[at-l] Tarp/Floorless Tent Question

[WenchAT03 at comcast.net (Teresa)]

I know that site selection is very important, but I found that on the AT,
"ideal" tent pads aren't always available, especially when you're not hiking
to anyplace in particular (shelter or official campsite), rather you're
hiking until you don't want to hike anymore.

How do you pick a spot that won't allow water to run in during heavy rain?
How dry will tyvek keep you and your belongings if the ground is soaked from
previous rain?

You've probably already figured it out, but I've never slept in a
tarp/floorless tent so maybe it's not as different from a tent as I'm
imagining.  I also know that in very hard rain, a tent might not stay dry
either, but I think it'd be "dryer" longer than in a tarp/floorless tent.

Any other wet weather tips for tarping are appreciated.

:-)
Tent Wench