catchdrain

from The Century Dictionary.

• noun A drain along the side of a canal or other conduit to catch the surplus water.
• noun A drain running along sloping ground to catch and convey the water flowing over the surface.

from the GNU version of the Collaborative International Dictionary of English.

• noun A ditch or drain along the side of a hill to catch the surface water; also, a ditch at the side of a canal to catch the surplus water.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A ditch or drain along the side of a hill or canal, to catch the surface water.