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Satellite-Based Studies of the 1997 Oder Flood Event in the Southern Baltic Sea
Satellite-Based Studies of the 1997 Oder Flood Event in the Southern Baltic Sea H. Siegel* and M. Gerth*
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n summer 1997 the largest Oder River flood of this century occurred from drainage area in Poland, the Czech Republic, and Germany. The spread of floodwater, which reached the Pomeranian Bight of the Baltic Sea end of July, was monitored and investigated by satellite data, shipborne measurements, and coupled biophysical modeling to document impacted coastal waters and to assess changes in water quality and influences on the ecosystem. This paper presents the spatial and temporal floodwater distribution patterns as determined from satellite data with different spectral ranges and spatial resolutions and the relationships of these flooding patterns to meteorological conditions and river discharge rates. The flooding patterns were observed using approximately 80 sea surface temperature (SST) maps, derived from infrared data of the Advanced Very High Resolution Radiometer (AVHRR) operating on weather satellites of the National Oceanic and Atmospheric Administration (NOAA). The flood period was characterized by dominant easterly winds transporting the floodwater along the German coast into the Arkona Sea. In situ measurements and data of the visible spectral range from NOAA weather satellites and the Indian satellites IRS-P3 and IRS-1C with ocean color sensors were used to study the distribution of water constituents in the Szczecin Lagoon and Pomeranian Bight and to verify results from the SSTs. The maximum horizontal extent of discharged river water covered an area larger than 3,000 km2 in the Pomeranian Bight and the southern Arkona Sea by 20 August. The eastern part of the Szczecin Lagoon was directly influenced by the floodwaters, leading to lower concentrations of water constituents and verifying the diluting effect.
* Baltic Sea Research Institute Warnemu¨nde, Seestr. 15, D-18119 Rostock, Germany Address correspondence to H. Siegel, Baltic Sea Research Institute, Seestrasse 15, 18119 Rostock, Germany. E-mail: herbert.siegel@ io-warnemuende.de Accepted 18 January 2000. REMOTE SENS. ENVIRON. 73:207–217 (2000) Elsevier Science Inc., 2000 655 Avenue of the Americas, New York, NY 10010
The in situ measurements in combination with the satellite observations provided an added assessment of potential impacts from floodwaters on the affected coastal ecosystem. Elsevier Science Inc., 2000
INTRODUCTION The Oder River provides the fifth-largest river runoff into the entire Baltic Sea and, in particular, the largest discharge of the southwestern part of the Baltic. The annual discharge amounts to about 18 km3 water originating from a drainage area of 119,000 km2. This region has been intensely studied in recent years to understand better the transport and turnover processes (Bodungen et al., 1995). Based on satellite-derived sea surface temperatures (SSTs), typical distribution patterns were derived for the eight compass point wind directions. These patterns helped to determine the main transport direction and to identify the accumulation areas for prevailing wind directions (Siegel et al., 1996; Siegel et al., 1999). Dominating westerly and easterly winds drive the river water eastward along the Polish coast into the Bay of Gdansk or northwestward along the German coast into the Arkona Sea. In summer 1997, the Oder River flooded during two periods of exceptionally heavy rainfall in July (Fuchs and Rapp, 1997; cf. Siegel et al., 1998b). During the first period (4 to 7 July), the rainfall maximum was centered in the mountains in the border region between Poland and the Czech Republic. In the second period (18 to 21 July) heavy rainfall covered the entire Oder River basin. In southern Poland, the rainfall rate in July exceeded more than five times the long-term mean values (Fuchs and Rapp, 1997). These extreme precipitations induced the highest flooding of the Oder River during the past 50 years with discharge rates in Eisenhu¨ttenstadt exceeding 2,500 m3/s (normally about 400 m3/s for the summer period) and with a 4-m increase in flood stage from mid 0034-4257/00/$–see front matter PII S0034-4257(00)00095-X
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July to 5 August. The flood reached the Szczecin Lagoon on 20 July (Rosenthal et al., 1998) and the Pomeranian Bight on 28 July (Mohrholz et al., 1998). For summer 1997 Mohrholz et al. (1998) calculated a total discharge of about 9 km3 in the three summer months (June to August) into the Pomeranian Bight, 6.5 km3 more than the long-term mean value during this season. The results from detailed interdisciplinary investigations of the Oder flood during the summer of 1997 included the measurement of river flows and nutrients (Mohrholz et al., 1998), the spread of the nutrient- and pollutant-loaded Oder plume forced by river discharge rates and the local wind (Siegel et al., 1998b), and effects on the ecosystem of the southern Baltic Sea (Humborg et al., 1998), have been previously published. The objectives of this paper is to document Oder River flood patterns and affected areas as determined from synoptic satellite data and demonstrate the important contributions satellite data can make to interdisciplinary investigations of such episodic events. AREA OF INVESTIGATION AND METHODS The study area (Fig. 1) covers the Oder River discharge area including the Oder estuary, the Pomeranian Bight, and the southwestern Baltic Sea. The Oder River represents the largest freshwater supply to the southwestern Baltic Sea and is located on the border between Poland and Germany. The drainage area, mainly located in Poland, also includes parts of the Czech Republic and Germany. The Oder water is transported through the Szczecin Lagoon and the three river branches Peenestrom (10%), Swina (80%), and Dziwna (10%) into the Pomeranian Bight. Bottom topography, the station net, and the location of the Oceanographic Data Aquisition System (ODAS) “Oder Bank” of the German Monitoring Network MARNET (Kru¨ger et al., 1996) are also shown in Fig. 1. The 20-m isobath is the northern extent of the Pomeranian Bight. Oder River water contains high concentrations of optically active water constituents that influence the ocean color, but a limited number of appropriate satellite sensors was available during the flood period in summer 1997. The ocean color sensor Multispectral Optical Sensor (MOS) on the Indian satellite IRS-P3 was operational, but was in a stellar mode from mid-June until 13 August. Thus prior to 13 August the investigation was based on data received from the NOAA weather satellites and the Indian satellite, IRS-1C. Variations in the distribution patterns of the Oder floodwater were observed daily using SST maps derived from National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) (1-km pixel resolution, cf. Siegel et al., 1994; Siegel et al., 1998b). Approximately 80 NOAA scenes, provided by the Federal Maritime and Hydrographic Agency (BSH) in Ham-
