I'm trying to pack super light, and wondering if a tent fly by itself works as a bivy? I've never tried camping with just a tarp though. The tent is a 1.8kg 1.5 man tubular style tent: two flexible poles of different lengths that don't cross. Oh, here's a photo of it.

Thoughts? Also, how do you stop water running along the ground from getting you wet? Dig a trench?