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Earthquakes, floods, landslides, and volcanic activity have resulted in catastrophicdam failures and devastating floods. To know the effects of a dam break, we have to know how the resulting flood will propagate.

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Dam-breaks and consequences.

X M Carreira

DAM BREAKS AND CONSEQUENCES Xos Manuel Carreira Rodrguez (notonlybridges@gmail.com)1. Introduction 2. Hydraulic flow models 2.1. 1D numerical modelling 2.2. 2D CFD techniques 3. Risk assesment 4. Examples 4.1. Historic dam failures 4.2. A recent case: the Aznalcllar case 5. Concluding remarks 6. References

1. INTRODUCTION The possibility of a devastating flood resulting from dam failure is a concern wherever these structures exist. Earthquakes, floods, landslides, and volcanic activity have resulted in catastrophic dam failures in a variety of environments. From 1946 to 1955, a total of 12 major dam failures were recorded and during the same period of time more than 2,000 dams were constructed worldwide. From years 1956 to 1965, a record of 24 failures and more than 2,500 new dams were constructed during the same period of time. [JANSEN88]. [JOHNILLES02] summarized 300 dam failures throughout the world. Dam failure can be primarily attributed to a number of major key factors including earthquake, differential settlement, seepage, overtopping, dam structure deterioration, rockslide, poor construction and sabotage [RICO08]. 2. HYDRALIC FLOW MODELS To know the effects of a dam break, we have to know how dams may break and how a flood will propagate. The damage parameter (flow velocity times water depth) deriving from a dam break flood proved to be a useful tool for estimating consequences of a dam failure (property damage and loss of life) as well as for emergency response planning. Prior to the preparation of any emergency response plan, the dam operator has to carry out a risk assessment study which provide information on the covered area: - the near safety zone, flooded in less than 15 minutes after the dam-break. - the remote area concerned by the submersion wave or the limit at which there is no significant danger for the populations.

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Dam-breaks and consequences.

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Numerical and physical models are used to answer these questions but the development of a dam break is a complicated extreme problem that contains a lot of uncertainties. 2.1.1D NUMERICAL MODELLING Even though, the probability of dam failure can be extremely low, but its occurrences can imply catastrophic consequences downstream, including loss of human lives, properties, natural resources and so on. Therefore, significant predictive data on hypothetical flood events such as flood flows, flow velocities, depths and flood wave arrival times at specific locations downstream of the dam become some of the most important pieces of information for disaster preparedness such as for the formulation of Emergency Response Plan (ERP) guidelines [TURA02]. General international practices on dam safety would include procedures that suit practical management of the dam conditions such as sending early warning and notification messages of emergency situation to the authorities, as well as information on inundation of critical areas for action in case of emergency. Generally, dam break analysis aims at predicting downstream hazard potential systematically in equitable approaches. Numerical modelling process simulations can be carried out based on the topography of a catchment area using an appropriate grid size of approximately 200 m. Generally, a scenario discharge may be assumed in the simulation and flood affected areas may be predicted over a distance of 25 km downstream of the dam, and 1 to 2 km in width [BOSS99]. Currently, there are a number of dam break simulation models widely used by researchers and consultants such as the national weather service dam break forecasting, Mike21 (Danish Hydraulic Institute), HEC-HMS/HEC-RAS flood hydrograph (U.S. Army Corps of Engineers), BOSS DAMBRK hydrodynamic flood routing and soil conservation service (SCS) TR#66 uniform dam failure hydrograph. Downstream hazards may include potential loss of human lives, properties (such as residences, commercial buildings, industrial facilities, croplands and pasturelands), infrastructures and utilities located downstream of the dam [TURA02]. The 1D modelling for the dam break hazard analyses is based on an implicit finite difference scheme. The cross-sections used in the model can be taken either from a GIS terrain model or they can be on-site measured cross-sections.

