Konrad Stopora1 1
Institut of Geology, TU Bergakademie Freiberg
Abstract. For the investigation in floodplains of the Elbe river the use of different disciplines is necessary. It is important to know the processes, like contamination with heavy metals (HM), sedimentation rate and distribution of a flood, in recent and relicted floods. The average discharge of the Elbe is ca. 600 m³/s and with a measured discharge from 4792 m³/s in summer 2002. Thereby the sedimentation rate depends on the position in the floodplain, there could be a range from 0.58 cm/a in a flood channel to 3 cm/a on an embankment erosion ridge and from 0,008 cm/event in depression to 0.8 cm/event in a edge of groyne field. The actual contamination is just determined in the overlaying 1-2 cm, coming from erosion of older contaminated soil or from industrial or private areas. The contamination is not strong enough to affect the pioneer plants or older plants, which used for feeding stuff. With this investigation it is possible to calculate sedimentation rates, flooded areas and the contamination for floods in the future
This paper is more or less a summary of different studies about floodplain deposits of the Elbe river. The investigation in processes of floodplains getting more and more interesting by the increase of the world temperature and so the increasing of natural hazards and also by the embanking of the rivers, in middle Europe, for example the high water in August 2002 of the Elbe river. The Elbe river is one of the biggest rivers in middle Europe, with a length of 1091 km and a catchment area of 148,268 km2. The high waters are results of the snowmelt in spring and often heavy rainfalls during the summer or autumn (Stachel B. et al. 2006; Markovic D. et al. 2006). By the investigation you can give general statements about the effects of flood induced processes for the regions close to the river. Especially for agricultural and populated areas it is imported to know the processes and the danger like the contamination with HM for soils or plants, the
over flooded areas and the spatial and temporal variability of sedimentation. The contamination depends on the local flooding conditions during phase of high water, on the quality of the eroded soils and of the chemical products in the flooded area. For example the Holocene soils along the river exhibit a high load of pollutants (Krüger F. et al. 2005). Therefore different disciplines of geology, hydrogeology and biology can be used. By using the isotopes 2
H and 18
O the role of infiltration by groundwater was investigated (Böhnke R. et al. 2002). The study of bioindicator-systems, like molluscs and pioneer vegetation can give informations about older floods and there contamination (Leyer I. 2006; Foeckler F. 2006). This aims and the correct classification of the generated soils will be done in the RIVA-project (Transfer and Futher Development of a Robust Indication System for Ecological Changes in Floodplain Systems). The calculation and the modeling of the sedimentation rate and the deposition and the analyzes of HM and other pollutants and the microbial diversity will be done in various single projects. By the investigation in floodplains it is important to know some facts before. The beginning of a flood is define when the discharge is higher as 1000 m³/s and the end when the discharge falls below (Büttner O. 2006). In this study short-term processes which occur to individual flooding events are investigated. In contrast to the medium term processes, which occur to an event during the last few decades until one century, or the long term processes which take place over one to several centuries. By the long-term processes you can generally divide between recent or relict floodplains by measuring the amount of sediment accumulation. With this method it is possible to get the placement of natural retention areas before the diking. So you get an accumulation rate from 1 mm/a by a mean density of 1-1.5 g/cm3
, this means an accumulation from 1500 g/m2
an in not drained depressions near to the river ( Fig. 1; Krüger F. 2006)
Fig. 1. Comparison of the surface above sea level in recent and relict floodplains between stream kilometer 472-485; MW = mean water level, SD = standard deviation (Krüger F. 2006).
There are two other studies for the sediment accumulation. The first one by Barthe et al. propose a rate of 3 cm/a on an embankment erosion ridge at stream kilometer 275 and the second one by the Hamburger Environment Authority with
0.58 cm /a in a flood channel by stream kilometer 485. This shows the range in the sedimentation rate depending of the position in the floodplain. The investigation in such long term processes yields to the question where are the retention areas and the old alluvial fans.
This question is answered by looking on the straitening and on the diking history. Since the building of dikes in the 12 century, they protect nearly 75 % of the alluvial areas against the flooding. This leads to a higher discharge, a higher flow velocity and to a higher erosion and limits flood related sedimentation processes of about only 25 % of the Elbe river. If there is a hazard it will be concentrate at this areas or it will become a crevasse at any other place (Krüger F. 2006). The mean discharge of the Elbe river lie between 600-700 m³/s and the maximum discharge, measured in the year 2002 with nearly 4792 m³/s at the gauge “Dresden” (Fig 2; LfuG Sachsen).
Fig. 2. Top – discharge stations at the Elbe river; down – discharge at gauge “Wittenberg” (Markovic D. et al. 2006)
2. Material and methods
2.1 Study area
The study area including the whole Elbe river between Dresden and Hamburg for analyzing the organic carbon, mercury, PCDD/Fs and DL-PCBs (Stachel B. et al. 2006). By this way three special areas have been selected to analyze the sedimentation rate, the allocation of molluscs, the microbial diversity and the pollution with heavy metals. For these three areas HENLE et al. (2006) define that it must be a seasonally flooded grassland under the direct influence of river and of an intermediate intensity of agriculture use covering approximately 70 % of today’s floodplain area. This seems to be the most common habitat type (Rinklebe J. et al. 2007), located in the middle Elbe River. This is the area “Schleusenheger Wiesen” between stream kilometer 242-244, the “Schöneberger Wiesen” between kilometer 283-286 and the area “Dornwerder” between kilometer 417-418. (Fig. 3)
Fig.3. Study areas, “Schöneberger Wiesen” near by Wörlitz, “Schleusenheger Wiesen” near by Steckby and “Dornwerder” near by Sandau.
