--- title: "Siphon Spillway | Technical Reference" slug: "siphon-spillway" description: "Find the data, theory and guidance for the siphon spillway: a short discharge conduit above the hydraulic grade line. " tags: ["1D river modelling", "weir", "siphon spillway", "conduit"] updated: 2026-10-16T18:25:48Z published: 2026-10-16T18:25:48Z canonical: "help.floodmodeller.com/siphon-spillway" stale: true --- > ## Documentation Index > Fetch the complete documentation index at: https://help.floodmodeller.com/llms.txt > Use this file to discover all available pages before exploring further. # Siphon Spillway This unit models flow through a self-priming siphon spillway. A siphon is essentially a short discharge conduit located above the hydraulic grade line. The existence of sub-atmospheric pressure allows water to be sucked up above the upstream free surface level before it is discharged at a lower level downstream. ## Data

Field in Data Entry Form

Description

Name in Datafile

Node 1

Upstream node label

Label1

Node 2

Downstream node label

Label2

Crest Level

level of weir crest (mAD)

Zc

Soffit Level

level of soffit of hood (mAD)

zsoff

Bore Area

bore area of syphon (m2)

Area

Max Hood Level

maximum hood level (for weir flow over hood) (mAD)

zmax

Weir Discharge Coefficient

Discharge coefficient for weir flow over crest

CWeir

Pipe Discharge Coefficient

Discharge coefficient for pipe or blackwater flow

Cfull

Modular Limit

Modular limit

m

Prime Level

level on the upstream side at which the syphon becomes fully primed (m AD)

zprime

 ## Theory and Guidance The Siphon Spillway unit models flow through a self-priming siphon spillway. A siphon is essentially a short discharge conduit located above the hydraulic grade line. The existence of sub-atmospheric pressure allows water to be sucked up above the upstream free surface level before it is discharged at a lower level downstream. Several types of flow are possible through a siphon spillway. If the upstream water level is below the soffit of the inlet but above the invert, gravity spillway flow occurs for which the equation for a broad crested weir is used. Once the inlet is submerged there is a transitional flow regime as the siphon becomes primed. This is modelled as a weighted average between the limiting cases of weir flow and pipe (or blackwater) flow to ensure continuity of flow. When the upstream water level exceeds the user defined primary level, pipe flow prevails. When the upstream level exceeds the maximum hood level, the flow is supplemented by spillage over the hood which is also modelled by the broad crested weir equation. ## Equations ### Mode 1 - Dry sill | Condition | y1 - zc < 0 | | --- | --- | | Equation | Q = 0 | ### Mode 2 - Free weir flow

Condition

y1 - Zc > 0

y1 £ Zsoff

( (y1 - Zc) / (y2 -Zc) ) > m

Equation

Q = 0.544 Cweir b Ög (y1 - zc)1.5

(1)

where:

b = breadth of siphon (normal to the flow) (m)

![RiverNodesimagesSiphon1.gif](https://cdn.document360.io/5074bc26-a9ef-4bf4-b1de-a5c6ce628aca/Images/Documentation/RiverNodesimagesSiphon1.gif?sv=2026-02-06&spr=https&st=2026-11-10T22%3A52%3A16Z&se=2026-11-10T23%3A52%3A16Z&sr=c&sp=r&sig=faKJWHJ9FHKKGFZDxJ9ob1TKZhgC5y3xzJ59Dl7G9m4%3D) Figure 1: Free Weir Flow (Mode 2) ### Mode 3 - Drowned weir flow

Condition

y1 - Zc> 0

y1 £ zsoff

( (y1 - Zc) / (y2 -Zc) ) £ m

Equation

Q = 0.544 Cweir b Ög (y1 - Zc)1.5 drownf

(2)

where:

drownf = (1 - (y2 - Zc) / (y1 - Zc)) / (1 - m)

 ### Mode 4 - Transitional flow regime

Condition

y1 > zsoff

y1 < zprime

Equation

Q = (y1 - Zc) (Qblack - Qweir) / (zprime - zsoff) + Qweir

(3)

where:

zprime = level on the upstream side at which the siphon becomes fully primed (mAD)

Qblack = 0.799 Cfull Area Ö(2g) (zprime - y2)0.5

Qweir is defined in equation (2)

 ### Mode 5 - Pipe or blackwater flow

Condition

y1 > zprime

y1 < zmax

Equation

Qblack = 0.799 Cfull Area Ö (2g) (y1 - y2)0.5

(4)

 ![RiverNodesimagesSiphon2.gif](https://cdn.document360.io/5074bc26-a9ef-4bf4-b1de-a5c6ce628aca/Images/Documentation/RiverNodesimagesSiphon2.gif?sv=2026-02-06&spr=https&st=2026-11-10T22%3A52%3A16Z&se=2026-11-10T23%3A52%3A16Z&sr=c&sp=r&sig=faKJWHJ9FHKKGFZDxJ9ob1TKZhgC5y3xzJ59Dl7G9m4%3D) Figure 2: Pipe or Blackwater Flow (Mode 5) ### Mode 6 - Free flow over hood and through siphon

Condition

y1 > zmax

( (y1 - zmax) / (y2 - zmax) ) > m

Equation

Q = Qblack + 0.544 Cweir b Ög (y1 - zmax)1.5

(5)

where:

Qblack is defined in equation (4)

 ### Mode 7 - Drowned flow over hood and through siphon

Condition

y1 > zmax

( (y1 - zmax) / (y2 - zmax) ) £ m

Equation

Q = Qblack + 0.544 Cweir b Ög (y1 - zmax)1.5 drownf

(6)

where:

drownf = (1 - (y2 - zmax) / (y1 - zmax)) / (1 - m)

Qblack is defined in equation (4)

 ## General Reverse flows are precluded for syphonic weirs in all run modes - they should never occur for any siphon operating as intended. Flood Modeller will stop running a simulation and produce an error message if this is predicted to occur. Priming usually occurs when the upstream water level has risen to not more than about one third of the throat height, but the precise level will depend on the particular siphon design. As the upstream water level reaches the priming level a sudden increase in flow is observed in practice. This increase in flow is modelled as a more gradual process in this unit by the assumption of a weighted linear average of weir and blackwater flow. ## Datafile Format Line 1 - keyword 'SYPHON' Line 2 - Label1, Label2 Line 3 - zc, zsoff, Area, zmax Line 4 - CWeir, Cfull, m, zprime ### Example ```plaintext SYPHON UNIT031     UNIT032     1.000     2.000     2.000     3.000     0.900     0.900     0.900     0.500 ```