I am thinking something like this, it can be repeated as many times as you wish:
~= water or magma
# = wall
X = floodgate
C = channel
^' = one time pressure plate or lever
= = path
~~~~~~~~~~~~
#X##########
#~~~~~~~~~#
#X#X#X#X#X##
=C=C=C=C=C=C=
^'#=#=#=#=#=##
===#===#===##
############
Sideways:
##X####X####X######
_ _ ____
\_/\_/ \_/\_/ \_/\_/
This system works as follows:
1. The channels are connected to some underground flushing unit. So as soon as the floodgates open, everyone in the channel gets flushed into a lake or pit. Forgotten beasts are going after the closest thing they can break, which is in my case a floodgate, so as soon as they destroy the floodgate the channel will be filled with water and they get flushed out. Adding a pressure plate in the channel insures that any critters will get a good hose. Dwarves will still be able to pass.
2. The upper left floodgate is connected to the one-time lever or switch. This is a safety device for reconstructing the floodgates. After a limited number of tries, the floodgates are gone and you need a way to construct new ones to ensure safety.
3. Restrict access to the roads on the bottom. If the dwarves find themselfs in front of a new watery channel, they will take that road instead.
4. This does not work against monsters that can swim.
5. This does not work against monsters that do not wander (they run after your dwarves instead)
A forgotten beast tries to get to the first floodgate they can find, so in theory will break a floodgate, causing the channel to fill with water, flushing the monster
(For added pleasure: Use magma).