The Collaborative International Dictionary of English v.0.48:

Batardeau \Ba`tar*deau"\, n. [F.]
   1. A cofferdam. --Brande & C.
      [1913 Webster]

   2. (Mil.) A wall built across the ditch of a fortification,
      with a sluice gate to regulate the height of water in the
      ditch on both sides of the wall.
      [1913 Webster] Batatas