Natural Hazard--Flash Flood

(Contemporary-woodcut-of-the-1607-flood-BristolChannel_UK_PublicDomain)

LOCATION: Outdoors, either urban or countryside.

OMEN/SIGN: The party sees a tremendous thunderstorm about a half-mile away. [Druids and Rangers may make a Passive Perception check to notice that the storm is uphill from the party's location.] A passing NPC (farmer or shopkeeper) jests to the party: "Man, I'm glad I wasn't caught in *that* downpour!"

THE FLOOD: Ten minutes (one turn) after seeing the thunderstorm, the flood water appears: a 1-inch high wave sweeping over the adventurers' feet. The water then rises at 1 foot per *round* until it is 12 feet deep. When it reaches 3 feet (for gnomes and halflings), 4 feet (for dwarves), or 5 feet (for others), apply drowning rules (remove boots/shoes or suffer a penalty to any swim/drowning check). DM's: Keep character encumbrance in mind--characters may need to shed armor and/or drop any heavy carried items in order to swim or climb to safety.

Wonders-of-the-Windie-Winter_London_1613_Folger-Shakespeare-Library (CC BY-SA 4.0)

OPTIONAL ADDED HAZARD: At DM discretion, an additional hazard is carried by the flood waters toward the hapless adventurers (the additional hazard may or may not be spotted one round in advance--DM option: roll for Surprise); the hazard may affect one party member selected at random, or a different hazard may affect each party member, or an additional hazard affects only those party members who did not make an offering to their god from their last treasure (or initial gold). Roll 1d12 to select hazard (or choose the most appropriate, or the most fun):

1. A Swarm of Rats, they want to climb you and cling to your head
2. A Swarm of Ants, (clinging to each other in a living raft), they want to climb you and enter your ears and nose,
3. A Poisonous Snake (paralysis poison, lasts 1 hour), or a Constrictor Snake, underwater, wraps around a foot/leg and tries to pull you under,
4. The flood waters push you toward a whirlpool that tries to suck you underwater (rural--into an underground stream, urban--into the sewer system, from which you are ejected 2 minutes later, 1/4 mile away, drowning damage = 20 - Constitution, DC 10 Const save for half damage)
5. A Giant Leech, underwater, anesthetic saliva means you may or may not feel it attach to you
6. A Decaying Corpse (roll for disease...or, could be a Zombie),
7. A large, terrifying mass of Humanoid Poop (roll for disease, Filth Fever!),
8. A frenzied Wolf/Wild Boar/Mountain Lion/Pit Bull--fight in the water!,
9. A large log bobs up from underwater and smacks you in the face, 1d6 bludgeon damage (DC 12 Dex save for half damage, minimum 1),
10. If characters removed boots/shoes, they step on a sharp stick (rural) or broken bottle (urban), 1d4 pierce damage; if they didn't remove boots/shoes, foot is caught in a tangle of vines or surface roots (rural) or sewer grate (urban)--DC 14 Dex check to free foot each round,
11. Two little kids, clinging to each other and clearly failing to stay afloat, are swept towards you--try to grab them (DC 12 check each round) and save them, or don't (and suffer a Curse from their god--you lose the Swim ability, if you have it, or if you don't, you have recurring nightmares of their watery deaths that prevent hp healing during Long Rests for 1d6 days,
12. You lucky dog! a buoyant log or door rushes by--if you can grab it (DC 12 Dex check), you can ride it to safety!

Deluge_Isaac_Major_1586-1630 (PublicDomain)

REWARD: The flood waters quickly recede 30 minutes (three turns) after they arrive. Looking down, a party member (selected at random) notices an item of treasure deposited by the flood, 1d12 feet from their location; roll 1d6 to determine treasure:

1. A cork-stoppered, green glass bottle filled with rare wine or whisky (25 gp),
2. A watertight, leather scroll case containing a scroll with a magic spell (single use) that gives the reader the Swim skill (permanent),
3. A glass-stoppered flask containing a Potion of Water Breathing,
4. A leather sack, cinched closed, half-filled with air, and half-filled with d100 gold pieces,
5. A bottle stoppered with a bloody fingertip--a hastily-scrawled note (adventure hook) inside reads: "Help me! I'm a prisoner of ..." [add name of foe and directions to the prisoner's location],
6. A cork-stoppered, dirty bottle with a treasure map (adventure hook) inside [of course, the treasure is located in a well-guarded dungeon/sewer lair nearby, or far away].

DM TIP: For added excitement, have the flood occur in the middle of a combat or tense/important conversation with important NPC's. If the party is on a small rise/hill, now they're trapped on a tiny island with their foes for 30 minutes.