burg, were received from two NOAA satellites (NOAA12 and NOAA-14). In addition, satellite data in the visible spectral range of the sensors NOAA-AVHRR and the Wide Field of View Sensor (WiFS) with a 185-m resolution on the Indian satellites IRS-P3 and IRS-1C were used to document the distribution of water constituents. The WiFS data were provided by the Remote Sensing Data Center of the German Aerospace Center (DLR) and EUROMAP in Neustrelitz. The concentration of suspended matter was derived by comparison with in situ shipboard measurements. Atmospheric corrected MOS data, provided by the Institute of Space Sensor Technology of the DLR in Berlin, were used to derive different water constituents on the basis of ground truth algorithms retrieved for the Pomeranian Bight as described by Siegel et al. (1998a). Based on near-real-time transfer of NOAA-SST images to research vessel “Professor Albrecht Penck” between 26 July and 13 August, the sampling station grids of the four surveys (Fig. 1) were adjusted to the patterns observed in the satellite data. During the ship surveys, vertical profiles of hydrographical, biogeochemical, and optical variables were recorded (Siegel et al., 1998b). Wind speed and direction were measured continuously at the ODAS station (Fig. 1). Discharge rates of the main outflow through the Swina river were computed by a barotropic model based on sea level records in the Szczecin Lagoon and in the Pomeranian Bight (Mohrholz et al., 1998). RESULTS AND DISCUSSION The spreading of the floodwater was observed by all available NOAA-SST images in relation to continuous wind measurements at the ODAS “Oder Bank” and modeled discharge rates provided by Mohrholz et al. (1998). During the preflood phase easterly winds were recorded until 19 July (Fig. 2b). After 2 days of northerly winds up to 14 m/s and an inflow of seawater into the Szczecin Lagoon, on 21 July the discharge increased from approximately 1,000 m3/s to 2,000 m3/s (Fig. 2c). This high discharge rate continued until 19 August with only short interruptions. The increased discharge was driven by the floodwater arriving in the eastern part of the Lagoon on 20 July (Rosenthal et al., 1998) and the Pomeranian Bight on 28 July (Mohrholz et al., 1998). The wind situation from 19 until 25 July was characterized by a northeasterly direction, inducing distribution patterns as seen in the SST image from 23 July in Fig. 3a. The warm Oder water was guided westward by a coastal jet associated with the cold upwelling off the Polish coast and then moved northwestward along the German islands Usedom and Ru¨gen into the Arkona Sea by the Ekman offshore transport. The wind turned to the west on 25 July and continued with this general pattern until 31 July. The associated onshore Ekman transport
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Figure 1. Map of the study area of the Pomeranian Bight (PB) including a map of the entire Baltic Sea, the positions of the in situ stations, and the location of the ODAS “Oder Bank” from the German Monitoring Network MARNET.
and eastward coastal jet along the Polish coast was seen as a band of warmer water in the SST map on 28 July as shown in Fig. 3a. On this day the floodwaters reached the Pomeranian Bight with concentrations of suspended matter higher than 16 g/m3. Wind speeds of up to 13 m/ s mixed the lagoon water and increased the sediment load up to 45 g/m3 in the Szczecin Lagoon (Rosenthal et al., 1998). This period was interrupted by a short episode of northeasterly winds on 30 July, leading to the formation of a new plume. The following period of weak winds from 2 to 4 August with varying wind directions between east and southwest produced an outflow of about 2,300 m3/s (Fig. 2c). From 4 to 7 August during northeasterly winds with a speed up to 10 m/s, a plume was observed in front of the Swina mouth close to the Usedom coast. The wind changed to the east during the following days, inducing offshore Ekman transport and an upwelling of cold water at the Polish coast, guiding the Swina outflow along the German coast and further into the Arkona Sea. The scenes from 8 and 9 August in Fig. 3a show the typical patterns observed in this period. The upwelling area north of Ru¨gen Island forced warmer water into the
Arkona Sea, and the upwelling area off the island Hiddensee limited the further penetration into the western Baltic Sea during this easterly wind period. An increasing southerly wind component starting on 12 August induced an upwelling off the Usedom coast, which continued in the following days (Figs. 3b, 4) and is seen also in the shipborne measurements in Fig. 5. A short episode of northwesterly winds on 14 and 15 August stopped the outflow of the Swina (⫺500 m3/s; cf. Fig. 2c) and induced a new plume with nearly the same discharge rate of about 2,000 m3/s. This plume was formed as a narrow transport region along the Polish coast as seen on 16 August (Fig. 6). No significant effects were observed on the open northwestern Pomeranian Bight. The southeasterly winds that followed reactivated the upwelling at the Polish coast and produced an offshore Ekman transport after 19 August and an upwelling area along the Usedom coast as shown in the SST from 20 August (Fig. 3b). The discharge rate was in the order of 1,000 m3/s and was observed until 25 August. With the change to southeasterly winds and the subsequent induced upwelling along the German coast the period of direct floodwater influence
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Figure 2. Wind speed (a) and direction (b) at the ODAS “Oder Bank” and the discharge rates through the Swina (c, calculated by Mohrholz et al., 1998) for the flood period July to August 1997.
