Analysis of dyke breaks during the 2002 flood in Saxony/Germany

Transcription

1 LARS 2007 Catchment and Lake Research Analysis of dyke breaks during the 2002 flood in Saxony/Germany H.-B. Horlacher, T. Heyer, D. Carstensen, U. Bielagk 1, E. Bielitz and U. Müller 2 1 Technical University Dresden, Institute for Hydraulic Engineering and Applied Hydromechanics 2 Dam Authority of the Free State of Saxony Abstract In August 2002 exceptional precipitation caused extreme floods in Saxonian Rivers. As a consequence only along the rivers Mulde and Elbe about 100 dyke breaks were reported. Intensive research was conducted in order to determine the main causes of failure, the failure mechanisms and to possibly describe the dam break chronology along both rivers. The paper reports about some of the results mainly focusing on the wide description of the dyke breaks in many respects, e.g. main causes of failure, circumstances of damage, direction of collapse. Introduction In August 2002 exceptional precipitation caused the occurrence of extreme flash and river floods in many Saxonian Rivers. In most cases record water levels were being observed. As a consequence flood protection devices were damaged resulting in heavy losses in economic terms and even human lives. Regarding the dykes only along the rivers Mulde and Elbe about 100 dyke breaks were reported on Saxonian territory. Elbe Vereinigte Mulde Zwickauer Mulde Freiberger Mulde Figure 1: General map of the study area 58

2 Catchment and Lake Research LARS 2007 Unfortunately only one week after the first flood another serious flood warning urged the authorities to immediately close all breaches with any materials available. Due to this time pressure a professional survey of the breaches could not be accomplished. However, in 2004 the Dam Authority of the Free State of Saxony contracted the Institute for Hydraulic Engineering and Applied Hydromechanics of the Technische Universität Dresden (IWD) to conduct subsequent analysis regarding the dyke breaks in order to outline the main failure causes and modes. The area under investigation stretched across the catchment areas of the rivers Vereinigte Mulde (including the tributaries Freiberger and Zwickauer Mulde) and Elbe on Saxonian territory (Fig. 1 & Table 1). Table 1: River reach lengths under investigation River Length [km] Elbe 175 Vereinigte Mulde 92 Freiberger Mulde 54 Zwickauer Mulde 11 First of all intensive inquiries had to be made due the time offset of the analysis to the flood event. Apart from the inquiries at the local authorities (Dam Authority of the Free State of Saxony, communities, municipalities), fire brigades, emergency forces and other potential eye witnesses were also interviewed over a period of one year in order to get the best overall picture for each dyke break. All of the relevant locations were investigated on site. As a result, various data was collected. All information was administrated in a database, which is for the purpose of a statistic analysis of the collected data. Hydrology Flood causes In the first two weeks of August 2002, exceptional rainfall occurred in the Elbe catchment. Thunderstorms with excessive precipitation were followed by less intense but long lasting rainfall. According to the classification of van Bebber, who categorised typical paths of low pressure areas, this weather situation was of type Vb (Fig. 2). This meteorological situation already caused several flash and river floods in central Europe in the past. Nevertheless, the 2002 event was extraordinary, since precipitation rates reached the all-time records in many locations. The rain gauge Zinnwald-Georgenfeld in the Ore Mountains has measured 312mm of precipitation in just 24 hours time. This is the highest value ever measured in Germany. Starting from the 12 th of August the area-wide precipitation generated flood situations in many Saxonian rivers. Especially the smaller mountain rivers showed their enormous destructive potential through the occurrence of flash floods. 59

3 LARS 2007 Catchment and Lake Research Figure 2: Path classification of low pressure areas after van Bebber (source: Elbe River Mainly because of the precipitation in the Moldau and Eger catchments on the 6th and 7th of August the water levels in the river Elbe rose rapidly. The Dresden gauge showed a water level rise of 400cm in only 3 days. On the 17 th of August the peak water level was reached with 940cm above datum so the maximum ever recorded value was exceeded by 63cm (Tab. 2). Table 2: Comparison of peak water levels at selected gauge stations along the Elbe River Gauge. HHW Year W peak Date ( past) cm cm cm Usti (CZ) /08/ Schoena /08/ Dresden /08/ Torgau /08/ According to the statistical analysis the related discharge of 4680m³/s is approximately equivalent to a 125-year flood. One should bear in mind that the designed flood for protection measures in Germany is usually set to a flood event with a statistic occurrence probability of 0,01 (1 in 100 years). Mulde river On the 11 th of August the discharge of the Mulde river was still average. Caused by the excessive precipitation on the 12 th and 13 th of August the storage capacity of the upriver area was quickly reached. Hence, the biggest share of the rain was transferred directly as overland flow. The Mulde River is divided into two rivers at the upper part, namely the Freiberger Mulde and the Zwickauer Mulde (see Figure 1). Coincidentally the discharge peaks of both tributaries simultaneously reached the confluence on the 13 th of August, thus a tremendous flood wave was creacted. Consequently the water levels of the river exceeded the historical records by up to 3 60

