5. Except as otherwise provided in this Act, all plots or parcels of land which, before the commencement of this Act, have been used for the purpose of obtaining earth or other materials for the repair of any public embankment, water-course or embanked tow-path as aforesaid, or which by agreement have been substituted for such lands, shall be deemed to be at the disposal of the Government or the Authority for such purpose, without payment of compensation for the use or removal of such earth or other materials. The Engineer may cause all such plots or parcels of land to be ascertained, surveyed and demarcated.