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Figure 3. (a) SST patterns during the Oder flood reflecting the spreading of the warm plume water from 23 July until 9 August. (b) SST patterns during the Oder flood reflecting the spreading of the warm plume water from 13 until 1 September.
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Figure 3. Continued.
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Figure 4. Comparison between SST (gray scale) and measured temperature (isolines) distribution on 12 August 1997.
came to an end. From 26 to 30 August the outflow was slowed by strong westerly winds as can be seen in Fig. 3b. The satellite monitoring of the Oder floodwaters has shown that the spreading was mainly limited to the western and central Pomeranian Bight and the southern Arkona Sea. The comparison of the SST patterns in relation to the local wind is consistent with the general discharge patterns described by Siegel et al. (1996). Therefore, it is concluded that the water temperature is an acceptable variable to identify the patterns of river water in cases where a shallow plume (5 m to 7 m, Siegel et al., 1998b) is warmer or heated faster than the surrounding area. This process is confirmed in Fig. 4 where in situ surface temperatures (isotherms) measured from 11 August 1997 18 UTC until 12 August 22 UTC are compared with the NOAA-SST map (gray scale) received on 12 August 2 UTC. The temperatures appear to be well matched in the area of the station net (indicated by triangles), especially in the maximum temperature and the upwelling features along the Usedom coast. The potential of satellite data is clearly seen in the enhanced investigation area as well as in the detection of the plume maximum extension and the driving upwelling system at the Polish coast. When the surface temperature is strongly influenced by solar heating, the open sea boundary of the river water cannot be clearly identified. In these cases, suspended and dissolved water constituents and their optical properties characterizing the river water and its ocean color can be used to identify this boundary. In Fig. 5 the horizontal distributions of temperature, salinity, chlorophyll, suspended particulate matter (SPM), transparency
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(Secchi depths), and absorption of yellow substances are compared for the fourth measuring period on 11–12 August. The river discharge can be identified by temperature of more than 22⬚C, salinities below 5 PSU, chlorophyll concentrations between 8 mg/m3 and 20 mg/m3, and by SPM of more than 5 g/m3 and 6 g/m3. Absorption values of yellow substances at a wavelength of 440 nm exceeded 0.3 m⫺1 to 0.6 m⫺1 and the Secchi depths were lower than 2 m, with some below 1 m. These typical values for the river plume water were observed in the entire western Pomeranian Bight to the boundary off northeastern Ru¨gen island. The distribution pattern consists of two plumes of discharge water around the Swina mouth and east of the entrance to the Greifswald Bay. This pattern is due to the pulsating outflow through the Swina (Fig. 2c) with low discharge rates of about 1,400 m3/s on 11 August and increasing rates on 12 August of up to 2,000 m3/s at noon and 2,400 m3/s in the evening. The plumes are separated by an upwelling located near Usedom island. Due to the diluting effects of the floodwaters during the four field surveys, the maximum concentrations of SPM decreased from values of 20 g/m3 found in the Pomeranian Bight on 28 July and on 5 August in the Szczecin Lagoon waters to values of 10 g/m3 on 11 and 12 August (Siegel et al. 1998b). Furthermore, the river water spread in the Pomeranian Bight was such that concentrations higher than 3 g/m3 were found during the first three surveys only in the western Bight close to the German coast, but covered the main part of the Bight on 11/12 August. The first ocean color image of MOS after the stellar mode from 16 August (Fig. 6b to 6d) represents a situation after 2 days of northerly to northwesterly winds that stopped the discharge (Fig. 2c) as described above. Winds from the west pushed a band of warm water along the Polish coast (Fig. 6a). High chlorophyll concentrations and absorption of yellow substances within the band (Figs. 6b, 6d) indicate the presence of river water. A plume in the Swina river mouth area close to Usedom Island indicates the new outflow event, which is more pronounced in the MOS-derived water constituents than in the SST map. Concentrations of chlorophyll and suspended matter as well as the absorption of yellow substances have the same magnitude as the ship measurements presented in Fig. 5. The maximum extension of the floodwater was reached around 20 August as seen in the SST maps in Fig. 3b. The change to more southeasterly wind directions exceeded the upwelling at the Usedom coast and a spreading of the discharge to the central part of the Pomeranian Bight. The concentration of suspended matter derived from IRS-P3-WIFS image on 21 August (Fig. 7) documents the extent of floodwaters in the western part of the Pomeranian Bight and the southern Arkona Sea, with an area of more than 3,000 km2. The highest concentrations of SPM at 8 g/m3 to 10 g/m3 were found
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Figure 5. Spread of temperature (a), salinity (b), chlorophyll (c), SPM (d), Secchi depth (e), and absorption of yellow substances (f) on 11/12 August 1997.
east of Ru¨gen island, where the flood plume is separated from the Arkona Sea water. Low concentrations in the coastal areas off northern Ru¨gen are a result of upwelling processes. This upwelling also occurred on the south coast in Greifswald Bay with lower water temperature, chlorophyll, and suspended matter (Fig. 6). In the Szczecin Lagoon, the transition area between the Oder River and the Pomeranian Bight, the concentration of water constituents, optical properties, and in some situations also the water temperature differed in
the eastern and western parts. The eastern part is covered by the Oder floodwater (10 g/m3 to 20g/m3), whereas old lagoon water with high concentrations of suspended materials (⬎30g/m3) still exists in the western part. The series of WiFS-derived suspended matter in Fig. 8 illustrate the diluting effect in the Szczecin Lagoon. After the floodwater arrived in the Lagoon, the strong mixing and outflow observed on 28 July was already decreasing the content of suspended matter on 30 July in the eastern part of the Lagoon. The western part
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Figure 6. Distribution of SST (a) and MOS-derived chlorophyll (b), SPM (c), and absorption of yellow substances (d) on 16th August 1997.
