RUDRAPRAYAG, India—Rescuers in northern India worked Monday to rescue more than three dozen power plant workers trapped in a tunnel after part of a Himalayan glacier broke off and sent a wall of water and debris rushing down a mountain in a disaster that has left at least 18 people dead and 165 missing. More than 2,000 members of the military, paramilitary groups, and police have been taking part in search-and-rescue operations in the northern state of Uttarakhand after Sunday’s flood, which destroyed one dam, damaged another, and washed homes downstream. Officials said the focus was on saving 37 workers who are stuck inside a tunnel at one of the affected hydropower plants. Heavy equipment was brought in to help clear the way through a 2.5-kilometer (1.5-mile)-long tunnel and reach the workers, who have been out of contact since the flood. “The tunnel is filled with debris, which has come …