Coastal erosion in Thailand is severe due to the deforestation of mangrove forests, which naturally dissipate waves, making it necessary to establish how to reproduce mangrove forests of a suitable size. First, a survey of mangrove forests was performed in 2012–2013 along the coast of the Gulf of Thailand, where mangrove vegetation has been affected by human activities. The survey showed that tentacle-root mangroves (Rhizophora mucronata) accounted for the largest proportion of the mangrove trees in the area. The distribution characteristics of the root number per mangrove tree and the diameters of root systems and main trunks were also clarified. The parametric results were used to make replicas of mangrove vegetation. Hydraulic experiments were carried out in a wave tank in 2014, and a calculation diagram for obtaining the wave transmission ratio was constructed, which was improved over 2015–2018. Moreover, experimental data of eight existing papers were used to increase the value and reliability of this diagram. As a result, a wave transmission ratio can be obtained as a function of the density of mangrove vegetation and the ratio of the incident wavelength to the offing direction width of a mangrove forest. Next, the wave transmission ratio of existing detached breakwaters made of geotextile sandbags was evaluated by simulation using the numerical model [V. T. Ca: VNU J. Sci., Earth Sci. 23 (2007) 160], then the required width of a mangrove forest for it to have the same effect as an existing detached breakwater was estimated. The wave transmission ratio of this breakwater was 0.33, and the offing direction width of the tentacle-root mangrove forest was 250 m with a tree density of 0.8 (the number of trees/m2). By performing verification simulations using experimental data, the high reproduction accuracy of the topographical changes obtained by the numerical model of Ca et al. was confirmed.