modelling a flood wave due to failure of flood protection ... · modelling a flood wave due to...

Univ.-Prof. Dr.-Ing. Holger Schüttrumpf

Dipl.-Ing. Sebastian Roger

Aachen, April 2008

German Research Foundation

Modelling a flood wave due to failure of flood protection measures Final Report Reference number: KO 1573/15-2

Contents I

Contents

1 General information .........................................................................................1

1.1 DFG reference number.......................................................................................1

1.2 Applicant.............................................................................................................1

1.3 Institute/department............................................................................................1

1.4 Project staff, periods of contribution ...................................................................1

1.5 Topic of the project .............................................................................................1

1.6 Period covered by the report, overall funding period..........................................1

1.7 Area of research, special field ............................................................................1

1.8 Areas of application ............................................................................................1

1.9 Involved co-operation partner.............................................................................1

1.10 List of publications resulting from this project.....................................................2

2 Progress report.................................................................................................3

2.1 Initial status and objectives.................................................................................3

2.2 Project developments .........................................................................................4

2.3 Presentation of results........................................................................................7

2.4 Outlook on future research .................................................................................9

2.5 Exploitation potential ........................................................................................10

2.6 (Interdisciplinary) contributions, involved researchers and qualifications.........11

3 Summary .........................................................................................................12

Modelling a flood wave due to failure of flood protection measures: KO 1573/15-2 1

1 General information

1.1 DFG reference number KO 1573/15-2

1.2 Applicant Jürgen Köngeter, Univ.-Prof. i.R. Dr.-Ing. Professor emeritus, former director of the institute

Mies-van-der-Rohe-Straße 1 52056 Aachen Tel.: +49 (0)241 80 25910 Fax: +49 (0)241 80 22348 e-mail: [email protected]

1.3 Institute/department Institute of Hydraulic Engineering and Water Resources Management (IWW) RWTH Aachen University

1.4 Project staff, periods of contribution Dr.-Ing. Sylvia Briechle (Dipl.-Ing. within period from 03/2005 to 02/2006) Dr.-Ing. Maren Niemeyer (born Harms) (Dipl.-Ing. within period from 03/2006 to 02/2007) Dipl.-Ing. Sebastian Roger (from 03/2005 to 02/2008)

1.5 Topic of the project Modelling a flood wave due to failure of flood protection measures

1.6 Period covered by the report, overall funding period Report period: from 03/2005 to 02/2008 Funding period: from 03/2005 to 02/2008

1.7 Area of research, special field Civil engineering, hydraulic engineering, fluid mechanics

Physical and numerical modelling

1.8 Areas of application (Public and private) flood protection, Risk Assessment fluid mechanics, insurance industry, civil protection, disaster control

1.9 Involved co-operation partner Dr. ir. Benjamin J. Dewals, Department ArGEnCo, University of Liege, Liege, Belgium

Univ.-Prof. Dr.rer.nat. Sebastian Noelle, Institute for Geometry and Practical Mathematics, Division of Numerical Mathematics, RWTH University

Prof. Dr.-Ing. habil. Helmut Martin, TU Dresden

Dr.-Ing. Dirk Schwanenberg, Deltares (former Delft hydraulics), The Netherlands


1.10 List of publications resulting from this project

Journal articles KUTSCHERA, G.; BACHMANN, D.; HUBER, N.P.; NIEMEYER, M; KÖNGETER, J. (2008): RAPID –

Ein Risk-Assessment-Verfahren für den technischen Hochwasserschutz. (in German) In: Wasserwirtschaft, Jg. 98, H. 1-2, pp. 43-48. – ISSN 0043-0978

HUBER, N.P., NIEMEYER, M, KÖNGETER, J., POLCZYK, H. (2005): Risikoaspekte in der DIN 19700: Eine exemplarische Betrachtung der Rurtalsperre. (in German) In: Die Was-serwirtschaft, Jg. 95, H. 1/2, pp. 24-30. – ISSN 0043-0978

NIEMEYER, M.; HUBER, N.P.; BRIECHLE, S.; KÖNGETER, J. (2005): Simulation damm- und deichbruchinduzierter Flutwellen. (in German) In: Österreichische Wasser- und Abfall-wirtschaft, Heft 1-2 Jänner/Februar 2005, 57. Jahrgang, Springer: Wien, NewYork, Ver-lagspostamt 1201 Wien, Zulassungsnummer: 02Z031194M

SCHWANENBERG, D.; HARMS, M. (2004): Discontinuous Galerkin Finite-Element Method for Transcritical Two-Dimensional Shallow Water Flows. In: Journal of Hydraulic Engineer-ing, Vol. 130, No. 5, May 1, 2004, ASCE, ISSN 0733-9429/2004/5-1-10

