At the last CAC meeting on 13Feb I asked the following question that has not appeared on the draft summary from PI.

Ie: During a storm event, when a street is flooded, how much water can flow thru a pick hole in a manhole cover?  What technology exists to prevent this?  What is the City currently doing about this?

Please provide an engineered calculation based on a 1 hour and 5 hour storm event assuming a manhole cover is completely submerged for this time period.

From the city of Kenosha Wisconsin study...

where water is allowed to pond over the manhole during heavy

rain or snow melt-off events. The results of the experiment were

very enlightening to say the least. With only 1-inch of water sitting

on top of a manhole cover, approximately 27 gallons per minute

(gpm) will flow through the manhole cover. Increase this depth to

10 inches and more than 60 gpm will flow through the manhole

cover. This discovery is quite alarming given the large number of

open-pick manholes that can be found throughout the sanitary

sewer collection system—many of them located in low-lying areas

of the roadways.

The Engineering Division decided to find out what difference it

would make, if any, if they were somehow able to plug the pick

hole of the manhole cover. They decided to use expandable foam

to accomplish this feat, as expandable foam is very affordable

and extremely easy to apply. The foam was sprayed into the pick

hole and allowed to harden. As soon as the foam had hardened

completely, the same experiment as before was repeated to see if

the foam plug would hold up under the pressure of the water, and

if there was any reduction in the amount of water flowing through

the manhole cover. This time the results were quite different.

With as much as a foot of water sitting over the manhole, only 4

gpm made its way through the manhole cover. When the depth of

water was reduced to 2-inches or less, only 1.7 gpm was observed

flowing through the manhole cover. These findings may lead to a

quick, interim fix for many of the manholes located in low-lying

areas of roadways. The permanent solution will be to replace all

the open-pick covers with solid covers and to replace the manhole

cover frames with frames that are less prone to leaking. This will

require a considerable amount of time and effort to accomplish.

Until then, it may be possible to temporarily plug these manhole

covers.

Michelle--were your neighbors trying to clear access to the storm drains? or to actual manholes? 

Mark--thank you for your research on the pick holes in manhole covers. It's very interesting and-- certainly with all the flooding in the streets today--very timely. I was helping a friend clean out his driveway today at the bottom of Chaucer Court and noticed there was a manhole cover nearby with 2 open holes in it. Of course,  a lot of water was flowing down these holes. If this is a sanitary sewer manhole then there is going to be a significant amount of stormwater entering this pipe. 

A follow-up question is: Has Ann Arbor filled any of their pickholes on manhole covers? Or replaced the covers with solid ones?

Frank,  I do remember the question regarding manhole cover pick holes.  I will insert it into the summary with a response.

Thank you Mr. Fleetham.  This is an example of why "meeting minutes" tend to be very beneficial to a project in lieu of meeting summaries that are issued many weeks after the attended meeting.

Can OHM provide the requested calculations to corroborate the data provided by Mr. Wagner?

There was a study done by Wayne County 5 years or more ago where they flood tested several manholes to attempt to measure the potential inflow when the manholes submerge under the Rouge River. These were manholes that I believe were bolted done with a mastic seal around the rim without pick holes.  I seem to recall that they found that when the seal was not seated properly, it was pretty amazing how much water would come in.  I seem to recall it being in the same range of the values Mark is describing. I will see if I can find some excerpts from that study.

Take care,

Robert

Can the Upper Mallet Study, or the Calibration Study, or the FDD study, or the FDD CAC or the TOAG all chip in and fund our own test here in Ann Arbor?

Maybe we could request some funding from the Art Bucket from Sewer Projects...

Those from Ann Arbor will understand the reference.

See attached spreadsheet using the orifice equation to compute flow rates through pick holes of various sizes, varying standing water head and hole count.  Greg said that most pick holes are either 3/4" or 1" in diameter, and most manholes have 2 pick holes.  I used these for the computations, but you can vary the assumptions in the spreadsheet.

I was not able to reproduce the Kenosha values with the orifice equation with only 2 pick holes.  When I increase the number of holes to 7 or 12 holes I can start to match the values.  Believe it or not, some utilities have perforated covers on their sanitary manholes.  Mark - does the references study discuss the type of covers being studied? Were they manholes with 2 pick holes, or more hole on a perforated cover?  Do they describe the size of the holes? Did they make any other changes to the MH cover, such as improving the seating of the cover on the rim?

Another item that Greg offered was that he thought that most of the City's pick holes were filled with a rubber stopper, and that the City had a program of plugging the pick holes. He was not 100% certain.  He thought that Troy Baughman would now. 

Lori - could you ask this question of the City and add it to the Q&A log?  Thanks.

Take care,

Robert

link to entire Kenosha article

This is Miller, yesterday.  This is of course, after the road reconstruction further west.  While we are the Sanitary Sewer Wet Weather CAC we are flooded with details of the relationship of our concern - sanitary sewer - with storm water [Wet Weather].

