Almost nothing about a bivouac on wet, sloping ground at dusk gets decided by the bivy. It gets decided by the ground under it: the grade, where the water goes when it arrives, what you put between your body and the soil, and what you can find to hold the shell down when the stakes won't bite. Choose the site well and a cheap shell gets you through the night dry. Choose it badly and the most expensive fabric on the market just wraps the problem around your sleeping bag and holds it there.
I have done this in the last twenty minutes of light often enough to have a routine, and it starts with a palm flat on the ground, not with the bag coming out of the pack. Press a hand into the duff and hold it for a slow count of five. If water squeezes up around your fingers, the spot is out, no matter how good it looks. Saturated soil does not care how waterproof the bottom of your shell is. It will push moisture up against it all night, and anything between you and the shell that can absorb water eventually will.
So put yourself where I usually am. Rain came through two hours ago and stopped. The light is going sideways through the trees and everything reads darker than it actually is. There is a slope above you, and you have maybe twenty minutes before the ground goes flat and featureless under a headlamp.
Read the ground while you still have daylight
A headlamp flattens topography. Under a beam, a wet slope and a dry one look like the same grey plate. So the survey has to happen now, before you switch it on.
Stand still and look uphill first. Whatever falls twenty feet above you arrives at your head around 2 a.m., and the source is almost never where you are standing. Look for a cutbank, a rock face, a gully, a roadbed, anything that concentrates flow instead of spreading it.
Then look at the ground itself. Wet soil reads darker and slightly glossier than dry, and moss goes a deeper green. On a slope, the wet streaks are not random. They trace the low line, and that line is where water is already choosing to travel. In flat evening light the low spots go dark first, which is a free contour map if you are paying attention.
Now walk a slow circle of ten or twelve paces and press your palm in three places. You are checking two things: how wet it is, and how deeply your hand sinks. The sinking tells you how much loft is under you, which matters as much as the moisture.
The three spots you walk past
Low spots first, anything that reads as the bottom of a bowl in all directions. Then drainage channels: the runout at the base of a gully, or the seam where two slopes converge. A wash that is bone dry at dusk can be running at midnight from rain that fell miles uphill.
The third one surprises people. The base of a single large tree. Water sheds off a crown and funnels straight down the trunk, so the drip line stays wet longer than the ground around it, and the roots under you make a lumpy bed on top of that. A dense stand of trees is a different story, since canopy interception genuinely keeps the forest floor drier than an open meadow, but a lone tree is just a funnel with a trunk.
Slope, drainage, and where the water leaves
The shape you want is anything with an outlet. A gentle grade that sheds, or the high side of a subtle crown, both work, because in either case water has a direction to leave. Concave is the enemy. A shallow depression that still drains downhill is usable; a shallow depression with no outlet is a bathtub. And 'it looks flat here' in a forest usually means the ground is flat because it is filling in.
Sleep with your head slightly uphill. Not because of blood flow or any of the usual talk, but because the highest point of your sleeping surface is the last place water reaches. If something does get under the shell at 3 a.m., you want it pooling at your feet, not wicking into your face and the hood of your bag.
Set the long axis diagonal to the fall line: head a few degrees uphill, feet a few degrees off. Straight up the fall line and you spend the night sliding down against the foot of the shell. Straight across it and you roll. Diagonal, you stay put.
Then rotate the whole thing so the opening ends up downwind and slightly off the flow line. Two things happen at once. Driving rain and spindrift do not get a straight shot into the hood, and the ventilation gap you are going to need sits on the lee side, where it also is not catching runoff. On a mylar shell with no breathable panel, that gap is not optional. Without it, your own moisture condenses on the inside of the fabric and you wake up wet from the inside out, which is worse than being rained on, because you cannot dry it.
What goes under you matters more than what goes over you
Ground pulls heat out of a body faster than air does, by a wide margin, and the underside of a bivy is essentially a plastic sheet with a heat path straight through it. So the layer under your body is doing more work than the shell above it.
