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[Pirate Bob]

I'm considering using the nearby brook to create some kind of water defense flood/cannon outside my exterior gates.
...
What other kinds of water defenses might be good?

Depending on the situation of your site, variants of "Load minecarts with water, fire it/them at invaders" may or may not be practical.  In some versions you could set up a self-contained system that would basically slam the cart against a fortification to launch the contents downrange; and Water [833] would be treated as a single projectile, doing considerable bludgeoning damage and knockback.  I'm not clear if that still works well; the search term you're probably looking for is "minecart shotgun".  The advantage of this system is that it can be set up to be fully automatic; pull the lever / trip the plate, firing continues until reset. 

Failing that, a possibly simpler and more deadly solution, but one requiring a lot more dwarf-powered resetting, is to just launch the water-filled minecarts at the bad guys directly.  Intermediate re-usability is to have a "doge 'em" trap that sends the water-filled minecarts back and forth or around and around, until they hit an invader trying to path through the death zone. 

With care, you could develop a system that works for water now, but can be upgraded to fire magma if and when you get around to creating a pump stack.  This would let you get the bugs out with a more forgiving fluid, and probably get you results sooner as well.

The minecart shotgun does still work, but it is somewhat fiddly to set up, and the cases which are known to work take considerable effort to build. QuantumMenace's masterpiece example uses all silver minecarts, a ton of power, and I believe would take several game years to build.  I have been working on figuring out the simplest possible method, but every time I think I have it worked out I encounter problems and it jams.  A few tips:
(1) Assuming you want the minecart going quite fast, you must collide it with ANOTHER STATIONARY MINECART to stop it and cause it to release its contents (water).  If you use a wall or fortification, a fast enough minecart can teleport through it and get stuck.
(2) The minecart needs to be going at least speed 70,000 to launch its contents when colliding with another minecart (or wall, but see above).  This means a max speed roller plus at least 5 impulse ramps.
(3) Assuming you want to drop the minecart through a hole after it is fired (this is the most popular reload design), it takes 8 ticks for the minecart to fall through the hole.  If another minecart collides with it from behind during this time, it gets stuck, and your cannon jams.  This means you need to regulate release of your minecarts to be slower than 8 ticks.  I believe raising them out of water using a medium roller should accomplish this, but I am having problems getting this to work, due mostly to
(4) You MUST ensure that any parts of your cannon which are supposed to be in 7/7 water are pressurized (with pumps) such that they remain at 7/7 at all times, including right after a minecart fills.  This part is really annoying, as things like floor hatches and I'm not sure what else will hold water above them at 7/7, but will NOT maintain pressure.  If a minecart goes through a square which is at less than 7/7, then the friction is far less, and it will be moving faster than the cart in front of it, which often results in unwanted collisions and jams.

TL:DR - I am pretty sure QuantumMenace's design will work in 40.24, but its really resource and labor intensive.  If I can figure out a simple design, I will post it, but I have been too busy to mess with this for a while.  This is my design thus far, but as I said I am not 100% sure it works.

Also, a drowning chamber is MUCH easier to build than a cannon, and makes things just as dead, so...

[Jigowah]

If you are too lazy to take the magma to the enemy, take the enemy to the magma.  This is exactly the kind of out-of-the-box thinking I needed.  The "High Altitude Magma Pool Drop w/ Drowning Enabled Entrance" is officially under construction.

[Broseph Stalin]

Spoiler (click to show/hide)

This design is basically a large pit with a small building in it protecting the central staircase. There's one lever to fill/stop filling and one to drain/stop draining. When the drain lever is off and the fill lever is on the pit becomes a huge pond that completely engulfs the entrance. When the fill lever is off and the drain lever is on the water drains out of the pond and turns it into a pit again.
I had alligators in the pond but that's optional.

[Miuramir]

If you are too lazy to take the magma to the enemy, take the enemy to the magma.  This is exactly the kind of out-of-the-box thinking I needed.  The "High Altitude Magma Pool Drop w/ Drowning Enabled Entrance" is officially under construction.

Further thoughts: IIRC the current version still handles falling (impact really) damage, mechanically, as "hit the falling thing with a giant imaginary cube hammer made up of what the floor is made of".  Since said virtual cube moves at a constant (virtual) impact velocity, using a denser floor (and thus virtual hammer) give more punch.  Applying a constructed floor of something denser than rock, and ideally magma safe for the optional follow-up "magma flush sorting" stage, would add both gratuitous aesthetics and actual killing power.  Presumably platinum would be the premium option

Having the magma separately controlled also allows using water pressure to *wash* foes into the chasm.  The deluge valves trip, they get washed into the chasm, fall dozens or hundreds of levels, and slam into a premium floor of polished platinum.  Then, depending on how one sets the levers, the bottom pumps either pull out the water, pull out the water then replace it with magma, or apply both for obsidianization in serious emergencies. 

