D. WERLE, T. C. MARTIN, AND K. HASAN

I

Flood and Coastal Zone Monitoring in Bangladesh withRadarsat ScanSAR: Technical Experience andInstitutional Challenges

Dirk Werle, Timothy C. Martin, and Khaled Hasan

n Bangladesh, river floods, monsoon rains, and tropical cyclones are an integral partof the annual hydrological cycle, regularly affecting the livelihood and well-being ofmore than 100 million people. During years of severe flooding, for example in 1987,1988, and 1998, the country was transformed by intermittent inland seas that occupiedas much as 60% of its land surface. Wide swath synthetic aperture radar (SAR) dataacquired by the Canadian Radarsat have shown considerable potential as a monitoringtool, particularly during the monsoon season, to assess flood extent and damage, as wellas flood impact on agricultural production, fisheries resources, and navigation. ForBangladesh to fully reap the benefits of wide swath SAR for flood monitoring andcoastal zone management, good institutional relations among the scientists, donoragencies, and the government of Bangladesh are critical. (Keywords: Bangladesh, Floodmonitoring, Radarsat ScanSAR.)

INTRODUCTIONBangladesh lies at the confluence of three major

rivers: the Ganges, the Brahmaputra-Jamuna, and theMeghna, which are all part of the Himalayan drainagesystem. Figure 1 shows the vast catchment area ofthese rivers. The country occupies only a very small,yet highly vulnerable portion of the system at its lowerend. More than 90% of the water originates frommainly monsoon precipitation outside its boundaries,while less than 10% is from the monsoon rains withinits borders.1 Each year, large amounts of sediments aredeposited along the river banks and in the floodplains,

148 JO

continuously shaping and reshaping the enormousdelta at the head of the Bay of Bengal.

Flood events have a profound influence on the lifeand welfare of many people in Bangladesh. The pop-ulation density on the vast, fertile floodplains and inthe coastal zone is one of the highest in the world, andpressure on resources is increasing owing to the expand-ing population and the developing economy. A mod-erate monsoon flood usually brings adequate crop yieldsand a successful catch from one of the world’s mostproductive floodplain fisheries. However, excessive

HNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (2000)

COASTAL ZONE MONITORING WITH RADARSAT SCANSAR

Radarsatsubscene

Figure 1. The Ganges, Brahmaputra-Jamuna, and Meghna river systems of Bangladeshin the regional context of the Himalayan drainage system and location of the Radarsatsubscene (insert, see Fig. 3). (Map courtesy of Ray Sterner, APL.)

flooding kills people and livestock and damages prop-erty; unusually light flooding leaves many areas withoutcrops, a drastically reduced fishery, and insufficientgroundwater levels to sustain irrigation throughout thedry season. Normal monsoon floods will inundate about30% of Bangladesh’s floodplains, but during excessiveflood years, such as 1988 and 1998, flooding can exceed60% of the entire land surface.

However, the actual extent, depth, and duration ofthe monsoon flood are largely unknown, because itbuilds up and recedes over a period of several monthsfrom May to October. Consequently, it is critical todevelop the means to accurately characterize andmonitor annual flooding for two reasons: to improveshort-term emergency response measures and to predicthow interventions on the landscape will impact thedepth and duration of the flood in the medium and longterm. This information could be very useful in support-ing the development of a long-term national watermanagement plan.2

The situation is somewhat more challenging consid-ering the nature, extent, and duration of flooding prob-lems in the coastal zone. The landfall of tropical cy-clones along the Bay of Bengal can result in extensivedamage within a very short time. The catastrophiccyclone of 1991, which killed more than 125,000 peo-ple, showed that storm and tidal surges affecting the flatcoastal plains and low-lying islands can reach 6 to 7 min height and advance many kilometers inland at arapid rate of about 2.5 m/s. Wind speeds can exceed70 m/s. Such events pose an enormous risk to the grow-ing population in these areas.

