The ornamental design for the drainage channel, as shown and described.
FIG. 1 is a top front isometric view of a drainage channel showing my new design;
FIG. 2 is an enlarged view of an end portion of the channel depicted in FIG. 1;
FIG. 3 is a bottom rear isometric view of a drainage channel showing my new design;
FIG. 4 is an enlarged view of an end portion of the channel depicted in FIG. 3;
FIG. 5 is a front elevational view of a drainage channel showing my new design;
FIG. 6 is a right side elevational view thereof; and,
FIG. 7 is an enlarged view of an end portion of the channel depicted in FIG. 5.
The left side elevational view of the drainage channel is a mirror image of the right side elevational view. The drainage channel is shown broken away with broken line interruptions in FIGS. 1, 3, and 5 in order to indicate that a specific length of drainage channel forms no part of the claimed design.