Dams are considered critical infrastructure under International Humanitarian Law because of the massive effect a breach or failure could have on the population and environment. For example, the failure of South Fork Dam in 1889 caused more than 2,200 deaths in Johnstown, Pa., USA. A similar failure could cause greater casualties and economic losses because of the many cities in the flood plains of dams. Were Aswan Dam in Egypt to fail, it would cause fatality to a large percentage of Egypt's population while sweeping many structures into the Mediterranean Sea.

The most common causes of dam failure are design errors, geological instability, extreme inflow, sub-standard materials/techniques and poor maintenance. However, an increase in global terrorism has imposed another potential cause of failure. The destruction of dams has the potential to cause catastrophic floods and put strain on electrical power supplies, which in turn could have adverse effects on many national critical infrastructure sectors. Thus, terrorists and other groups seeking to cause mass casualties or economic disruption or attract media attention could target dams.

The effects of explosives on dams are not well-understood, and most dam owners/engineers lack the expertise to estimate the vulnerability of dam infrastructure to such attacks. Knowledge of these effects will undoubtedly lead to implementation of mitigation strategies aimed at limiting damage to dam infrastructure and its consequential effects on downstream communities and the electrical power system. Although little test data is available to aid the dam owner/designer in identifying vulnerabilities and establishing the amount of explosives that can compromise the many critical dam infrastructure systems, data from attacks on dams during conflicts or wars are available. Investigations into these attacks provide insight into the robustness of dams to attacks with various amounts of explosives at a number of locations on the dam structure.

Attacks on dams

A literature review reveals only a few attempts by terrorists or violent protestors to attack dam infrastructure. One dam targeted by terrorists is Chingaza in Colombia, which supplies water to the city of Bogota. The dam is gravel fill with a concrete face and impounds a reservoir with a capacity of 223 million cubic meters. The Revolutionary Armed Forces of Colombia (FARC) have attacked several dams and aqueducts. In January 2002, FARC detonated an explosive device in a gate valve inside a tunnel in Chingaza Dam in an attempt to breach the structure and disrupt water supply to Bogota and flood the city of Villavicencio downstream of the dam. However, the attack was not successful and the dam was not breached.

In July 2011, the Indian Army intercepted a terrorist threat to Bhakra Nangal Dam in Himachal Pradesh, India, a concrete gravity structure that holds the distinction of being the world's highest straight gravity dam. The dam, on the Satluj River, impounds water for a 1,325 MW hydroelectric project. Reports indicated two terrorist groups were planning strikes during the monsoon, when the water level behind the dam is at its highest, to cause maximum damage to downstream structures and people. Luckily, no such attack was carried out.

With regard to embankment dams, the below case studies present information on two attacks and the accompanying damage.

Sorpe Dam

Sorpe Dam in North Rhine-Westphalia, Germany, was constructed from 1922 to 1935. It is an earthfill embankment with a watertight concrete core wall. The dam crest is 700 m long, and the height above the valley floor is 60 m, with a maximum water depth of 57 m. The dam is 10 m wide at the crest and 307 m wide at the base, with 1:2.25 and 1:2.50 slopes of the upstream and downstream faces, respectively.

 
This dry model embankment dam was built to help further understanding of the effects of explosives on embankment dams.

Sorpe Dam was attacked during Operation Chastise (Dambusters Raid) of WWII. The air attack was carried out on May 17, 1943, by a squadron of the Royal Air Force using the Upkeep (bouncing) Bomb developed for that purpose. This bomb was 1.5 m long and 1.3 m in diameter and was filled with 3,600 kg of RDX explosive. Sorpe Dam suffered two hits on its crest that resulted in craters about 12 m deep. The dam structure, however, did not fail. Shortly after the Dambusters Raid, the water level in Sorpe Reservoir was lowered as a precautionary measure.

The attacks on Sorpe Dam were repeated several times in 1944 by the Allied Forces. On October 16, 1944, an attack on Sorpe Dam resulted in 11 direct hits. Although 12 m-deep and 25 to 30 m-diameter craters were formed, the embankment dam was not breached. The second wave of attacks used 5,500 kg "Tallboy" bombs, which also did not breach the dam. After the war, the dam was repaired and the craters filled. However, in the 1950s serious leakage problems were discovered and more remedial work was authorized. In 1958, when the dam was partially drained for repairs, an unexploded Tallboy was found buried in the embankment dam and removed. Sorpe Dam was repaired and put back into service and remains in operation today.

Peruca Dam

Peruca Dam is on the Cetina River in the Republic of Croatia. It is a 425 m-long and 60 m-high rockfill dam with a convex upstream axis. Peruca Dam was constructed between 1955 and 1960.

During the Balkan wars, the area around the dam was occupied by the Yugoslav Army and later by Serbian Forces, who planted 20 to 30 tons of TNT in five locations in the walls of the spillway structure and inspection gallery. On several occasions, Serbian Forces threatened to destroy the dam using these explosives and closed the spillway gate, power tunnels and outlets and maintained the water level as high as possible so that maximum damage would result.

In 1992, a United Nations Protection Force took control of the dam and lowered the water level to the design elevation. On January 28, 1993, Serbian Forces retook the dam and detonated the explosives in an attempt to destroy Peruca Dam and flood the villages and hydroelectric power station downstream.

But the effects of the detonations were not sufficient to breach the dam or lead to overtopping and subsequent erosion and failure of the structure. The explosions did leave two large craters at the abutments. The elevation of the rim of the crater was only 300 mm from the water level in the reservoir. The inspection gallery was also heavily damaged at the abutments and completely destroyed at the deepest section in the middle of the dam. A total settlement of about 1.55 m of the crest was recorded about ten months after the explosion.