The investigation on the dispersal and temporal variability of sediment depositions were carried out in the floodplain “Schöneberger Deich”, which covered an area of 2 km2
, between stream kilometer 435-440 (Fig. 2; Baborowski M. et al., 2007; Büttner O. et al. 2006), which is included in the investigation area to specify the sedimentation rate and the distribution in grain size located between stream kilometer 435-523.
To specify the sedimentation rate and the distribution in grain size an area between stream kilometer 435-523 were chosen (Fig. 4; Krüger F. et al. 2006)
Fig.4. Left picture - Study areas, between stream kilometer 435-523 to calculate the sedimentation rates; the grain size distribution and the dispersal and temporal variability of sediment depositions at “Schöneberger Deich” (Krüger F. et al. 2006); right picture – study area between stream kilometer 436-440 showing the position of the sediment traps for grain size analyzes (Büttner O. et al. 2006).
2.2 Sampling methods
The analysis of the sedimentation rate is called an event-related method and is a short-term process, which were determine by synthetic turf traps with a size of 30*40 cm, trying to imitate natural soils. Therefore typical riparian landscapes or morphological points were chosen to place the sediment traps, like plateaus, depressions, flood channels, old arms, grassland close to the river or in distance to the river. The traps were placed shortly before the flooding at the point and were taken away after the ending of the flooding period, in the mean time water samples were taken daily at different places to control the contamination of heavy metals or other pollutants like arsenic or mercury (Krüger F. et al. 2005, 2006). After the recovering of the sedimentation traps, they sediment were air dried and beaten by using a plastic rod. The finely ground material were putting for 24 h in a water filled plastic container for settling. After removing the supernatant the sediment were oven tried by a temperature of 105°C for 24 h to specify the dry weight of the material afterward few samples were analyzed after DIN 18123-4 and 19683-2 to determine the distribution in grain size.
The problem in the sedimentation rate analyzes is the difference in the suspended load material, which is depending on internal processes like erosion, displacement, production and the sedimentation it self. To be protected for this problems it is good to have informations about the morphology in the near to the river, about the surface and the source area. (Büttner O. et al. 2006; Krüger F. et al. 2005, 2006). For the analysis of heavy metals, arsenic and silica the X-ray fluorescence (EDXRF) method was used. The total organic carbon was analyzed in triplicate with a C (H) NS analyzer. With these methods you can determine the Si/Al-ratio, which can give informations about the sand, silt and clay content (Tab.1; Krüger F. et al. 2005; Baborowski M. et al. 2007)
Table 1. Examples for positions of sampling points for flood induced HM and Arsenic (Krüger F. et al. 2005)
For the investigation in molluscs as bioindicators 60 samples has been taken in “Schöneberger Wiesen” (36 samples); “Schleusenheger Wiesen” (12 samples) and “Dornwerder” (12 samples), with a 1000 cm2
steal frame and a sampling depth of 5 cm, all the samples were put into air permeable bags. A mechanic sieve separate the molluscs from soil and vegetation parts, after that the molluscs were tried and sort out manually. They were stored in 70 % alcohol and were identified and counted.
This process was repeated four times during 1998 and 1999 in spring and autumn (Foeckler F. et al. 2006). For the analyze of the microorganism 3 samples were taken and pooled to one sample, after removing visible roots, macro fauna and fresh litter. The material was sieved to 2 mm and well homogenized. Than the sample was divided for physical and chemical analysis, an air-dried sample is needed and for microbiological analyzes, a frozen sample at –20°C is needed (Rinklebe J. et al. 2006).
3.1 Sedimentation rate and spatial and temporal variability
By comparing the results of different methods (Table 2), it point out, that the investigation in short-term processes with sedimentation traps is maybe the
accurate and best comparable method with the best disintegration of different morphology places (Table 3). By this method a spatial distribution of different places and also the discharge and by accurate sampling also the contamination level will be achieved. With this information it is possible to create a model. Table 2. Sedimentation rates and sediment loads registered with different methods
Table 3. Sedimentation rates, sediment load and discharge in the floodplain “Schöneberger Deich” between stream kilometer 435-440 at selected location and in different time slices. SL = sediment load (dry matter in g/cm²); nsl = no sediment load(Krüger F. et al. 2005; 2006).
