The enlarged district and the trustees thereof shall possess all the powers and perform all the duties throughout such enlarged territory previously vested in and imposed upon said district, and the trustees thereof within the original territory of said sanitary drainage district; and no proceedings taken under sections 31-525 and 31-526 for the enlargement of any sanitary drainage district shall be construed as impairing or suspending in any manner the powers and jurisdiction of any sanitary drainage district previously existing or of the trustees thereof. The trustees of such sanitary district shall have power by resolution to define the flood area of any stream or watercourse within said district and to exclude from such area the construction of buildings or other improvements which, if constructed, might interfere with the flow of flood waters. To determine such flood area the trustees shall cause a survey to be made of the watershed in the vicinity and highwater marks reached by floods in the past, and compute the volume of water flowing in the valley under flood conditions and, based thereon, together with such other engineering data as may be available, specify the area requisite for confining flood waters within reasonable dike embankments set back from the natural banks of the stream. Upon such area having been determined the district may acquire either a fee title to the property within such area or a perpetual easement therein for carrying flood waters and the construction of dikes, by purchase or condemnation proceedings. Any property within such flood area owned by the state or governmental subdivision of the state shall be available for the purpose of establishing and maintaining suitable control of flood waters without compensation for such use.