Flash Flood rules

Today, I've got some rules for a “special event” which I designed for another game of the Ichar IV campaign.

One of the scenarii, using the Combat Patrol format, was supposed to take place in the Hive’s sewer system. The Imperials were trying to place charges into a brood-nest hidden within the sewer system. If the Imperials failed then the ‘Nids would gain a fourth stratagem for the next game: Sewer Rats.

To add a level of randomness to this game, it was decided that the sewer systems were badly damaged and that flash floods frequently occurred. I was thus tasked with coming up with rules for such an event...

I'm only posting the rules without a full scenario because they are actually usable regardless of the mission you pick (adding a twist to the events).
I've added a few comments (between brackets) here and there to help you understand the reason behind some of the rules and thus use them more easily. But the translation from french proved harder than I anticipated and there may still be some parts which are not clear enough. If that's the case, please let me know, I'll edit accordingly.

Without further delay, here are the rules as we played them:

[You can have the pdf version (without the additional comments) here (right click and "save as"). ]


++ FLASH FLOOD RULES ++

Decide prior to the game where the flood will originate from. [A corner of the table does the trick nicely. If you pick a central point, then you can cut the resolution of the flood effects in two, getting one half of the table done before dealing with the other.]
It is important to note which parts of the system will remain out of reach of the flood (gantries, etc...).
Grates, sluice-gates and the eventual cave-ins in the system will count as Impassable during the flood (even if they only count as Difficult terrain during the rest of the game).

The flood may occur at any moment starting with turn 2. On turn 2 the flood occurs on a 6+, on turn 3 on a 4+, on turn 4 on 2+ and it takes place on turn 5 if it didn’t earlier...

When the flood occurs roll for each model separately against their Strength, starting with the furthest from the origin point of the flood (in case of doubt roll a d6 to decide which model must test first), if the roll is failed then the model looses its footing and is carried away by the rushing water. [It may seem counter-intuitive to start by the model furthest from the flood's point of origin, but it has its own game logic: "bumped" models --see below.]

The model is carried away by 1d6". If the result of the roll is a 6 then it is carried for another d6", if that result is another 6 then roll another d6, etc... Roll until the model either stops or comes into contact with Impassable terrain. [This represents the tunnel-fighter trying to gain hold of something to stop being carried further by the torrents of water.]
If the corridor turns, then the model's movement follows the twists and bends. If a corridor branches, then roll 1d6 to decide along which branch the model is carried away.

If, while carried away, a model comes into base contact ("bumps") with another one, on a 4+ the "bumped" model is knocked off its feet by the other and is carried away by the water. [This means that a model which was once carried away and succeeded in stopping its movement can still be carried away further because one of his squad mates bumps into him --and is the reason why you should start with the model furthest from the flood source.]

All models which loose their footing and get carried suffer a S1 hit without save, +1S for each full 3" moved (hence S2 for 3" to 6", S3 for 6" to 9", etc...). A model which is stopped by Impassable terrain suffers an additional S3 hit. [This represents the fighter hitting the walls, pipes and grates while fighting to stop his momentum and the possibility that he's knocked-out or even drowns.]

Once the flood took place, every part of the board which was flooded counts as Difficult terrain for the rest of the game.

It is evident that the flood will cause chaos and force the majority of the models to break unit coherency.
All models out of coherency must use their maximum movement to restore unit coherency and cannot do anything else but run/fleet during the shooting phase. They may however fight in CC if charged (but can't charge).

A unit which has less than 50% of its (surviving) members in unit coherency is scattered and cannot act normally (they remain only able to move towards their nearest squad mates by their maximum movement, run/fleet and fight if charged).
Once part of a unit has lost the scattered status, it may act normally, but all the members of the unit still out of coherency will continue to be only able to try to join with their squad mates.

If a single model or a scattered unit can't restore unit coherency without having to go within 1" of an enemy unit/model then 2 possibilities arise:
1) if the enemy unit is itself scattered, move the unit/model by its full movement "through" the enemy models, any model which could not go fully through the unit and more than 1" away by the end of the shooting phase instantly dies. Once all the models have been moved, fight a single round of CC considering all models for both units were engaged. Remove casualties as usual. [This represents the men/xenos fighting their way through the relative opposition of enemies without coordination.]
2) if the enemy unit is not scattered then move the models into base contact with the enemy unit and fight a round of CC as usual, the enemy counting as having charged! If the model/unit survives the round of CC then leave it locked in CC with the enemy unit. On the enemy's turn another round of CC will be made and if the unit/model survives then you'll be allowed to break combat on your movement turn without any penalty. [This represents the men/xenos fighting their way through organised opposition.]

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As you can guess, these rules transform the game into pandemonium as soon as the flood occurred. They were very funny to play with and we’ve used them a few other times since then with great pleasure…

The fact that one of my friends, the one who’s tasked with all the terrain building for our campaigns, built a beautiful modular sewer system table for us played a big part in us re-using these rules. But you don’t really need to build a table just for this, you may simply build “corridors” by placing rectangles of cardboard on a table and with a bit of imagination you’ve got a playable sewer system table ;)

What I like the most about these rules is that, for at least 2 turns after the flood, the game feels very different to play, emulating quite well the complete anarchy that would result in such a situation.

I must recommend however not to play it with a format bigger than Combat Patrol, because the model by model nature of the flood rules themselves would become to heavy to manage if you’ve got more than a few squads each to move around on the table.

I hope you’ll have as much fun with these as we did!