Figure 7. Distribution of SST (a) and WIFS-derived suspended matter (b) on 21 August 1997 during maximum extension.
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Figure 8. Suspended matter distribution in the Szczecin Lagoon derived from WiFS sensors of the Indian satellites IRS-P3 and IRS-1C.
was excluded from this water exchange. The Oder water crossed the Lagoon directly. The images from 9, 16, and 21 August in Fig. 8b to 8d show that the inflow of Oder floodwater into the western part started at the southern coast of the Lagoon. On 21 August the Oder water reached the center of the western part. CONCLUSIONS The combination of synoptic remote sensing data, shipborne measurements, and numerical modeling allowed a consistent description of the largest flood in the drainage area of the Oder River in this century. On the basis of satellite data of different spectral ranges and spatial resolutions the river plume was monitored in terms of SST and water constituents over 5 weeks in summer 1997, including nonsampling periods and regions. The in situ measurements revealed the vertical structure used in the assessment of effects and consequences on the ecosystem. The main transport direction along the German coast into the Arkona Sea during dominant low easterly winds as well as the variable responses to short-term changes were identified using approximately 80 SST im-
ages in relation to local wind and discharge rates. Satellite-derived maps of water constituents showed the maximum horizontal extent of total suspended matter in the western Pomeranian Bight and the southern Arkona Sea covering more than 3,000 km2 around 20 August. The direct influence of river discharge at the German coast was reduced by the upwelling processes due to southeasterly winds. WiFS-retrieved suspended matter concentrations demonstrated that the western part of the Szczecin Lagoon was mainly excluded from the water exchange and a dilution effect occurred in the eastern part. From the large number of chemical and biological measurements (e.g., nutrients, trace metals, and chlorinated carbons), it can be concluded that the strong precipitation led to a dilution effect in the Oder River water such that the floodwater contained concentrations in a range typical with annual spring flows. Thus there were no long-lasting flood effects and harmful impacts on the ecosystem of the Pomeranian Bight and the entire Baltic Sea. The authors would like to thank Mrs. G. Tschersich of the BSH Hamburg for providing the NOAA-AVHRR images. The IRSP3-WIFS data were kindly provided by the Remote Sensing
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Center Neustrelitz of the DLR and the atmospheric corrected MOS scene by H. Krawczyk DLR-Institute of Space Technology Berlin. The authors thank V. Mohrholz for river discharge rates, B. Davenport and F. Tanis for editorial support, and the reviewers for helpful comments. The investigation was supported by projects of the DLR (50 EE 92168).
REFERENCES Bodungen, B. V., Graeve, M., Kube, J., Lass, H.-U., MeyerHarms, B., Mumm, N., Nagel, K., Pollehne, F., Powilleit, M., Reckermann, M., Sattler, C., Siegel, H., and Wodarg, D. (1995), Stoff-Flu¨sse am Grenzfluß—Transport- und Umsatz¨ bergangsbereich zwischen Odera¨stuar und prozesse im U Pommerscher Bucht (TRUMP). Geowissenschaften 13: 479–485. Fuchs, T., and Rapp, J. (1997), Zwei außergewo¨hnlich starke Regenepisoden als Ursache des Oderhochwassers im Juli 1997. Beilage zur Wetterkarte des DWD 158:1–4. Humborg, C., Nausch, G., Neumann, T., Pollehne, F., and Wasmund, N. (1998), The exceptional Oder flood in summer 1997—The fate of nutrients and particulate organic matter. Dt. Hydrogr. Z. 2/3:169–181. Kru¨ger, S., Roeder, W., Wlost, K.-P. (1996), The IOW—Baltic Stations DARSS SILL & ODER-BANK—Part of the German Marine Monitoring Network—MARNET. In Proc. Bal-
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tic Marine Science Conference, 21–26 October 1996, Rønne, Denmark. Mohrholz, V., Pastuszak, M., Sitek, S., Nagel, K., and Lass, H.-U. (1998), The exceptional Oder flood in summer 1997—Riverine mass and nutrient transport into the Pomeranian Bight. Dt. Hydrogr. Z. 2/3:129–144. Rosenthal, W., Wolf, T., Witte, G., Buchholz, W., and Rybaczok, P. (1998), Measured and modelled water transport in the Odra estuary for the period July and August 1997. Dt. Hydrogr. Z. 2/3:215–230. Siegel, H., Gerth, M., and Mutzke, A. (1999), Dynamics of the Oder river plume in the southern Baltic Sea—Satellite data and numerical modelling, Cont. Shelf Res. 19:1143–1159. Siegel, H., Gerth, M., and Ohde, T. (1998a), Case 1 and Case 2 Algorithms for MOS-IRS and their application to different regions, In Proc. 2nd Int. Workshop on MOS-IRS and Ocean Colour, 28–30 April 1997, Berlin, pp. 155–162. Siegel, H., Gerth, M., Rudloff, R., and Tschersich, G. (1994), Dynamical features in the western Baltic Sea investigated by NOAA-AVHRR-Data. Dt. Hydrogr. Z. 3:191–209. Siegel, H., Gerth, M., and Schmidt, T. (1996), Water exchange in the Pomeranian Bight investigated by satellite data and shipborne measurements, Cont. Shelf Res. 16:1793–1817. Siegel, H., Mattha¨us, W., Bruhn, R., Gerth, M., Nausch, G., Neumann, T., and Pohl, C. (1998b), The exceptional Oder flood in summer 1997—Distribution of the Oder discharge in the Pomeranian Bight. Dt. Hydrogr. Z. 2/3:145–167.
