NEAR REAL TIME FLOOD MONITORING IN P.R. CHINA DURING THE 2005 AND 2006 FLOOD AND TYPHOONS SEASONS BASED ON ENVISAT ASAR MEDIUM AND HIGH

RESOLUTION IMAGES

YESOU Hervé (1), LI Jiren (2) , MALOSTI Rita (3), ANDREOLI Rémi (1), HUANG Shifeng (2) , XIN Jingfeng (2),CATTANEO Fabrizia (3)

1 SERTIT, Université Louis Pasteur, Strasbourg, France, Email : herve@sertit.u-strasbg.fr

2 IWHR, Ministry Water Resource, Beijing, China, Email: lijiren@iwhr.com, huangs@iwhr.com, xinjf@iwhr.com

3 SERCO SpA, support to the European Space Agency (ESA ESRIN), Frascati (RM), Italy, Email: Rita.Malosti@esa.int, Fabrizia.Cattaneo@esa.int

Abstract: The Flood Dragon project enhances the Envisat contribution for natural disaster monitoring such as flooding. During the 2005 and 2006 Chinese flood seasons, over the 23 attempted NRT exploitations of Envisat, 19 were successful, allowing exploitation of emergency programming procedures as well as Rolling Archive capabilities (based on ordinary long term acquisition scheduling). Obtained results are illustrated and lessons are discussed in terms of programming, downloading, processing, and images type and format. Keywords: Flood mapping, monitoring, Envisat, ASAR medium resolution, Rolling Archive, NRT

1. INTRODUCTION China is one of the countries in which flood occurs most frequently in the world and with the current economic growth, flood disaster causes more and more economic losses. The first objective of the Flood Dragon project is to explore Envisat ASAR and MERIS spatial and temporal resolution potential for rapid flood mapping and flood monitoring [1]. The SAR potential for flood mapping and monitoring, thanks to its all weather capabilities but also to its potentially large coverage with a relative high resolution, has been evaluated during demonstration projects and tested in operational condition, including numerous “Charter Space and Major Disaster” actions [2-8]. One of the major Flood Dragon goals is to provide data exploitation guidelines and recommendations for system’s improvements for a Near Real Time exploitation of high, medium and low resolution ASAR products for flood monitoring. The aim of this paper is to present the improvements achieved in Near Real Time exploitation of Envisat

ASAR images for flood mapping, in terms of programming, downloading and processing, within the Flood Dragon project. The results obtained in 20 successful NRT actions during the 2005 and 2006 flood and typhoon seasons, based over Chinese major flood prone areas, are illustrated and lessons discussed.

2. METHODOLOGICAL PART

2.1. NRT data acquisition mode and Rolling Archive exploitation Within the Dragon Programme, ESA exceptionally granted to the Flood project the possibility of requesting a maximum of 5 Envisat ASAR acquisitions at very short notice ("rush planning", i.e. 3 days notice, later referred to as "emergency" planning as well). These acquisitions are supplementary to the ordinary acquisition planning, which, again exceptionally in the frame of the Dragon Programme, is performed on a 3 Envisat cycles basis (i.e. every 35 by 3 days). The rush planning is usually reserved to the "International Space and Major Disaster” charter actions and to commercial Users (Category 2). In order to trigger an emergency request in case of flooding events, the Flood Dragon PI, Pr LI Jiren, has to provide the acquisitions details to ESA at least 3 full working days before sensing time [9]; clearly, this is not so obvious in case of severe flood. For medium resolution products (IMM APM and WSM), the processed data is nominally available to the Dragon partners within the day of the acquisition while the high resolution products (IMP and APP) are downloadable one or two days later, both from the ESA Rolling Archive website. Since 2006, Envisat data could also be acquired by the Beijing Ground Station.