4 Catchment and Lake Research LARS 2007 metres. The statistical analysis of the peak discharges along the river Mulde which yielded to recurrence intervals between years. Therefore, flood protection measures along the river were significantly overloaded. Data base With the objective of clearly storing and arranging the collected data a relational thematically structured data base was established. Having in mind the parameters that could influence the dyke s failure mechanisms, a large number of attributes were defined within a record, e.g. geographic location, geometry, distance of toe to the main river bed (direct dykes, with floodplain), dyke structure (homogeneous, heterogeneous), subsoil structure, vegetation, preexisting damage caused by animals, breach geometry, approximate time of failure, etc. Before data entered into the data base it was necessary to verify the reported dyke failures. Since there was no fixed definition when a dyke actually has failed, the following criteria were defined to determine the set of dyke failures: sudden or gradual development of a breach due to a critical combination of parameters not manually initiated free surface flow through the breach with the water level below original dyke crest in the long run The verification procedure identified 84 records sets that fulfilled these criteria (Fig. 3). 310 reported damages 184 small and medium damages 126 large damages 42 other failures 84 dyke breaks 33 other damages 2 false reports 7 dyke openings (manmade) flood protection walls railroad dams bridge bearings... Figure 3: Verification outcome of reported damages From the mathematical point of view a large number of statistical evaluations are possible, since beside single parameter evaluations, multiple parameter analysis can also be conducted for the whole record. Restrictions derive from the varying information content in the records and from physically reasonable combinations of parameters. In the following only selected evaluations will be presented. 61

5 LARS 2007 Catchment and Lake Research Analysis Chronology of failures The determination of the dyke failure chronology turned out to be very difficult, since most dyke breaks occurred in rural areas. Only in very few cases the exact moment of failure could be appointed through the reports of eye witnesses. However, by analysing aerial photographs it was possible to determine dates of failure for all dyke breaks. It showed that the frequency of occurrence correlates directly with the transition of the flood waves in the rivers Mulde and Elbe (Fig. 4). Figure 4: Frequency of dyke breaks related to stage hydrographs of the 2002 flood Comparing the stage hydrographs in the Mulde River the influence of the dyke breaks should be noticed. It appears that most dyke breaks occurred after the designed water levels were exceeded so that most dykes can achieve their design goals. Due to limited knowledge about the moment of failure and the scarcely available stage hydrographs along both rivers, more detailed conclusions were not possible. Degree of damage Different breach shapes will develop depending on the failure mechanism, the duration, direction and intensity of flow through the breach and on the conditions of the dyke body and subsoil. Regarding their cross sections (perpendicular to the dyke s axis) the breaches were classified into three groups partial damage (a), total damage (b), total damage with large scours (c) (Fig. 5). As a result almost two-third of the dyke breaks were assigned to the set total damages with scour (Fig. 6 & Tab. 3). Especially along the river Mulde, where the water levels far surpassed the designed values, this breach type was determinant. 62

6 Catchment and Lake Research LARS 2007 Figure 5: Breach classification regarding cross section profiles Table 3: Frequency of dyke breaks classified after degree of damage Degree of Sum Elbe Mulde Freiberger Damage Mulde partial total total with scour Sum Partial damage Total damage Total damage with scour Figure 6: Breach classification, criteria: degree of damage Dyke overtopping As mentioned before large-scale dyke overtopping occurred during the 2002 flood. Overtopped dyke sections could be located by analysing air photographs taken during the flood event. The analysis showed that in 74 cases (88%) dykes broke in overtopped sections. It should be noted that although the dykes were overtopped over long stretches the breach itself, they were developed more or less only punctually (Fig. 7). Hence, the existence of certain additional conditions is responsible for the breach development on a specific location. Regarding dyke failure mechanisms the overtopping of the dyke may trigger several types of failure, e.g. outer erosion of inner slope, infiltration and others (Kortenhaus & Oumeraci 2002, Lecointe 1998, Wahl 1998). Further detection of the main failure mechanism initiated by overtopping can only 63