Several programs have attempted to characterizeflooding events in the Ganges, Brahmaputra-Jamuna,

JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (2000)

and Meghna river systems by usinghydrologic mathematical models.Since there is only a limitedamount of reliable field informa-tion for verification purposes, theutility of these models for charac-terizing floodplain dynamics is de-batable. Some cloud-free, high-res-olution optical satellite images ofthe monsoon floods are on recordfor part of the country. However,they are of very limited use, be-cause there are only a few to coverthe complex floodplain environ-mental conditions in both spaceand time. Although several studiesof the coastal zone have investigat-ed the usefulness of remote sensingfor shoreline characterization andchange detection, there has beenno attempt to date to utilize satel-lite remote sensing to capture the

floods resulting from storm surges. This situation hasbeen largely due to the short duration of these eventsand the rather long response times for remote sensingdata acquisition and delivery.

Recently, wide swath synthetic aperture radar(SAR), and more specifically, the 500-km-wide Scan-SAR mode of the Canadian Radarsat,3 has become afocal point of several studies investigating more reliablemethods of mapping and monitoring floods in Bang-ladesh.4–8 These investigations have been performed asmodestly funded R&D and demonstration projects. Asa developing country, Bangladesh has had little expe-rience with the application of modern satellite radartechnology and must rely almost entirely on foreignfinancial resources if it desires to implement and sup-port the systematic and operational use of this technol-ogy. In this article, we discuss ScanSAR in the contextof recent satellite radar work in Bangladesh. Then weoutline potential applications of wide swath SAR froma resource management and emergency response pointof view. The article concludes with observations andcomments on current opportunities and difficulties inadapting imaging radar technology within the frame-work of institutional, financial, and human resourcedevelopment in Bangladesh.

WIDE SWATH SAR FOR FLOODMONITORING

Experimentally, wide swath SAR imagery has beenapplied to flood monitoring in Bangladesh ever sincethe Radarsat ScanSAR product became commerciallyavailable in 1996. As indicated in Table 1, more than

149

D. WERLE, T. C. MARTIN, AND K. HASAN

50% of all Radarsat scenes acquired over Bangladeshduring the first 3 years of satellite operations were ac-quired in ScanSAR mode, and approximately one-thirdof all scenes were acquired in fine mode. One investi-gation compared fine mode and ScanSAR narrow im-agery of the same area during the height of the monsoonflood of 1996.4 The digital image analysis revealed thatthe ScanSAR classification accuracy of flooded andnonflooded areas during times of extensive flooding wasvery encouraging. In fact, it was comparable to theaccuracy achieved with fine (resolution) mode. Forexample, in almost 90% of all cases, the ScanSARnarrow (300-km swath width) and fine mode classifi-cations for flooded terrain during 19 and 20 August1996 were in agreement with the ground reference data.The result also compares favorably with previous Eu-ropean Remote Sensing satellite (ERS)-1 SAR dataanalysis, which provided accuracies ranging between 70and 90% for a variety of flooded terrain features duringthe monsoon flood of 1993.4,9

These findings have implications for the radar ap-plication potential and the choice of suitable radar dataproducts, particularly with regard to nationwide andmultitemporal data requirements in Bangladesh. Theresult is also attractive to potential operational usersand investigators familiar with the 100-km swath (orless) radar imagery, which was the norm for satelliteSAR coverage before the introduction of ScanSAR.The spatial coverages for both the 100-km-wide ERS-1 and -2 SAR or Radarsat standard mode and the�300-km-wide ScanSAR narrow swath were com-pared. It was found that the ERS completes nationwidecoverage after approximately 4 weeks, involving sixorbital passes and the equivalent of 25 scenes with anominal resolution of 25 m.10 In the case of ScanSARnarrow, nationwide coverage can be completed within

Table 1. Radarsat scenes acquired over Bangladesh,1996–1998.

Mode,swath width (km);resolution (m) 1996 1997 1998 Total

Fine, 50; 10 6 6 7 19Standard, 100; 25 2 2 2 6Wide, 150; 25 1 1 0 2ScanSAR, 300; 50 5 8 12 25ScanSAR, 500; 100 2 2 3 7Total number

of scenes 16 19 24 59(Source: personal communication, A. Ramirez, Radarsat Inter-national, Feb 1999.)

150 JO

1 week, involving either two orbital passes with fourscenes at a 50-m resolution or only a single pass andone scene in ScanSAR wide mode with a 500-km swathand a 100-m resolution.11 Therefore, because of itsmonitoring capability, data handling, and the financialperspective, the ScanSAR data product offers morefrequent and better synoptic coverage (albeit at reducedresolution) at substantial cost savings.

