Underwater explosion
Overview
 
An underwater explosion, also known as an UNDEX, is an explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

 beneath the surface of water. The type of explosion may be chemical
Explosive material
An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...

 or nuclear
Nuclear explosive
A nuclear explosive is an explosive device that derives its energy from nuclear reactions. Almost all nuclear explosive devices that have been designed and produced are nuclear weapons intended for warfare....

. They are categorized in accordance with their depth beneath the water's surface, because this has a strong influence on their effects.
The effects of an underwater explosion depend on a number of things, including distance from the explosion, the energy of the explosion, the depth of the explosion, and the depth of the water.

Underwater explosions are categorized by the depth of the explosion.
Encyclopedia
An underwater explosion, also known as an UNDEX, is an explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

 beneath the surface of water. The type of explosion may be chemical
Explosive material
An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...

 or nuclear
Nuclear explosive
A nuclear explosive is an explosive device that derives its energy from nuclear reactions. Almost all nuclear explosive devices that have been designed and produced are nuclear weapons intended for warfare....

. They are categorized in accordance with their depth beneath the water's surface, because this has a strong influence on their effects.

Effects

The effects of an underwater explosion depend on a number of things, including distance from the explosion, the energy of the explosion, the depth of the explosion, and the depth of the water.

Underwater explosions are categorized by the depth of the explosion. Shallow underwater explosions are those where a crater
Subsidence crater
A subsidence crater is a hole or depression left on the surface of an area which has had an underground explosion. Many such craters are present at the Nevada Test Site, which is no longer in use for nuclear testing....

 formed at the water's surface is large in comparison with the depth of the explosion. Deep underwater explosions are those where the crater is small in comparison with the depth of the explosion.

Shallow underwater explosion

An example of a shallow underwater explosion is the BAKER nuclear test at Bikini Atoll
Bikini Atoll
Bikini Atoll is an atoll, listed as a World Heritage Site, in the Micronesian Islands of the Pacific Ocean, part of Republic of the Marshall Islands....

 in July 1946, which was part of Operation Crossroads
Operation Crossroads
Operation Crossroads was a series of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. It was the first test of a nuclear weapon after the Trinity nuclear test in July 1945...

. A 20 kiloton warhead was detonated in a lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

 which was approximately 200 ft (61 m) deep. The first effect was illumination of the water because of the underwater fireball. A rapidly expanding gas bubble created a shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 that caused an expanding ring of apparently dark water at the surface, called the slick, followed by an expanding ring of apparently white water, called the crack. A mound of water and spray, called the spray dome, formed at the water's surface which became more columnar as it rose. When the rising gas bubble broke the surface, it created a shock wave in the air as well. Water vapor in the air condensed as a result of a Prandtl-Glauert singularity
Prandtl-Glauert singularity
The Prandtl–Glauert singularity is the prediction by the Prandtl–Glauert transformation that infinite pressure conditions would be experienced by an aircraft as it approaches the speed of sound. Because it is invalid to apply the transformation at these speeds, the predicted singularity does not...

, making a spherical cloud that marked the location of the shock wave. Water filling the cavity formed by the bubble caused a hollow column of water, called the chimney or plume, to rise 6000 ft (1,828.8 m) in the air and break through the top of the cloud. A series of surface waves
Ocean surface wave
In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the...

 moved outwards from the center. The first wave was about 94 ft (28.7 m) high at 1000 ft (304.8 m) from the center. Other waves followed, and at further distances some of these were higher than the first wave. For example, at 22000 ft (6,705.6 m) from the center, the ninth wave was the highest at 6 ft (1.8 m). Gravity caused the column to fall to the surface and caused a cloud of mist to move outwards rapidly from the base of the column, called the base surge. The ultimate size of the base surge was 3.5 mi (5.6 km) in diameter and 1800 ft (548.6 m) high. The base surge rose from the surface and merged with other products of the explosion, to form clouds which produced moderate to heavy rainfall for nearly one hour.

Deep underwater explosion

An example of a deep underwater explosion is the WAHOO test, which was carried out in 1958 as part of Operation Hardtack I
Operation Hardtack I
Operation Hardtack I was a series of 35 nuclear tests conducted by the United States in 1958 in the Pacific Ocean.Operation Newsreel was a series of three high-altitude nuclear tests conducted as part of Hardtack I. The individual tests in the series were Orange‎, Teak‎ and Yucca‎.-Test Blasts:-...

. The nuclear device was detonated at a depth of 500 ft (152.4 m) in deep water. There was little evidence of a fireball. The spray dome rose to a height of 900 ft (274.3 m). Gas from the bubble broke through the spray dome to form jets which shot out in all directions and reached heights of up to 1700 ft (518.2 m). The base surge at its maximum size was 2.5 mi (4 km) in diameter and 1000 ft (304.8 m) high.

The heights of surface waves generated by deep underwater explosions are greater because more energy is delivered to the water. During the Cold War, underwater explosions were thought to operate under the same principles as tsunamis, potentially increasing dramatically in height as they move over shallow water, and flooding the land beyond the shoreline. However, later research and analysis suggested that water waves generated by explosions were different from those generated by tsunamis and landslides. Méhauté et. al. conclude in their 1996 overview Water Waves Generated by Underwater Explosion that the surface waves from even a very large offshore undersea explosion would expend most of its energy on the continental shelf, resulting in coastal flooding no worse than that from a bad storm.

If a deep underwater explosion occurs at a sufficient depth, the rising gas bubble can over expand because the gas pressure falls below the pressure of the surrounding water. This causes the bubble to collapse, which causes a second shock wave and bubble expansion. This may be repeated, though there are unlikely to be more than three expansions. An example is the WIGWAM
Operation Wigwam
Operation Wigwam involved a single test of the Mark 90 Betty nuclear bomb. It was conducted between Operation Teapot and Operation Redwing on May 14, 1955, about 500 miles southwest of San Diego, California. 6,800 personnel aboard 30 ships were involved in Wigwam...

 test, which was carried out in 1955. The nuclear device was detonated at a depth of 2000 ft (609.6 m).

Other effects

The detonation of an explosive charge underwater results in an initial high-velocity shockwave through the water, in movement or displacement of the water itself and in the formation of a high-pressure bubble of high-temperature gas. This bubble expands rapidly until it either vents to the surface or until its internal pressure is exceeded by that of the water surrounding it. (The volumetric expansion of the bubble also leads to a drop in internal temperature in accordance with Charles’ Law.) At this point, as noted above, the overexpanded bubble collapses into itself, leading again to a rise in bubble pressure and internal temperature until such time as the bubble pressure exceeds water pressure. The bubble again expands, although to a rather smaller size. A second shockwave is produced by this expansion, although it will be less intense and of rather greater duration than the first. With each cycle, the bubble moves upwards until it eventually vents or dissipates into a mass of smaller bubbles. The number of cycles, while generally low, is difficult to predict; they and the overall effects, depend on explosion depth (and thus water pressure), the size and nature of the explosive charge and the presence, composition and distance of reflecting surfaces such as the seabed, surface, thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...

s, etc. This phenomenon has been extensively used in antiship warhead
Anti-ship missile
Anti-ship missiles are guided missiles that are designed for use against ships and large boats. Most anti-ship missiles are of the sea-skimming type, many use a combination of inertial guidance and radar homing...

design since an underwater explosion (particularly one underneath a hull) can produce greater damage than an above-surface one of the same explosive size. Initial damage to a target will be caused by the first shockwave; this damage will be amplified by the subsequent physical movement of water and by the repeated secondary shockwaves or bubble pulse. Additionally, charge detonation away from the target can result in damage over a larger hull area.
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