud M., Boissin B., Perret L, E. Boussarie E., Gleyzes A., "The Pleiades Optical High Resolution Program," Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B (see also 6. Fraipont P. de, S. Clandillon, R. Andreoli & H. Yésou 2006 : Apport des données Formosat pour la gestion de risques. Exemple d exploitation en cartographie rapide. Geo événement, Paris, Porte de Versailles, mai 2006 Finally there is a great expect for the further and fruitful synergistic exploitation of SAR and optical data applied to flood extent mapping, to dike breaks mapping and the monitoring of flood water dynamics. Acknowledgement: Authors would like to thank all the Charter actors: the different PM for their confidence in our team. We also would like to thank the technical staff at ESA, CNES, CSA, Radarsat int and SPOT Image involved in the rush mode programming and crisis images dissemination scheme during these Charter actions. 5. References 1 Yésou H., Clandillon S., Allenbach B., Bestault C., Fraipont de P. 2003: A constellation of advantages with SPOT SWIR and VHR SPOT 5 data for flood extent mapping during the September 2002 Gard event (France). IGARSS 2003 Toulouse. 2 Laugier O., Fellah K., Tholey N., Meyer C., de Fraipont P., 1997, High Temporal Detection and Monitoring of Flood Zone Dynamic using ERS Data around Catastrophic Natural Events: the 1993 and 1994 Camargue Flood Events, Proceedings of the third ERS Symposium, ESA SP-414, Vol. 1, Fellah K., Stock L., Axes F., Bach H., Ebel U. Grabak O. de Fraipont P. 2003: Toward an operational EO service for flood monitoring. IGARSS 2003 Toulouse, France 4.. Schneiderhan, T; Hoffmann, J. Huber, M. Zwenzner, H., Voigt, S., 2007: Use of ENVISAT ASAR and ERS SAR Data for