Given the spatial and temporal scales of flooding inBangladesh, ScanSAR appears to be the preferred SARimaging mode to effectively monitor the flood dynam-ics weekly during the monsoon period. However, inBangladesh, such high-frequency ScanSAR data acqui-sition and data handling capability has not yet beentested; previous R&D investigations and demonstra-tion projects relied on the 24-day Radarsat repeat cyclein order to keep wide-area coverage and radar illumi-nation conditions at a constant.

From a technical point of view, the value of theRadarsat-1 ScanSAR imagery acquired over Bang-ladesh to date has been hampered by two factors: (1)the previously uncalibrated beams of the 8-bit dataproduct limited its use to analog interpretation andchange detection exercises rather than quantitativedigital analysis, and (2) the analog-to-digital converter(ADC) problem resulted in a loss of image quality andvalue, when widespread flooding was involved. TheADC effect was associated repeatedly with flooding inthe Sylhet Depression in the northeastern portion ofthe country. The ADC effect is usually experienced incoastal areas with large open water surfaces in the near-range portion of the radar beam.12 (See also Vachon etal., this issue.)

APPLICATION POTENTIALFrom the operational point of view, the use of wide

swath SAR data for monsoon flood monitoring inBangladesh has proven to be practical for three reasons.First, it provides synoptic and sufficiently frequentcoverage on a nationwide scale, unaffected by cloudcover. Second, the digital data format of the Radarsatsystem is potentially conducive to fast delivery, i.e.,within hours of data acquisition. Third, ScanSAR hasperformed a useful pathfinder role for follow-up andmore detailed area coverage. For instance, the buildupand oscillations of the severe 1998 flood levels exceed-ed what is considered the danger level for an un-precedented 65 days. The hydrographic data shown inFig. 2 demonstrate that the rising and falling waterlevels of the 1998 peak flood can be monitored by theScanSAR, although only a few scenes had been re-quested by the authorities in Bangladesh during thelatter stage of the 1998 flood.

Data acquisition opportunities generally occur every4 to 5 days, whereas individual flood peaks appear to

HNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (2000)

be more than 10 days apart. Within that time periodof individual flood peaks, there is also opportunity toschedule data acquisitions at fine or standard mode tocover affected areas in greater spatial detail. Figure 2bshows that actual Radarsat ScanSAR data acquisitionsin August and September coincided with flood condi-tions above the danger level and the peak flood event.Such imagery provides a unique and invaluable, almostnationwide, overview of the flood extent at a time ofgreatest calamity (Fig. 3). Subsequent image analysesand time series prepared for briefings to government

Figure 2. Comparison of water levels of the Jamuna River atBahadurabad during (a) a normal year (Jan–Dec 1997) and(b) the exceptional 1988 and 1998 monsoon floods (Jun–Nov). Thedanger level (19.5 m) as well as several Radarsat ScanSAR dataacquisition dates for the 1998 flood season are indicated (PWD =Public Works Department data). (Source: EGIS Project, Dhaka,Bangladesh, 1998.)

21

20

19

18

17

16Wat

er le

vel (

m +

PW

D)

Danger level

Radarsat acquisitions in 1998

Jun Jul Aug Sep OctMonth

0

5

10

15

20

25

Wat

er le

vel (

m +

PW

D)

Jan Apr Jul Oct1997

(a)

(b)

1998 (flood year)

Lower panel1988 (flood year)

1997 (normal year)

1998 (flood year)

1988 (flood year)25 August 1998

JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (

COASTAL ZONE MONITORING WITH RADARSAT SCANSAR

officials made a convincing case for the usefulness ofwide swath SAR for flood assessment in Bangladesh.