Conference proceedings ROGER, S.; DEWALS, B.J.; SCHÜTTRUMPF, H.; PIROTTON, M. (2008): Simulating dike-break in-

duced flows: Model tests vs. 2D-numerics. Accepted for FLOODrisk Conference, 30 September – 2 October 2008, Oxford, UK

SCHÜTTRUMPF, H.; GEISENHAINER, P.; POHL, M. (2008): Analysis of dike failures at the coast and in estuaries. Accepted for 31st Int. Conf. on Coastal Engineering, 31 August – 5 September 2008, Hamburg, Germany

ROGER, S.; BÜSSE, E.; KÖNGETER, J. (2006): Dike-break induced flood wave propagation. Proc. of the 7th Int. Conf. on Hydroinformatics, 4–8 September 2006, Acropolis, Nice, France, Vol. 2, pp. 1131-1138, Research Publishing Services. – ISBN 81-903170-1-6

HARMS, M.; BRIECHLE, S.; KÖNGETER, J.; SCHWANENBERG, D. (2004): Dike-Break Induced Flow: Validation of Numerical Simulations and Case Study. Proc. of the 2nd Int. Conf. on Fluvial Hydraulics (River Flow 2004), 23-25 June 2004, Naples, Italy, Vol. 2, pp. 937-944, Balkema. – ISBN 90-5809-658-0

BRIECHLE, S.; JOEPPEN, A.; KÖNGETER, J. (2004): Physical Model Tests for Dike-Break In-duced, Two-Dimensional Flood Wave Propagation. Proc. of the 2nd Int. Conf. on Fluvial Hydraulics (River Flow 2004), 23-25 June 2004, Naples, Italy, Vol. 2, pp. 959-966, Bal-kema. – ISBN 90-5809-658-0

Books, monographs, institute notes NIEMEYER, M. (2007): Einfluss der Breschenbildung auf die Flutwellenausbreitung bei Damm-

und Deichbrüchen. (in German) Aachen: Shaker. – ISBN 978-3-8322-7132-9 BRIECHLE, S. (2006): Die flächenhafte Ausbreitung der Flutwelle nach Versagen von Hoch-

wasserschutzeinrichtungen an Fließgewässern. (in German) Aachen: Shaker. – ISBN 978-3-8322-6674-5

KÖNGETER, J.; BRIECHLE, S., HUBER, N.P. (2005): Contributions in Mobile Hochwasser-schutzsysteme: Grundlagen für Planung und Einsatz. (in German) Bund der Ingenieu-re für Wasserwirtschaft, Abfallwirtschaft und Kulturbau (BWK) e.V., Bulletin 6. – ISBN 3-936015-19-8

ROGER, S. (2008): Hybride Modellierung einer deichbruchinduzierten Strömung in direkter Umgebung einer idealisierten Bresche. (in progress)


2 Progress report

2.1 Initial status and objectives Dikes or (mobile) walls are essential parts of flood protection conceptions along rivers to de-fend densely populated or industrially used areas from flooding. In case of a failure of these protective structures a flood wave is initiated into the hinterland and may cause disastrous damage. Massive flood events and recurring dike breaks indicate that inland flood protection may be vulnerable and the resulting risk has to be assessed (see 2.5). In this context, the discharge through the breach of a collapsed dike section significantly affects the final water level, its rising speed, the duration of the flood event, and the extent of the inundated areas in the floodplain. The static impact (slow increase of the water level in the whole floodplain) basically depends on the total volume of water entering the floodplain over a long period (steady-state conditions), while during the first transient phase of a dike break, the combina-tion of flow velocities and water depths within the flood wave induces dynamic damages above all near by the breach location. Compared to the endurance of the whole flood event this period is often rather short but also of great danger for people and property in the af-fected area.

The wide knowledge as regards dam-break induced flow can not be transferred to a flood wave scenario due to failure of flood protection measures without restrictions. In contrast to the flood wave propagation initiated by dam breaks, a dike-break induced flow is influenced by the main flow direction of the river parallel to the protective structure. The permanent mo-mentum causes an asymmetric flood wave propagation into the hinterland (fig. 1). Moreover, unlike reservoirs, the river bed will not empty: the persisting flood discharge in a river leads to a fixed water level in the breach after a certain time (final steady state). This state has to be considered when focussing on the long-term, large-scale inundation and the resulting static impact in the whole floodplain.

