The answer to the question of "What are containment bunds?" would simply be that they are a retaining wall that is constructed to prevent the passage or flow of any type of material due to a breach or failure of a prohibitive barrier.
For example, in a situation where caustic chemicals would be stored, a containment bund would be a secondary system of containment that would protect an environment should a breach occur in the primary containment system. Bunding, or the construction of a bund wall, is a process that is expressly designed to prevent such spills, which can and do occur.
This term can also be applied to dikes, but it is most often used to refer to liquid containment where spillage and leaks from tanks, pipes, or any primary barrier could cause significant damage.
Most often, the liquid that is contained in these types of tanks are very caustic and dangerous if they were to leak or spill out of their primary container. The liquid could be dangerous either by chemistry, or by sheer force, such as by water alone.
Typically, a bund will be large enough and will have enough strength to contain the contents of an entire storage tank, although some regulations require bunds to be able to contain a volume of up to a third more content than the tank is capable of storing. If there are more than one tank sharing a bund, then the requirements are calculated based on the storage contents of the largest tank that is involved.
A common form of construction for a a containment bund is to build a wall of concrete around the tank, with the area having a concrete floor. Then on the outside of the wall, an earthen berm is put into place.
While there are many liquids that can be contained by concrete, it does not work well at all with other liquids. For example concrete would not be a good choice for many acids. In many situations, using earth for a berm is not recommended either, as it can be compromised by many things, including the weather. Sometimes liners can be used to lessen the possibility of permeability, but even that can break down over time.
In smaller tanks, you will find containers that are constructed of plastic or steel. In the final analysis, the type of container will be based upon the chemical makeup of the liquid, the cost of the material, and the density of the liquid.
As an example, plastic containers will not be able to hold liquids of higher densities at higher wall levels.
If the bunding area is large, and openly exposed, there will probably need to be a sump pump type of arrangement or similar systems so as to be able to remove precipitation, as well as being able to transfer the liquid to another container. In the case of rainwater, it must be assumed that the rainwater is toxic due to its contact with the toxic liquid itself.
If there is a roof, or covering over the container, proper ventilation must be provided, particular if the substance being stored is flammable, and there must be ladders or steps provided for people to quickly escape, if need be.
If the liquid itself is not caustic, or it is not likely that failure will occur, the bund can simply be designed to contain small leaks from valves and hoses, for example. The bund will not be designed to contain the entire volume of stored material, and the bund itself could simply be a small lip, or hump around the concrete floor. In defining the question as to "what are containment bunds," it could be said that they contain materials that breach or leak inappropriately, and are constructed with different materials, based upon the substances being stored.
Some bunds are temporary, and can be for use with short-term storage. Also if it is necessary for vehicle access to the inside of a bund, a lip or hump is ideal. Some materials are constructed so the edge of the bund contracts when a vehicle drives over it and the pops back to its original shape again when the vehicle passes.
Most countries have regulations concerning bunding according to the types of materials that are being stored and their toxicity. So to answer the question of "what are containment bunds?" the answer would be similar in most countries around the world.