I like the idea of being able to utterly destroy the equipment of invading armies, except that it means you would lose the metal junk that they had, which means you wouldn't be able to smelt it for free iron. (Of course, you need the economy for that...)
Unfortunately, there are a few problems with your plan:
Dwarves are incapable of opening a door which has been hooked to a lever. Thus, your refuse room would be utterly inaccessible to them unless the door was open all the time. And if the refuse room door was closed, and the bridge raised, and the farm floodgates closed, the goblins would just stand outside wondering how to get in.
Secondly, there is much here that could be improved or simplified:
1. Why bother with an elaborate flood scheme when you could just crush the enemy outright? Leave an unlocked door near the bridge or something, and lock it when they get near. Then crush them.
2. I would suggest that you put your waste management area somewhere else entirely, and use a layout that prevents miasma unhappiness in your refuse-haulers (Ignore the map description and just look at the PoI description and the image, of course).
3. Floodgate farming is impractical and un-necessary (in the immortal words of SHODAN), especially if it takes you that many steps to ready your farms each year. A much safer, quicker, and more reliable way is to do this:
1. Dig out one or two spaces beside the river bank for a good distance up or down the river.
2. Create a 6x10 room, or several connected rooms, near the river. Design it, and cut out extra rock along the river, such that you only have one tile of rock between the room and the riverbank tiles.
3. Smooth and then build fortifications on those wall tiles.
You don't need to hook up any levers or build any floodgates, and you don't really even need safety channels or doors outside the farms. You could build double or triple doors to reduce the likelihood of the natural floods getting your hallway floors wet, but you don't really NEED to.
Now to get your farms ready, wait for the natural river flood to safely turn those rooms' floors into mud, without any risk of drowning any dwarves or pulling them into the river, and then follow this 1-step program:
1. Start farming.
P.S. Simplicity is nice. I have a world-flooder which can flood either water or magma, and it has only three levers. One for the outside floodgate, one for the water pump, and one for the magma pump. My world anti-flood generator also only has two levers. All the floodgates powering all of my defensive-channel-re-flooders are all hooked up to a single lever.
[ August 07, 2007: Message edited by: Shadowlord ]