en Water-Coastal Water-Coastal Research Paper Research Paper The concept of Geosynthetic Cellular Systems (GCS) has recently emerged as a new method in construction of breakwaters and coastal protective structures. The method potentially has significant advantages compared to conventional systems from the standpoint of constructability, cost effectiveness, and environmental considerations. This paper presents the results of physical model testing on the hydraulic responses of GCS structures under wave action. A series of model tests were carried out in a wave flume on GCS models with different shapes and soil types, subjected to various wave characteristics. Horizontal wave forces acting on the models were measured at different elevations. The maximum horizontal force in each test was calculated and compared with conventional formula of predicting wave pressure on breakwaters. The results show that Goda’s equation overestimates the hydrodynamic water pressure on these structures. This can be attributed to the influence of seeping water through the GCS models because of relative permeability of the GCS. Geosynthetic cellular system (GCS), Breakwater physical modeling, Flume, Hydrodynamic wave pressure 251 260 http://ijce.iust.ac.ir/browse.php?a_code=A-10-736-1&slc_lang=en&sid=1 H. Ghiassian h_ghiassian@iust.ac.ir 180031947532846005751 180031947532846005751 Yes IRAN UNIVERSITY OF SCIENCE AND TECHNOLOGY M. Jalili jalili@iust.ac.ir 180031947532846005752 180031947532846005752 No IRAN UNIVERSITY OF SCIENCE AND TECHNOLOGY I. Rahmani 180031947532846005753 180031947532846005753 No Transportation Research Institute M. M. Madani 180031947532846005754 180031947532846005754 No Islamic Azad University, Central Branch,