The board of directors or such drainage district is hereby empowered to grant easements in or to such forty-foot strip for such use and occupancy as will not interfere with the right of the state to the use of the same for the prevention of overflow, and if at any point along such harbor line the full width of forty feet is not required for the construction of levees or other improvements to prevent overflow, the board of directors of such drainage district may relinquish such part as shall be deemed necessary.