; and of which such sums as are necessary to cover one-half of the costs of construction, replacement, rehabilitation, and expansion of inland waterways projects shall be derived from the Inland Waterways Trust Fund, except as otherwise specifically provided for in law: Provided , That the Secretary may initiate up to, but not more than, two new construction starts during fiscal year 2018: Provided further , That the new construction starts shall consist of two projects where the majority of the benefits are derived from navigation transportation savings, flood and storm damage reduction, or environmental restoration: Provided further , That for new construction projects, project cost sharing agreements shall be executed as soon as practicable but no later than August 31, 2018: Provided further , That no allocation for a new start shall be considered final and no work allowance shall be made until the Secretary provides to the Committees on Appropriations of both Houses of Congress an out-year funding scenario demonstrating the affordability of the selected new starts and the impacts on other projects: Provided further , That the Secretary may not deviate from the new starts proposed in the work plan, once the plan has been submitted to the Committees on Appropriations of both Houses of Congress. Mississippi river and tributaries For expenses necessary for flood damage reduction projects and related efforts in the Mississippi River alluvial valley below Cape Girardeau, Missouri, as authorized by law, $301,000,000, to remain available until expended, of which such sums as are necessary to cover the Federal share of eligible operation and maintenance costs for inland harbors shall be derived from the Harbor Maintenance Trust Fund. Operation and maintenance For expenses necessary for the operation, maintenance, and care of existing river and harbor, flood and storm damage reduction, aquatic ecosystem restoration, and related projects authorized by law