Systematic and operational use of satellite radar ispotentially of direct benefit to government and nongov-ernment organizations, planners, researchers and aca-demics, donor-funded programs, and developmentprojects.13 Several potential Radarsat applications haveemerged. Table 2 indicates that flood monitoring is themain application of Radarsat, followed by agriculturalplanning (etc.), river dynamics and monitoring, andoffshore environmental surveillance. However, the stateof readiness to integrate radar data into ongoing oper-ational activities is generally low. Activities related tomonitoring monsoon floods and river morphology arethe only exceptions. Here, ScanSAR data have highvalue as a support tool for strategic decision making andnationwide flood emergency response measures. Apartfrom emergency situations, wide swath radar is equallyimportant for water resources analysis and managementwhere it helps to address three critical issues: (1) sea-sonal hydrological change in the floodplain on a region-al scale; (2) the relationships among hydrological re-gime, vegetation cover, land use, and inland fisheries;and (3) predictions (through coupling with hydrody-namic models) of the effects of interventions such asflood embankments on floodplain hydrology.

By contrast, it is very difficult to capture with ex-isting satellite SAR systems the actual flooding relatedto storm surge events lasting on the order of a few days,or, in extreme cases, only hours. The limitations aretwofold: (1) relatively long satellite revisit cycle(s) andactivation times to actually capture the event and relaythe image data in a timely fashion; and, to some extent,(2) discrimination of nonflooded terrain versus inun-dated coastal land surfaces under usually high windspeed conditions. The chances for the latter situationincrease with the use of large radar incidence angles;small incidence angles favor detection of wave andwind fields. Under these conditions, it would be moredifficult to distinguish their radar-bright signature fromthose of the coastal land areas.

Table 2. Assessment of Radarsat ScanSAR application potential and state of readiness in Bangladesh.

Radarsat ScanSAR application Potential State of readiness

Monsoon flood monitoring and disaster management High ModerateAgriculture planning, management, and damage assessment Moderate LowInland fishery, aquaculture, and wetland resource management High LowEnvironmental planning for infrastructure and energy Low LowRiver dynamics and monitoring Moderate ModerateCoastal geomorphology; storm surge monitoring Low LowOffshore environmental surveillance Moderate Low

2000) 151

D. WERLE, T. C. MARTIN, AND K. HASAN

Figure 3. Multitemporal Radarsat composite SAR image map of central Bangladesh constructed from ScanSAR wide data acquiredduring severe monsoon flooding on 25 August 1998, and ScanSAR narrow data acquired during the dry season on 21 March 1999.See Ref. 13 for detail, Fig. 1 for regional context. (Source: SPARRSO/CIDA/CCRS/Ærde, © CSA, 1998 and 1999.)

100 km

Normally open water

Monsoon flooding

Inundated vegetation

Settlements

100 km

Normally open water

Monsoon flooding

Inundated vegetation

Settlements

152 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (2000)

COASTAL ZONE MONITORING WITH RADARSAT SCANSAR

The current 48-h response time for Radarsat-1 SARdata acquisition programming is inadequate for suddenevents like storm surges, assuming that the area ofinterest is readily accessible by the radar. Also, theweekly coverage of the Bay of Bengal by Radarsat isinsufficient for making a substantive contribution toshort-term emergency response operations. However,the sea surface conditions (e.g., wind and wave fieldsrelated to these destructive events) could potentiallybe assessed even within the current time constraints.More imaging opportunities may present themselveswith the denser coverage pattern afforded by acoordinated constellation of wide swath SAR satellites(e.g., the Radarsat-1 and -2 and the Advanced SAR[ASAR] onboard the planned European Envisat).

ADAPTATION FOR OPERATIONAL USEIn Bangladesh, several research and demonstration

projects have been successfully carried out over the past5 years involving satellite radar data as the primarysource of input. However, ScanSAR imagery has notbeen used operationally for monitoring flood events,land use, and the coastal areas of the country. Theprocess of integrating the radar data stream into day-to-day resource management and emergency responseoperations has been hampered by both institutional andtechnical constraints. Effective and widespread use ofScanSAR-type data products hinges on resolving ad-ministrative and technical problems.

The designated national governmental focal pointfor remote sensing matters in Bangladesh is the SpaceResearch and Remote Sensing Organization. It is re-sponsible to the Ministry of Defense, but also is in-volved with applications in resource management andenvironmental monitoring. In addition to the localgovernment activities, several international donor in-itiatives have been undertaken. Examples include theU.S. Agency for International Development–fundedFlood Action Program 19 Geographical InformationSystem (GIS) Project and the Dutch-funded Environ-mental GIS Project (EGIS II), supporting the Ministryof Water Resources and other agencies. Thus, as inmany other developing countries, aerospace and re-mote sensing affairs of Bangladesh are officially withinthe civilian and military domains of government, butare opening up to participation by developmentprojects and private sector companies.