The order that works: ground sheet or the shell's own floor, then your pad, then you. If you have a closed-cell foam pad, that is your foundation, and nothing you pile up from the forest beats it, because foam resists compression. Duff, pine needles, dry leaves, dead grass: all of it loses most of its loft within a minute of your weight settling onto it. A three-inch pile flattens to something closer to half an inch.
That does not make the duff useless, it makes it an addition rather than a substitute. Take dead and downed material only, gather it from a wide area instead of stripping one patch, and scatter it back in the morning so the site looks like nothing happened. If the material is damp, leave it there. Wet duff conducts heat away about as well as bare ground, and it soaks whatever is above it.
Watch for hardware under a thin shell, too. Pine cones, sharp twigs, a broken branch stub. Any of those will find your hip through a mylar layer. If the ground is littered with them, put the foam down first and the duff on top of the foam rather than under it.
If you have no pad at all, the pack becomes your insulation. Empty most of it, keep the soft items in the middle, and lay it back-panel-down under your torso. It adds about as much as a thin foam pad, which is to say not much, but it puts a barrier directly under your core, and your core is what you are protecting. Spare layers, a rope, your boots. Anything you can flatten under your hips and feet is worth the thirty seconds it takes.
When the ground will not hold a stake
Loose duff and pine needles have almost no shear strength. A stake pushed into them holds until the first real gust and then releases, usually at the worst hour. You have three better options.
Rock. Wrap the guyline around a fist-sized rock, set the rock in a slight dip, and pile two or three more on top of it. This is a deadman anchor, just built from stone instead of wood.
Tree. Loop the line around a trunk and back to itself, and pick a trunk thick enough that the cord will not bite through the bark when it loads. Cinching cord directly onto a thin tree leaves a scar and slips anyway.
Trekking pole. Buried horizontally, a pole becomes a deadman with a very long lever, and the grip gives you a clean attachment point. That same pole is also the best prop for the head of a bivy if the shell has a liftable hood. Plant it, drape the hood over it, and you have turned a flat sack into something with an air gap over your face. That gap is where condensation goes instead of onto your bag.
On bare rock or a saturated meadow with nothing to tie to, weight is the last resort: pack at the head, boots at the feet, rocks on the corners. A shell that is not anchored flaps, and flapping does two bad things. It pumps your warm air out, and it keeps you awake. A night without sleep in a wet bivy is the actual failure mode.
Getting in is a technique, not luck
The interior of a bivy is essentially a vapor barrier, and once mud and water get inside, they stay inside for the rest of the night. So the entry sequence is worth thinking about before you are tired.
Sit down on your pack or a sit pad rather than on the ground. Take your boots off while your feet are still on something clean, and set them somewhere they will not roll into a puddle.
Lay the shell out, open it, and scoot in from the head end with your feet last. If you have to come in feet first from the mud, roll the foot of the shell under itself as you go so the inside stays on the dry side.
Get out of the day's socks before you get in. A pair of socks you have hiked in all day holds a startling amount of water, and it pulls heat out of your feet all night. This is the cheapest warmth you will get, and it has nothing to do with the bivy.
If your layers are soaked, do not bring them in with you. Hang them on a branch or stuff them in an outer pocket. A shell with no breathable panel and wet clothing inside it is a cold, wet bag by midnight. Change into dry layers quickly and get sealed up.
Before you close the hood, wipe the outside of the shell with a bandana if it is muddy, or you will carry grit in with you. Then leave that downwind vent gap open two or three inches. A hood cracked open on the lee side stays dry; a hood closed tight soaks you from the inside.
The order that works at dusk
Ground first, shell second, bag third, body last, and the ground part happens while you can still see it.
Walk a circle, look uphill, palm the ground in three places, and pick a spot with a grade and an outlet. Avoid the low spots, the channels, and the lone tree. Lay your insulation in the right order. Anchor to rock, root, or pole. Set the long axis diagonal to the slope with the opening downwind, and get in without carrying the ground in with you.
That is a fifteen-minute process and none of it happens inside the bivy. The shell's only real job is to keep the sky off you. Everything else, the warmth and the dryness and whether you sleep at all, comes from what you did to the ground before you unrolled it.
