The Role of Breach Mechanics in Dam Failures

Breach Mechanics in Overtopped Earthen Embankments Explained

When thinking of dams, what comes to mind? A dam is a large structure holding a considerable amount of water to the point of forming a lake-like body. Hydropower generating plants may also come to mind as man has used these water systems in the past 2 centuries to generate electricity. But how many times do you think of the possible dam failures? It’s probably happened in your country in the past; can you remember?

Let’s see how the breach mechanics works and what’s its role in the dam failures. The release of impounded water can be catastrophic to a large number of people. Just to demonstrate this: when the Banqiao Reservoir Dam in China failed back in 1975, over 171,000 people lost their lives. From this single event, it is estimated that an average of 11 million people ended up losing their homes.

This is just one case but these dam failures have happened all over the world. As we continue, we will take a tour down history lane as we see how the ancient world dealt with this. We’ll also discuss some of the causes.

Can’t They Be 100% Sure When Constructing Dams?

Like with any other masterpiece, the architects desire 100% efficiency as engineers try to bring this into perfection. Nonetheless, there are some inevitable factors that even the most profound engineer cannot tackle or accurately predict. This is especially so in the last century which has experienced the largest number of dam breaches in history.

Initially, they desired to achieve a project that would outlive hundreds of years. However, recent weather changes and adverse climatic patterns have resulted in the global warming crisis. These changes have not only brought about the dangers of flooding but have also caused overtopping and piping dam failures.

This has led the mechanical engineers in the past 50 years to develop models of dam failures due to earthen embankments and piping. Moreover, these techniques have also shown – to a near approximation – the consequential outflow that would occur during breach hydrographs.

The best way to illustrate these models is through a time-to-peak and a peak flow. They fail to give a proper outline of the time history together with the outflow hydrograph that is needed for flood routing. These predictions are made through;

  • Parametric models
  • Empirical relationships
  • Physically-based numerical models
  • Dimensionless models.

The physical-based math models have become more popular due to the combination of analytical and numerical solutions. That notwithstanding, the physical process which governs embankment breaching has to be characterized as Priori to develop such methods. Once these models have produced an estimate, flood routing algorithms are used to determine the extent of flooding downstream. They will then determine the extent of flooding that would be likely to occur in densely populated regions.

The Essence of Prediction and Estimation

The most definitive aspect of such predictive estimates is the development of inundation maps. These are maps that describe the extent of flooding that would occur from a hypothetical dam failure and its critical appurtenant structure. They are used to;

  • Enhance proper preparedness under such circumstances.
  • Enable flood risk analysis which is important for mitigation and planning.
  • Develop a timely response through quick collaborations with local communities.
  • Assess the extent of probable damage and put recovery measures.
  • Identify wastelands and hazardous spill cleanup as part of Environmental and Ecological Assessments.

Modern-day computing has gone a long way in the development of such maps. It has been possible to develop them even in areas of complex topography with urban centers and valleys in a matter of minutes. These simulations are obtained from supercomputers with preinstalled multiprocessing systems that can handle massive flood routing algorithms and prevent dam failures. This has gone a long way to eradicate the former systems of developing inundation maps with multiple flooding scenarios.

These maps become viable during such occurrences of dam failures. They are more effective if they can provide the residents enough time to escape the coming disaster. In as much as it may be possible to assess the downstream outflow, it is even more crucial to predict the hydrograph right from the flood source.

Factors Governing Breach Mechanics

  1. Soil erosion
  2. Sudden collapse mechanism
  3. Hydrodynamics
  4. Reservoir routing
  5. Geotechnical processes.

When all these factors are put together, the environmental geologist and engineer have to scratch their heads harder. At present, it is not computationally feasible or mathematically possible to incorporate all these factors in a physically-based numerical model. They only consider the most dominant factors and assume the rest or simplify them. This has resulted in substantial uncertainties in the predicting process of these hydrographs.

