Flow velocity of water

from Wikipedia, the free encyclopedia

In watercourses, the flow velocity is the average speed at which the water in a river moves through its bed . The flow velocities in rivers vary between 0.1 and 6 m / s. If a river leads to high water, the flow speed increases with the flow cross-section, i.e. the discharge disproportionately. With regard to the erosion effect and the destructive power of floods, it should be noted that the kinetic energy of a given amount of water increases as the square of the velocity.

Basics

The flow velocity varies over the cross section (flow profile). It generally increases due to the decreasing friction towards the middle of the stream and with increasing height above ground. In river loops, however, the streamline shifts from the center towards the outer arch. The discharge is the integral of the flow velocity over the cross section, see volume flow . The flow velocity is the discharge divided by the cross-sectional area.

The flow speed of a body of water as an open channel , but also in vadose caves, depends on the following factors:

  • Slope of the water . The greater the gradient, the greater the flow velocity.
  • Discontinuities (= jumps, see waterfall ). The water hits the ground or the surface of the water vertically and therefore cannot produce any directional horizontal movement. The height of fall is lost to the rest of the watercourse as a height difference. Therefore, flow, at the same height difference, flows with a uniform gradient faster than rivers with water cases, weirs or barrages .
  • Flow resistance of the bed. Are there many obstacles in a body of water such as B. rocks or structures (bridge piers), there is turbulence, which causes the water to lose kinetic energy. Therefore the speed is also lower here.

The situation is similar in closed channels (phreatic caves, underground sections), but completely different in groundwater or seepage sections . If the flow velocity is zero, we speak of dead water .

determination

The flow velocity is estimated by:

  • Direct measurement of the flow profile:
  • Measurement of the flow with a known cross-section:
If you let a body of water flow through a pipe or a weir sluice, you can measure the flow there. The flow velocity then results from the flow and the cross section of the water away from the measuring system, for example in the underwater area of ​​a barrage.
  • Calculation:
Empirical flow formulas take the relationship between flow profile, gradient and friction into account.

Individual evidence

  1. ↑ Hans Georg Wunderlich: Introduction to Geology, Volume I, Exogenous Dynamics , Bibliographisches Institut Mannheim / Vienna / Zurich, BI-Wissenschaftsverlag, Mannheim, 1968, p. 78