Figure 1: Difference between dike-break and dam-break induced flow

The objective of the research project is to jointly enhance the prediction accuracy regarding flood wave propagation due to failure of flood protection measures and to quantify the re-maining uncertainties. That is to develop a reliable forecasting tool for engineering purpose whose accuracy and limitations are well-known in advance. The related dike-break induced flows can be characterized as large-scale, long-term but distinctly transient, 3D, turbulent and multiphase (air entrainment, sediment transport) flows with complex free-surface behav-iour upon a heterogeneous topography (initially dry, geometrically complicated, covered by buildings and plants). There is a lack of knowledge as regards these types of flood waves, which goes beyond the scope of assured scientific findings. The existing measured data is not sufficient due to the danger1, rareness and unpredictability of such events. That is why numerical models can not be properly validated and therefore may not calculate reliable 1 The safety of threatened people and property as well as the security of the gauging staff during the event has priority, which complicates complex measuring campaigns close to a breach.


simulation results for complex scenarios. Circumventing the expense of a full-sized proto-type, a bench-scale model may provide experimental data, which is recorded with sophisti-cated measurement techniques to explore flow effects and to validate numerical models. In an experimental model natural phenomena are scaled down and idealised resulting in differ-ences between modelled and real procedures. A numerical model contains simplifications in the mathematical description and numerical treatment of flow phenomena.

To solve this problem, experimental (theme A) and numerical (theme B) models are com-bined in a hybrid approach to benefit from each other by exploiting their respective strengths and advantages: on the one hand, the accuracy of numerical forecasts as regards the ex-perimental flow can be directly quantified by measurements. On the other hand, numerical simulations complement the hydraulic model tests by calculating many scenarios with differ-ent configurations, geometries and boundary conditions. This includes less simplified practi-cal applications. The combination enables selective improvements and optimizations in nu-merical methods. The forecast quality is directly quantifiable by comparisons between simu-lation results (coming from different numerical schemes and mathematical approaches) and measured data in terms of flow discharges, water depths, flow velocities, and arrival times (depending on the fact whether the first dynamic phase or the final steady state is evaluated).

2.2 Project developments

Scale model tests (theme A): The idealised experimental set-up in (fig.2) takes into account specific boundary conditions of a dike-break induced flow (BRIECHLE ET AL., 2004). It consists of a channel (width 1 m) with a pneumatically driven gate at one bank and an adjacent flat propagation area (3.5 x 4.0 m²) made of glass. The entire opening of the flap takes less than 0.3 s, which represents the worst case scenario of a sudden total failure. Moreover, the opening mechanism is a combi-nation of rotation and pull to minimize the influence on the freed water column when the wave is initiated. As opposed to flumes, the water here propagates in all directions and falls off the glass plate freely at three edges. The bottom of the propagation area is made of glass to minimize roughness effects and to enable laser measurements from below the plate. Initial water levels in the channel (0.3 – 0.5 m), channel discharges (0.1 – 0.3 m³/s), and the breach width (0.3 – 0.7 m) are varied in each model run.

The inflow is controlled via an ultrasonic discharge-measuring device. A weir at the end of the channel is calibrated for different crest heights and ensures the initial water depth. The steady-state breach discharge is indirectly calculated as the difference between model inflow and balanced channel outflow over the adjusted weir. Due to strong temporal and spatial variations of the initiated wave and air entrainment, non-intrusive measuring techniques are necessary, which provide high frequencies and stability towards dynamically changing water levels. Thus, water depths are detected by ultrasonic sensors in a high frequency of 50 Hz. In case of transient measurements the sensors are triggered by the opening mechanism. A CCD-camera takes 50 pictures per second recording the front of the wave, which is identified by self-aeration processes and image processing. Velocity profiles are sampled using a con-ventional 1D laser-Doppler anemometer (LDA) which is mounted beneath the glass plate on an automatic traversing unit. All these measuring techniques are well-tried while a particle tracking velocimetry (PTV) to determine surface speeds failed due to extreme requirements in terms of image solution and frequency.

For all the configurations flow depths and arrival times of the transient wave are detected in a raster (Δx = Δy = 0.2 m) on the glass plate. Together with top-view photos by a CCD-camera and digital image processing the propagation of the wave front (contour line, velocity, ap-proaching time) is scanned continuously. With respect to the steady state, water levels were also recorded in the channel, in the body of the wave in a refined raster (Δx = Δy = 0.1 m), and in the rotation range of the flap gate (up to y = 0.8 m). Horizontal velocities over depth


are measured at three cross-sections (y = 0.25, 0.30, 0.35 m) near the breach in a dense raster (Δx = 0.05 m, Δz = 0.01 m) within the wave. These profiles provide a distribution of momentum correction coefficients (BOUSSINESQ coefficients) to be exploited for 2D numerical modelling. The resulting amount of data is evaluated, analysed and manageable filed.