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

2.2. Water extraction methods Flood inundation mapping corresponds in fact to a surface water extraction exploiting the low backscattering characteristic of water. Therefore in case of windy condition, or partially submerged vegetation, the backscattered signal can be affected, by presenting higher backscattering values [1, 6]. Semi-automatic classical water extraction techniques, such as crisis image binarization through thresholding, or change detection technique applied on a pair of reference/crisis of SAR image, can fail. A target oriented approach, based on an innovative image processing technique, has been applied during some NRT flood mapping actions [10] and it is presented below.

Figure 1: Flood map set up over the Harbin city neighbourhood, Heilongjiang province, exploiting target oriented approach. NRT product was derived from Envisat medium resolution products, combining ascending and descending data acquired the 28th of July 2005

3. NRT ACTIONS DURING THE 2005 FLOOD SEASON IN CHINA The 2005 flood season in China was dramatic, beginning very early in the year with major events in early and end of June. The summer floods in 2005 have affected 90 million people in China, a minimum of 764 people have been killed with other 191 missing, more than 3.72 million people have been evacuated to safer places. About 7 million hectares of crops were destroyed as well as 702,000 houses over 27 regions involving 5.79 billion USD of direct economic losses. The worst-hit regions have been in Southern and Eastern parts of China, including the provinces of Guangxi,

Fujian, Hunan, Guangdong, Jiangxi, Sichuan, Anhui and Henan. The Heilongjiang province (Northeast of China) has been affected in July and August 2005 by floods as well as by a critical benzene pollution episode in November 2005, occurred in the Songhua river. Between June and November 2005, 16 flood rapid mapping actions in NRT have been carried out by Flood Dragon team. These NRT cases have exploited both the emergency programming possibilities and the Rolling Archive capabilities. Over the 13 emergency planning requests, 7 were successful in term of acquisition, downloading, processing and dissemination (Fig. 2, Tab. I).

Figure 2: Major flood prone in China and 2005 NRT Flood Dragon actions A few requests were not taken in account due to delay in requesting the rush programming activation, others were in conflict with commercial and/or Charter requests (data was made accessible to the Flood Dragon team through the Rolling Archive) and 2 failed due to technical constraints. Part of these NRT actions was carried over Flood Dragon test sites, Poyang and Dongting lakes in Central Chain

and Nen-Songhua rivers in Northeast China, but also on other major flood prone areas in China such as the Huaihe River and the Peal River watershed, and on the South coast of China, affected by typhoons (Fig. 2).

Location Province action Date Date type Dongting

Poyang lake Hunan Jiangxi RA 01-05 IMM

WSM Wuzhou Guangxi NRT 23-06- APP Wuzhou Guangxi NRT 27-06- APP

Huaihe river Anhui, NRT 11-07 Dazhou Sichuan NRT 11-07 App Nen and Songhua Rivers

Jilin, Heilongjiang

NRT RA 25-07 APM,

IMM

Songhua tributary Heilongjiang NRT 4-08

Huaihe Anhui NRT 5-08 APP Shanghai Shanghai NRT 5-08 APP

Zhangzhou Fujian RA APP, APM

Fushun Liaoning NRT 16-08 APP Huaihe Anhui NRT 29-08 Huaihe River Anhui, RA APP

Huaihe River Anhui, NRT 05-09 APP

Yangjiang Guangdu NRT 23-09 APP

Songhua Heilongjiang NRT 24-11 MERIS FR

Table I: List of flood mapping actions carried during 2005, exploiting emergency programming (NRT) as well as systematic Rolling Archive (RA) products. Unsuccessful requests are indicated in grey

