Internal Drops
Dissipating energy where inlets enter well above the outlet.
Where an inlet enters a maintenance hole well above the outlet, flow falling through the structure causes turbulence, spray, odour release and damage to the benching. An internal or external drop introduces the flow at a controlled level instead.
Typical arrangements are an internal drop pipe, a backdrop with a stilling area, or an external drop structure. The choice depends on the height of fall, the pipe diameters and the available space.
Key design considerations
- Height of fall that triggers a drop, and the authority's preferred arrangement.
- Internal drop pipe with a smooth entry and a controlled outlet at the channel.
- Backdrop arrangement, with a stilling area and access for maintenance.
- Ventilation, since drops release hydrogen sulfide and odour at the structure.
- Material selection for the drop, given higher velocities and turbulence.
- Maintainability: drops readily block and must be accessible for cleaning.
Design checks
- Drop provided where the inlet fall exceeds the authority limit.
- Drop arrangement detailed with entry and outlet levels.
- Venting addressed where a significant fall is present.
Standards and authority references
- Authority standard drop details
- WSA 02 (current edition)
- Authority odour and corrosion guidance
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Trigger heights and preferred arrangements are authority specific.
- Drop hydraulics are not resolved by gravity pipe equations; the structure is selected from standard details.
This reference is general engineering information. Confirm requirements against WSA 02, the current Australian Standards and the local water authority's supplement before use in design, and obtain project-specific advice for the conditions at hand.