Saemangeum Seawall

The Saemangeum Seawall, on the south-west coast of the Korean peninsula, is the world's longest man-made dyke, measuring 33 kilometres (21 mi). It runs between two headlands, and separates the Yellow Sea and the former Saemangeum estuary. It was built to reclaim land for both agriculture and urban uses, including industrial uses, and 401 km2 of the estuary was planned to be developed into an artificial lake (118 km2) and reclaimed land (283 km2) that would provide nearly 10% of South Korea's total rice production. It is said to be the world's largest wetland reclamation, and the various controversies, court cases and commissions led in part to the Wetland Conservation Act of 1999 (amended 2014), which should both prevent the future loss of wetlands and help with wetland restoration.

Dam data from Wikidata (CC0) plus curated additions. Summary from Wikipedia (CC BY-SA).