Sec. 4. IMPROVEMENT OF LAND; TITLE TO LAND. The City of Corpus Christi, its agents or assigns shall improve such portions of the land covered by said patent or any corrected patent as such city, its agents or assigns, deems suitable and proper therefor. Such improvement shall consist of the raising or filling to a height of at least three (3) feet above the level of mean high tide, except for such part as may be devoted to channels, canals, or waterways. Title to any portion of such land (except that devoted to channels, canals, or waterways) that has not been so improved by filling to such height before July 1, 1977, shall revert to the State of Texas, and from and after that date neither said city nor its assigns shall have any right, title, claim, or interest to such portion which has not been so improved. No title shall revert, however, to the State of Texas as to any portion or portions which are filled to such height before July 1, 1977, including portions which are devoted to channels, canals, or waterways appurtenant to or used in connection with any portion so improved.