The lack of a bathtub floor is one of the chiefest comparative drawbacks of tarp camping vs tenting. I have been toying with this idea for some time. I used to usually collect some suitably lengthed dead branches and drape the edges of the tarp over them on the appropriate uphill side if rain threatened to inundate the ground.
I played with various means of suspending the edges of the tarp with mitten hooks attached to the tarp. This works but is awkward and slow with my arthritic fingers, then I thought, what if I made an inflatable tube which circumnavigates the tarp? I thought this was a genius idea till I Googled it and found someone else had already been there before me. https://www.esvocampingshop.com/en/air-barrier-tent-ground-sheet-inflatable/ Imitation is the sincerest form of flattery. I did come up with the idea independently though. Theirs is quite heavy and only really suited to car camping not hiking.
Before I ever looked to see if there was such a thing I was thinking mylar or silnylon (both possibilities still – further experimentation needed), then I hit on the DIY packraft site I posted about here: http://www.theultralighthiker.com/new-diy-pack-raft/ and realised they could supply the materials for the tube and valve and that I could simply sew this to the edge of my 1.3 oz/yd2 silnylon tarp then seam seal the join.
Unfortunately the lightest heat sealable material (eg from http://www.seattlefabrics.com/nylons.html) is (I believe) 3 oz/yd2. I would need a tube 22’ long to circle double (7’ x4’) groundsheet. If I wanted the tube to be 2” in diameter, this would mean the tube would be in excess of 11ft2 or 1.3yds x 3 = 4 oz plus the 1.3 oz/yd2 silnylon 3yds x 1.3 = 3.9 Total 7.9oz or approx 240 grams. Good, but too heavy. If I can make the whole thing out of silnylon the first figure will become (1.3 x 1.3) 1.69 oz giving a total of 5.6 oz – or approx 160 grams. Much better.
A silnylon dry bag seems to hold air quite well though it is not designed to, so I suspect that if I glue up a tube of silnylon it will serve quite well, even if I have to add additional silicon as in this post: http://www.theultralighthiker.com/waterproofing-tent-floors-and-ground-sheets/ The thing is people use the heat sealable materials because they need strength, but with this ground sheet, the inflatable barrier does not have to take any stress, so glued/welded silnylon might work well.
Why not try it yourself, and get back to me?
PS: This groundsheet will go very well with this tarp:
I realise this inflatable tube could be added to my Holeless poncho to make it into a better groundsheet: http://www.theultralighthiker.com/hole-less-ponchoshelter/ eg for my http://www.theultralighthiker.com/the-deer-hunters-tent/
PS: As with my other design ideas, feel free to make one yourself but if you want to manufacture them I would appreciate some credit.