Possible Causes of Dam Failures and Breach Embankment

  1. Changes in water levels may result in geological instability. This may occur during filling or as a result of inadequate surveys.
  2. The spillway design error.
  3. Extreme inflow into the dams.
  4. Computer or human errors during the design process.
  5. Earthquakes.
  6. Piping or internal erosion which is more prevalent in earthen dams.
  7. Poor maintenance of the general dam and outlet pipes.
  8. Sub-standardized construction materials and cheap techniques during installation.
  9. Reduction of spillway flow when the dam crest height goes down.
  10. Deliberate breaching which led the Geneva Convention to initiate the 1977 Protocol I amendment. This barred such attacks from occurring again if such dangerous water forces would lead to a massive civilian loss.

History of Dam Failures in Ancient Civilization

You may think this menace of dam failures started back in the 20th–Century when man entered the age of rediscovering the peak of civilization. This is not the case, nonetheless. Dams and other hydraulic engineering systems in the ancient world were used to serve the water supply problem as well as enable agriculture to continue. It led to the development of many historic and prehistoric civilizations in Asia, Africa, and Europe.

Some of these dams include the 5m-high masonry gravity and earthen dam located in the Black Desert (presently Jordan) which served the Jews from the 4th-Century BC. It was meant to retain water from a stream runoff hence assist in cultivating land downstream.

The Dam of the Pagans (Sadd el Kafara) was a rock-fill dam 14m-high that had a gravel/silty-sand core outstretched with a limestone slope. It is assumed to be the world’s oldest dam and is found in Memphis – Egypt, specifically at the Wadi el Garawi (2650 BC).

Even with all their magnificence, they could not endure the fury of floods. All the Bronze Age dams are believed to have been overtopped after the great floods which swept the entire earth.

The majority of ancient dams have their origins in Africa. In Egypt, canals were dug to form a succession of basins. These protected the Egyptians from high flooding as the waters of the Nile were then directed towards the Birket Qarun, presently called Lake Moeris. These waters were high enough and if the need arose, it could still be redirected back into the Nile.

An earth dam was constructed in 2300 BC which focused on diverting the floodwaters into the depression. A second dam would then rechanneled the waters back into the Nile. This was made possible through a succession of annual breaches before and after the floods. Once breached, they would immediately begin reconstruction for the following year.

With time, the water levels dropped and this system was abated. It was during the Graeco-Roman era that a dike was constructed leading to the depression as a land reclamation project. More arable land was created which fostered further cultivation.

Irrigation Systems in the ancient Mā’rib of Southern Arabia began around 4000BC. The Great Mā’rib Dam was built by the Sabaeans who greatly relied on flood irrigation. This was overtopped in the 6th-Century AD by floods and met its demise in the 7th-Century AD through Sayl al Arim translated as ‘barrier flood’.

The oldest dam in Anatolia, present-day Turkey was the Karakuyu dam which existed during the Hittite period (2000-700 BC). Seepage is attributed to its failure on the embankment’s bottom outlet.

There is still the remarkable Marduk Dam near Samaria on the Tigris River. It had extraordinary longevity from around 2000BC-1256AD when it was finally breached and left to utter ruin. It survived the different empires that had arose during this long period including the Sassanid Empire, Romans, Greeks, Persians, Chaldeans, and even the mighty Assyrians.

The East was not left behind. At the cradle of Chinese civilization were two primary rivers – the Huang He (Yellow River) and the Chang Jiang (Yangtze River). These have been a cause of both happiness and sorrow to these early inhabitants. It was during the rule of Emperor Yau that dikes and dams were built in the river’s lower reaches to redirect the water. That notwithstanding, the Yellow River is said to have shifted its course about 26 times. This led to the overtopping of these dikes and millions of lives lost during the years.

This is just a small account of dams in the ancient world. There are more stories to be told and more failures that have happened over the years that can assist us in making more sound decisions. By now you should have a clear understanding of the problem at hand and a grand view of how far-reaching its effects can be to an economy and a people.

Lindsey Angus Hughson