The Orifice models flow through an orifice, short culvert, flood relief arch, outfall or inverted syphon using either the equations for weir control or surcharged flow depending on the upstream and downstream water levels.
Field in Data Entry Form 
Description 
Name in Datafile 
Upstream 
Upstream node label 
Label1 
Downstream 
Downstream node label 
Label2 
Throat Invert Level 
invert level of culvert (m AD) 
zinv 
Throat Soffit Level 
soffit level of culvert (m AD) 
zsoff 
Bore Area 
bore area of culvert (m2) 
Area 
Upstream Sill Level 
level of sill on upstream side of structure (m AD) 
zcup 
Downstream Sill Level 
level of sill on downstream side of structure (m AD) 
zcdn 
Opening type 
'FLAPPED'  does not allow reverse flows: 'OPEN  allows bidirectional flow 

Discharge coefficient: Weir Flow 
Discharge coefficient for weir flow over sill (typical value=1) 
Cweir 
Discharge coefficient: Surcharged Flow 
Discharge coefficient for surcharged flow (typical value=1) 
Cfull 
Modular Limit 
Modular limit (eg 0.8) 
m 
The Orifice models flow through an orifice, short culvert, flood relief arch, outfall or inverted syphon using either the equations for weir control or surcharged flow depending on the upstream and downstream water levels.
Flood relief arches in the approach roads of bridges, short culverts under causeways, outfalls through longitudinal river embankments and inverted syphons are modelled using variants of the orifice equations and/or the broad crested weir equation.
Five possible modes of flow are considered, including the cases where there is no flow. In the case where both upstream and downstream water levels are below the sill level, indeterminacy may occur  if there is another structure upstream with a closed gate, the direct method will show the stage calculated as 9999 to indicate indeterminacy.
The culvert or orifice cross section is assumed to be rectangular.
Reverse flow can also be modelled in unsteady mode and pseudo timestepping steady mode but not currently for the direct steady method.
Condition 
y1  zcup < 0 
Equation 
Q = 0 
Condition 
For flapped gates: y2  zcdn > y1  zcdn For inverted syphons: 2/3(y1zinv) < h and (y2zinv) < h 
Equation 
Q = 0 
Condition 
2/3(y1  zinv) < h (y2  zinv) < h (y2  zcup) / (y1 zcup)) < m 
Equation 
Q =(2/3)1.5 Ög Cweir b (y1  zcup)1.5 where: b = breadth of culvert (normal to the flow) (m) (=Area/h) 
Condition 
2/3(y1  zinv) < h y2  zinv < h (y2  zcup) / (y1  zcup) > m 
Equation 
Q = (2/3)1.5 Ög Cweir b (y1  zcup)1.5 drownf where: drownf = Ö[(1  (y2  zcup)/(y1  zcup)) / (1  m)] or drownf = (1  (y2  zcup)/(y1  zcup))/ (0.3* (1  m)) if the first formula for drownf gives drownf<0.3 
Condition 
2/3(y1  zinv) > h or y2  zinv > h 
Equation 
Qori = 0.799 Ö(2g) Cfull Area (y1  zinv  hx)0.5 where: Area = bore area of culvert (m2) hx = max (0.8h, y2  zinv) or for inverted syphons: Q = min (Qsyph, Qori) where: Qsyph is defined as in Mode 2  Free Weir Flow Equation Qori is defined as in Mode 4  Orifice flow 
The transition from free to drowned weir flow is smooth because the modular limit remains fixed throughout the computation and is thus independent of the calculated upstream water depth.
Reverse flow is allowed when the Flapped field is set to Open but not when set to Flapped. Reverse flow is not allowed in the direct steady method of calculation.
For mode 4, the direct steady method always assumes the flow is governed by the orifice equation.
The second form of the drownf equation in mode 3 (drowned weir) flow is an approximation used to avoid an infinite derivative as the downstream and upstream levels equalise. It is a linearisation of the drowning function between drownf=0.3 and drownf=0.0.
Only the upstream sill level is used in determining whether flow is possible. Thus if the upstream sill is lower than the invert level of the culvert, flow is possible even when the upstream water level is below the invert. It is recommended that the sill levels should normally be at or above the invert level to avoid unexpected results.
Line 1  keyword:'ORIFICE', 'INVERTED SYPHON', 'OUTFALL' or 'FLOOD RELIEF ARCH'
Line 2  keyword: 'FLAPPED' or 'OPEN'
Line 3  Label1, Label2
Line 4  zinv, zsoff, Area, zcup, zcdn
Line 5  Cweir, Cfull, m