Water roller
A roller of water is a special flow of water that occurs in flowing waters such as streams and small rivers , which, especially in mountains, repeatedly leads to life-threatening situations and deaths. This causes a backflow on the water surface. The water roller is a form of hydraulic jump (also called alternating jump ), as it occurs with high Froude numbers .
background
Rollers differ in how strongly they hold objects or swimmers, as well as in their drainage. A roller with "free drainage" is used when the (roller) water flowing back flows back into the main flow on the sides of the roller. Such a roller is usually roughly in the shape of a downstream “V” or arc: the sides of the roller bend downstream. Objects or swimmers will therefore be washed (downstream) to the sides of the roller and relatively soon out of the roller.
However, if the outflow is dammed up, as is the case with upstream roller sides or, for example, at the stilling basins of weirs , then a back-dammed alternating jump occurs. There is a risk to life for swimmers here: The backward movement of the water pulls a swimmer upstream (in the case of weirs: towards the threshold). There the rolling motion of the water pulls the float down; a short distance downstream, the swimmer is usually driven upwards again, but is immediately pulled back upstream and again under the water (washing machine effect). In addition, falling water - for example at weirs - becomes saturated with air , which leads to significantly less buoyancy in the basin-like area below ; swimmers and objects are drawn into the area of lower buoyancy.
Even good swimmers may no longer be able to free themselves against the current on their own from this rolling movement of the water, so that ultimately there is a risk of drowning from exhaustion. Therefore, a swimmer who has been pulled into a roller should try to take a breath and dive down as deep as possible. There he can use the current close to the ground that carries the water out of the roller. As soon as the current turns upwards, he can try to free himself sideways (downstream) from the roller.
It should be noted that water rolling can occur very suddenly even at low tide if the amount of flowing water increases sharply in a short time due to heavy rainfall in the mountains.
water sports
Large water rollers are a popular destination for play boaters and rafters in particular who surf on them . Free drainage is also important for paddlers on rollers with a stronger backflow. If a kayak is held on to the side by a small roller, you have to cant properly to avoid capsizing . This is not so easy with large rollers. If the boat is held for a long time, there are often repeated turns, which alternately capsize and right the boat.
literature
- Anita Rogacs, Cole Marr, Anizka Garcia: Drowning Machines Low-Head Dam Hydraulics and Hazard Remediation Options. 2005. ( PDF; 3.5 MB ( Memento from March 4, 2016 in the Internet Archive ))
- Reinhard Wacker (Red.): Safety in canoeing. (= Series of publications by the German Canoe Association 5). Volume 2. German Canoe Association - Wirtschafts-und-Verlags-GmbH, Duisburg 1986, ISBN 3-924580-13-8 .
- Gerhard H. Jirka, Cornelia Lang: Introduction to channel hydraulics. 2nd Edition. Universitätsverlag, Karlsruhe 2009, ISBN 978-3-86644-363-1 .
Individual evidence
- ↑ Death trap water roller: How to escape the deadly current on merkur-online.de, report on a laboratory demonstration in Kulisch.