Dash 8 Series Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Flight Crew Requested ATC Assistance / Clarification; General Physical Injury / Incapacitation
Weather
Flight was smooth during the cruise portion of the flight.  As we started our descent from 17000 feet it was also smooth and no turbulence was noticed. After we finished the before landing checklist and level at  4000 ft  approximately 50 miles south of ZZZ we encountered a pocket of heavy turbulent air and the aircraft suddenly dropped 800 ft per minute and lost about 400-500 ft in altitude. Concerned about the passengers and FA; I tried calling the FA in the cabin but she did not answer.  So I called again and she answered the phone and right away I knew she was in pain due to her voice. She told me she hurt her leg really bad and she couldn't walk. I [advised ATC] and asked for medical personnel to meet the plane at the gate.
Upon encountering the turbulence; I; the pilot flying at the time; disconnected the autopilot and switched to hand flying as a result of the turbulence. The captain; the pilot monitoring at the time; called the Flight Attendant to make sure everything in the cabin was ok; and received no answer. He called a second time and the Flight Attendant informed him that she had badly injured her leg while returning to her jumpseat; and was seated in an empty row of passenger seats near the front of the cabin. The captain decided to declare an emergency and request medical personnel meet the plane. We switched roles; I became pilot monitoring and he became pilot flying; and we continued the flight the remainder of the way to ZZZ without incident.
DHC-8 flight crew reported turbulence during descent that injured the Flight Attendant.