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n summer 1997 the largest Oder River flood of this century occurred from drainage area in Poland, the Czech Republic, and Germany. The spread of floodwater, which reached the Pomeranian Bight of the Baltic Sea end of July, was monitored and investigated by satellite data, shipborne measurements, and coupled biophysical modeling to document impacted coastal waters and to assess changes in water quality and influences on the ecosystem. This paper presents the spatial and temporal floodwater distribution patterns as determined from satellite data with different spectral ranges and spatial resolutions and the relationships of these flooding patterns to meteorological conditions and river discharge rates. The flooding patterns were observed using approximately 80 sea surface temperature (SST) maps, derived from infrared data of the Advanced Very High Resolution Radiometer (AVHRR) operating on weather satellites of the National Oceanic and Atmospheric Administration (NOAA). The flood period was characterized by dominant easterly winds transporting the floodwater along the German coast into the Arkona Sea. In situ measurements and data of the visible spectral range from NOAA weather satellites and the Indian satellites IRS-P3 and IRS-1C with ocean color sensors were used to study the distribution of water constituents in the Szczecin Lagoon and Pomeranian Bight and to verify results from the SSTs. The maximum horizontal extent of discharged river water covered an area larger than 3,000 km2 in the Pomeranian Bight and the southern Arkona Sea by 20 August. The eastern part of the Szczecin Lagoon was directly influenced by the floodwaters, leading to lower concentrations of water constituents and verifying the diluting effect.
* Baltic Sea Research Institute Warnemu¨nde, Seestr. 15, D-18119 Rostock, Germany Address correspondence to H. Siegel, Baltic Sea Research Institute, Seestrasse 15, 18119 Rostock, Germany. E-mail: herbert.siegel@ io-warnemuende.de Accepted 18 January 2000. REMOTE SENS. ENVIRON. 73:207–217 (2000) Elsevier Science Inc., 2000 655 Avenue of the Americas, New York, NY 10010
The in situ measurements in combination with the satellite observations provided an added assessment of potential impacts from floodwaters on the affected coastal ecosystem. Elsevier Science Inc., 2000
INTRODUCTION The Oder River provides the fifth-largest river runoff into the entire Baltic Sea and, in particular, the largest discharge of the southwestern part of the Baltic. The annual discharge amounts to about 18 km3 water originating from a drainage area of 119,000 km2. This region has been intensely studied in recent years to understand better the transport and turnover processes (Bodungen et al., 1995). Based on satellite-derived sea surface temperatures (SSTs), typical distribution patterns were derived for the eight compass point wind directions. These patterns helped to determine the main transport direction and to identify the accumulation areas for prevailing wind directions (Siegel et al., 1996; Siegel et al., 1999). Dominating westerly and easterly winds drive the river water eastward along the Polish coast into the Bay of Gdansk or northwestward along the German coast into the Arkona Sea. In summer 1997, the Oder River flooded during two periods of exceptionally heavy rainfall in July (Fuchs and Rapp, 1997; cf. Siegel et al., 1998b). During the first period (4 to 7 July), the rainfall maximum was centered in the mountains in the border region between Poland and the Czech Republic. In the second period (18 to 21 July) heavy rainfall covered the entire Oder River basin. In southern Poland, the rainfall rate in July exceeded more than five times the long-term mean values (Fuchs and Rapp, 1997). These extreme precipitations induced the highest flooding of the Oder River during the past 50 years with discharge rates in Eisenhu¨ttenstadt exceeding 2,500 m3/s (normally about 400 m3/s for the summer period) and with a 4-m increase in flood stage from mid 0034-4257/00/$–see front matter PII S0034-4257(00)00095-X
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July to 5 August. The flood reached the Szczecin Lagoon on 20 July (Rosenthal et al., 1998) and the Pomeranian Bight on 28 July (Mohrholz et al., 1998). For summer 1997 Mohrholz et al. (1998) calculated a total discharge of about 9 km3 in the three summer months (June to August) into the Pomeranian Bight, 6.5 km3 more than the long-term mean value during this season. The results from detailed interdisciplinary investigations of the Oder flood during the summer of 1997 included the measurement of river flows and nutrients (Mohrholz et al., 1998), the spread of the nutrient- and pollutant-loaded Oder plume forced by river discharge rates and the local wind (Siegel et al., 1998b), and effects on the ecosystem of the southern Baltic Sea (Humborg et al., 1998), have been previously published. The objectives of this paper is to document Oder River flood patterns and affected areas as determined from synoptic satellite data and demonstrate the important contributions satellite data can make to interdisciplinary investigations of such episodic events. AREA OF INVESTIGATION AND METHODS The study area (Fig. 1) covers the Oder River discharge area including the Oder estuary, the Pomeranian Bight, and the southwestern Baltic Sea. The Oder River represents the largest freshwater supply to the southwestern Baltic Sea and is located on the border between Poland and Germany. The drainage area, mainly located in Poland, also includes parts of the Czech Republic and Germany. The Oder water is transported through the Szczecin Lagoon and the three river branches Peenestrom (10%), Swina (80%), and Dziwna (10%) into the Pomeranian Bight. Bottom topography, the station net, and the location of the Oceanographic Data Aquisition System (ODAS) “Oder Bank” of the German Monitoring Network MARNET (Kru¨ger et al., 1996) are also shown in Fig. 1. The 20-m isobath is the northern extent of the Pomeranian Bight. Oder River water contains high concentrations of optically active water constituents that influence the ocean color, but a limited number of appropriate satellite sensors was available during the flood period in summer 1997. The ocean color sensor Multispectral Optical Sensor (MOS) on the Indian satellite IRS-P3 was operational, but was in a stellar mode from mid-June until 13 August. Thus prior to 13 August the investigation was based on data received from the NOAA weather satellites and the Indian satellite, IRS-1C. Variations in the distribution patterns of the Oder floodwater were observed daily using SST maps derived from National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) (1-km pixel resolution, cf. Siegel et al., 1994; Siegel et al., 1998b). Approximately 80 NOAA scenes, provided by the Federal Maritime and Hydrographic Agency (BSH) in Ham-
