The governing body shall be the sole judges of the necessity for and the expediency of the making of the improvements mentioned in Kan. Stat. Ann. §§ 12-694 and the manner of the payment of the cost thereof as provided for in Kan. Stat. Ann. §§ 14-701a and 14-701b, and this act shall be liberally construed to encourage the improvements of such natural watercourses and to protect lands and public and private property from damage and from injury by overflow, and to promote the public health, convenience and welfare, and no suit, action or proceeding of any kind in any wise challenging the validity of any proceedings instituted or carried on under and by virtue of this act, or to set aside any general or special tax or assessment or to enjoin the making of the levy or collection of any such tax or assessment or any installment thereof herein provided for, or questioning the validity of any bond issued pursuant thereto, shall be brought after the expiration of thirty days from the time the amount due on such lots, pieces and parcels of land for special assessment is ascertained. If any improvement herein provided for shall cross or interfere with any highway or street of any county or adjoining city the engineer of such county or adjoining city shall have authority to require such construction at said point as will safeguard such county or city against damage. Nothing in this act shall be construed as authorizing a discharge of sewage into any natural watercourse or changing the general laws in that regard.