Continuing from the above, there are only four nature-based scenarios I can imagine that could cause inundation with a wave of up to 100 ft high.
The first and perhaps most obvious is a tsunami from a major undersea megathrust earthquake, not too far from the coast. That is theoretically
possible even along that stretch of California, but far more likely further north in the PNW region, where tsunamis of that height along the Cascadia
Subduction Zone are known to have occurred in the past.
The second and probably much less likely along the US west coast region would be a massive storm, but the wind force (speeds and persistence) required
to create such a wave are something I've never heard of.
The third but unlikely cause would be a very large volcanic eruption/explosion, possibly even thousands of miles away but within a volcano or volcanic
chain that is coastal or even in an island chain. However, for such an event to send a 100-ft-high tsunami into the US west coast, it would have to be
even more devastating closer to its source and there are probably no known volcanoes on the Pacific Rim capable of erupting that violently.
Just a note on this point: tsunamis from volcanic explosions (eg Thera [Santorini] and Krakatau [Krakatoa]) can be very
severe, but they tend
to be much more local in their effects. This is mainly because the wave length of the tsunami is much shorter than those generated by megathrust
quakes and overall, the body of water moved is smaller and hence has less energy. I won't go into all the technicalities but that's the basics of it.
Short summary: a huge tsunami from a volcanic explosion is very unlikely in the US west coast region.
The fourth is one that we might not normally even consider, because it involves a combination of nature and humanity and for the most part, we've
focused on inundation from tsunamis.
Up in the hills and mountains inland from LA, there are several reservoirs and as you can imagine, their capacity is huge. It has to be to supply the
needs of one of the largest urban regions on the planet, especially as there is a lack of very large rivers with enough free flow to otherwise supply
the needs of humans and industry year-round.
Here's what I'm thinking: perhaps the wall of water rushing down that channel doesn't come up from the sea, but comes down from a broken dam. There
are many faults in the region and more that are probably not even known, simply because they haven't been active. A few hundred years of quiet means
nothing with some faults.
Known fault or not, it would only take one of them to let go in a big way near a dam and it might fail and release its millions of cubic yards of
water in one huge flood. In that case, it would follow the path of least resistance -- and those canals are designed the same way. Logically, a flood
from a dam could wind up reaching the same piece of lower-lying land where the canals are.
The truly scary thing about this scenario is that it would not even need to be a "huge" quake. A moderately severe quake that might not do more than
rattle the windows in downtown LA could be enough to destroy the integrity of a dam some dozens of miles away in the hills if the epicenter is close
to the dam and it's a fairly shallow quake. And once a major dam starts to go then there's little hope of stopping it failing completely.
edit on 28/3/13 by JustMike because: Added note re volcanic explosions and their tsunamis versus megathrust quake tsunamis.