Further refinement / complication: Instead of platinum floor, could one do a platinum bridge?  Under it would be a further short drop direct to the Magma Flow tiles, destroyer of all things.  One would keep the bridges deployed most days for the ability to collect the useful bits of goblinite; but in case of serious attack (husks or undead, presumably, or something with really alarming syndrome dust), you have the dwarves open the lower bridges and the foes plummet directly to ultimate magma, do not pass Go, do not collect 200☼

Side thought: I think the DF community would benefit from adopting the horological term "complication".  It implies a feature that actually does something, but in many cases (pun not originally intended, but I'll take it) is designed in more to show off the fact that you *can* design in such a feature, than for the actual value of what it does.  It's a nice bridge between "feature" and "decor". 

[Albedo]

Since it's on topic, what's the best way to drain a drowning chamber if you want to collect the goblinite?

Grates are reported to (occasionally?) let items thru via "flow", and also are subject to BD's, so I'd rather avoid those if I can help it (unless that "report" is flawed?).

Bank of pumps the best/fastest plan (if a bit more complex)?

[milo christiansen]

Use floor grates and fill/empty through the floor. Filling is done with pumps and a pressure tower and emptying is just a mater of opening the bridge.

Basically you make a three z cistern under your drowning chamber, put a bridge between the top level and the bottom two levels, and pump from the bottom.

This way you can drain instantly by opening the bridge to allow the water to fall from the top to the bottom. Filling is done by pumping from the bottom of the cistern into "pressure towers" that are level with the top of the drowning chamber and connected to the top level of the cistern, this then forces water to flow up from the floor more-or-less evenly under pressure (obviously more pressure towers means faster flow).

If building destroyers are a problem you will want to do something else...

[Albedo]

And won't lose items thru the grates due to flow since there is no flow as water drops in level - nice!

[milo christiansen]

And no muddy floors

[Bakaridjan]

I found this thread looking for some ideas for water defenses. I really feel like water, and particularly water pressure, is under-utilized as a defense. I honestly haven't messed around with drowning chambers much because they seem to have limited application and don't take care of the gnarliest attackers. I think I'm going to experiment with an underground (this isn't Human Fortress after all) water canon that will be powered by nothing but a large reservoir that pressurizes the water to shoot it down a long single-wide corridor. Everything not killed by the pressure will get flushed down to where I have magma to cleanse all the dross from the goblinite. It will be a long haul to get it back up, but if it's just the iron, it shouldn't be too bad. Everything will be gravity fed and the water will drain into a pond in the third cavern and off the map. The reservoir will be filled from the aquifer so there should be no need for pumps, just a few bridge/gates, levers and pressure plates.

I'll probably try some experimenting (no claims here to doing real "science") with dumping stone and maybe spiked metal balls in the path of the water to see what sort of effect they have when pushed by pressurized water into hordes of cadavers. 

[Shadow_Hornet]

In my own chasm based defenses I would have a series of 5 or 7 large bridges set to retract at the pull of one lever. The fact I use a series of them means that even though bridges don't work on titans I can still trap a titan in my hallway of bridges. The Titan would be standing on one bridge while all the bridges behind and in front retract leaving it nowhere to go. I think a bonus with this kind of trap is that if a Titan shows up that can shoot webs or something I will have time to think about what I want to do with it.

[Uggh]

To collect all goblinite, build a room large enough to collect all the water from the drowning chamber at level 1-2 depth.If your chamber is for example 3x3,a 56 tile room will do. This could even be an extension of the drowning chamber on the same level, saving some space.

For all water based defenses, be prepared for winter if you are in a freezing environment. While freezing can cancel your defenses and only invite some goblins to a skating party, you can also use it to crush amphibious creatures that got trapped in a drowning chamber. Cave crocs, for example...

[Bakaridjan]

I got my water cannon all set up and was happy to see an undead siege show up with the next season. It took them forever to filter into the trap, but I was very happy with the way it functions. Water was moving about 20 squares a tick. But then I got the big disappointment. The corpses seemed totally unfazed by the moving water. They just stood still while 7/7 water filled the hallway they were in in seconds. That's a major disappointment because undead seem to be the only sieges I'm getting at the moment. I guess it proves, what was already known, magma is always the best solution.

[Larix]

More importantly, water pressure isn't really modelled in the game: water that sloshes from one place to another can occasionally push items a tile or two, but "pressurised" fluids simply teleport to the nearest open space without affecting intervening tiles - they will not move items or units. "Washing away" only happens when water can spread out and doesn't pressure-teleport.

If you want to throw enemies around with water, your best bet is building a minecart water-gun. The water blobs thrown by them are not treated as fluid, but as pseudo-solid projectiles.

[Bakaridjan]

I'm kinda disappointed. I had tried using a pressure washer before, to push siegers into a pit and it worked pretty well. I thought it was the pressure that was washing them in, but it was probably the flow that was doing it. I think I'll give up on the water thing and go back to the more reliable magma. I do wish that the water physics were a bit better though, because you could do some pretty amazing things with water-pressure. Especially if hydraulic pressure were modeled. MMMmmmm geyser defenses....

[AbanShakehandles]

MMMmmmm geyser defenses....

http://mkv25.net/dfma/movie-2362-superdefenses
Frogwarrior's MAGMA LAND MINES!
(The tour ends and the carnage begins around 0:45)

Original thread here: http://www.bay12forums.com/smf/index.php?topic=91789.0