Interesting problem, here is a partial brain-dump of ideas to consider.

Check a range of inlet loss coefficients, junction loss coefficients, etc. to see if the HGL could conceivably be above the rim due to highly turbulent conditions, or if the pipes are submerged at both ends.  Have the pipe inlet conditions been field verified and matched to the coefficients used in the models?

Did the hydraulic models include accurate representation of all the losses, inlets, junctions, bends, etc?  If you crank up all the loss coefficients, and the models still don't show a problem, then maybe a TV survey is in order.

Could there be something stuck in one of the pipes?  

Another, less-likely, issue could be pipe venting.  If both ends of a pipe run are submerged, and the downstream water surface is less than the upstream pipe invert, and normal flow is less than pipefull, then the pipe may be starved for air (in the "midsection"), which could result in pulsing/oscillating flow, which can produce unexpected HGLs.  Hard to assess, but easy to fix.....

Your post implies that air pressure fluctuations could be the culprit - I've never heard of this & find it hard to believe so long as the lids have some kind of significant openings, e.g. even if a small hole for a lid puller/hook passes a lot of air!  (Hmm, I wonder if they whistle like a tea kettle just before they pop?) If the lids are solid with no openings then maybe a grated lid or supplemental venting would help.

Good luck!

Thanks for the input.  This is a fairly interesting problem.  We do have the system scheduled for a visual inspection through the offending section.  I really am hoping to see a blockage in the system, but from what I could visually inspect in the field, I don’t think that is the case.  We also have a field verified as built survey on the system so I believe the model’s physical input parameters are correct.

While modeling the system, I did bump up the junction losses until the system failed (K>4.0) but this seemed unreasonably high, especially since I calculated them conservatively at around K=2.5 (I used Christodoulou’s equation for plunging losses).  

The pipe venting issue somewhat goes hand in hand with the pressure oscillations issue (article “Pressure Oscillation Induced by Trapped Air in a Trunk-Shaft Sewer System, 2001).  I can’t seem to find any way to analyze this numerically.

Well, hopefully the inspection will unearth something interesting.

Jim Lee, P.E.
City of Tallahassee
Stormwater Division

I think what is happening is 'air pressure moving the manhole covers. The water is going down the pipe but the speed of the air leaving, venting, to make space for the water is blowing off the covers. If water is coming  out, then the water can't force the air out, so it is seeking the next best exit, the cover.  You may have to install two or three oversize manholes to allow for the venting. Try using some manhole covers that are really round catchbasin grate to see if it is just air.

Jim,
To what degree are the lids vented & what specific type/style of lids are being blown?

And, how much flow and what size pipes are we talking about here?

Even if the lids were replaced to pass more air flow, if the problem is that the pipes are "air-starved" as noted in my previous post, flow could be very unsteady.  This could present problems such as extreme vibrations (leading to increased maintenance, etc.) in addition to blowing lids.  Flow could be unsteady from siphon conditions forming then disrupting repeatedly; it is doubtful any hydraulic models take this into account, vented pipes are likely assumed in most models.

As noted before, a pipe could require venting if the following conditions are met:
1.  Downstream end submerged
2.  Upstream end submerged
3.  Downstream water surface elevation is less than the pipe crown at the upstream end (entrance).  

I recommend checking for this condition using a flow rate similar to what you've observed to cause problems, and somewhat conservative loss coefficients.

A solution could be to install a plate that partially obstructs the top several inches of the pipe at the entrance, and to add a vent pipe that feeds the air pocket that is thereby maintained at the top of the pipe.  This is the solution i've seen used effectively on continuous welded HDPE stormwater "tightlines" placed on steep slopes.


BLT

BLT,

Thanks for all the good suggestions.  The lids are normal 2’ Dia (195 lb) lids.  The pipes are 36” RCP and an approximate flow is 90 cfs (12.5 fps full flow velocity).  Unfortunately, I or the Streets and Drainage folks have not first hand witnessed the actual removal as it is happening.  But, we do have eyewitnesses that explain that water is flowing out of the manholes when the lids come off.  Also, I placed the lids back on myself after a large rainfall.

 I think you are on the right track on air starvation causing oscillation and irregular flow.  If it were air pressure only, then you wouldn’t expect to see water flowing out of the manhole.  The upstream inlet (headwall) is submerged during the event, and I would expect that at least three of the manholes have a submerged inlet condition.

Jim

Jim Lee, P.E.
City of Tallahassee
Stormwater Division

I did some analysis on a similar type system, although much larger.  Our analysis showed that due to a steep pipe slope, the pipe would not always flow full.  However, due to tailwater conditions caused by the outfall into a detention basin, the pipe seals and a hydraulic jump would occur at some location in the pipe upstream of the outlet.  When the jump forms, there would be a transient wave that was speculated could blow the pipe or pop manholes.  Our proposed solution was to provide oversized, grated inlets on all the manholes within some distance upstream of the outlet.