Flap gate losses

[MitchHeineman]

Discussed in #170

^{Originally posted by MitchHeineman April 16, 2024}
SWMM inherently applies the Armco equation for losses across flap gates associated with weirs and orifices. However, unless I'm not looking closely enough, it does not assign comparable losses to flap gates on pipes or outfalls. This seems inconsistent to me. While the user can apply a form loss coefficient to the pipe above an outfall or to a pipe with an internal flap gate, that's a slightly different equation.

[MitchHeineman]

A bit more information... The inconsistency appears to date to at least SWMM4 which includes the Armco equation for weirs (bound.for), but not orifices. The term was added to orifices in SWMM5 (link.c). I suspect the original addition in 4 was to meet a specific need, and no one has ever considered the issue of consistency across different types of tide gates, no less the various places one can represent a gate in SWMM (orifice, weir, pipe, outlet, outfall)

[dickinsonre]

It was not intended to be used at outfalls in SWMM4, as those were not deemed the same as the flap gates for the weirs. I probably added the outfall flap gate so you can blame me,

[cbuahin]

@dickinsonre , to clarify, are you saying the Armco would be inappropriate to apply to outfall flap gates?

[dickinsonre]

More research needs to be done about that - I do remember the outfall gates in Pittsburh so it might be good for those gates.

AI comment - The Armco flap gate equation was originally developed for specific conditions and gate types. To understand if it's appropriate for outfall flap gates, we need to examine the fundamental differences between the conditions these equations were developed for and the conditions at outfalls.
The Armco equation was developed for irrigation gates under controlled conditions. It assumes:

A constant upstream head
Clean water conditions
Standardized gate dimensions and materials
Specific mounting configurations
Controlled downstream conditions

Outfall flap gates, however, operate in a very different environment:

They experience varying upstream heads due to storm events
They deal with debris-laden water and sediment
They come in various sizes and designs
They often experience tidal or river backwater effects
They may be submerged or partially submerged
They frequently encounter turbulent flow conditions

These differences can significantly affect the discharge coefficient and the overall hydraulic behavior of the gate. The Armco equation might not capture these real-world complexities accurately, potentially leading to either over- or under-estimation of flow rates.
A more appropriate approach for outfall flap gates would be to:

Use manufacturer-specific discharge coefficients when available
Consider using more general orifice equations with modified coefficients
Account for tailwater effects explicitly
Include safety factors to compensate for debris and maintenance conditions