Water Resources Development Act of 2020 This bill modifies water resources infrastructure funding and addresses Army Corps of Engineers projects for navigation, flood damage reduction, and ecosystem restoration. The bill removes certain funds appropriated for harbor operations and maintenance from annual discretionary spending limits. This allows additional funds to be appropriated for these purposes without the amounts counting against annual discretionary spending limits. Further, the bill increases the percentage of available funds allocated annually to emerging harbors (i.e., those that handle less than 1 million tons of cargo annually). Additionally, the bill modifies cost-sharing requirements for the construction of inland waterway projects for FY2021-FY2027. Further, the Corps of Engineers must establish a pilot program to address the flood and storm damage risk reduction needs of economically disadvantaged communities. The Corps of Engineers may also construct projects for flood and coastal storm risk management in areas that have had been impacted by repetitive flooding. The Corps of Engineers must also identify any engineering or maintenance defects for levees in the national database. The bill also creates a process to de-authorize projects that are no longer viable for construction and authorizes new projects, including projects for navigation, flood damage reduction, shoreline protection, and ecosystem restoration. In addition, the Government Accountability Office must study (1) consideration by the Corps of Engineers of natural and nature-based features during project planning, (2) mitigation of the impact of water resources development projects, and (3) the operation and maintenance needs of federally authorized harbors and inland harbors.