What is the low-cost safe solution for creating an estuary in sea?

Does anyone have a low-cost and safe solution for creating an estuary in sea for a river for storing blackish water for treatment? The treated water would be used in a Newly Developed Industrial City of an area of around 30,000 acres.

The river delta had already created an island which is blocking about 60% length of the open sea. Only 40% area is to be embanked.