3.1 The June 2005 Wuzhou flood mapping action One of the most important actions was carried in late June 2005, during the Wuzhou flood [9]. Heavy rains affected the Guangxi Zhuang Autonomous Region in the Southwest of China. Floodwaters forced the mass overnight evacuation of residents in low-lying areas of the industrial city of Wuzhou where the Xijiang river had reached 25.74 meters by 21st of June, more than 8 meters higher than the warning level over passing dikes. The 23rd of June, a NRT Envisat acquisition was requested in an AP mode (HH/HV polarization), which appears to be most suitable for water mapping, and scheduled for the 25th of June, first potential revisit over the area of interest. Due to an “International Charter Space and Major Disasters” ongoing action, the requested format was not acquired. A IM mode (VV polarization) was acquired instead, and made accessible to the Flood Dragon team. The systematically produced medium

resolution IMM image was put on the Rolling Archive on the Saturday 25th of June, mid day. This IMM data was downloaded at IWHR and processed both in China and Europe (Fig.7 to 9). The obtained results are very similar, highlighting the flooded low valleys near the Xijiang River as well on the border of its tributaries. In addition, exploiting its socio-economic database, IWHR was able to timely provide to the Ministry a first precise impact of the flood. The on request high resolution PRI images were made available on Monday 27th afternoon during the Second Dragon Symposium, which most of the Flood Dragon team attended in Santorini Island (Greece).

Figure 3: Detailed flooded impact map derived from

ASAR IMP image dressed by Flood Dragon team

3.2 Benzene slick NRT mapping on the Songhua River At the end of November 2005, a dramatic benzene pollution episode affected the Songhua River (Heilongjiang Province). This event was due to explosions at the petrochemical plant in Jilin city, occurred on the 13th of November 2005. The blasts created a 80 km long toxic spill, consisting in an estimated 100 tons of toxic substances (a mixture of benzene, aniline and nitrobenzene) in the Songhua River.

The 22nd of November , official media confirmed the Jilin explosion and the Songhua River pollution and the following day this information was widely broadcasted. By the 24th of November, a rush programming request was sent to ESA and accepted. A MERIS FR acquisition was scheduled for the 29th November 2005. Firstly, the Reduced Resolution MERIS data was downloaded from the Rolling Archive, on the same day as the acquisition. The 1st December 2005 the Full Resolution MERIS product was as well made available

online. Due to the presence of floating ice on the Songhua River and due to the fact that the benzene slick did not rise the surface of free water but rather laid a few centimetres/decimetres under it, it was not possible to characterize and to map the slick. A situation map showing benzene slick crossing dates for several cities on the Songhua, from Bayan (P.R. China) to Khabarovsk (Russia), was produced the 2nd December 2005 and sent to our Chinese Partners (Fig. 4).

Figure 4: Benzene pollution situation map : location of the benzene slick during the days of crisis. .

4. NRT ACTIONS DURING THE 2006 FLOOD SEASON IN CHINA In 2006, severe floods occurred in the Pearl River and the Minjiang River in Fujian, main stream of the Huai River. Two significant typhoons, Billis and Samoi, hit the Chinese coasts and mainland in July and August 2006. The resulting floods and landslides have killed more than

1,200 people; crops on at least 10 million hectares of farmland have been destroyed and 0.65 million houses ruined. Direct economic losses were estimated to 13 billion USD. Between June and October 2006, 8 flood

rapid mapping actions have been carried out by Flood Dragon team1, exploiting Envisat data (Fig 1, tab II).

Location Province action Date Date type Mingjiang

river Fujian RA 10-06 IMP

Mingjiang river Fujian NRT 10-06 APP

Huaihe river Anhui, RA 04-07 WSM Chongqing Chongqing RA 09-07- APP

Fujian Zheijiang NRT 12-07- WSM

Yangjiang Guangdou RA 31-07 APP

Fuzhou Zhejiang Fujian NRT 15-08 IMP

Songhua Heilongjiang RA 28-08 MERIS FR

Table II: List of flood mapping actions carried during 2006, exploiting emergency programming (NRT) as well as systematic Rolling Archive (RA) products

