Even though it looks like they are building a wall around Barra, this is just sand dredged out of the lagoon behind and eventually will be spread down the beach where needed. Probably to be washed away next summer.
Barra de Navidad is built on a sandbar between Bahia de Navidad and the Laguna de Navidad in an area of Mexico's west coast known as the Costalegre (Costa Alegre) - the Happy Coast