Those portions of the East and Hudson Rivers in New York County, State of New York, lying shoreward of a line within the United States Pierhead Line as it exists on August 13, 1968, and bounded on the north by the north side of Spring Street extended westerly and the south side of Robert F. Wagner, Senior Place extended eastwardly, are hereby declared to be nonnavigable waters of the United States within the meaning of the laws of the United States. This declaration shall apply only to portions of the above-described area which are bulkheaded and filled. Plans for bulkheading and filling shall be approved by the Secretary of the Army, acting through the Chief of Engineers, on the basis of engineering studies to determine the location and structural stability of the bulkheading and filling in order to preserve and maintain the remaining navigable waterway. Local interests shall reimburse the Federal Government for any engineering costs incurred under this section.