Figure 2: Scale Model set-up

After each model run there remains a thin water layer on the glass plate causing significant changes in the wave behaviour. The effect of a dry versus a wet propagation area is further investigated beyond the original work schedule. For test runs on a dry bed the glass plate has to be drawn off by a huge squeegee every time. The examination of roughness, which is changed by using sand of different grain size on the propagation area, shows that the water layer and roughness structures interact with each other. As regards the execution of the scale model tests, the drying of the propagation area between two test runs is strongly rec-ommended. A complete drying of the sandy surface is impossible in the execution of the tests. Hence, a separate quantification of roughness effects for the transient phase is impos-sible. Therefore, they are not investigated in detail as originally formulated in the application. However, the influence of roughness is minimised by using the glazed plate and the focus may be put on other phenomena. In the work schedule (theme A) it has been assumed that a first formulation of a simple forecasting tool in terms of flood wave propagation would be based on empirical and statistical approaches. Actually, in the course of the work a semi-analytical procedure has been developed, predicting water levels on the main axis rectangu-lar to the breach (BRIECHLE, 2006). Moreover, a formula to forecast the contour of the wave front from top-view has been derived.


Numerical modelling (theme B): Firstly, numerical simulations of the experimental flow configurations (theme A) have been performed with two different 2D models solving the depth-averaged shallow water equations (SWE): on the one hand DGFLOW is applied, a total variation diminishing (TVD) RUNGE KUTTA discontinuous GALERKIN (RKDG) finite element (FE) method on unstructured triangu-lar meshes, developed at RWTH Aachen University (SCHWANENBERG & HARMS, 2004). A slope limiter is adopted on every intermediate time step of the RK method, introducing a se-lective amount of dissipation to obtain stability at shocks. On the other hand WOLF2D is used, a finite volume (FV) scheme involving a flux vector splitting (FVS) method on a mul-tiblock structured grid, developed at the University of Liege (see 1.9). Variable reconstruction at cells interfaces is performed linearly, in conjunction with slope limiting, leading to second order spatial accuracy. Both schemes are well suited to handle transient, rapidly varying dam-break induced flows with steep gradients over a dry bed. Now they are applied to the dike-break induced flow conditions in the experimental test arrangement.

In addition to the hydraulic (discharge, initial water level) and geometric (breach width) boundary and initial conditions, several numerical parameters are examined. A sensitivity analysis is performed with DGFLOW as regards parameters related to the steep gradients at the front of the propagating wave (minimal allowed water depth in combination with parame-ters of the limiting function in the TVD-scheme). A preliminary study has yielded the required fineness of the numerical grid in order to obtain an almost mesh independent solution and to minimize the discretization error. Besides the spatial resolution also the explicit temporal scheme has been investigated by methodically changing the CFL-number. Notwithstanding the original work schedule, but according to the procedure in theme A, roughness coeffi-cients have been changed systematically but no new approaches to account for dynamic roughness effects are implemented. Additional simulations are performed with WOLF2D in-troducing non-uniform BOUSSINESQ-coefficients (for uneven horizontal velocity distribution over depth) as well as different turbulence closure schemes to analyze whether these ap-proaches affect the computed 2D solutions compared to the measurements.

Subsequently, a fully 3D model has been set up: the commercial code Star-CD is based on the REYNOLDS-averaged NAVIER STOKES equations (RANS) and accounts for the free surface location via a volume of fluid method. Roughness and turbulence effects are considered via a wall-functional approach and a k-ε-turbulence model, respectively. Once again the different scenarios of the scale model tests are computed in 3D. All results are compared with the measurements and exploited in detail in terms of flow depths, flow velocities, modelling of the wave front (contour lines, approaching times), and calculation of the breach discharge (ROGER, 2008). In some cases (dispersion, dissipation), the 3D information is used to esti-mate appropriate settings for the 2D approaches (ROGER ET AL., 2008).

Specific difficulties occur with respect to the modelling of the flow split into breach discharge and the discharge in the downstream channel. Particularly, in combination with the down-stream boundary condition, this additional degree of freedom leads to numerical problems (ROGER ET AL., 2006). In both 2D codes the calibrated weir formulas for different crest heights (theme A) have been implemented while the complete weir is geometrically discre-tized for the 3D computations. Thus, the dynamic of the weir discharge can be modelled, depending on the actual water level in the channel and the resulting backwater effects affect-ing the flow resistance of the downstream channel reach. The direction change of momen-tum from the main flow direction of the channel to the breach main axis is closely related to the interaction of flow split and the downstream boundary. It seems that also in real large-scale applications the breach discharge is strongly affected by this interplay.


2.3 Presentation of results

Scale model tests (theme A): A huge amount of data has been collected and concentrated in a data pool. In the framework of diverse test runs specific phenomena and boundary conditions of a dike-break induced flow have been modelled successfully. By changing the test conditions systematically the analysis of isolated influencing factors is enabled. The measured data provides a worthwhile basis for establishing, validating and improving numerical models and simulations, which is the objective of the second project part (theme B). The interpretation of the measurements and the developed formula yields the conclusion that the following governing assumptions are largely correct: a dike-break wave may be divided into sub-processes, which can be su-perposed, as for example positive and negative dam-beak waves for the lateral propagation and the inflow from the channel. For the first time a semi-analytical method of calculation has been set up to predict water levels on the propagation area which accords well with the measured data. Furthermore, an empirical formulation to track the changes in contour of the propagating wave front has been derived from experimental data. Both formulas are user-friendly and suitable for a quick estimation of the wave propagation. Flow depths and dis-tances between the wave front and the breach main axis are both functions which depend on the initial water level in the channel, breach width, distance to the breach, and elapsed time in relation to the failure. All derivations and remaining differences (coming from the complex initial phase, dynamic effects, hydrostatic pressure assumption, superposition of sub-processes, and the wave deflection due permanent channel discharge) are broadly dis-cussed (BRIECHLE, 2006).

