Mangrove forests play a significant role in stabilizing coastal regions, preventing flooding, and providing great potential for sedimentation. Our recent field trip revealed that the mangrove forest was being destroyed at an unprecedented alarm rate and that portions of Hai Phong Beach were eroding quickly. In locations at Do Son Beach, mangroves have even disappeared. It appears that the vegetative belt is insufficient to “trap” sediment on the shore. The abundant healthy vegetation that protects the shore from erosion should, in theory, be residence to a diverse range of living plants and animals. The wider the mangrove forest can be, the stronger the ability to accumulate shore. However, a minimum mangrove belt width is necessarily investigated for flood protection and coastal accretion. Rehabilitating the vegetation in front of the sea dike and identifying the minimum mangrove width by modelling simulation are the main objectives to give Hai Phong Province sufficient resilience against flooding. The research carried out a couple of models SWAN-SWASH, taking SS- and IG-waves into account to simulate wave attenuation by vegetation for three different coastline regions. The critical range of the mangrove width is found from 400 to 700 m.