In this paper, a few problems and points of sediment erosion control works with groundsels are discussed by means of numerical simulation of a long-term bed variation and a short-term bed variation under no sediment supply. On a long-term basis, groundsels have to reduce remarkably erosion depth as well as erosion volume at an equilibrium bed condition. So, the effect of groundsels on erosion depth was investigated with the simulation of the bed variation of a river for a time-series of water discharge with a duration of 100 years. The result has pointed out that a series of groundsels can reduce the erosion volume, but not the erosion depth because the river bed with non uniform materials is not much eroded even without the groundsels due to the development of armor coat. An increase in groundsels can not always, therefore, improve efficiency on reducing erosion depth. On a short-term basis, a role of groundsels is to protect river beds from too much erosion by a heavy flood. The simulation of bed variation for a hydrograph with a design discharge has shown that the erosion depth could be thoroughly reduced if a series of groundsels is placed only in the most upstream section with three times length of the erosion region predicted in the case of no groundsel.