PLANT COMMUNITIES RESPONSE TO FLOODWATER CONDITIONS IN ŁAWKI MARSH IN THE RIVER BIEBRZA LOWER BASIN, POLAND

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1 International conference Towards natural flood reduction strategies, Warsaw, 6-13 September 2003 PLANT COMMUNITIES RESPONSE TO FLOODWATER CONDITIONS IN ŁAWKI MARSH IN THE RIVER BIEBRZA LOWER BASIN, POLAND Tomasz Okruszko 1, Wiesław Dębek 2, Michał Wasilewicz 1 Abstract: Like other areas of its type wetlands of the Biebrza Valley, one of the most valuable wetlands complexes in Central Europe, are undergoing dynamic change. The most apparent process is plant community succession which is leading to expansion of bushes and reeds. The process is largely the result of discontinuation of harvesting and grazing of wetland meadows. Another important factor, probably also associated with the process, is decrease in moisture content of wetland sites, the result of human action and climatic change. This impact needs to be better understood, being crucial in planning active protection of the Biebrza wetlands, at present protected as a National Park and registered as a Ramsar site. The area is of international importance as a bird refuge. Plant communities response to floodwater conditions in Ławki Marsh in the River Biebrza Lower Basin, Poland CONTEXT The aim of the present study is to establish whether sites in the lower basin of the Biebrza valley are registering change in moisture content and whether the process is related to water level fluctuation in the river, flood events in particular. The area covered by the research is the Bagno Ławki fen lying in the lower basin of the Biebrza valley, an area particularly rich in nature values (fig. 1). Studies focussed on sites situated along a transect following the line of the Honczarowska Grobla dirt road which cuts across the Bagno Ławki fen in the southern area of the lower basin of the Biebrza. East to west, between the uplands, stretches a level valley peatland, its width reaching no less than 13 km. The area is probably the largest continuous peatland complex in the valley of a lowland river in Poland [DEMBEK, 2000]. A largest area in the Bagno Ławki fen is occupied by extensive peatland tracts of the floodplain. The channel zone of the river is bordered by a slightly folded zone of alluvial deposits and mud flats cut by abandoned channels. A detailed description of the geomorphology of the valley and processes shaping sites in the Lower Biebrza Basin is given in works by OKRUSZKO [1973] and ŻUREK [1991]. Analysis of present-day hydrological situation in the sites bordering the study transect based on water level readouts made daily at the water-level gauge profile at Burzyn. The Burzyn water-level gauge profile is situated 8.5 km upstream of Biebrza s confluence with the Narew closing a catchment some km 2 in area. The water-level gauge was installed in June 1930 but the present study bases on a record for period because the observation series was interrupted by the war years. The series of daily water level readouts available for the period in question includes recordings. 1 Department of Hydraulic Engineering and Environmental Recultivation, Warsaw Agricultural University, Nowoursynowska St. 166, Warsaw, ph/fax Department of Nature Protection in Rural Areas, Institute for Land Reclamation and Grassland Farming (IMUZ), Falenty, Raszyn, Poland

2 2 Plant communities response to floodwater conditions in Ławki Marsh Prior to subjecting hydrological data to analysis it was necessary to account for the lowered of the water level in the river. This was done by taking a measurement of the water level in the Biebrza during the high water period at the stretch of the river between Burzyn and Mocarze. CONDITIONS OF HYDROLOGICAL FEEDING IN SITES OF THE LOWER BIEBRZA BASIN The river channel of the lower Biebrza is constantly being formed by natural processes of erosion and accumulation nothing was done to stabilise the course of the river. The channel at this point is meters wide, well-defined, not overgrown by vegetation, cutting fairly deeply into the valley floor, especially in its southern stretch, near the confluence with the Narew. At some distance to the main channel are found numerous abandoned channels in varying degrees of overgrowth. The southern part of the Bagno Ławki fen where the Biebrza flows close to the margin of the upland there is a network of overgrown drainage ditches, most of them dating to the period. In their alignment the majority of these ditches follow the NE-SW axis, with a gradient towards the Biebrza. The ditches are between 1 and 3 meters wide, most often, less than 1.5 meters. At present in many places they are completely choked by vegetation and the flow in them is almost at a standstill. A major factor responsible for shaping site conditions in the basin of the lower Biebrza is groundwater. Subterranean waters are responsible for paludification of much of the peatland area lying at a greater distance from the river channel. The valley aquifer is fed by waters flowing from two directions from the north, i.e. from the area of the middle basin of the Biebrza and from the east, i.e. from the upland. Figure 1. The Lower Basin of Biebrza River hydrographic network and localization of the transect VEGETATION IN THE STUDY TRANSECT AREA Generally speaking [OŚWIT, 1968; PAŁCZYŃSKI, 1988] vegetation in the study area forms a number of distinct zones, described below from the river channel outwards:

