Dams are the primary structures used to store water on the planet. However, the failure of embankment dams can cause catastrophic disasters.
It is a known fact that overtopping is one of the leading causes of the failure of embankment dams.
Therefore different protection systems and overflow strategies have been put forward for embankment dams to safeguard them against overtopping. This paper will look at the overtopping failure mechanism and focus on the different overtopping protection systems.
Failure Mechanism of Embankments Due To Overtopping
At least two types of failure can destroy the earth dam during overtopping and include mass slip and surface erosion. Surface erosion of the downstream slope causes the breach of the body of the dam. The erosion starts when the tractive shear stress becomes more than the resistance that keeps the materials in place.
In the case of cohesive soils, the breach happens by head cutting. The head cut starts typically from the downstream toe and develops upstream until the dam’s crest is breached.
However, mass slip is a particular process. Mass slip results from an immense pressure field being created in the downstream slope overflow due to filter materials and proper drainage not being embedded in the dam.
The Methods of Embankment Overtopping Protection
There are at least four different ways of protecting against overflow and overtopping. They include increasing the dam’s reservoir capacity, heightening the dam, using the coverage systems, and using the fuse plugs. Some of the approaches are ways used to prevent overtopping.
You can select one of these approaches in a project depending on various factors such as geology, economic issues, topography, and level of risk downstream. One major disadvantage of preventive ways is that they are usually uneconomical, and their coverage isn’t only cost-effective but can also be used as spillways. This enhances the capacity of existing spillways.
Coverage Overtopping Protection Systems
The coverage overtopping protection systems like filters have been used extensively in recent years for embankment management in Australia and the United States. These systems include riprap, gabion, reinforced earth, pre-cast concrete blocks, vegetation & timber cribs, and RCC (Roller Compacted Concrete).
These coverage systems also have several advantages: energy dissipation, delaying dam destruction, using diversion dams, and utilizing as spillways. Let’s look at coverage systems below.
Vegetation and Timber Cribs
Vegetation is among the best methods of overtopping protection. It can stand against overflows that have a maximum velocity of 1.5 m/s.
Timber cribs have also been in widespread use, especially in Russia, since the 18th century. However, timber cribs don’t have enough strength against heavy flows. You’ll get them being used in small levees.
Reinforced Earth
The first reinforced earth stepped overflow was used in the Prins dam in South Africa in 1920. Since then, the system has been used in over 50 projects, most of which are small levees and dams.
For instance, in the Toonumbar dam, the slopping bars are applied to prevent the destruction of upper rocks. There was also a surface mesh used to prevent the movement of fine materials.
Riprap
Riprap is made up of rocks and rubble stones that have proper granulometry and size. Even though riprap is one of the most cost-effective overflows, it isn’t effectively safe against overtopping.
Some experimental tests were done by the CSU (Colorado State University) and the USBR (United States Bureau of Reclamation) on ripraps, which observed some instabilities during overtopping. The resistance of riprap to overflow depends on the hydraulic gradient, material properties, and the rate of overflow discharge.
The design of riprap depends on the mass median diameter (d50) of rocks. The thickness of riprap is considered to be 1.5 to 3 times the d50. The rounded rocks are the most effective to use in ripraps.
You should understand that the stability factor of riprap has an inverse relationship to the coefficient of uniformity (Cu). Poorly graded materials are more resistant to overflow, although they get destroyed suddenly.
You can use the rocks with either steel or concrete. The rocks are deployed on the concrete slabs and anchored to the body of the dam by pedestals. However, this method is only effective in small dams.
Gabions
Gabions are also among the most effective approaches that you can use to protect against overtopping. They are mainly used in the downstream slope.
Pre-Cast Concrete Blocks
Reinforced concrete blocks were initially applied for overtopping protection. However, investigations revealed fluctuations in the pressure of the turbulent flow leading to instability of the blocks. It was therefore recommended to either overlap the elements or apply wedge-shaped blocks.
The idea of employing wedge-shaped blocks as crest spillways for embankment dams was first presented in the early 1970s by Professor Gordienko. The systems allowed an overflow discharge of 60 sq. m/sec and a velocity of 23 m/s. There are further studies by Pravdivets to diminish the shortages of Gordienko’s design.
Several overflow embankments were built under wedge-shaped blocks with an overflow discharge of below 5 sq. m/sec.
For instance, the stepped spillway on the Brushes Clough dam in England was developed according to the guidelines of wedge-shaped blocks. There was a spillway build on the downstream slope to increase the capacity of the existing spillway. Its maximum capacity stands at 3.66 sq. m/sec.
The most recent studies concerning the hydraulics of flow over the wedge-shaped concrete block spillways have been performed by Relvas and Pinheiro. They include hydrodynamic pressure on the blocks, inception point and air concentration of the flow, and velocity distribution and the rate of energy dissipation and the steps.
Another study by Sabbagh and Misaghian investigated the influence of the flow infiltration through the wedge-shaped concrete block spillways on the downstream slope stability of the earth overflow dams using finite element modeling. Their findings showed that if materials with appropriate permeability coefficients are not chosen, it will reduce the stability of the downstream slope. This means that the wedge-shaped concrete block spillways will not save the dam for all the overtopping duration.
Roller Compacted Concrete (RCC)
Roller compacted concrete has been in use since the late 1970s, especially in the United States. The method has been employed to protect against overstepping / spillway in at least 100 dams.
The main difference between the RCC and wedge-shaped concrete block is displayed in its low permeability. This makes RCC systems pass overflow for a long time without considerable water infiltration to the downstream slope.
However, RCC protection isn’t based on any analytical or mathematical relationships but rather on construction conditions. It must have proper weight, continuity, and durability for resisting erosion, weathering, and movements.
RCC is placed on the downstream slope from toe to crest in layers with a thickness of between 0.3 to .6 m and a minimum width of 2.5 m.
If you wish to protect RCC, it would be best to place a layer of ordinary concrete on its surface. It’s also essential to use a suitable drainage system under the layers. This releases pore pressure, thus restricting the separation of the RCC layer.
It is also essential to completely seal the dam’s crest, considering that these issues will prevent the erosion of materials underneath. For instance, there was an erosion of the underneath material of the spillway on the left abutment due to improper sealing.
Any safe design of an RCC overtopping protection system should involve an approach apron resistant to erosion, a proper filter/drainage system, and an upstream cut-off wall.
Conclusion
In recent years, there has been a reexamination of several dams’ design flood, and the calculation results for most of them were more than the initial values used to design the dams. In most cases, the occurrence of the revised design floods could potentially lead to overtopping.
This has led to the development of several overtopping protection systems. Selecting the perfect system will depend on several factors, including the geometry of the dam’s location, the revised design flood, economic issues, and the capacity of the spillway.
In most cases, considering the economic and safety aspects, concrete stepped spillways would be the best option above other systems. However, as per past experiences, proper flow conditions, regular maintenance, good construction quality, and a good design are necessary for a safe operation.