THE INFLUENCE OF INITIAL FLOW CONDITIONS ON THE PROPAGATION OF DAM BREAK WAVES

 

Rosi Liem ¹, JÜrgen KÖngeter ²

 

Research Engineer ¹, Professor ²

Institute of Hydraulic Engineering and Water Resources Management (IWW), RWTH Aachen, Versuchshalle Kreuzherrenstrasse, 52056 Aachen, Germany

Tel.: 0049-241-807778, Fax: 0049-241-8888275, E-Mail: liem@iww.rwth-aachen.de

 

 

Abstract

The sudden break of a dam or other hydraulic constructions which ought to protect dry regions from flooding initializes the propagation of an unsteady flood wave. Several former investigations have been conducted assuming that the discharge into the emptying reservoir equals zero and therefore water levels within the reservoir will decrease immediately. At the Institute of Hydraulic Engineering and Water Resources Management (IWW) at Aachen University of Technology experimental investigations on dambreak waves have been carried out considering stable water levels in the reservoir and also regarding different discharges into the reservoir. These three dimensional investigations were required to predict the propagation of a flood wave caused by a failure of flood protection walls along rivers in Germany. Thus the river functions as the reservoir in a dambreak problem. The obtained data indicated constant propagation velocities not influenced by the amount of discharge within the river but a significant increase in water levels within the flood wave as the discharge is raised. The construction of the model, the conducted experiments as well as results obtained from water level analysis are presented in this paper.

 

Keywords: Flood Wave Propagation, Unsteady Flow, Physical Model

 

Introduction

In Germany there are many urban regions strongly affected by flooding of rivers. To avoid the unpleasant appearance of high walls and levees along riversides, mobile flood protection systems have been designed, which are set up any time a flood is forecasted. Due to the short amount of time after the flood is forecasted, there are many workers engaged to fix the system. Because of the time pressure mistakes are made easily. Moreover it has been noticed, that people passing by are not aware of the risk of flooding and have taken away screws or other constructional elements of the system. Therefore mobile flood protection systems are not regarded as safe as permanent constructions, which seems to be even worse, since they are mainly used in densily populated areas.

To judge the risk of direct flood damage on human beings and the surrounding in case of a failure experimental investigation had to be carried out. Former investigations on dam break were not reliable since the main flow of the river behind the flood protection walls causes initial flow conditions to differ from those in the reservoir behind a dam. The main differences are pictured in figure 1.

 

 

Figure 1 : Comparison between flooding caused by a dambreak and a wall failure

 

In contrast to flow conditions in a reservoir the volume V_R and the water level h_w in a river do not decrease progressively after the failure, but stay at a constant level because of the very high discharge in the river during a flood. Hence it can be assumed that water levels and velocity profiles within the propagating flood wave will differ as well. Moreover the flood wave after a failure might not move symmetrically as found after a dam break, but propagate rather into the direction of the main stream of the river. In former investigation dambreak has usually been treated as a two-dimensional problem because of the symmetrical appearance of the propagating wave. Experiments were mostly carried out in open channels, as for instance found in Hager et al. (1996) and Bell et al. (1992). Since the discharge into the reservoir is much lower than the dicharge passing the broken dam, it has always been neglected and set to zero.

Three-dimensional studies on flood wave propagation caused by dambreak were carried out by Tingsanchali et al. (1993) and Bechteler et al. (1992) who prooved the symmetrical shape of the propagating wave and used their measuring data to validate numerical models. Initial flow conditions were set as in open-channel investigations. However, Bechteler et al. (1992) used a very long open channel as a reservoir to cause a slow decrease of the water level.

 

Experimental Setup

 

Physical Model

A three-dimensional physical model was constructed integrating an aluminium gate of 0.3 m height and 0.6 m width between the river section and the propagation area which can be seen in figure 2.

A water propagation area of 3.5 m x 8.5 m is provided. To avoid unintential backflow effects the area is surrounded by small channels where the water can run off directly towards the outflow. The gate is controled by an electronical opening mechanism.

The slope of the propagation area is S=0.05 %, the Manning roughness during all investigations is n=0.01. The chosen parameters S and n will enable comparison to results from experimental dambreak investigations conducted by Tingsanchali et al. in 1993.

 

Figure 2 : Physical model

 

Measuring Techniques

Velocities and water levels were recorded continuously by an advanced measuring system. The components of the measuring system to obtain water levels had to be developed, since the strong unsteady flow within the propagating wave requires instrumentation providing high frequencies and stability towards dynamically changing water levels. There were two different measuring systems to obtain water levels.

