"violent riots" is correct and usable in written English.
You can use it when referring to a specific instance of riots that have become aggressive, violent, and destructive. For example: "Riots have broken out in many cities across the country, and authorities are struggling to contain the violent riots."
Julia Preston (NYT) CANADA: VIOLENT RIOTS -- The police charged 112 demonstrators with taking part in a riot after an outbreak of youth violence in Quebec.
The demonstrations in Lhasa turned from peaceful protests into violent riots as ethnic Tibetans set fire to Chinese-owned businesses.
The incident occurred during violent riots at the Australian-run regional processing and detention centre in February 2014.
Mr. Khan's agreement is likely to further ease tension in Herat, where violent riots took place two weeks ago.
Tibet has been closed to domestic and foreign tourists since March 16, two days after violent riots erupted in Lhasa.
Pakistan's Islamist parties organized street protests and threatened violent riots if Raymond Davis was not tried and hanged for his crimes.
I would like to point out that these were not peaceful protesters; armed with dynamite, rifles and Molotov cocktails, they organised violent riots.
Touré's coup was generally welcomed because of Traoré's repressive policies, which had led to popular unrest, often manifested in violent riots, in 1990 91.
The recent murder of an ethnic Russian football fan in Moscow by north Caucasians sparked violent riots by ultranationalists and skinheads near the Kremlin.
The statement called on religious mullahs to publicly denounce the movie; such condemnations in the past have led to violent riots, particularly after Friday Prayer.
The Gujjars of Rajasthan, for instance, held violent riots two years ago to protest the government's refusal to declare them as "most backward".
