The seacoast may be a nice place to go for a vacation, but it’s
no place to be when a wall of water slams into the shore. A tidal
wave is created by some cataclysmic occurrence on the ocean
floor, such as an earthquake or the eruption of an undersea volcano.
The resulting shock wave does not dissipate relatively rapidly,
as is the case when such a force moves through solid matter.

    Instead, the ocean water in the immediate vicinity absorbs the
force, which is then passed on to the surrounding water with relatively
little decrease in intensity. Unless and until something gets
in its way, a tidal wave can travel for thousands of miles and still
possess enormous force when it hits land. Imagine the “ripple effect’’
that occurs when you drop a pebble into a pond, multiply
that by a few million times in terms of speed and volume of water
affected, and you have a rough approximation of what a fairly
docile tidal wave is like.

    A character who is within sight of the shore and is looking out to
sea will notice the oncoming wall of water, and recognize it for
what it is, when it is one-half mile away from the shore. This much
warning may provide time to take some precaution (aerial movement,
magical protection, etc.) before the wave hits land - but
time is at a premium: The velocity of the wave will be at least 150
miles per hour, which means that even in the best case less than
30 seconds will pass from the time it is sighted until the time it hits
the shore. The wave will be at least 75 feet tall and 500 feet long
when it hits, which translates to avolume of almost 30 million gallons
for the smallest tidal wave possible.

    A tidal wave may rush inland for several miles before its force is
spent. (Its velocity will diminish rapidly after it hits land, but several
million gallons of water moving at “only” 20 miles per hour
can still wreak a lot of havoc.) How far inland it moves and how
rapidly it dissipates are dependent upon the terrain along the
coastline: If an ocean ends at the base of a cliff that rises 5,000
feet above the water, people standing a few hundred yards from
the edge of the cliff might not even get wet -- but if the terrain 20
miles inland is still only a few feet above sea level, people standing
there have almost as much to fear as those who live on the
coast itself.

    A character directly in the path of a tidal wave will die - it’s that
simple. If he survives the buffeting he will take from being hit by
the water, he will at least be knocked unconscious, and he will
drown shortly thereafter. If a character is hit by the wave after it
has traveled some distance inland and lost a lot of its height and
velocity, he can probably survive by “going with the flow” as long
as he has proficiency in swimming. A character who cannot
swim, or who cannot stay afloat, is in mortal danger even if he is
hit by the wave at the very end of its inland incursion. The particular
circumstances will dictate what happens in any situation, and
it is up to the Dungeon Master to exercise his judgment when
such judgment is called for.