A group of chavs. (otherwise known as pikeys, townies, scums of the earth etc.) Often a group that is following a regular person, nerd, emo or even lesser chav in order to inflict physical pain upon that person.
Can also be used when walking towards a group of chavs.
Ben: Uh-oh! Chavalanche!
Peri: We were totally bunned by a chavalanche last week.