I am currently writing a book about people with powers. One of these characters have a power to make portals, and I am wondering what would make it realistic for said character to survive hitting the floor with a lot of momentum. (You can bend reality a little, it is a book!)
There's a YouTube channel called Today I Found Out that occasionally asks and answers questions like this, and they recently did one on falling out of an airplane without a parachute.
And the answer to the question of what you need for a fall at terminal velocity to be at least theoretically survivable turns out to be: 'Not a whole lot.' At terminal velocity, stretching out the distance you decelerate over about a meter is enough to turn your odds from 'certain to pancake' to 'broken bones and a hospital, if you're lucky'; it's just a matter of finding a way to get that extra meter or two to slow down.
People who have historically fallen out of airplanes and survived usually did so because they were still in their chair, and the chair hit something on the way down that caused it to slow gradually rather than instantly. For instance, one person survived because they hit a snow-covered slope, going in mostly the same direction they were, so they slid for a while. Another survived because their chair went through a dense jungle canopy, and the branches bled off a lot of the speed. Of course, neither one of those people would have survived if the chair wasn't there to take the hit, so you may need to get a little creative.
Can the character open a portal which points in arbitrary directions? If falling, the character only has to open the portal so that the exit points upwards. Then then enter the portal, and come out going upwards. Gravity slows them down, and when they reach the highest point and start to descend again, they open a portal directly under them with the exit just above the ground, and they fall out and hit the ground going at a very reasonable speed. Timed correctly, they can probably land on their feet and walk off.
If they cannot point the portal in a new direction, so they always head downwards, taey should set the portal up with the exit directly above the entrance, with some very soft material in the middle to gradually bleed off speed. For example, if you can locate a column of air which is rapidly rising, say above a large fan such as those skydiving trainer machines, then repeatedly falling from top to bottom through the air column would get you down to safe speeds. Even repeatedly falling through trees, hay, snowdrifts, a kids ball pond, etc would work. If you can rapidly relocate the portal you could do this in a few inches of snow, just keep repositioning the portals so it seems like you have an infinitly deep snowbank.
The human body can be very robust and there are known situations where people have survived free falls. In any scenario, what you want to do is reduce the impact, or better yet, the rate of deceleration your body will experience when it hits the ground and inevitably stops moving.
So here are a bunch of things that you could do
There is a huge difference between landing on concrete, water, and a hay bale. The softer and voluminous the material, the better chance it has of decelerating you over a longer period of time and hence ensuring you survive the landing. Mythbusters has actually run a test that hitting water at high speeds would be like hitting concrete. Luckily, hitting water is better... but still a very risky business. So if your human can aim for something soft, something that will break or injur him but not kill him outright, they will have a better change (e.g. Going through a tin roof into a hay bale could save them, because the tin roof absorbs some of the impact, and the hay absorbs a bit more. Your human is still however seriously injured).
In addition to this, landing on angled ground is a huge factor in surviving a fall. This way you can redirect some of your speed into a sideways motion instead of straight into the ground with no where to go. Landing on a slope or positioning yourself so you are moving horizontally can be a great way to disperse the impact force.
Landing posture is also an important part. Landing head first is a sure fire way to make sure you never survive. Landing on your legs however will ensure that your legs break first, dispersing some of that energy and giving you more time to decelerate as your body collapses on itself. Ideally you want to use your entire body as one huge spring to help redirect that energy and provide yourself more time to slow down. The aim would be to land on your heels (we are trying to come in at an angle here), bend your knees and assume a rolling motion like you see in those parkour videos. I'm at bit dubious as to weather you would be able to move fast enough (its just on the edge of believable) or if you would just snap your legs like twigs.
Moving onto the actual part where you are in the air. Your posture is going to be huge in determining your terminal velocity. Spread yourself out to catch as much of the air as you can to slow yourself down. You however don't want to spin out or lose control, otherwise you might have trouble performing positioning yourself or steering yourself towards a suitable landing spot.
The final thing left in your control is having a makeshift parachute. Similar to how a sugarglider uses its skin folds to help glide, you want to use anything you can get your hands on to help act as a parachute and decelerate you. A giant piece of card board (Again performed on Mythbusters) could easily be life saving. Outside of that, using your clothes, opening those buttoned T-shirts and taking your pants off to drag in the wind could all help slow you down just enough to survive.
The power he has to create portals also lets him create an energy field around his body that acts like a full-body airbag, causing the deceleration to begin slightly before he impacts a surface, decreasing the deceleration to a survivable level.
Alternatively, his power might simply make him momentarily invulnerable during decelerations once he reaches a certain speed, so that when he hits the ground, he doesn’t break- the ground does, and he walks out of the resulting crater unharmed.
As FlyingLemmingSoup mentioned above, stretching out the time for falling is probably the best way to go if you can't reverse the direction of the portal.
One solution relies on clear-headedness while falling, really good timing, and the ability to rapidly make new portals.
If you're in an area without anything to cushion your fall, use portals to turn the ground into a makeshift cushion. Start by getting into a good landing posture, wait until just after your feet hit the ground and you've slowed slightly, but haven't injured yourself yet (this is where timing is important), then open a portal directly below you that leads to your right and about a foot up. Repeat this process, bleeding off a little bit of speed every time, until you come to a safe stop.
If you want it to look cool(er), but take more concentration, reduce the vertical distance that the portal moves, and horizontally move back and forth between the same few location. It will look a little "glitchy" from the outside, but you'll effectively slow yourself.
A variation of the above that relies more on personal strength/agility, rather than portal-making ability, is to place a portal on the ground and another one slightly above it, with no need to move them until you've stopped. This requires the portal edges to not be sharp. You'll also want to be wearing gloves.
When you pass through the ground portal, use your hands to lightly "push" against the ground on either edge of the portal, then immediately retract them (don't want to break your arms/wrists/fingers). Repeat this on every pass through the portal. As long as you exert more force on the ground than the additional speed you gain from falling you'll eventually stop. This is effectively like falling between two infinite ladders, using the rungs to slow your fall. Gloves are very important here (especially if the ground is rough) as the friction and force will do a number on the skin of your hands otherwise.