burg, were received from two NOAA satellites (NOAA12 and NOAA-14). In addition, satellite data in the visible spectral range of the sensors NOAA-AVHRR and the Wide Field of View Sensor (WiFS) with a 185-m resolution on the Indian satellites IRS-P3 and IRS-1C were used to document the distribution of water constituents. The WiFS data were provided by the Remote Sensing Data Center of the German Aerospace Center (DLR) and EUROMAP in Neustrelitz. The concentration of suspended matter was derived by comparison with in situ shipboard measurements. Atmospheric corrected MOS data, provided by the Institute of Space Sensor Technology of the DLR in Berlin, were used to derive different water constituents on the basis of ground truth algorithms retrieved for the Pomeranian Bight as described by Siegel et al. (1998a). Based on near-real-time transfer of NOAA-SST images to research vessel “Professor Albrecht Penck” between 26 July and 13 August, the sampling station grids of the four surveys (Fig. 1) were adjusted to the patterns observed in the satellite data. During the ship surveys, vertical profiles of hydrographical, biogeochemical, and optical variables were recorded (Siegel et al., 1998b). Wind speed and direction were measured continuously at the ODAS station (Fig. 1). Discharge rates of the main outflow through the Swina river were computed by a barotropic model based on sea level records in the Szczecin Lagoon and in the Pomeranian Bight (Mohrholz et al., 1998). RESULTS AND DISCUSSION The spreading of the floodwater was observed by all available NOAA-SST images in relation to continuous wind measurements at the ODAS “Oder Bank” and modeled discharge rates provided by Mohrholz et al. (1998). During the preflood phase easterly winds were recorded until 19 July (Fig. 2b). After 2 days of northerly winds up to 14 m/s and an inflow of seawater into the Szczecin Lagoon, on 21 July the discharge increased from approximately 1,000 m3/s to 2,000 m3/s (Fig. 2c). This high discharge rate continued until 19 August with only short interruptions. The increased discharge was driven by the floodwater arriving in the eastern part of the Lagoon on 20 July (Rosenthal et al., 1998) and the Pomeranian Bight on 28 July (Mohrholz et al., 1998). The wind situation from 19 until 25 July was characterized by a northeasterly direction, inducing distribution patterns as seen in the SST image from 23 July in Fig. 3a. The warm Oder water was guided westward by a coastal jet associated with the cold upwelling off the Polish coast and then moved northwestward along the German islands Usedom and Ru¨gen into the Arkona Sea by the Ekman offshore transport. The wind turned to the west on 25 July and continued with this general pattern until 31 July. The associated onshore Ekman transport
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Figure 1. Map of the study area of the Pomeranian Bight (PB) including a map of the entire Baltic Sea, the positions of the in situ stations, and the location of the ODAS “Oder Bank” from the German Monitoring Network MARNET.
and eastward coastal jet along the Polish coast was seen as a band of warmer water in the SST map on 28 July as shown in Fig. 3a. On this day the floodwaters reached the Pomeranian Bight with concentrations of suspended matter higher than 16 g/m3. Wind speeds of up to 13 m/ s mixed the lagoon water and increased the sediment load up to 45 g/m3 in the Szczecin Lagoon (Rosenthal et al., 1998). This period was interrupted by a short episode of northeasterly winds on 30 July, leading to the formation of a new plume. The following period of weak winds from 2 to 4 August with varying wind directions between east and southwest produced an outflow of about 2,300 m3/s (Fig. 2c). From 4 to 7 August during northeasterly winds with a speed up to 10 m/s, a plume was observed in front of the Swina mouth close to the Usedom coast. The wind changed to the east during the following days, inducing offshore Ekman transport and an upwelling of cold water at the Polish coast, guiding the Swina outflow along the German coast and further into the Arkona Sea. The scenes from 8 and 9 August in Fig. 3a show the typical patterns observed in this period. The upwelling area north of Ru¨gen Island forced warmer water into the
Arkona Sea, and the upwelling area off the island Hiddensee limited the further penetration into the western Baltic Sea during this easterly wind period. An increasing southerly wind component starting on 12 August induced an upwelling off the Usedom coast, which continued in the following days (Figs. 3b, 4) and is seen also in the shipborne measurements in Fig. 5. A short episode of northwesterly winds on 14 and 15 August stopped the outflow of the Swina (⫺500 m3/s; cf. Fig. 2c) and induced a new plume with nearly the same discharge rate of about 2,000 m3/s. This plume was formed as a narrow transport region along the Polish coast as seen on 16 August (Fig. 6). No significant effects were observed on the open northwestern Pomeranian Bight. The southeasterly winds that followed reactivated the upwelling at the Polish coast and produced an offshore Ekman transport after 19 August and an upwelling area along the Usedom coast as shown in the SST from 20 August (Fig. 3b). The discharge rate was in the order of 1,000 m3/s and was observed until 25 August. With the change to southeasterly winds and the subsequent induced upwelling along the German coast the period of direct floodwater influence
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Figure 2. Wind speed (a) and direction (b) at the ODAS “Oder Bank” and the discharge rates through the Swina (c, calculated by Mohrholz et al., 1998) for the flood period July to August 1997.
