The results of numerical analysis indicated that the velocity inside mat decreases when the velocity at the upper boundary becomes large. A small amount of circulating flow takes place with the occurrence of vertical flow, which seems to relate to the diffusion of suspended sediment. Sediment concentration near the bed, averaged over a wave period, is about 1-40% of that without mat. The maximum Shields parameters are about 0.16-0.65 and 0.04-0.13 with and without the mats, respectively. The maximum Shields parameter without mats is nearly equal to the critical condition of eelgrass growth obtained by Nakase et al. Meanwhile, with mats it is less than the critical value obtained by Nakase et al.The sand-control mat presented in this study is demonstrated to be one effective means for the development of eelgrass beds.