2.2. CFD TECHNIQUES In the case of very complicated topography, the use of a 2-dimensional model seems to be more reasonable than the use of a 1D model. One-dimensional model needs a lot of experience since the cross-sections have to be put at the right locations. The use of a 2-dimensional models is more straightforward. The impact flow on a vertical wall resulting from a dam break problem can be simulated using a Navier-Stokes (NS) solver. The NS solver uses an Eulerian Finite Volume Method (FVM) along with a volume of fluid (VOF) scheme for phase interface capturing. One of the most common Computational fluid dynamics (CFD) packages for simulations of free surface problems is

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Dam-breaks and consequences.

X M Carreira

FLUENT [FLUENT]. Results show favorable agreement with experiments before water impact on the wall. However, both impact pressure and free surface elevations after the impact depart from the experiments significantly. Hence the code is assessed to be good only for qualitative studies. In particular, we have examined the classical dam break problem and subsequent water impact on a plane vertical wall. The FLUENT results for the initial stages of the problem closely agreed with other numerical techniques and experimental results. However, there was some disagreement in water tip location between numerical results and experiments. This is perhaps due to the imperfect initial conditions and some physical effects not numerically modeled. The water impact pressure was numerically measured and compared with experiments. Although the first peak agrees with the experimental measurements of [ZHOU99], the second peak was largely underestimated. This suggests that FLUENT is acceptable for qualitative studies only. In general, the problem after the initial impact could not be modeled with the desired accuracy. Further research is needed to strengthen the features of the software which are not suited for these types of applications. Free-surface reconstruction (complex geometry) including fluid discontinuity and the treatment of entrained air are some of the areas that require further investigations.With careful modelling and accurate data the results of different modelling approaches may be relatively close each other. However, there is a lot of uncertainties in the modelling and specially in the onedimensional flow modelling where the user of the model can have a significant effect on the results by selecting the locations of cross-sections carelessly.

3. RISK ASSESMENT Risk assessment is the process of deciding whether existing risks are tolerable and present risk control measures are adequate and if not, whether alternative risk control measures are required. Risk assessment incorporates, as inputs, the outputs of the risk analysis and risk evaluation phases. Risk assessment involves judgements on the taking of risk and all parties must recognize that the adverse consequences might materialize and owners will be required to deal effectively with the consequences of a dam failure. In 1988 the U.S. Department of the Interior [USDI98] classified downstream hazards in terms of two major potential adverse impacts on: 1) the number of human lives in jeopardy and 2) economic losses (such as properties, infrastructures, outstanding natural resources and other developments) downstream of the dam. Based on Downstream Hazard Classification Guidelines published by USDI, downstream hazards may further be classified as low for zero live loss associated with minimal economic loss; as

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significant for 1-6 lives in jeopardy associated with appreciable economic loss and as high or >6 lives in jeopardy associated with excessive economic loss. Downstream hazards can further be categorized into: 1) low danger zone, 2) high danger zone, and 3) judgement zones. The judgement zone could be determined from depth-velocity danger level relationship for 1) adults, 2) children, 3) houses and 4) passenger vehicles. For instance, a depth of flooding >1.00 m associated with flow velocity of > 3.0 m/s is considered as high danger level for adults, children, houses and passengers. Downstream risks (consequences of dam failure) have been estimated concerning property (infrastructure, buildings, agriculture) and loss of life. The most severe damage from the flood wave would be caused to buildings such as residences, industrial and business buildings, offices and stores. A dam breach flood would also affect bridges, the telecommunications network, the power grid, the water supply and sewage system, the street and road systems, the railway stations, traffic and agriculture. Depending on the severity of the dam break flood event only a certain part of the population will be confronted directly with the flood, i.e. population at risk near the flood inundation boundaries. The loss of life evaluation should be prepared with different impacts on population at risk, different dam break cases and different warning and emergency/rescue scenarios in mind (hours in a day, workday and weekend, and seasons). There are two ways to conduct an evaluation of the loss of life potential caused by a dam failure: One way is to use observations on life-loss associated with dam failures in the past and deal with the problem on statistical base. The other way is to model the expected flood event and its impact to the popu