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Figure 3. (a) SST patterns during the Oder flood reflecting the spreading of the warm plume water from 23 July until 9 August. (b) SST patterns during the Oder flood reflecting the spreading of the warm plume water from 13 until 1 September.
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Figure 3. Continued.
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Figure 4. Comparison between SST (gray scale) and measured temperature (isolines) distribution on 12 August 1997.
came to an end. From 26 to 30 August the outflow was slowed by strong westerly winds as can be seen in Fig. 3b. The satellite monitoring of the Oder floodwaters has shown that the spreading was mainly limited to the western and central Pomeranian Bight and the southern Arkona Sea. The comparison of the SST patterns in relation to the local wind is consistent with the general discharge patterns described by Siegel et al. (1996). Therefore, it is concluded that the water temperature is an acceptable variable to identify the patterns of river water in cases where a shallow plume (5 m to 7 m, Siegel et al., 1998b) is warmer or heated faster than the surrounding area. This process is confirmed in Fig. 4 where in situ surface temperatures (isotherms) measured from 11 August 1997 18 UTC until 12 August 22 UTC are compared with the NOAA-SST map (gray scale) received on 12 August 2 UTC. The temperatures appear to be well matched in the area of the station net (indicated by triangles), especially in the maximum temperature and the upwelling features along the Usedom coast. The potential of satellite data is clearly seen in the enhanced investigation area as well as in the detection of the plume maximum extension and the driving upwelling system at the Polish coast. When the surface temperature is strongly influenced by solar heating, the open sea boundary of the river water cannot be clearly identified. In these cases, suspended and dissolved water constituents and their optical properties characterizing the river water and its ocean color can be used to identify this boundary. In Fig. 5 the horizontal distributions of temperature, salinity, chlorophyll, suspended particulate matter (SPM), transparency
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(Secchi depths), and absorption of yellow substances are compared for the fourth measuring period on 11–12 August. The river discharge can be identified by temperature of more than 22⬚C, salinities below 5 PSU, chlorophyll concentrations between 8 mg/m3 and 20 mg/m3, and by SPM of more than 5 g/m3 and 6 g/m3. Absorption values of yellow substances at a wavelength of 440 nm exceeded 0.3 m⫺1 to 0.6 m⫺1 and the Secchi depths were lower than 2 m, with some below 1 m. These typical values for the river plume water were observed in the entire western Pomeranian Bight to the boundary off northeastern Ru¨gen island. The distribution pattern consists of two plumes of discharge water around the Swina mouth and east of the entrance to the Greifswald Bay. This pattern is due to the pulsating outflow through the Swina (Fig. 2c) with low discharge rates of about 1,400 m3/s on 11 August and increasing rates on 12 August of up to 2,000 m3/s at noon and 2,400 m3/s in the evening. The plumes are separated by an upwelling located near Usedom island. Due to the diluting effects of the floodwaters during the four field surveys, the maximum concentrations of SPM decreased from values of 20 g/m3 found in the Pomeranian Bight on 28 July and on 5 August in the Szczecin Lagoon waters to values of 10 g/m3 on 11 and 12 August (Siegel et al. 1998b). Furthermore, the river water spread in the Pomeranian Bight was such that concentrations higher than 3 g/m3 were found during the first three surveys only in the western Bight close to the German coast, but covered the main part of the Bight on 11/12 August. The first ocean color image of MOS after the stellar mode from 16 August (Fig. 6b to 6d) represents a situation after 2 days of northerly to northwesterly winds that stopped the discharge (Fig. 2c) as described above. Winds from the west pushed a band of warm water along the Polish coast (Fig. 6a). High chlorophyll concentrations and absorption of yellow substances within the band (Figs. 6b, 6d) indicate the presence of river water. A plume in the Swina river mouth area close to Usedom Island indicates the new outflow event, which is more pronounced in the MOS-derived water constituents than in the SST map. Concentrations of chlorophyll and suspended matter as well as the absorption of yellow substances have the same magnitude as the ship measurements presented in Fig. 5. The maximum extension of the floodwater was reached around 20 August as seen in the SST maps in Fig. 3b. The change to more southeasterly wind directions exceeded the upwelling at the Usedom coast and a spreading of the discharge to the central part of the Pomeranian Bight. The concentration of suspended matter derived from IRS-P3-WIFS image on 21 August (Fig. 7) documents the extent of floodwaters in the western part of the Pomeranian Bight and the southern Arkona Sea, with an area of more than 3,000 km2. The highest concentrations of SPM at 8 g/m3 to 10 g/m3 were found
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Figure 5. Spread of temperature (a), salinity (b), chlorophyll (c), SPM (d), Secchi depth (e), and absorption of yellow substances (f) on 11/12 August 1997.