Here is an MLive article yesterday by Ryan Stanton.  Thank you Ryan.  http://www.mlive.com/news/ann-arbor/index.ssf/2014/02/ann_arbor_adopts_new_green_str.html#incart_riv…

RE: Manholes, here's info from the report on "alternate options"  Certainly looks like something we should be considering.

Thanks for more examples of this working.

1.      Scio Township, Michigan – Population 20,000.  Scio Township performed a pilot manhole rehabilitation program in 2006 that involved 48 manholes.  The manholes were located in a low-lying wetland area in which the manholes would regularly become inundated with stormwater and the manhole rims would become submerged by several feet. The manholes were rehabilitated by raising the rim of the manhole by several feet to be above the water level during storms and water proofing the manholes. Flow evaluations were performed using pre- and post-rehabilitation flow data.  The evaluation showed that approximately 75% of the peak flows were removed. This high rate of peak flow removal was considered unusual for manhole rehabilitation, and it was suspected that it was the result of the condition and location of the manholes in a wetland area. A paper on the rehabilitation and flow evaluation was published in 2008 and is available. 

Attached is the report of the flood test performed by Wayne County in 2008.  The report concludes that MHs in the floodplain that get submerged under water are prone to river inflow.  The ranges tested were 1 and 2 foot of submergence, which results in 22 and 36 gpm inflow.

Take care,

Robert

I added a computation to the MH pick hole flow spreadsheet (attached) to examine the number of pick holes that would match the Wayne County results.  The analysis result is that the equivalent cross section area of opening from two (2)  1-inch pick holes or three (3) 3/4-inch pick holes would generate the flows from the Wayne County test. 

Take care,

Robert

Attached photo shows water flowing into manhole labelled "sewer" near my home. It was not raining (maybe mist) at the time. Manhole is not near the bottom of the hill, so water from yesterday's melting does not pool here, but flows down hill across manhole. Some was flowing in - more than a trickle, but not pouring in rapidly, either (WAG maybe 1 gpm). I did not try to find any manholes at the bottom of the hill under snow, ice, or water.  If we can get 2 gpm  savings in heavy rain by plugging selected manholes, this seems worthwhile and low cost, even if it does not cure the problem by itself. Although, at our tech briefing, Nick mentioned corrosive gases that need to vent from the system.

By the way, water flowing down the street today is not entering the manhole depicted.

Michelle had 2 links, one article about the good work that had been done and one video showing flooding, on Miller.  Are these 2 different sections of road or the same?  

Thanks for getting back to me.  The article does talk about improvements on Miller also, the picture of rain gardens is Miller.  I don't know if all of the same things were done on Miller (as pictured on Madison), but improvements were made and the flooding is still occurring, per the video...... That's why I wondered if it was the same section of road, that work had been done OR if it was a section, where work had NOT been done.  Hopefully it's not the new improved section..... Miller is a very long road, not just a couple of blocks.  So it's probably a different section.  

The video is not the same section of Miller that was just reconstructed last year. The section shown in the video is near Miller & Chapin.  This area is in the flood plain, and is a known area of flooding issues. Also, I hear that inlets blocked with snow and ice were either causing or exacerbating the problem there.

As far as Miller and Madison, some similar things were done on each of the projects. But each project is different - different street characteristics, different soils, etc., so the treatments that are designed for handling stormwater are different for each.

Thanks !

Michelle asked this question at the beginning of this thread: Does the City dedicate any resources to going out and doing this themselves in big water events?  It is ice now, but there are also leaves, etc.  If not, why not?

[CITY]

A. Yes, when a large storm event is forecasted, city field crews will visit a number of key locations throughout the city to ensure the inlets are clear of debris.  Unfortunately, with over 15,000 inlets located citywide, not all locations can be visited.  

Judy asked this question: Has Ann Arbor filled any of their pickholes on manhole covers? Or replaced the covers with solid ones?

[CITY]

A. The City has sealed manholes in the past in areas of known street flooding issues, and is continually reviewing and implementing sealing measures, where possible. This is something that can be considered as part of the alternatives analysis.

Thank you Lori.  To the City:  "has sealed manholes in the past in areas of known street flooding issues".  I will check that "floodplain" area myself, at the dip in Miller near Chapin. 

"Continually reviewing and implementing sealing measures".  Is a record kept by the department that reviews and implements?  Thanks.

In response to Michelle's question, "Is a record kept by the department that reviews and implements?" 

Thanks Lori

There are ten (10) manhole covers labeled SEWER on Miller where the traintracks pass over - the site of the flooding, stormwater overrun, shown previously.  Each of those manhole covers have two (2) pickholes.  None of those pickholes are sealed.  All are open.