• Created by Unknown User (ooms), last modified on 31-01-2010

Flat V weirs are used to calculate the flow of a river or stream. Predominantly, Flat V weirs are used where the flow rates are low and river sections quite narrow.
Note: When the downstream level of water in the river should be taken into account for backwater correction, use the flatVWeirBackwater transformation instead of the flatVWeir transformation.

Input
  1. headLevel: is the upstream level of water in the river measured from the top of the crest at the bottom of the V.
  2. type: type can be 'simple' or 'crest_tapping'. With 'crest_tapping' the pressure tapping measurements are taken and used for the flow calculation.
Coefficient set
  1. pUpValue: is the distance in metres from the bottom of the river to the top of the crest.
  2. width: is the width if the weir crest in metres. Note that there can only be one crest at a Flat V weir.
  3. cSlope: is the slope of the "V" at the crest. The slope is the ratio of the horizontal distance over the vertical distance one metre (1m) expressed as a number.
  4. sSlope: is the side slope of the weir. Most Flat V weirs don't have side slope. The slope is the ratio of the horizontal distance over the vertical distance one metre (1m) expressed as a number. There are two different calculation methods for Flat V weirs that are identified by either having the s-slope equal to 9999, or an actual value.
Output
  1. discharge: discharge of the weir.
Description

Calculates discharge of a flat v weir. The flow calculations are done using measurements taken at the weir.
The following formula is used:

Parameters:

  • hr = reference level
  • P = height of weir or height of vertex above bottom
  • L = length of weir
  • b = width of weir opening of width of triangle
  • h1 = upstream water level
  • H1 = upstream energy level
  • H2 = downstream energy level
  • htr = height of triangle
  • B = width of channel
  • ha = height of opening

For h1 <= htr :

  • Q = Cg . Cd . m . H1^(5/2)
  • Cg = 4/5 sqrt(g)
  • Cd = 0.615 for m <= 15
  • Cd = 0.620 for 15 < m < 30
  • Cd = 0.625 for m >= 30
  • m = b/2htr

For h1 > htr :

  • Q = Cg . Cd . m . (H1^(5/2) - (h1 - htr)^(5/2))
  • Cd = 0.620 for m <= 15
  • Cd = 0.625 for 15 < m < 30
  • Cd = 0.630 for m >= 30

The modular limit is:

  • for h1 <= htr h2/h1 = 0.70
  • for h1 > htr h2/h1 = 0.75

If the modular limit is exceeded a missing value will be entered for the discharge.

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