3 Tomasz Okruszko, Wiesław Dębek, Michał Wasilewicz 3 Reed associations proper of alliance Phragmition a zone subjected to intensiveflooding occupied by a single species Phragmites australis, Typha sp., Schoenoplectus sp or Glyceria aquatica, established on the river banks and in the numerous abandoned channels; Tall sedge communities of alliance Magnocaricion growing in compact tussocks, occupying a zone of less long-lived and shallower inundation; Low sedge and moss and moss sedge communities of class Scheuchzerio-Caricetea fuscae subjected to sporadic flooding; Alder communities of alliance Alnion glutinosae affected by lateral flow, flooded by spring melt water, occupying the valley margin. The distribution of vegetation zones in the study area in detail is shown in fig. 2.

4 International conference Towards natural flood reduction strategies, Warsaw, 6-13 September 2003 Fig. 2. Plant communities in [OŚWIT, 1968] and 2000 after MATUSZKIEWICZ [2000], peat stratygraphy and actually soils in the investigated transect; 1 moorsh, 2 moss peat, 3 sedge-moss peat, 4 sedge peat, 5 reed peat, 6 alder peat, 7 willow peat, 8 gyttja, 9 mud, 10 alluvial deposit, 11 mudy sand, 12 sand, 13 Phragmition, 14 Magnocaricion, 15 Caricion fuscae, 16 Caricion lasiocarpae, 17 Arrhenatherion, 18 Alnion glutinosae, 19 groundwater level, 20 peat boring

5 International conference Towards natural flood reduction strategies, Warsaw, 6-13 September 2003 VEGETATION CHANGE In view of possible differences in evaluating the distance from the river by OŚWIT [1973] and MATUSZ- KIEWICZ [2000], and the lapse of 35 years between the two surveys, it was decided for the purpose of the present analysis to disregard communities forming smaller patches and limit the comparison to communities with major importance for the valley. In the river channel zone the beds of Caricetum gracilis described by OŚWIT [1973] has become much narrower, partly displaced by association Caricetum appropinquatae. This shift suggests lowered floodwater dynamics and the rise of water conditions proper to areas with sluggish water flow. Water flow is probably inhibited by vegetation which is no longer harvested for hay. In the zone bordering on the dry ground area, subfossil communities of tall sedges with Carex elata and Carex appropinquata present in the deposit are replaced by low sedge communities, during the 1960s, association Caricetum diandrae with a substantial proportion of Caricetum diandrae subvariant Carex appropinquata. At present the dominant association is Carici-Agrostietum caninae variant with Carex diandra, suggesting albeit not definitely a certain dehydration of the site. The zone beyond the dry ground area continues to be occupied by association Caricetum diandrae; it is worth noting that sub-association Caricetosum chordorrhizae with Drepanocladus vernicosus in this community - obviously paludic and peat-forming includes a higher proportion of tussock Carex appropinquata, a community less involved in the process of peat-formation. This is proof on the one hand of discontinued harvesting, on the other of a slight destabilization of formerly continuous hydration of this area. Summing up the above data it appears that the riparian communities of rushes proper continue to occupy a site typical for them. Tall sedge communities of Caricetum elatae, Caricetum gracilis and Caricetum appropinquatae as a result of harvesting have colonised the area of former reed beds. Subsequently the zone has bee occupied by communities of mosses and low sedges dominated by Carex diandra. This shift suggests decreasing in water table fluctuation and impoverishment of sites as a result of reduced inundation (over several centuries). In comparison to moss and low sedge communities beyond the dry ground area, communities found closer to the river are characterised by the presence of small patches of Carex elata, and consequently, by a slightly more nutrient rich, hydrologically more dynamic site. The development of rush communities and bushes is associated with discontinuation of harvesting of the zone during the recent few decades. A shallow tract of lower-lying ground in the dry ground area is under grassland having variable moisture adapted to hydration levels lower than typical of waterlogged sites formerly found in this area. Immediately beyond the dry ground area in the direction of the valley margin is found a small area of sedge vegetation, mainly with Carex appropinquata and Carex elata, beyond which are found moss and low sedge communities which eventually develop into moss and low sedge peat. Moving still farther away from the river channel there are reed beds growing on peat of the described type. This makes the area especially valuable apparently exhibiting a stability of a specific type of plant community throughout the entire time of formation of the peatland. Even farther from the river channel the valley is under alder communities growing on soils developed from tall sedge, which is evidence of discontinuation of harvesting in the area and dehydration of this zone as compared to earlier site conditions. An evident indication that the zone used to be harvested for hay is the presence of the remains of a wet grassland, to this day surviving in small isolated patches. HYDROLOGY OF SITE AND VEGETATION ZONES The present analysis focuses on high water levels when water leaves the river channel and spreads over the floodplain. At the water-level gauge section at Burzyn this takes place when the gauge readouts is over 280 cm. The bed full water level is the same with the top of the channel margin.

