Section 31. The department may make surveys and improvements for the preservation of harbors and may repair damages occasioned by storms or other destructive agencies along the coast line or river banks of the commonwealth, and may take by eminent domain under chapter seventy-nine, or acquire by purchase or otherwise, in the name and behalf of the commonwealth, any land or materials necessary for making such improvements or repairs. No contract made under the authority of this section shall be valid until approved in writing by the governor and council.