7 LARS 2007 Catchment and Lake Research be done on the basis of measured or observed data. In most cases such information was not available. Direction of failure Overloaded river embankments usually fail from river to land since this is in most cases the direction of the pressure gradients in the dyke body and the subsoil. The analysis of dyke failures along the river Mulde lead to the somewhat surprising result, that 15 (26%) out of 57 dyke breaks failed in the opposite direction. This phenomenon can be explained as follows: landside failures are caused by normal waterside failures in upstream river sections. The area behind the dyke then becomes flooded. If the spreading of the water is limited through the topography of the flooded area ( polder region ), the water level in that area can rise very quickly. The water head in the polder region can be higher than the actual water level in the river profile. riverside landside Figure 7: Overtopping and final breach extend (Mulde River) landside riverside Figure 9: Landside failure near Grubnitz (Mulde River) 64 Figure 8: Landside and waterside failure near Wurzen (Mulde River)

8 Catchment and Lake Research LARS 2007 During the 2002 flood the landside dyke overtopping with overflow depths of about 1m had been observed whereas the water level in the river bed was still some decimetres below the dyke crest. Landside overtopping of dykes mainly occurred in corners of the polder area, which where for instance formed through the dyke connection to land of higher elevation or at connections of dykes with bridge bearings. On the Elbe river all embankments were failed from the waterside to landside since the dyke protected territory is extremely flat. As a consequence one single dyke failure usually caused the irrigation of large areas. Near the town of Torgau one breach with a width of about 340m flooded an area of over 20km². Failure Modes The failure of river embankments is in most cases the consequence of a time depending combination of several causes and effects. Event trees are one theoretic model to map the natural processes onto a theoretical background as for instance quoted by Foster & Fell (2000) for the special case of failure due to internal erosion. Concerning the dyke breaks under investigation the problem posed was the subsequent determination of the main failure modes, or even the single events which lead to failure. In order to achieve reliable results the presence of direct observations was required, which was rarely the case. Therefore in most cases a combined parametric-subjective analysis was conducted. Given the limited project frame a detailed analysis was not feasible. The assignment of the main failure causes, which are presented in the following gives only a rough estimation about the prevailing types of failure. The number of possible types of failure depends on the structure and the location of the dyke and is therefore variable. Kortenhaus & Oumeraci (2002) give an overview about the possible types of failure and name about 20 mechanisms. Although it is mainly focused on sea dykes, most outlined failure types are also relevant for river embankments. With regard to the large number of breaches and to the uncertainty in reasoning due to the limited amount of information about the main failure types, only four general classes were defined (Fig. 14). failure of... dyke body subsoil stability failure external erosion internal erosion subsoil failure - sliding of inner slope -sliding of outer slope - erosion of the surface layers through: - overtopping - piping - suffusion -concentrated leaks - heave -base failure -piping -sliding of dyke body -infiltration... - turbulence behind buildings - damage through floating debris Figure 10: Classification of main failure modes 65

9 LARS 2007 Catchment and Lake Research Conclusion The 2002 flood relentlessly revealed the weak points of the existing flood protection systems along many rivers in Saxony. Regarding the river embankments over 100 failures were reported, many of them were investigated within this project. Developing a dyke break data base is the major achievement of this project. The detailed compilation of over 80 failures during one flood event is considered extremely valuable for practical and even more for scientific purposes. Further research activities might build up on this information since the records might be used for calibration, validation and verification purposes, e.g. in breaching models or risk assessments. References Bielagk, U Diploma thesis: Analyse der Deichbrüche an Mulde und Elbe während des Hochwassers 2002 im Bereich Sachsen. Institut für Wasserbau und Technische Hydromechanik (IWD), Technische Universität Dresden, Germany Carstensen, D. & Horlacher, H.-B. et al Bewirtschaftungsmaßnahmen zum Erhalt der Schutzfunktion der Deichanlagen in Sachsen. Institut für Wasserbau und Technische Hydromechanik (IWD), Technische Universität Dresden, Germany Foster, M. & Fell, R Use of event trees to estimate the probability of failure of embankment dams by internal erosion and piping. In Proceedings of the 20 th International Congress on Large Dams (ICOLD), Beijing, Vol. 1, pp Kortenhaus, A. & Oumeraci, H Probabilistische Bemessungsmethoden für Seedeiche (ProDeich), Leichtweiss-Institut für Wasserbau, Technische Universität Braunschweig, Germany Lecointe, G. D Breaching Mechanisms of Embankments. MSc project. School of Civil Engineering, The University of Birmingham, United Kingdom Singh, V. P Dam Breach Modeling Technology. Kluwer Academic Publishers, Dordrecht, The Netherlands Wahl, T Prediction of Embankment Dam Breach Parameters: A Needs Assessment, IAHR Congress San Francisco, USA, August 1997 LfUG Vorläufiger Kurzbericht über die meteorologisch-hydrologische Situation beim Hochwasser im August Landesamt für Umwelt und Geologie, Dresden, Germany 66

10 Catchment and Lake Research LARS 2007 Notes: 67