So my main question is, how do we retain the water that flows into this depression so that it will only be released at a controlled rate after the storm so that the downstream area is not flooded? What structures need to be built? Should an enclosure parallel to the river be built, which shall be closed so that the water does not come back to the river...etc. This is where I need help in conceptualization because I am not an engineer and don't have an idea about it.