Table 2 shows that the sediment input range between 0.07-21 kg/(m²*a) and the sedimentation rate range between 0.007-1.5 cm/a, which show that the sedimentation rate in floodplains are not uniform. Thereby the highest sedimentation rate take place in a groyne edge with a maximum of 0.8 cm/event or with a sediment load of 8.3 kg/m². The lowest rate sedimentate in a flood channel with a maximum of 0.04 cm/event or 0.4 kg/m² cause of the high erosion rate in such a flood channel by a maximum discharge of 3800 m²/s. The mean sedimentation on recent alluvial plain is in an order of 0.23-0.35 kg/m² (Stachel B. et al. 2006) or 0.02-0.08 cm/a (Krüger F. et al. 2006). The calculated remobilization of old stillwater sediments is in an order of 4.6 kt/km (Stachel B. et al. 2006). It can be said that high average sedimentation rates of more than 0.2-1.5
cm/a are mainly restricted to channel locations more or less in the near of the river, by a mean elevation of 0,5 m above the mean water level. This area account nearly 30% of the investigated floodplains (Krüger F. et al. 2006), but in general there is no evidence for a correlation between ground level elevation and sediment input (Baborowski M. et al. 2007). By modeling on the basic of this facts, it can be said that the deposition of fine grained material is higher in abandoned channels than in the higher floodplain area liable to rapid overflow. The fraction of transported sediment increasing with the discharge and the velocity in such abandoned channels (Büttner O. et al. 2006). Between ¼ and 1/3 of the deposit material will be sedimentated in quite regions. Generally there is no relation of the distance to the river and the sedimentary input (Baborowski M. et al. 2007).
3.2 Contamination with HM, arsenic, mercury and other pollutants
The content of HM, arsenic and mercury strongly depends of the metal mobility in soils, from the plant species or just from the sampled organic matter and by the sediment itself, cause HM tend to be absorbed by the fine sediment fraction (Büttner O. et al. 2006). Therefore mixed samples was used for the analyzes. The plant availability of trace metals decreases from Cd > Zn > Cu and Pb > As > Hg ( Fig. 5).A correlation between the total content and the plant available fraction exists for Cu and As, not for Cd, Zn, Pb and Hg (Gröngröft A. et al. 2005).
Fig. 5. Plant available trace elements (Gröngröft A. et al. 2005).
The elements Cd, Zn and Pb also show a connection between the pH-value and the mobility of this elements. For example the solubility of Pb increase under pH 4.5. Figure 6 show the concentration of trace metals in the floodplain “Schöneberger Deich”, it is conspicuous that the HM concentration correlate with TOC (total organic carbon), cause the organic material acting as the main binding product. Also the HM concentration interact with the elevation above the mean water level, there is no correlation with the distance to the river but with the position of the section at the river length.
Fig. 6. HM contents in the floodplain “Schöneberger Deich” between stream kilometer 435-440 ( Krüger F. et al. 2005)
The investigation also show that the HM concentration, in top soils, on farmland is lower than on grassland, caused by the higher position and so a lower sedimentation rate of the farmland ( Krüger F. et al. 2005). This fits to the fact that the concentration is higher in topographically lower positions like depressions, flood channels or low lying terraces (Rinklebe J. et al. 2007). The concentration of HM downstream of the Mulde inflow is significant higher than upstream, maybe caused by a higher erosive remobilization of contaminated soils ( Stachel B. et al. 2006). The sediments are still contaminated with PCDD/Fs, which have only an anthropogenic origin. In a project, 55% of the sampled feeding stuff was contaminated with PCDD/Fs but there is no correlation between PCDD/F in the soils and the concentration of PCDD/Fs in the plants. The pollution with HM, arsenic and mercury is often controlled by the erosion of older contaminated soils or in the case of PCDD/Fs and PCDD/F by the humans.
This investigation show a relationship between the distribution of the bioindicators and the habitats. The main controlling factors are the soil humidity and the flooding duration, but also the vegetation. The moisture requirement, the flooding tolerance, the tolerance against intensification of agricultural use and grazing are the factors which closely related to the gradient of environmental conditions.
Fig. 7. Simultaneous ordination of the environmental parameters (top, abbreviations see text), the species distribution (middle) and their biological and ecological traits (bottom) along the first axis of an RLQ analysis.
The analysis of the sedimentation rate and the sediment load yields to the conclusion that it is strongly depend on the position in the floodplain. The values range from 0.58 cm /a in a flood channel to 3 cm/a on an embankment erosion ridge. The mean sedimentation on recent alluvial plain is in an order of 0.02-0.08 cm/a (Krüger F. et al. 2006). This show that the influence of HM, arsenic or other pollutants of soil and plants can not be so high, cause of the small amount of accumulated sediment. The contamination adjust over more decades and is not increasing now, it is just a variation by erosion and relocation of the older contaminated soils. The distribution in grain size is not depending on the distance to the river, more or less it depend on the morphological position and on the vegetation. With the bioindicators it is easily to say something about the position of relicted floods, about the contamination and the grain size(Fig.7; Foeckler F. et al. 2006). The Investigation in groundwater analysis show that the floodplain can flooded in two ways, by the river it self and by the rising of the groundwater. Several investigation shows that in most of the cases the flooding starts with such process and later on by the river (Böhnke R. et al. 2002). All this informations are very important to calculate the accumulation space of coming floods and the contamination of the soils.
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