A flow control system includes a movable riser with multiple flow rate restrictors within a stationary riser that is interfaced to a drainage system. The movable riser is buoyed by float(s) attached to the movable riser. As the fluid level around the flow control system changes, the movable riser tracks the changes, thereby raising and lowering the flow rate restrictors. Since the flow rate restrictors have differing areas in the horizontal plane, an interstitial opening between the outer edge of each flow rate restrictor and the inner perimeter of the stationary riser differs. The flow rate is constant and proportional to the depth of the fluid over the interstitial opening with the least area. The flow rate remains constant until that flow rate restrictor creating the smallest interstitial opening lifts above the upper edge of the stationary riser at which time, the next flow rate restrictor determines the flow rate.