JAKARTA, Indonesia—Torrential rains triggered flash floods on Indonesia’s main island of Java on Thursday that left at least 11 people missing, officials said. The National Disaster Mitigation Agency said rivers on the slopes of Mount Arjuno overflowed their banks and inundated five hamlets in Kota Batu, a city in East Java province. It said 15 people were swept away and four were later rescued. The agency chief, Ganip Warsito, said heavy rains are expected to continue and increase until February, partly because of a La Nina weather pattern. Rescuers are still searching for the 11 missing people, said Achmad Choirur Rochim, head of the emergency relief division at the local disaster agency. Relief efforts were hampered by power outages and blocked roads covered with thick mud and debris. Photos and videos released by the agency showed a damaged bridge, and cars and houses covered in thick mud. Authorities were still …