Water levels close to the gate were measured by a controler sensing the water surface with an electrode system. Changes of the water level up to 0.7 m/s can be perceived with a frequency of 600 Hz. Using this system only the water surface was slightly touched. There is actually no disturbance of the flow.

Water levels further away from the gate were measured by capacity probes, consisting of two conducting poles, which have to stand within the current. As the water depth grows the electrical capacity of a probe increases as well. Water levels were obtained at a frequency of 200 Hz.

To proove the reliability of both measuring systems there were two checking points in the measuring grid, where both systems were applied.

 

RESULTS

Data was obtained from 72 measuring points covering 1 m x 1.5 m of the propagation area. In figure 3 the acquired propagation times of the wave front close to the gate are plotted. As already expected the flood wave does not spread symmetrically but rather in the direction of the main flow.

 

Figure 3 : Propagating pattern of the flood wave

 

Moreover compared to Tingsanchali et al. (1993) the wave front moves faster. Considering the same bottom roughness and slope a distance of 1 m to the gate was already reached after 0.7 s whereas 0.85 s was derived from a usual dambreak experiment regarding to the dimensions used in Tingsanchali et al. (1993). This can be caused by the remaining water level in the reservoir, since experiments for different discharges into the reservoir indicated identical propagating times of the wave front.

Water depths within the propagating flood wave are significantly effected by variing discharges. As shown in figure 4 water depths rise with increasing discharge.

It can also be noticed that there are two momentum effecting the direction of the flood wave induced by the opening direction of the gate and by the main flow direction within the reservoir. These two directions of the flow wind up gradually into one flow direction only influenced by the flow direction within the reservoir. Steady flow characteristics within the flood wave are attained after four to five seconds also depending on the discharge within the reservoir.

 

Figure 4 : Water depths within the flood wave after 4 s

 

Conclusions AND OUTLOOK

Doing comparisons to results from former investigation turned out to be very difficult, since measuring frequencies have not been as high as for the conducted experiments. Since frequencies in former investigations have been between 1 and 2 Hz compared to a frequency of at least 200 Hz, it is hard to tell, if differences in the propagating time are caused by inaccurate measurements or by initial flow conditions. An exact comparison between the propagation of dambreak waves and waves initialized by wall failures along rivers will only be possible by carrying out dambreak studies again using the same measuring equipment.

Still the influence of initial flow conditions on dambreak waves has clearly been prooved by the conducted experiments. The influence of initial flow conditions will be even more meaningful in smaller reservoirs and rivers during flood events, when flow conditions differ significantly from those of former experimental investigations.

To achieve more accurate relationships between flood wave characteristics after a dam break and initial flow conditions within the reservoir it will be reasonable to investigate velocity profiles within the river and the spreading wave, which is currently done.

Besides water depths especially the acceleration of the wave front initializing strong forces within the flood wave can already endanger people at low water levels to fall down and drown. Therefore the investigation of velocity profiles is even more meaningful to predict certain risks caused by failures of dams, dikes or similar constructions.

To predict the further extense of water influenced by different initial flow conditions a numerical model for the city of Cologne along the river Rhine will be set up, considering measuring data from physical investigations as boundary conditions. In this way case studies can be carried out handling the influence of variating slopes an bottom roughnesses as well. For validation the surrounding area close to the gate of the physical model is simulated at the moment and will be compared to the obtained measuring data.

 

References

[1]    Bechteler, W.; H. Kulisch, M. Nujic (1992) "2-D Dam-Break Flooding Waves- Comparison between Experimental and Calculated Results"; Proceedings of the 3rd International Conference on Flood and Flood Management, Florence

[2]    Bell, S.W.; R.C. Elliot, M.H. Chaudhry (1992) "Experimental results of two-dimensional dam-break flows", Journal of Hydraulic Research, Vol. 30, No. 2

[3]    Hager, W. H.; Lauber, G. (1996) "Hydraulische Experimente zum Talsperrenbruchproblem"; Schweizer Ingenieur und Architekt, Heft 24, Jahrgang 114

[4]    Tingsanchali, T.; W. Rattanapitikon, W. (1993).; "2-D mathematical modelling for dam break wave propagation in supercritical and subcritical flows"; Proceedings of the 25th IAHR Congress in Tokyo, pp. 25 - 32