I realize there are a hundred variables that apply here but how deep can you go and “safely” cross a creek? Velocity of the water is probably most important. I’ve gone neck deep on the Florida Trail and knee deep on the Arizona Trail. Deepest I went on the PCT was knee deep near Muir Trail Ranch. Wear your shoes and use your poles?
bowlegs
The variables are what matters. A fast stream with a dangerous runout and a heavy backpack could be dangerous for a 100-pound out of shape person even if it’s only ankle deep. If the person is strong and smart and not carrying a pack with water that is placid, they can swim in 2-mile deep water and be safe.
If you mean a typical PCT thru doing a Sierra creek, in shape, 30-lb. pack, sure of themselves, i would say you’re probably hitting the danger zone when it’s about mid-thigh deep. With poles and somebody else around to help fish you out downstream, maybe waist-deep. Sierra streams in June are fast enough though that anything deeper might be a bad idea.
But really, too many variables to say…
markv
once it reaches crotch level, it starts to push on your whole body & can start to lift you off your feet in fast rapids even b4 that! On bear creek where trail crossed was ridiculous fast & deep; but just a few yds downstream it was only just above ankle deep next to a huge boulder that collected gravel around it in its downstreamside eddy!!!
gingerbreadman
You’re right about the speed. I believe the pressure that fluid exerts on a body is a function of the cube of the speed. So if Creek A is twice as fast as Creek B, Creek A’s force is 8X greater. If it’s three times as fast, it’s 27X greater.
I had to back off a knee-deep ford in steep ravine in a flash flood once, it was that fast.
Garlic
On the Florida Trail we used our poles all the time to check water depth ahead of us. Sometimes we could detour around a deep spot. Lots of standing water there so velocity was never a concern.
bowlegs
In the Sierra’s I’d agree with the analysis of markv and gingerbreadman. Another factor that is vital is the temperature of the water, which comes from snow melt. Within a few seconds of entering the water you loose feeling in the legs and they become hard to control or use. That, coupled by fast moving water and that awful pain in your bones when the feeling comes back is often enough to make your subconscious detour you from going to the crotch level ?.
magic
Garlic
The force of water is a function of the square of the speed.
Drag load (the force of the water on you) = 1/2 * Coefficient of drag * Density of water * speed ^2
Cd and density are constants (more or less) in this case.
The cube function with speed is for power output of a windmill or power required to drive a boat, for example.
Power required = Force * speed, with force being a function of speed ^2, that makes Power a function of the speed cubed.
Token Civilian
Thanks for the clarification on the equations. I’d based my post on power required for bicycling in the wind, not for standing still in a stream. Makes perfect sense and it’s good to know.
Garlic
This is interesting. So in the cycling example, if it takes 200 watts to pedal 15mph into a 5mph headwind, then everything else remaining constant, does it take the cube of 200 watts to pedal 15mph into a 10 mph headwind? In other words, if the speed of the wind doubles, and its force is thus 8 times greater, then does the counter force of the cyclist also have to increase eightfold in order to maintain the same forward speed?
blisterfree
Other forces on a bicycle like rolling friction, bearing friction, losses in the frame and tires, etc. probably remain nearly constant, so total power needed is not cubed–just the portion needed to fight the wind.
That’s one reason why the national speed limit of 55 mph was enacted during the 70’s oil embargo. Again, there are many more forces on a car with an internal combusion engine than wind, but it is noticeable.
Garlic
blisterfree,
Drag is the force exerted on an object having a relative velocity to the air/liquid. The force is a square of the velocity. The power needed to overcome this drag is a cube of the velocity. So in your first example, the velocity would be 20 mph, in your second it would be 25mph. So if it takes 200 watts to go 20 then it would take 391 watts to go the 25mph in your second example.
gg-man
Thanks guys. Knowledge is power, as they say. The more watts the better. Now if only there were a way to reduce friction in the synaptic connections and to vastly increase storage capacity.
blisterfre
When I’ve had difficulty with stream crossings it’s been due to suddenly finding myself in an unseen hole in the middle of the stream. Initial easy movement leads to overconfidence and/or inattention. Instead of the initial very slow exploratory pace, I tend to try to speed things up, and then slide or step into an unseen deep hole where all the force equations described above are instantly magnified. If the water is turbulent, you won’t be able to see the bottom and must proceed slowly by feel - all the way across.
booger
Try to time the crossing for early in the morning. Run off will be at the lowest point. Good advice I read on another forum.
ed