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

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

Mode 2 - Free weir flow

Figure 1: Free Weir Flow (Mode 2)

Mode 3 - Drowned weir flow

Mode 4 - Transitional flow regime

Mode 5 - Pipe or blackwater flow

Figure 2: Pipe or Blackwater Flow (Mode 5)

 Mode 6 - Free flow over hood and through siphon

Mode 7 - Drowned flow over hood and through siphon

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 - z_c, z_soff, Area, z_max

Line 4 - C_Weir, C_full, m, z_prime

Example

SYPHON
UNIT031     UNIT032
     1.000     2.000     2.000     3.000
     0.900     0.900     0.900     0.500