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CivilKit/Calculators/Hydraulics / weirs

hydraulics / weirs

Rectangular sharp-crested weir-flow calculator.

Estimate free-flow discharge from the crest width, upstream head, and an explicit discharge coefficient.

read the result

Head drives the discharge strongly.

For a rectangular sharp-crested weir under free-flow conditions, the discharge varies with the three-halves power of the upstream head. Keeping the coefficient visible prevents a nominal width and head from looking more precise than the hydraulic arrangement allows.

Governing relationship

The screen uses Q = (2/3)Cdb√(2g)H3/2, where Cd is the entered discharge coefficient, b is crest width, H is head above the crest, and g is 9.81 m/s2.

What still needs a full hydraulic design

  • This is a free-flow sharp-crested rectangular-weir screen. Submergence, end contractions, nappe aeration, approach velocity, crest thickness, and side-wall geometry are not solved.
  • Measure head at a location where the approach flow is representative and keep the head datum consistent with the coefficient basis.
  • Check the applicable laboratory relation, local standard, tailwater, debris, freeboard, and downstream energy conditions before using the result for a structure.