Weir / Flume Open-Channel Flow
Open-channel flow over a 90-degree V-notch or rectangular (Francis) weir in cfs, GPM.
Example
You enter
- Weir type vnotch90
- Head over crest H (ft) 0.5
You get
- Flow (cfs) 0.446
- Flow (GPM) 200.3
Checked against published worked examples from: USBR Water Measurement Manual (V-notch / Francis weirs).
Details, formula, and sources
ScopeOpen-channel flow over a 90-degree V-notch or rectangular (Francis) weir in cfs, GPM, and MGD from the head over the crest and an editable weir coefficient.
Formula or table90-degree V-notch: Q = 2.49*H^2.48. Rectangular Francis: Q = 3.33*(L-0.2H)*H^1.5 (contracted) or 3.33*L*H^1.5 (suppressed); with the approach width B and weir height P entered, Q = C L [(H + hv)^1.5 - hv^1.5], hv = V^2/(2g), V = Q/(B (P + H)), iterated. 1 cfs = 448.831 GPM.
Source and editionPer the USBR Water Measurement Manual (public domain) - V-notch and Francis rectangular-weir equations and Kindsvater-Carter / Francis coefficients, by name.
Free accessFree at usbr.gov/tsc/techreferences/mands/wmm; the user confirms the calibrated weir coefficient.
What governsEstimate. AHJ and licensed professional govern.
API field namesweir_type, head_ft, flow_cfs, flow_gpm
- Weir condition ventilated, sharp-crested, free-flow (fully contracted for the V-notch and contracted rectangular; end contractions suppressed for the suppressed rectangular)USBR Water Measurement Manual