The path from applied research and development tooperational use is further complicated by the fact thatcivilian government institutions involved with floodforecasting and damage assessment in Bangladesh arenot financially able to invest in an operational, radar-based flood monitoring system. The financial resourcesneeded for the adaptation and implementation of

JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (2

much-needed technologies are very scarce. Manygovernment agencies and potential radar remote sens-ing users operate on a very low budget. Apart from theactual acquisition of SAR data, there are other majorcost factors to consider, including human resources andtraining, reliable software and hardware, archival anddistribution facilities, as well as regular maintenance tosustain an operational system.

Despite current limitations, international donors areenthusiastic about the potential of radar remote sensingtechnology. During the devastating flood of 1998,United Nations and Canadian International Develop-ment Agency funds supported initiatives led by theEGIS II Project and by the Space Research and RemoteSensing Organization. Both institutions were able todemonstrate the effectiveness of multitemporal Ra-darsat ScanSAR data as a flood-mapping tool. Manygovernmental and nongovernmental organizationswere found to be ardent users of thematic maps of the1998 flood extent and applied the data in their disastermanagement and postflood relief and rehabilitationefforts. As a result, institutes such as the Flood Fore-casting and Warning Center and the Surface WaterModeling Center are interested in satellite SAR as aunique data source to validate and calibrate theirmathematical models. Furthermore, cooperationamong donors, government organizations, and the pri-vate sector has recently led to the temporary installa-tion of a European-built portable ground station inDhaka. The facility is capable of receiving 100-kmwide swaths of ERS-2 SAR data in near–real time fora 9-month trial period during the 1999 monsoon sea-son. Such initiatives are instrumental in building oper-ational SAR monitoring capacity and experience inBangladesh.

With European and Canadian commitments tolaunch their Envisat and Radarsat-2 satellites in the2000 and 2002 time frames, respectively, ScanSARdata continuity appears to be established for at leastanother 5 to 7 years. This is important and reassuringfor users who plan to integrate these data into theiroperations. From a technical perspective, the main con-straint appears to be the timely transfer of ScanSARimagery to and within Bangladesh during times ofemergency. Whereas the SAR data can be processedwithin less than 2 h of reception at the Canadian DataProcessing Facility, the limited bandwidth of Bang-ladesh telecommunications lines currently restrictsprompt delivery of the imagery to the user. This delayand subsequent lag time for image processing, analysis,and information product generation compromise theoperational utility of the data. The value of the imageryfor short-term emergency response measures is signifi-cantly diminished after a few days, and in the caseof coastal tidal surges, even after several hours from

000) 153

D. WERLE, T. C. MARTIN, AND K. HASAN

data acquisition. It is expected that new compressiontechniques implemented at the Canadian facility willsubstantially improve the data transmission times andthus reduce or alleviate the processing problem forBangladesh users.

CONCLUSIONCurrently available wide swath SAR data, such as

Radarsat ScanSAR imagery, are well suited for appli-cation in Bangladesh to address urgent environmentalmonitoring requirements, particularly for regularly oc-curring monsoon floods, and to monitor the annualhydrological cycle. Research and demonstrationprojects have shown that the information derived fromsuch imagery can be very valuable to operational usersfor planning flood-related emergency response mea-sures; monitoring flood dynamics, inland fisheries, andaquaculture; and validating and calibrating hydrologi-cal models. The successful monitoring activities basedon Radarsat ScanSAR during the devastating flood of1998 highlighted the value of SAR technology, notonly for potential user agencies, but also for their in-stitutional backers within government and internation-al donor organizations. Several implementation issuesremain unresolved, including financial and institution-al arrangements, as well as technical difficulties asso-ciated with the timely transfer of SAR data processedin near–real time to support operational emergencyresponse activities for flooded areas and coastal stormsurges in Bangladesh.

