Section 21. The amount of tidewater displaced by any structure below high water mark, or any filling of flats, shall be ascertained by the department, which shall require the person, his heirs or assignees who are responsible for such displacement to make compensation therefor by excavating, under its direction, between high and low water mark in some part of the same harbor a basin for a quantity of water equal to that displaced; or by paying to the commonwealth, in lieu of such excavation, an amount assessed by the department, or by improving the harbor in any other manner satisfactory to the department. The department shall by regulation determine a schedule of rates of assessment to be charged pursuant to this section per cubic yard of water displaced. In determining said rate of assessment the department may consider the following factors: the costs incurred by the department in the licensing of tidelands; the costs typically associated with excavation or improvement of harbors or basins as permitted in lieu of payment of an assessment under this section; and the level of assessments historically charged in the commonwealth or in other states with respect to the displacement of tidewater. An assessment for the tidewater which has been displaced may be recovered in contract in the name of the state treasurer.