Setbacks. Within the WPO District, no principal structure shall be located within 100 feet of a creek, stream or waterbody, and no accessory structure 200 square feet or greater shall be located within 50 feet of a watercourse. These setbacks shall not apply to docks, piers, bridges, and other structures, which, by their nature, must be located on, adjacent to, or over the watercourse. For purposes of determining setbacks, measurements shall be horizontal distances measured from the top of the bank or mean high-water line, as appropriate. For lots in existence prior to the effective date of this chapter, and for any project for which an environmental impact statement has been prepared, the City Planning Board may modify these setback requirements, provided that the City Planning Board finds that the proposed construction will comply with § 300-46F(2).