Gravel is a material with multiple purposes in construction and infrastructure. Its usage ranges from construc-tion of roads, building navigational channels to land reclaimation. Gravel can be obtained through multiple methods, such as dredging with ships. One of such dredging ship is the trailing suction hopper dredger (TSHD). However, in the seabed the gravel is thoroughly mixed with sand, thus during the collection process the unwanted sand is pumped up with the gravel. It is not uncommon to receive up to 80% sand and only 20% gravel. In order to use the storage of the ship more efficiently, the gravel is separated from the sand through an on board sieve installation. The sand is released back into the water and the gravel is collected in the hopper. This has several disadvantages, such as wasting energy by pumping unwanted sand which is returned back to the seabed. The turbidity in the seabed that occurs by releasing the fine sand which settles slowly to the seabed. And the requirement of a large sieve installation to separate the gravel from the sand, which reduces the already limited space of a ship. In order to limit the mentioned disadvantages, it would be favourable to bring the separation process at or near the seafloor. Through a literature study the best underwater separa-tor has been selected. From this it is concluded that the usage of a jet that pushes fine sediment out is the best for underwater separation. The fine sediment is pushed to the upper part of the pipe, whilst the coarse particles resist the jet and remain at the lower part of the pipe. The fine particles at the top are sucked in by a retour pipe, which guides them down to the seabed where it is released with a lowered environmental impact. Further research on this concept is done through the use of computer simulations which models the conditions using a drift-flux solver. The concept has been tested with different configurations to determine the influence of several input variables. The chosen variables are the inlet velocity, jet velocity, sediment size and sediment concentration. For these configurations the separation efficiency of the system is determined by analysing the retained gravel and the filtered sand. The containment of gravel is generally over 90% and highly satisfactory. The maximum separation of sand is 50%. These results indicate the viability of the system and thus research on improvements are recommended to further increase the systems effectiveness and efficiency.