Make sure you have an adequately sized grate over the drain. On the other hand, if you have too open of a grate, you'll get bandages and stuff going down the drain. The drain itself should be properly designed with a trap that is easily accessible so you can pull the grate off and clean it on a regular basis. Water lines should be buried below the frost line and deep enough, especially if they're under driveways or areas where heavy vehicles travel, so they won't crack from surface pressure. Concrete drainage pipe can be used for this.
Every barn usually has two wash racks, and there's a drainage system underneath all that.
The lines are about three to four feet deep, and we haven't had any problems with crushing the lines. Local statutes might dictate where the drainage lines should lead. Elsewhere, drainage, especially for outside racks, might simply consist of grading that leads to ditch. Although it's not necessary, it is nice, particularly in colder areas, to have hot water available in the wash rack. Your horse doesn't like it any more than you do.