Dykes linked to the Han River estuary in the central Vietnamese city of Da Nang are being threatened by a rash of illegal dredging.
The crews of these boats are carrying the stolen sand to an unidentified construction site.
The problem has continued for months and, so far, the authorities have done nothing to stop the practice.
Every monrning, around ten boats anchor outside the Tien Sa Port. Each boat is capable of carrying around 120 cubic meters of sand. After loading up, the ships all head to a nearby construction site.
The primary investor in the project has yet to be identified.
Nguyen Tinh, a local fisherman, said he sees four to six boats digging sand in the area, every day.
"They've been dredging more and more over the past two months. Some days, I see 12 boats," Tinh said. "People here have been really worried as the activity may create underwater holes in the surrounding dykes, resulting in massive erosion."
The dykes were built during the French occupation and were recently repaired with roughly VND1.1 trillion (US$53.7 million).
Huynh Van Thang, deputy director of the Da Nang Department of Agriculture and Rural Development, said the boats have violated the dykes' protection zone.
In the long-term, these encroachments could divert the flow of water, putting the dykes in danger and adversely affect boat traffic in the area, he added.