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View Full Version : Tunnel flooding with water, how fast, what happens?



Gnaritas
2010-10-04, 06:57 AM
D&D 3.5

The party has travelled up a tunnel last session (10 foot diameter, slightly increasing angle). The tunnel was eventually blocked by rocks, behind the rocks is water (they know).
In the tunnel are two larger areas (say 50 foot cubes, with the tunnels halfway). One of those larger areas is directly against the blocked tunnel-part.
The tunnel itself is like a 20 minute walk.

So, when they remove the blocks...what happens? Does the impact of the water hurt them? Will the rocks propel towards them at some point? Will they be able to outrun the water, taking into account that there are two larger areas that need to be filled?

How can i make this exciting for the players. I guess i can let them make balance checks if they try to run "downhill" on moist, smooth underground.
If they get caught in the current, they start drowning (they can prolly hold their breath for like 3 minutes), eventually they will get smashed into the 2nd chamber or end of the tunnel, getting damage along the way.

Quirp
2010-10-04, 07:35 AM
The problem is that we need to know how high the water is on the outside, since higher water means higher pressure means faster flooding.

Halaster
2010-10-04, 07:57 AM
It also depends on what is on the lower end of the tunnel. If it is open to, say, the outside world, the water will run out there and, for example, not fill up the two larger areas. Again, depending on how much water there is.

TheStranger
2010-10-04, 08:07 AM
The short answer: whatever you want to happen.

In reality, it depends on a lot of factors, including the depth of the water on the other side, the integrity of the pile of rocks, and how they go about breaking down the barrier.

For instance, if they create a small hole, but somehow leave the rest of the barrier intact, a relatively manageable amount of water will come through, and they'll have more than enough time to get out of the room. However, if they attack the barrier indiscriminately, and it fails all at once, they'll be instantly engulfed by a wall of water and boulders, causing a huge amount of bludgeoning damage as they are thrown against walls and crushed by rocks. If they survive that, they'll probably drown.

The best solution, assuming you want to create the cinematic effect of the characters running down a tunnel with a wall of water behind them, is to handwave a lot of the details. As the characters break through the wall, water begins gushing through the first small hole they make. They can see that the flow of water is increasing as the pressure tears more rocks loose from the sides of the hole, possibly doing damage to any character dumb enough to get in the way. If they have any sense at all, they'll run like hell right about then (whether they have any sense is debatable, since you seem to assume they'll go ahead and destroy a dam from the downstream side).

While they're running, you can tell them that they hear a crashing/roaring sound from the tunnel behind them - hopefully they'll realize that's the sound of the dam failing completely. If they don't pick up on it, give them a basic knowledge check to figure it out, because that's their cue to start panicking. They won't be able to see the water coming yet, so it's up to you to sell them on the idea that they'll be in a lot of trouble if they don't get to a safe place. At this point, you might have them make a few checks to get through difficult parts of the tunnel. Be sure to mention these as they're first walking up to the dam, though - it'll seem like flavor text on the way in, and make it seem less arbitrary on the way out. If they make all the checks, they should get out of the tunnel just in time to reach a safe place and watch the certain death they avoided. If they don't quite make it, have the water do some damage based on how close they got. If they fail miserably, the damage should be enough to have a reasonable chance of killing them.

If you don't like character death hinging on a few skill checks, you could have some kind of encounter as they're running away from the water. Maybe a recurring villain, or just something large and hungry, has tracked them into the tunnel, thinking it has them cornered. Regardless, it should be completely unconcerned by the oncoming flood, either because it's too dumb, too committed to killing the PCs, or because it can teleport itself out. Decide ahead of time exactly how many rounds the PCs can afford to waste on this encounter without getting caught by the water; if they push past and keep running, they should be fine, but if they take more than a few rounds to fight, they'll get inundated, either while they fight or after, just before they get out of the tunnel.

Since this is a static encounter, though, be prepared for creative thinking on the part of the PCs. The dangerous part doesn't start until they break through the wall, so they have plenty of time to plan. What will happen, for instance, if they go into a rope trick instead of running from the water? If they come up with something that sounds workable, I would suggest rewarding creative thinking rather than railroading them into the big tunnel scene.