6 6 Plant communities response to floodwater conditions in Ławki Marsh [Byczkowski, 1996]. Out of total water level readouts recorded over the 53 year study period 33.03% exceeded the value for the bed full water level. The average flood duration period was 120 days; it is worth noting that from 1970 onwards this period has become longer. The average for is 86, for as much as 150 days (fig. 3). This growing duration of flooding in the basin of the lower Biebrza was due to the occurrence during the 1970s of a series of very wet years with high rainfall and snowy winters. There may have been additional impact from drainage projects carried extensively in the basin of the upper Biebrza in and resulted in accelerated outflow of water from that area. days Fig. 3. Number of the days during a year with water level above flood surface (floods) No visible change is registered with regard to readouts corresponding to peak water levels and dates of their occurrence. High water subsidence occurs in the end of May mid-june period, ie six weeks after the peak period. Trampled turf vegetation and rush communities proper. This zone, the closest to the river channel, is most frequently subjected to flooding. Some 700 meters in width, it is largely under reed bed communities, in general submerged by waters of the main channel and old channels the year round. For this reason it was thought unnecessary to analyse flood extent and frequency in this zone. Tall sedge communities. The width of this zone is ca 600 m. In the 53 year study period this zone was flooded 50 times (94% of the observation period), ie, practically every year. Average flood duration in a hydrological year was 120 days, maximum duration days. A great flood registered at the turn of 1980 and 1981 lasted uninterrupted for 320 days. Also noted were flood events of a single days duration. During the study period a floodless period occurred once in every 15 or 16 years. Not all flood events registered in a given year lasted uninterrupted not infrequently there were several episodes. In some years there were as many as six or seven, most frequently, just one (38.9% of all floods) or two episodes (24.07%). Maximum duration of a single continuous flood in a hydrological year was 220 days, average duration of an uninterrupted flood in a year some 54 days. Moss - /low/ sedge communities bordering on the dry ground area. The width of this zone is ca 2000 m. Lying on slightly higher ground than the river, the area is only rarely reached by floodwater 21% of all recorded flood events. During the 53 year study period flooding was registered 38 times, ie 72% of observation years. Consequently, on the average in three out of four years the floodwaters reached the zone in question. The longest period without flooding in this zone is two years. This happened twice. The period under discussion registered occurrence of a series of nine years of regular floods in succession. Most often a year with a flood may be seen to alternate with a floodless one. Flood duration was on the average 26 days, at the most, 174 days. Grassland with variable moisture and moist grassland growing on the Pogorzały dry ground. The width of this zone is about 2000 m. On the average four days in a year its surface is flooded by

