Headwall property sheet

Headwall property sheet

Headwalls can be used in cases where an open channel enters a pipe and overflows can occur over the top of the headwall.

The inputs to the property sheet for a headwall consist of its name (of up to 10 characters), an entrance loss factor and an overflow level.

A K factor representing the entrance loss is required. This is multiplied by the downstream velocity head in the same way as for a pit pressure change. It can include losses due to bends and grates over pipe entrances and exits.

There is no need to define the downstream pipe in the Headwall property sheet, as this will be detailed in the pipe's property sheet. Assuming that an overflow can occur, an overflow level (in m AHD) must be specified.

Entrance loss factors, K (entry), are detailed in the following table intended for culverts:

Circular Concrete Pipes

K Value

Projecting from fill, socket end

0.2

Projecting from fill, square cut end

0.5

Headwall or headwall and wingwalls

Socket end of pipe

Square edge

Rounded (radius = 1/12 D)

 

0.2

0.5

0.2

Mitred to conform to fill slope

0.7

End-section conforming to fill slope (Standard precast)

0.5

Bevelled edges, 33.7 or 45 degree bevels

0.2

Side-tapered or slope-tapered inlets

0.2

Pipe, or Pipe-Arch Corrugated Steel

 

Projecting from fill

0.9

Headwall or headwall and wingwalls, square edge

0.5

Mitred to conform to fill slope

0.7

End-section conforming to fill slope (Standard prefab)

0.5

Bevelled edges, 33.7 or 45 degree bevels

0.25

Side-tapered or slope-tapered inlets

0.2

Box, Reinforced Concrete

 

Headwall

Square-edged on 3 edges

Rounded on 3 edges to radius of 1/12 barrel dimension, or bevelled edges on 3 sides

 

0.5

0.2

Wingwalls at 30 to 75 degrees to barrel

Square-edges at crown

Crown edge rounded to radius of 1/12 barrel dimension,

or bevelled top edge

 

0.4

0.2

Wingwalls at 10 to 25 degrees to barrel

Square-edges at crown

 

0.5

Wingwalls parallel (extension of sides)

Square-edged at crown

 

0.7

Side-tapered or slope-tapered inlet

0.2

Projecting

Square edge (estimated)

Bevelled edges, 33.7 or 45 degree bevels

 

0.7

0.2

(Source: AUSTROADS, 1994)

Additional losses can be added to these to allow for bends, grates over entrances or exits or other features that cause head losses along the pipe.

It will also be necessary to define an overflow route from the headwall, which will have the same form as that for a detention basin. The weir characteristics or elevation-discharge relationship nominated will define the headwater levels when overflows occur along the route.


References:
  1. AUSTROADS (1994) Waterway Design Manual, Sydney
  2. Boyd, M.B. (1986) Head-Discharge Relations for Culverts, Monier Rocla Technical Journal
  3. Henderson, F.M. (1966) Open Channel Flow, Macmillan, New York


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