Figure 3: Exemplary measured water levels for a thin water layer (dotted line) and dry bed (solid line)

If a thin water layer remains between two tests, the wave front is significantly altered: the wave is decelerated, there is much air entrainment, and the peak piles up unlike the dry case. Figure 6 shows the differences between wet and dry bed, where the envelope of both cases is adumbrated for orientation (BRIECHLE ET AL., 2004). The classical approach by Stoker with an initial water level on the propagation area fails and underestimates the wave front velocities. However, it confirms the occurrence of an unfixed hydraulic jump with a roller bucket at the wave front. The application of the model after Ritter is recommended for safety reasons because it assumes too high wave peaks (BRIECHLE, 2006).


Numerical modelling (theme B): All numerical simulations of the different experimental configurations reproduce the basic flow pattern satisfactorily (fig. 3), which confirms the convergence and general applicability of the methods on dike-break type of problems. At first the initial state is computed with a closed gate which corresponds to a simple open channel flow with a sharp crested weir at the outflow. According to the opening mechanism in the scale model tests, an immediate and complete failure of flood protection measures with a rectangular breach is realised in the nu-merical computations by changing the respective boundary condition at the breach location. The whole flooding event is modelled in a single stable run without oscillations including both, the river and the flood wave propagation on the floodplain. The resulting deviations from the performed measurements (theme A) are analysed focussing – like in the parallel literature review – on the simplifications of the mathematical model or the numerical ap-proach. The latter has to deal with complex free surface structures, steep gradients, flow over initially dry bed, and friction at the wave front. As regards the mathematical models the different simulations enable direct comparisons between the assumptions of the SWE and the less simplified RANS in relation to the collected experimental data.

Figure 3: Numerical simulation of experimental dike-break induced flow: SWE (left) and RANS (right)

First shallow water results indicate that the deflection of the flood wave, the water levels, and the horizontal flow velocities within the wave are numerically overestimated. Higher water levels in the channel yield a greater outflow at the downstream weir and at the same time an underestimation of breach discharge (ROGER ET AL., 2006), though the qualitative flow split for different configurations is modelled correctly. The computed dynamic behaviour of the wave front in terms of arrival times and shape from top-view shows little differences com-pared to measured data (HARMS ET AL., 2004; NIEMEYER, 2007). In consistency with theoreti-cal hypothesis, the 3D results fit better the scale model data than the shallow water ap-proaches (ROGER, 2008). By identifying the significant weak points of the SWE concerning that type of flow the priorities of developing and implementing enhanced approaches in shal-low water models are elaborated (ROGER ET AL., 2008). The relevance of an optimisation is evaluated by the influence of the analysed effect on the experimental flow. The objective is to comprehend the fluid dynamics in the near field of the breach and to determine a destination from this area where the mathematical simplifications are negligible. The large-scale and long-term inundation of a floodplain in practical applications is strongly affected by the steady-state discharge through the breach. Thus, its correct calculation is the decisive pa-rameter to evaluate model modifications. Comparative simulations including realistic scenar-ios with complex topography are used to demonstrate the accuracy, feasibility, robustness, and practical applicability of the numerical approaches (NIEMEYER ET AL., 2005; KUTSCHERA ET AL., 2008).


2.4 Outlook on future research

Scale model tests (theme A): In principle, the experimental set-up enables additional non-intrusive measurements via laser Doppler velocimetry (LDV). Face to face with the breach location there is window in the side channel wall. A strong 3D laser with the capacity to penetrate more than one meter of water may detect 3D velocities in the channel close to the breach and within the wave. This yields insight in terms of deflection of momentum and the flow split into breach discharge and the discharge in the downstream channel. Moreover, the new data enables to estimate the flow resistance of the downstream reach compared to the breach and helps to learn more about turbulent and shear effects in the transient, 3D, turbulent and multiphase dike-break induced flow. Besides the knowledge of the flow regime in the channel, the measurement of vertical velocity components would become possible. Installing a measuring system like this and performing respective experimental investigation seem to be worthwhile, but on the other hand they are very costly also in terms of labour and time.