east of Ru¨gen island, where the flood plume is separated from the Arkona Sea water. Low concentrations in the coastal areas off northern Ru¨gen are a result of upwelling processes. This upwelling also occurred on the south coast in Greifswald Bay with lower water temperature, chlorophyll, and suspended matter (Fig. 6). In the Szczecin Lagoon, the transition area between the Oder River and the Pomeranian Bight, the concentration of water constituents, optical properties, and in some situations also the water temperature differed in
the eastern and western parts. The eastern part is covered by the Oder floodwater (10 g/m3 to 20g/m3), whereas old lagoon water with high concentrations of suspended materials (⬎30g/m3) still exists in the western part. The series of WiFS-derived suspended matter in Fig. 8 illustrate the diluting effect in the Szczecin Lagoon. After the floodwater arrived in the Lagoon, the strong mixing and outflow observed on 28 July was already decreasing the content of suspended matter on 30 July in the eastern part of the Lagoon. The western part
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Figure 6. Distribution of SST (a) and MOS-derived chlorophyll (b), SPM (c), and absorption of yellow substances (d) on 16th August 1997.
Figure 7. Distribution of SST (a) and WIFS-derived suspended matter (b) on 21 August 1997 during maximum extension.
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Figure 8. Suspended matter distribution in the Szczecin Lagoon derived from WiFS sensors of the Indian satellites IRS-P3 and IRS-1C.
was excluded from this water exchange. The Oder water crossed the Lagoon directly. The images from 9, 16, and 21 August in Fig. 8b to 8d show that the inflow of Oder floodwater into the western part started at the southern coast of the Lagoon. On 21 August the Oder water reached the center of the western part. CONCLUSIONS The combination of synoptic remote sensing data, shipborne measurements, and numerical modeling allowed a consistent description of the largest flood in the drainage area of the Oder River in this century. On the basis of satellite data of different spectral ranges and spatial resolutions the river plume was monitored in terms of SST and water constituents over 5 weeks in summer 1997, including nonsampling periods and regions. The in situ measurements revealed the vertical structure used in the assessment of effects and consequences on the ecosystem. The main transport direction along the German coast into the Arkona Sea during dominant low easterly winds as well as the variable responses to short-term changes were identified using approximately 80 SST im-
ages in relation to local wind and discharge rates. Satellite-derived maps of water constituents showed the maximum horizontal extent of total suspended matter in the western Pomeranian Bight and the southern Arkona Sea covering more than 3,000 km2 around 20 August. The direct influence of river discharge at the German coast was reduced by the upwelling processes due to southeasterly winds. WiFS-retrieved suspended matter concentrations demonstrated that the western part of the Szczecin Lagoon was mainly excluded from the water exchange and a dilution effect occurred in the eastern part. From the large number of chemical and biological measurements (e.g., nutrients, trace metals, and chlorinated carbons), it can be concluded that the strong precipitation led to a dilution effect in the Oder River water such that the floodwater contained concentrations in a range typical with annual spring flows. Thus there were no long-lasting flood effects and harmful impacts on the ecosystem of the Pomeranian Bight and the entire Baltic Sea. The authors would like to thank Mrs. G. Tschersich of the BSH Hamburg for providing the NOAA-AVHRR images. The IRSP3-WIFS data were kindly provided by the Remote Sensing
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Center Neustrelitz of the DLR and the atmospheric corrected MOS scene by H. Krawczyk DLR-Institute of Space Technology Berlin. The authors thank V. Mohrholz for river discharge rates, B. Davenport and F. Tanis for editorial support, and the reviewers for helpful comments. The investigation was supported by projects of the DLR (50 EE 92168).
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tic Marine Science Conference, 21–26 October 1996, Rønne, Denmark. Mohrholz, V., Pastuszak, M., Sitek, S., Nagel, K., and Lass, H.-U. (1998), The exceptional Oder flood in summer 1997—Riverine mass and nutrient transport into the Pomeranian Bight. Dt. Hydrogr. Z. 2/3:129–144. Rosenthal, W., Wolf, T., Witte, G., Buchholz, W., and Rybaczok, P. (1998), Measured and modelled water transport in the Odra estuary for the period July and August 1997. Dt. Hydrogr. Z. 2/3:215–230. Siegel, H., Gerth, M., and Mutzke, A. (1999), Dynamics of the Oder river plume in the southern Baltic Sea—Satellite data and numerical modelling, Cont. Shelf Res. 19:1143–1159. Siegel, H., Gerth, M., and Ohde, T. (1998a), Case 1 and Case 2 Algorithms for MOS-IRS and their application to different regions, In Proc. 2nd Int. Workshop on MOS-IRS and Ocean Colour, 28–30 April 1997, Berlin, pp. 155–162. Siegel, H., Gerth, M., Rudloff, R., and Tschersich, G. (1994), Dynamical features in the western Baltic Sea investigated by NOAA-AVHRR-Data. Dt. Hydrogr. Z. 3:191–209. Siegel, H., Gerth, M., and Schmidt, T. (1996), Water exchange in the Pomeranian Bight investigated by satellite data and shipborne measurements, Cont. Shelf Res. 16:1793–1817. Siegel, H., Mattha¨us, W., Bruhn, R., Gerth, M., Nausch, G., Neumann, T., and Pohl, C. (1998b), The exceptional Oder flood in summer 1997—Distribution of the Oder discharge in the Pomeranian Bight. Dt. Hydrogr. Z. 2/3:145–167.