The "skip to the end" version:
If you build a moat, connected to an infinite water source, make sure you install an emergency shut-off floodgate, and make sure you have a means of draining it if you need to, ideally to an infinite sink like a chasm. Also if your moat ever freezes, make sure you use restricted zones to prevent dwarves from walking on the ice.
Long version:
I am still fairly new at this. This is my first big fortress and I'm still waiting on my first siege, which at a population over 100 and wealth of about 800K I think should be coming very soon. I had a near disaster last night, which would have been a total loss had it not been for one clumsy dwarf who got himself killed years prior.
Around my entrance I wanted to install a moat. Not around the entire fortress, but just a circle, where half of it would go into the cliff face and the other half would stick out with drawbridges and defensive towers in the middle. To enter the fortress you would have to cross the moat twice to get in and I had separate levers set up for the outer bridges and inner bridge. This worked fine and it looked cool.
The first winter after building it, the whole thing froze. This would have been fine but the dwarves started using the ice like solid ground and the day it thawed out, naturally someone fell in and drowned. Lesson learned. Use restricted zones to prevent idiots from walking on the ice. This is also the point when it occured to me that it might be handy to be able to shut off the source. That project would have to wait until the following winter when the next freeze came. I quickly dug out the ice, installed some floodgates, and hooked up a lever before the water thawed. This is what would eventually save my entire fortress from total disaster.
Inside my front entrance, past the moat, I thought it might be handy to have an area that in case my front defenses failed, I could flood with water to prevent any further incursion. So I channeled out the hallway that would get flooded, hooked up a floodgate, and my first ever screw pump(!) that would pump the water back into the moat. This all worked great. I was very proud of myself.
At some point I figured what I really needed was a waterfall over my entryway. I decided to pump water from the moat, up 3 levels, then dump it on the path below. In retrospect, I probably could have dumped it right over the inner drawbridge and maybe I wouldn't have needed floor grates or a drain at all. Nevertheless, I went ahead with the grates. I also wanted to connect the drain to the moat so that the water would go back in. Here's where it started getting complicated.
I figured I needed to drain my moat low enough so that a miner could go in and dig out the drain tunnel. So I dug down really deep and made a large reservoir, 2 levels deep (should have made it much bigger), then dug straight up to that flood hallway that I had built in my entry corridor and installed a floor hatch. So I shut off the main floodgate to the moat, flooded my hallway, opened the hatch and the moat (very slowly) drained into my reservoir. Awesome! My miner went in and dug out the drain tunnel I needed, I closed the drain and refilled the moat.
The problem occured when I realized that a single floor grate was not even close to being sufficient for the waterfall. Water went everywhere. So in my floor grate ignorance I channeled the space N, S, E, and W of the grate that I had already installed. Oops! The grate collapsed. No big deal right? It just fell down into the drain tunnel, (which happens to be connected to the moat, which is connected to the infinite water source), right? Nope, it punched a hole in the floor down into the tunnel below. The tunnel that I had built for soldiers to get to the defense towers out front. Unfortunately there were so many stones in the drain area that I didn't notice the hole.
So I continued mucking around with the floor grates, having trouble figuring out where I could build them that they wouldn't fall, didn't notice the 6's and 5's in my moat near where I was working. Until I got a weird message that I had never seen before. "So-and-so cancels job, looking for infant." Huh? Never seen that one before? So I go to the unit page to find said infant. Oh there he is, in the housing area... under 3 feet of water! I had been tooling around for a half hour while my fortress was quietly filling up with water!
Fortunately once I figured out where the leak was, I was able to shut off the water, and open the hatch to help drain the moat into the reservoir instead of the housing level. I also dug an additional reservoir to help reduce the water rushing down my main stairwell. I was also fortunate that the housing level was not the bottom. I had been doing a lot of mining below that and the mine soaked up most of the water. Only reason the housing area got wet is because that is where the stairwell goes from 2x2 to a single stair below that.
So my story has a happy ending. Nobody died. I just made a really big mess. But if I hadn't retroactively installed that floodgate cutting off the water source, it would be a different story.
Also I was left with a very full reservoir underground which I would later attempt to drain into the chasm I discovered. More hijinks would ensue, but that is another story.
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Topic: Moat hijinks (Read 1175 times)