Yesterday I snowshoed the one mile to Skyline Lake from Stevens Pass in Washington to meet up with a handful of Mountaineers to finish off the last requirement of the Advanced Scramble Course. We had to spend the night in emergency bivys and live to tell about it. Mountaineers are required to carry an emergency bivy system as part of the 10 Essentials, and this was a chance to see if what we carried actually worked. We were allowed to bring sleeping bags and stoves, which we normally don't carry on day trips.
I dug a trench near a tree and then rigged up my homemade silnylon tarp over it, attaching it to the trees and trekking poles and sticks jammed into the snow. I made the tarp out of red silnylon, because it is meant to be used in emergencies, and the obnoxious red would make it easier for someone to find me. I was not pleased with the rigging, because I had wanted something more elegant I guess, but it worked fine. The picture doesn't show all the ties grounded yet, but ultimately I had 8 ties - 1 at each corner and 1 in the middle of each center. The tarp measured 5' X 7' and tapered to 4' at the other end.
Temperatures were in the low 20's overnight, with frequent wind gusts around 19 mph and an inch of new snow. I had to tap the tarp occasionally during the night to get the snow to slide off. I lined the trench with a stretched-out ULA rainskirt, then a prolite 3 short thermarest, and 2 1/8" GG thinlight closed cell foam pads. I put my WM Ultralite bag into my MB bivy sack.
I wore my BPL woolie longjohns, MEC Schoeller pants, merino l/s hoodie, thruhiker vest, and Patagonia micropuff jacket. I didn't need the BPL balaclava, so it was part of my pillow system.
I was warm and comfortable all night, even though I tend to be a cold sleeper, and even though the tarp was drafty because I'd dug the trench too wide. I was grateful the tarp held up fine against the frequent wind gusts. As it happens, the 4 of us who "trenched" were much more comfortable than the 3 instructors who used tents. They froze all night. They were also amazed my backpack total weight was only 15 pounds.
Another pleasant surprise was how well my supercat alcohol stove worked. At first I used my aluminum shovel as a platform, but the stove seemed slow. So I switched to using a hardware cloth platform I'd made, and it worked great. The stove had just enough air under it to keep from getting too cold, and the hardware cloth didn't get so hot that the whole setup would start sinking down to bedrock.
So with a little more practice, I hope to get the tarp idea working better, and I will continue to use the alcohol stove. A good time was had by all.