The horribly painful rash one gets after tripping and falling, or 'pavement surfing' as it's often called.
"I fell on my face and now I have gravel rash on my face, down my arm, down my leg and on my arse"
You trip, you hurtle towards the ground, you skid over 25 yards on your face, you're pavement surfing, dude.
person 1: Where'd you get the gravel rash
person 2: drank 352 double vodkas and surfed the pavement.