Wash. Rev. Code § 35.56.010
If the city council or commission of any city of the first or second class in this state deems it necessary or expedient on account of the public health, sanitation, the general welfare, or other cause, to fill or raise the grade or elevation of any marshlands, swamplands, tidelands or lands commonly known as tideflats, or any other lands situated within the limits of such city and to clear and prepare said lands for such filling it may do so by proceeding in accordance with the provisions of this chapter.
For the purpose of filling and raising the grade or elevation of such lands and to secure material therefor and to provide for the proper drainage thereof after such fill has been effected, the city council or commission may acquire rights-of-way (and where necessary or desirable, may vacate, use and appropriate streets and alleys for such purposes) and lay out, build, construct and maintain over and across such lowlands, canals or artificial waterways of at least sufficient width, depth and length to provide and afford the quantity of earth, dirt and material required to complete such fill, and with the earth, dirt and material removed in digging and constructing such canals and waterways, fill and raise the grade or elevation of such marshlands, swamplands, tidelands or tideflats; and such canals or waterways shall be constructed of such width and depth (provided that all the earth, dirt and other suitable material removed in constructing the same shall be used to fill the lowlands as herein provided) as will make them available, convenient and suitable to provide water frontage for landings, wharves and other conveniences of navigation and commerce for the use and benefit of the city and the public. If canals or waterways are to be constructed as herein provided, such city may construct and maintain the necessary bridges over and across the same; such canals or waterways shall be forever under the control of such city and shall be and become public thoroughfares and waterways for the use and benefit of commerce, shipping, the city and the public generally.
The expense of making such improvement and in doing, accomplishing and effecting all the work provided for in this chapter including the cost of making compensation for property taken or damaged, and all other cost and expense incidental to such improvement, shall be assessed to the property benefited, except such amount of such expense as the city council or commission, in its discretion, may direct to be paid out of the current or general expense fund.
RCW 35.56.010