Failure mechanisms of flood protection structures have yet not been considered. Above all the influence of a breach development on the extent of the inundated areas and the dynamic damage has to be evaluated. Apart from the erosion, transport and deposition of dike mate-rial there is one phenomenon in the hinterland previously neglected: scours, resulting from an interaction between soil, breach and water. In this context, scours can be defined as flood wave induced lowering of the sediment bed in the floodplain in the vicinity of the breach loca-tion. Hitherto, this effect has been consciously neglected until now. After knowing of the flow conditions in the idealised model, a near-natural modelling may be performed by adapting a movable bed on the propagation area. Synergy effects are exploited by reconstructing the existing model. The glass plate may be substituted by a box of solid, erodible material. After model runs the changes in bed elevation can be recorded in a pre-defined raster.

Numerical modelling (theme B): At present a fully 3D simulation of a complete real dike break scenario is not possible due to the computational costs and the huge amount of data to be interpreted. The outcome of the comparative numerical simulations shows that most of the differences originate from the hy-drostatic pressure assumption when neglecting vertical velocities and momentum. There are several ways to account for some of the neglected effects in extended shallow water equa-tions (EWE). However, additional equations lead again to higher computation costs. A more effective way would be to consider the shape of the free surface as a criterion to include a pressure distribution over depth via appropriate source terms. In a first step the measured shape of the steady state could be used to derive appropriate formulations. The co-operation partner for example has shown expertise in this strategy by adopting the bed curvature of a spillway in suitable curvilinear coordinates. A further improvement may be implemented by means of parameterized velocity depth-profiles, which are solved maximizing the breach discharge due to hydraulic drop. The 3D measurements and numerical computations have shown change in velocity direction and recirculation zones on both sides of the breach. If these regions are continuous and identifiable a multi-layer shallow water approach is also possible. Another way might be the so-called adaptive modelling strategy, where suitable models (1D, 2D, 3D) are chosen and coupled automatically in the whole domain depending on certain criteria, e.g. gradients. So far, the current hybrid approach is a manual reference example to develop and validate coupled or adaptive simulations with respect to dike-break induced flow. Exploiting further the combination of experimental and numerical works, mor-phodynamic simulations are required to compute scour geometries and to include the varying topography in the flood wave simulation.


2.5 Exploitation potential The scientific outcome can be exploited not regarding patent or industrial joint ventures but in terms of selective applications within existing methods. The importance of the research topic for flood control strategies is emphasized by the economic relevance which makes a precise forecast of the flood wave propagation desirable. There is a change of mind currently taking place in society from a safety-conscious to a risk-based view. Absolute security can not be guaranteed and society has to accept a residual risk. Engineer standards for German dams begin to take residual risks into consideration (HUBER ET AL., 2005). The basic principle of this approach can be transferred to rivers and dikes with a risk assessment procedure for technical flood protection measures like RAPID in KUTSCHERA ET AL. (2008). The determina-tion, evaluation and mitigation of the risk provoked by floods (mathematically obtained by multiplying the probability of a failure by the extent of the potential damage on people and property caused in the event of a collapse) involves the identification of inundated areas as well as water depths and flow velocities of an initiated flood wave. Based on the experiences of flood events at the Rivers Rhine, Danube and Elbe, a risk analysis has to be accomplished above all in densely populated areas along large rivers. Beside the damage calculation, the results of flood wave computations are used to manage the residual risk. The determination of evacuation zones/routes, the coordination of disaster control programs and emergency measures as well as the land use planning are important for risk mitigation. The simulation of different failure scenarios may provide civil protection authorities with valuable information about the availability of important infrastructure (main routes, telecommunications) and ad-vance warning deadlines in terms of arrival times and main flow directions. Within the scope of a risk assessment procedure for protective measures, the required hydraulic input data, i.e. the wave propagation in space and time, may be provided by inundation modelling.

Mobile protective structures gain in importance in the context of flood protection conception above all in urban areas. Moreover, the owners of private objects tend to protect their prop-erty with a separate mobile flood defence. The conducted studies support the design and risk assessment of such mobile elements, particularly by means of providing a simple computa-tion formula and optimising the numerical forecasting tools. First findings have been intro-duced into a German bulletin dealing with planning and application of mobile flood protection measures (KÖNGETER & BRIECHLE, 2005). The insurance industry estimate damage catego-ries of objects depending on inundation probabilities and resulting water levels. Up to know, rather simple models are frequently used. Enhanced hydrodynamic simulations may provide a more reliable calculation basis in terms of flow depths. Furthermore, the modelling of flood wave propagation may be exploited to quantify dynamic damages instead of evaluating only the static material damage. Buildings for example meet with total loss for certain damage parameters (flow depth times velocity). In the course of police actions to assure public safety the damage parameter for human beings matters concerning the specification of safety zones behind protective structures which should not be entered. This question arises not only for the protection against vandalism, sabotage and flood tourists searching souvenirs. It is also interesting to know where people are knocked over or harmed in case of a failure in order to staff the protection line reasonably.