Figure 5: Extract of a NRT product derived from Envisat over Guandng province after Billis Typhoon Due to the importance of typhoons, these actions have been mostly concentrated over South Eastern coastal provinces of China and neighbouring provinces. It must be said that, in this dramatic occasion, ESA staff were very responsive. As the Billis typhoon was progressing rapidly towards the Chinese coast, a 7 minutes ASAR pass was programmed; regardless of the short notice, the planning was achieved. In addition, due to the potentially dramatic situation, two Envisat commercial customers

1 In addition to the Envisat NRT actions, IWHR carried out 2 rapid mapping studies based on Radarsat data exploitation during the Billis event

accepted to give priority to the Flood Dragon project request, thanks to the ESA and Beijing Ground Station contacts. A Wide Swath pass was acquired two days later and exploited in the Medium Resolution format (Fig. 5). At the end of August 2006, another incident hit the Heilongjiang province; a MERIS FR pass was acquired within the scheduled programming and generated in rush mode. Unfortunately, the area of interest was covered by clouds.

5. CONCLUSION AND PERSPECTIVES It is remarkable that this Dragon project has been able to carry over 20 rapid mapping actions, in the past 2 years, for flood mapping and pollution incident monitoring. It shall be noted that during the first year of the project, over the 16 attempts, 11 were successful and 5 failed. Reasons for failing are mostly related to delays in information dissemination or technical constrains. The 3 working days needed for acquisition scheduling, are often reason for rejection, as it is not always obvious to forecast a flood event up to five days in advance. It took time to get accustomed to the defined rules for emergency request; as the project progresses, application of procedures and data exchange improve greatly, resulting in less rejections. The experience gained during the first year of monitoring allowed a higher rate of successful emergency requests. The data acquired as part of the ordinary acquisition planning increased between 2005 and 2006: in 2005 rush programming accounted for 75% of the total amount of acquisitions (leaving only 25% to the ordinary planning), while in 2006 the data acquired as part of the ordinary scheme, represented 62% of the total. It is clear that a more systematic long term acquisition plan over sensitive areas can be a powerful tool for EO data provision. Based on such systematic programming, monitoring of large flood prone areas could be envisaged in Asia as well as Africa and South America. For this reason, the future Sentinel-1 constellation, with its background Wide Swath mission mode and a daily revisit, will insure EO data provision. In terms of Envisat data exploitation for flood monitoring and mapping, only ASAR data that have been requested; no MERIS FR data have been exploited for this application so far. An assessment of the potential of MERIS FR data for NRT action should be considered for the 2007 flooding season, over Chinese areas where simultaneous acquisition of ASAR and MERIS FR are technically feasible. This would allow exploring all the potential of the Envisat imaging sensors. For ASAR data, the obtained results confirm previous studies [8].

Medium resolution ASAR product, with a pixel spacing of 75m, can provide valuable information covering very large areas with a correct thematic accuracy [11]. While the medium resolution products provide information over natural and agricultural large areas, the high resolution mode is more suitable in case of flood affecting urban areas. In terms of polarization, the HH scheme has been requested for WSM data and either HH or HH/HV schemes for IM/AP images. The VV scheme was instead not taken into account when planning an emergency request, even thou most of the existing archive was acquired with this scheme. We are approaching the third year of disaster monitoring in P.R. China: during the upcoming flood and typhoon seasons, Envisat ASAR medium and high resolution products will be exploited. In the near future, possibly in the framework of a Dragon II Programme, new investigations could be developed following two approaches: a first one consisting in the systematic flood monitoring, and the second corresponding to the launch in orbit of new sensors. The latter would help carrying out an assessment of SAR bands synergy for flood monitoring, with focus on water elements identification, such as partially inundated sandy islands. Acknowledgement: The Flood Dragon project Team would like to thank ESA for allowing access to rush programming for emergency, and the Dragon team for their interest and confidence. Authors would also like to associate to this paper IWHR staff involved in the NRT actions: Ms. Li Lin, Ms. Xu Mei, Dr. Sun Tao and Mr. Li Xiao-Tao.

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