This link explains the spillway:
http://www.mvn.usace.army.mil/pao/bcarre/bcarre.htm

Bonne Carre Spillway

Bonne Carre Spillway

Spillway needles on cart.

Spillway looking to the Mississippi River.

Spillway looking to the Mississippi River.

Spillway looking to Lake Pontchatrain.

Ship on the Mississippi River at Ft. Jackson.