In German legislation according to the “act to improve preventive flood control” of 3 May 2005 potential inundated areas have to be mapped. These are regions which might be flooded in case of a failure of protective structures like dikes. On the European level accord-ing to the directive 2007/60/EC of 23 October 2007 “on the assessment and management of flood risks”, regions with a significant flood risk should be indentified in a hazard mapping. In addition the European Commission in 2006 has proposed a directive based on the “pro-gramme for critical infrastructure protection”, further promoting a risk discussion. In annex 2 of the underlying Green Paper the flood protection is explicitly mentioned. In either case, the identification of relevant areas by a reliable modelling of flood wave propagation due to a failure of flood protection measures can make important contributions.


2.6 (Interdisciplinary) contributions, qualifications of involved researchers

Applicant: Jürgen Köngeter, Univ.-Prof. i.R. Dr.-Ing. Professor emeritus, former director of the Institute for Hydraulic Engineering and Water Re-sources Management, RWTH Aachen University

Director of the institute (since October 2007): Holger Schüttrumpf, Univ.-Prof. Dr.-Ing. Co-ordination, surveillance, advice

Project staff Dr.-Ing. Sylvia Briechle (theme A) Dr.-Ing. Maren Niemeyer (born Harms) (contributions theme B) Dipl.-Ing. Sebastian Roger (theme A+B)

Co-operation with other scientists Shallow water free-surface flows are mathematically formulated by coupled hyperbolic differ-ential equations. That type of equation system is currently under investigation in the disci-pline of numerical mathematics. Applications and possibilities of comparison are eagerly re-quired. A common publication along with Univ.-Prof. Dr.rer.nat. Sebastian Noelle (Institute for Geometry and Practical Mathematics, Division of Numerical Mathematics, RWTH University) in a journal focussing on mathematics is intended, dealing with studies about well-balanced and TVD-techniques. Moreover, Prof. Noelle is supervisor and reviewer of a doctoral thesis in context of the project (ROGER, 2008). Further research activities concentrate on the adap-tive modelling strategy, where suitable models (1D, 2D, 3D) are chosen and coupled auto-matically in the whole domain depending on certain criteria, e.g. gradients. So far, the current hybrid approach is a manual reference example to validate coupled or adaptive simulations with respect to dike-break induced flow. Further following personal communication and help-ful suggestions are gratefully acknowledged:

• Univ.-Prof. Dr. Marek Behr, chair for computational analysis of technical systems, RWTH Aachen University (grid generation, free surface flows)

• Dr.-Ing. Ulf Sickmühler, CD-adapco, Nürnberg (solution mapping for transient simula-tions in Star-CD)

• Prof. Dr.-Ing. habil. Helmut Martin, TU Dresden (scale model tests and flood wave propagation, supervisor BRIECHLE, 2006)

• Dr.-Ing. Carsten Thorenz, Federal waterways engineering and research institute (BAW), Karlsruhe (VOF-method in Comet, the forerunner model of STAR-CD)

• Dejana Djordjević M.Sc., University of Belgrade, Serbia (experimental dam break simulations)

• Dr.-Ing. Dirk Schwanenberg, Deltares (former Delft hydraulics), The Netherlands (common implementations in the shallow water code of DGFlow)

• Prof. Hubert Chanson, University of Queensland, Brisbane, Australia (exchange on flood wave propagation)

The main co-operation has been established with Dr ir Benjamin J. Dewals, University of Liege. During his research fellowship (Belgian Fund for Scientific Research) he worked for 6 months together with Dipl.-Ing. Sebastian Roger in Aachen on the simulation of dike-break induced flows focussing on the assumptions of shallow water flows. The modelling tech-niques of WOLF2D (Liege) and DGFLOW (Aachen) are investigated (ROGER ET AL., 2008).


Degree, doctoral or professorial dissertations

Diploma theses (in German) VAN WICKEREN, D. (2005): Geschwindigkeitsermittlung nach Versagen einer Hochwasser-

schutzeinrichtung. BÜSSE, E. (2005): Zweidimensionale tiefengemittelte Simulation von Deichbruchströmungen KPALOBI, C. (2005): Flutwellenausbreitung an einem physikalischen Modell - Einfluss der

Rauheit auf Wellenausbreitung und Skalierbarkeit der Modellergebnisse LOBECKE, I. (2004): Dreidimensionale Flutwellenausbreitung - Untersuchung der Flutwellen-

front und Skalierung der Ergebnisse

Doctoral theses NIEMEYER, M. (2007): Einfluss der Breschenbildung auf die Flutwellenausbreitung bei Damm-

und Deichbrüchen. (in German) Aachen: Shaker. – ISBN 978-3-8322-7132-9 (Notes of the Institute of Hydraulic Engineering and Water Resources Management, RWTH Aachen University: doctoral thesis, full digital version http://darwin.bth.rwth-aachen.de/opus3/volltexte/2007/2012/)