REFERENCES1Huq, S., and Rahman, A. A., “An Environmental Profile of Bangladesh,” inEnvironment and Development in Bangladesh, Vols. 1 and 2, A. Rahman, R.Haider, S. Huq, and E. Jansen (eds.), The University Press Ltd., Dhaka,Bangladesh, pp. 38–70 (1994).

2Water Resources Planning Organization (WARPO)/Ministry of Water Re-sources, National Water Management Plan, Inception Report, Vol. 1: MainVolume, Dhaka, Bangladesh (1998).

3Parashar, S. (guest ed.), Special Issue: RADARSAT, Can. J. Remote Sens.19(4), 278–395 (1993).

154 JO

4Martin, T. C., Hasan, K., Shireen, A., and Werle, D., “RADARSAT Data forMonitoring Flood Conditions in Bangladesh—An Assessment of EarlyResults,” in Proc. Int. Symp. Geomatics in the Era of RADARSAT (GER ‘97)(on CD-ROM), Natural Resources Canada/National Defence Canada, Ot-tawa (25–30 May 1997).

5EGIS Project, “Mapping and Monitoring Floods with RADARSAT Images,”Environment and GIS Support Project for Water Sector Planning/Delft Hydraulics,Water Resources Planning Organization (WARPO), Ministry of WaterResources, Dhaka, Bangladesh (1997).

6Hasan, K., Martin, T. C., Shireen, S. A., and Choudhury, A. M. “Dynamicsof Monsoonal Flooding in Bangladesh Using RADARSAT Imagery,” in Proc.Application Development and Research Opportunity (ADRO) (on CD-ROM),Canadian Space Agency, Saint-Hubert, Canada (1999).

7Ali, A., Quadir, D. A., Sardar, A. M., Rahman, H., Islam, Z., and Shahid, A.,“Flood Monitoring and Damage Assessment in Bangladesh UsingRADARSAT Data,” in Proc. Application Development and Research Opportunity(ADRO) (on CD-ROM), Canadian Space Agency, Saint-Hubert, Canada(1999).

8Flood Action Program (FAP) 19 GIS, Satellite-Based Radar for FloodMonitoring in Bangladesh, USAID Irrigation Support Project for Africa and theNear-east (ISPAN) and Flood Plan Coordination Organization (FPCO),Ministry of Water Resources, Dhaka, Bangladesh (1995).

9Martin, T. C., Haque, I., Hassan, A., Werle, D., and Tittley, B., “Multi-Temporal Satellite Radar Imagery for Flood Monitoring in Bangladesh,” inProc. 26th Int. Symp. on Remote Sensing of Environment/Information Tools forSustainable Development, Vancouver, BC, Canada, pp. 353–358 (25–29 Mar1996).

10European Space Agency, Proc. First ERS Thematic Working Group Meeting onFlood Monitoring, ESRIN, Frascati, Italy (26–27 Jun 1995).

11Radarsat International (RSI), RADARSAT Illuminated—Your Guide ToProducts and Services, Vancouver, BC, Canada (1995).

12Vachon, P. W., Gray, A. L., Livingstone, C. E., and Luscombe, A. P.,“Adaptive Compensation of Radarsat SAR Analog-to-Digital ConverterSaturation Power Loss,” in Proc. Int. Symp. Geomatics in the Era ofRADARSAT (GER ’97) (on CD-ROM), Ottawa, Canada (25–30 May 1997).

13Werle, D., “Bangladesh 1998 Flood: Remote Sensing Emergency,” ÆrdeEnvironmental Research Report to the Canada Centre for Remote Sensing (CCRS)and the Canadian International Development Agency (CIDA), Halifax, NovaScotia, Canada (1999).

THE AUTHORS

DIRK WERLE is affiliated with Ærde Environmental Researchin Halifax, Nova Scotia, Canada. His e-mail address isdwerle@fox.nstn.ca.

TIMOTHY C. MARTIN is affiliated with EGIS II in Dhaka,Bangladesh. His e-mail address is tmartin@ezlink.com.

KHALED HASAN is affiliated with EGIS II in Dhaka,Bangladesh. His e-mail address is khasan@cegisbd.com.

HNS HOPKINS APL TECHNICAL DIGEST, VOLUME 21, NUMBER 1 (2000)