Whitewater#Holes

As water roller is called a special, resulting in flowing waters such as streams and small rivers flow of water, which always results in particular in the mountains to life-threatening situations and even deaths. It comes at the water surface to the back flow. The water roller is a form of hydraulic jump (also hydraulic jump ), as occurs at high Froude numbers.

Background

Rolls differ in how strongly they hold objects or float, as well as in its outflow. A roller with "free drainage " is used when the back flowing (rolling ) of water flowing back to the side of the roller in the main flow. Such a roller usually has roughly the shape of a downstream -facing "V" s or arc: The sides of the roller turn downstream to. Objects or floats are therefore (downstream) washed out at the sides of the roll, and relatively quickly from the roll.

If an outflow but dammed, as in upstream -facing rolling pages or for example at the stilling basin of weirs, there occurs a back jammed hydraulic jump. Here is for swimmers of fatal injury: The backward movement of the water pulls a float upstream ( at weirs: the threshold down ). There, the rolling motion of the water pulls the float down; a short distance downstream of the float is as a rule driven back up to but right after that again moved back to upstream and under water (washing machine effect). On top of that cascading water - for example, at weirs - saturated with air, including leads in the beck similar range to a much lesser buoyancy; in the region of the lower buoyancy float and objects are drawn.

Even good swimmers can free itself under certain circumstances against the current no longer pull itself out of the rolling motion of the water, so that eventually threatens a drowning from exhaustion. Therefore, should a swimmer who was pulled into a roll, trying to catch his breath and dive so deep it goes down. There he can take advantage of the near-surface flow, leading out the water from the roller. Once the flow veers to the top, he can try sideways ( downstream ) to free themselves from the roller.

It should be noted that water rolls can also occur at low water very suddenly when greatly increased by heavy rains in the mountains, the amount of water flow in a short time.

Water

Suitable rolls are popular destination for whitewater kayakers (especially game boaters ) and Rafter to surf on them. When rolling with a greater return current free discharge is also important for paddlers. When a boat is held by a roller, it usually capsizes. If the boat is held longer, it often leads to repeated rotations that can capsize the boat alternately upright again. If canoeists get into a roll with a greater return current, they sometimes manage to get in that they stretch out the capsizing their hands with the paddle to the river bottom; when near the ground, leading out of the roller flow acquires the paddle, the kayaker can be pulled from the roller downstream. In rolls with stronger return flow remains only the possibility to leave the boat and diving to the ground to take advantage of this flow.