BRIECHLE, S. (2006): Die flächenhafte Ausbreitung der Flutwelle nach Versagen von Hoch-wasserschutzeinrichtungen an Fließgewässern. (in German) Aachen: Shaker. – ISBN 978-3-8322-6674-5 (Notes of the Institute of Hydraulic Engineering and Water Re-sources Management, RWTH Aachen University: doctoral thesis, full digital version http://darwin.bth.rwth-aachen.de/opus3/volltexte/2006/1540/)

ROGER, S. (2008): Hybride Modellierung einer deichbruchinduzierten Strömung in direkter Umgebung einer idealisierten Bresche. (In Progress)

3 Summary The failure of flood protection measures (e.g. dikes, mobile walls) releases a wave into the hinterland which may cause extensive damage in the floodplain. In contrast to the flood wave propagation initiated by dam breaks the knowledge of so-called dike-break induced flows can not be called satisfactory. By means of literature research the most influencing variables are identified. A combination of physical (theme A) and numerical (theme B) modelling is used to quantify the impact of the determining factors as regards a reasonable forecast of dis-charges, water depths and flow velocities of flood waves initiated by dike breaks.

An idealised physical model was constructed to examine experimentally dike-break induced flows taking into account the specific boundary conditions of that type of flow. Initial water levels and discharges in the channel as well as the breach width are systematically varied in the scale model test (theme A). A novel semi-analytical formula was developed via measured data to predict the water levels on the main axis rectangular to the breach. It is based on the assumption that the lateral propagation of a dike-break wave can be described as a super-position of well-known sub-processes, namely positive and negative dam-break waves. The approach is applicable in engineering practice and accords well to the measured data. More-over, the shape of the wave front from top view is approximated. The remaining differences coming from the underlying assumptions are broadly discussed (BRIECHLE ET AL., 2004, BRIECHLE 2006). Altogether, the scale model tests yield an overall picture, which can be used to effectively forecast the flood wave propagation. Special attention so far is paid on the first transient phase as regards the propagation and arrival times of the wave front. Furthermore, water levels, horizontal velocities over depth in the body of the wave as well as discharges through the breach are detected for the final steady state. Once again, the hydrodynamic and geometric boundary conditions are methodically changed (ROGER, 2008). The measured data provides a worthwhile basis for establishing, validating and improving numerical models and simulations, which is the objective of the second project part (theme B).


According the experimental configurations the flow is computed via two different depth-integrated shallow water models and a fully 3D free surface code based on the RANS. The basic flow pattern is reproduced well by all three simulation results. In addition to the hydrau-lic and geometric conditions several numerical parameters are examined. It was possible to compute the whole flood event when releasing the dike-break induced wave from the initial open channel flow with a closed flap gate up to the final steady state in a single stable run without oscillations. The resulting deviations from the performed measurements are analysed focussing – like in the parallel literature review – on the simplifications of the mathematical model or the numerical approach. The latter has to deal with complex free surface structures, steep gradients, flow over initially dry bed, and friction at the wave front. As regards the mathematical models the different simulations enable direct comparisons between the as-sumptions of the SWE and the less simplified RANS in relation to the collected experimental data. In consistency with theoretical hypothesis the 3D results fit better the scale model data (ROGER, 2008). By identifying the significant weak points of the SWE concerning that type of flow the priorities of developing and implementing enhanced approaches in shallow water models are elaborated (ROGER ET AL., 2008). The relevance of an optimisation is evaluated by the influence of the analysed effect on the experimental flow. The large-scale and long-term inundation of a floodplain in practical applications is strongly affected by the steady-state discharge through the breach. Thus, its correct calculation is the decisive parameter to evaluate model modifications. Comparative simulations including realistic scenarios with complex topography are used to demonstrate the accuracy, feasibility, robustness, and prac-tical applicability of the numerical approaches (NIEMEYER ET AL., 2005; KUTSCHERA ET AL., 2008).

The research findings expand into risk assessment studies especially into the calculation of potential damage and vulnerability. Application and exploitation potential can be found in cost estimations used by the insurance industry, evacuation mapping for civil protection, disaster control, emergency management, flood hazard and inundation mapping, set-up of flood pro-tection conceptions, and in the designing and dimensioning of protective structures. Hence, reliable simulation results are eagerly required to fulfil legal and economic demands.

Aachen, 30th of April 2008

Sebastian Roger Dipl.-Ing., Project staff

Jürgen Köngeter Univ.-Prof. i.R. Dr.-Ing. (Professor emiritus, former director), Applicant

Holger Schüttrumpf Univ.-Prof. Dr.-Ing., Director of the institute

modelling a flood wave due to failure of flood protection ... · modelling a flood wave due to...

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