Dokkum The Dokkumergrootdiep and the Dokkumerdiep was a connection for Dokkum to open sea. Dokkum lost its function as a port town in the 16th and 17th centuries as the connection silted up and was eventually closed with a dam and lock at Dokkumer Nieuwe Zijlen. (c) 10 Aug 2022 Aerophoto-Schiphol | aerial view 309309