The right, power and authority to acquire the necessary and needed rights-of-way for any and all purposes now existing by law or created by this act, may be acquired by the commissioners of any diking district over, across and upon any land, or interest therein, of the state of Washington or any county of this state, and streets, avenues, alleys or public places of any city, town or municipal corporation of this state: PROVIDED, HOWEVER, That the construction of such dike or dikes shall not have the effect of impairing any right, power or authority now existing on the part of any city or town to construct in, upon, underneath, above or across such dike or dikes, sewers, water pipes, mains, or the granting of any franchise thereon, or the improvement by way of planking, replanking, paving, repaving or any other power, right or authority which but for this act such city or town would have in or to such street, avenue, alley or public place; except, however, that such right, power or authority on behalf of such city or town shall not be exercised either by such city or town or by any person, persons, firms or corporations to whom it might grant any right or franchise, which will materially impair the efficiency of such dike or dikes. The provisions of this section as regards said system of dikes to be located within the boundaries of any incorporated city or town shall apply to the extension or enlargement of any dike or dikes already existing upon, over and across any street, avenue, alley or public place of any city or town, as well as the original construction thereof.