A 1:15-scale, geometrically undistorted, physical hydraulic model was used to develop horizontal deaeration channels for the District of Columbia Water and Sewer Authority who plans to construct facilities to manage their combined sewer overflows for the Anacostia River basin in Washington D.C. Due to geotechnically weak soil conditions, conventional horizontal deaeration tunnels are too expensive to construct. Therefore, use of vertical construction shafts is preferred. The dropstructure scheme developed comprises a tangential inlet structure similar to one used in Milwaukee, a unidirectional circular deaeration channel that resembles a horse-racing track, and a discharge orifice through which the combined overflow discharges into the main storage tunnel. The circular deaeration channel was found to perform well in deaerating dropshaft flows. However, air-core vortices were found to develop over the discharge orifice. To suppress formation of air-core vortices, triangular, vertical baffles with corner fillets were successfully utilized around the discharge orifice.