7 Tomasz Okruszko, Wiesław Dębek, Michał Wasilewicz 7 river water. Maximum flood duration was 44 days. Only twice during 53 years the flood occurred in two and three episodes. During the remaining years it occurred uninterrupted (30% years) or did not occur at all (68%). Moss-low sedge communities beyond the dry ground area. The width of this zone is about 1700 m. The area was flooded by river water only four times during the 53 hydrological years. The longest flood occurred in 1979 and lasted 15 days. Bushes and alder communities. The width of this zone up to the Carska Droga road is around 2600 m. During the analysed period floodwaters reached this area only once at the time of the great flood of 1979 at which time its maximum range included a part of this zone measuring several hundred meters. THE RELATIONSHIP BETWEEN FLUVIAL - AND GROUNDWATER Data on water level fluctuation in individual plant communities studied by OŚWIT [1973] using readouts from observation wells on the whole agree with results obtained in the present study. OŚWIT [1973] recorded only the level of water above or below the surface without determining its origin, whereas a flood is produced by both surface (fluvial, precipitation) and subterranean waters. It may be assumed that the greater the discrepancy between the results recorded by OŚWIT [1973] (measured from the level of the ground surface) and those obtained in the present study, the greater the participation of groundwater in the feeding of a given community. For exemplification let us compare the results of water level frequencies at 10 cm above the ground surface in communities Phragmition and Magnocaricion, by OŚWIT [1973] recorded for different communities as ca. 7%. According to our data the same frequency of water level occurrence at the height of 10 cm is just below 7%. The above figures suggest that the area in question is flooded for th3e most part by river water. But when figures corresponding to flooding frequency in the central section of the valley occupied by communities of alliance Caricion lasiocarpae (acc. to OŚWIT [1973] 52.4%) are compared against values obtained for the same zone in the present study (0.14%) one notes a serious discrepancy. This is because the feeding of this zone by fluvial water is practically too small to see. Consequently, flood frequencies to a large extent are the result of intensive underground feeding. In some plant communities the discrepancy obtained from the comparison of flooding frequency is slight but flooding by river water is nevertheless sporadic. This situation applies to grasslands with variable moisture from alliance Arrhenatherion (typical for dry ground). The figure for flood frequency to the level of 10 centimetres given by OŚWIT [1973] is 1.5%, according to the authors of the present study 1.11%. The area in question is one subject to reduced flooding, both by river and ground water. CONCLUSIONS 1. Analysis of the frequency of flooding with surface water when compared against readouts made in observation wells confirmed the assumption that the central section of the valley in the lower basin of the Biebrza is waterlogged largely due to the action of groundwater. 2. Analysis of water level readouts from the 50 years period from the water-level gauge at Burzyn did not indicate the reduction of river flooding. The period of flooding duration has extended, the peak levels and the peak water period are unchanged but the time of high water subsidence has shifted. 3. The decrease in the moisture level of the valley is not caused by the change in river water dynamics. Taking into consideration the low probability of reduced feeding of the valley from subterranean aquifers the most probable cause of the described phenomenon must be the reduction of snowfall and associated melt water essential for feeding the central and marginal section of the valley not subjected to flooding by river water. 4. Vegetation change observed over the last 35 years has led to increased participation of Carex appropinquata within the community of Caricetum diandrae in the zone closer to the river. This

8 8 Plant communities response to floodwater conditions in Ławki Marsh suggest increase in the amplitude of water level fluctuation with a tendency of the water level to drop lower down during the dry period. The zone beyond the dry ground area at a greater distance from the river features moss low sedge communities showing greater stability. 5. To establish the cause of change in a part of plant communities and the soil root zone indicating periodical drying out of sites it is necessary to make a study of the dynamics of change of snow retention and changes in feeding of the Bagno Ławki fen by water from this source. REFERENCES Byczkowski A., Hydrologia. Warszawa: SGGW. Dembek W., Wybrane aspekty zróżnicowania torfowisk w młodo- i staroglacjalnych krajobrazach Polski Wschodniej. Rozpr. Habil. Falenty: Wydaw. IMUZ pp Kondracki J., Geografia regionalna Polski. Warszawa: PWN pp Matuszkiewicz A., Szczegółowy profil roślinności wzdłuż Grobli Honczarowskiej. maszyn. Nowiński M., Polskie zbiorowiska trawiaste i turzycowe. Warszawa: PWRiL pp Okruszko H., Przyrodniczo-rolnicza charakterystyka doliny Biebrzy. Zesz. Probl. Post. Nauk Rol. z. 134 Oświt J., Strefowy układ zbiorowisk roślinnych jako odzwierciedlenie stosunków wodnych w dolinie dolnej Biebrzy. Zesz. Probl. Post. Nauk Rol. z. 83 p Oświt J., Warunki rozwoju torfowisk w dolinie Dolnej Biebrzy na tle stosunków wodnych. Rocz. Nauk Rol. ser. D z. 143 pp. 80. Oświt J., Żurek S., Rekonstrukcja rozwoju zabagnień w pradolinie Biebrzy. Zesz. Nauk AR Wroc. 38 p Pałczyński A., Bagna jaćwieskie. Pradolina Biebrzy. Warszawa: PWN pp Szafer W., Zarzycki K., Szata roślinna Polski, T. 1, 2. Pr. zbior. Red. Warszawa: PWN pp Żurek S., Związek procesu zatorfienia z elementami środowiska przyrodniczego wschodniej Polski. Rocz. Nauk. Rol. Ser.

APPLICATION OF NEWTONIAN NUDGING DATA ASSIMILATION METHOD IN HYDRODYNAMIC MODEL OF FLOOD FLOW IN THE LOWER BIEBRZA BASIN

Studia Geotechnica et Mechanica, Vol. XXXIV, No., 01 DOI: 10577/sgm0108 APPLICATION OF NEWTONIAN NUDGING DATA ASSIMILATION METHOD IN HYDRODYNAMIC MODEL OF FLOOD FLOW IN THE LOWER BIEBRZA BASIN DOROTA MIROSŁAW-ŚWIĄTEK