Geotextile mattress with sloping curtain (GMSC), as a flexible structure filled with dredged sediment, may be an advisable alternative countermeasure against erosion since it can make deposited dunes form naturally at upstream and downstream sides of GMSC when installed in erosive flow. GMSC owns environmental friendly benefits, and have demonstrably lower construction and lifetime costs than those of hard structures. The effectiveness of GMSC has been confirmed by intuitionistic scouring and depositing tests on erodible beds in the previous studies. Based on the simulations with software FLOW-3D, a 3D numerical model was built for the flow with GMSC installed on unmovable beds, and the hydrodynamic changes around GMSC was investigated. The results show that GMSC can effectively improve the structure of flow near bed surface. The sand-pass opening, through which the bottom current could pass with high density of sediment, is reasonable for the vertical distribution of suspended sediment in river flow, and makes it possible that the sediments in the bottom flow may be trapped and the bed terrain could be steady in a long term, even alluvium formed locally. The simulation results can visually represent the 3D hydrodynamic characteristics of flow around GMSC, which may be effective to improve GMSC techniques in the potential applications in engineering. With respect to the sediment trapping mechanism around the GMSC structure, GMSC can be used in more complex hydrodynamic conditions such as reciprocating flow. If the main current direction of most threating flow in the marine environment is not changing obviously in a year, GMSC can be used to protect many kinds of offshore structures such as pipeline, platforms, submarine cables, etc.

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