HomeMy WebLinkAbout2002-10-02 OPH Memo to CC Re 9th Edition Lift Station Emergency Overflow Ci o
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Park Heights
14168 57 Street N.• Box 2007 • Oak Park Heights, MN 55082 • Phone (651) 439 -4439 • Fax 439 -0574
Interoffice Memo
To: Mayor and City Council Via Friday Packets October 4, 2002
From: Public Works Director, Jay Johnson, PE
City Engineer, Dennis Postler, PE
CC: City Administrator, Building Official
Date: 10/02/02
Re: 9th Edition Lift Station Emergency Overflow
Conclusion: The construction of an overflow is technically feasible and would cost
approximately $5,500. The elevation of the manhole in the intersection of 53 Street
and Obrien Avenue is approximately 1.4 feet higher than the basement elevation at
14245 53 Street and this home does have an automatic backflow preventor that is
not 100% effective. Mayor Beaudet was provided with the attached email that has the
elevation data and some possible solutions to the homeowner. The homeowners are
working with their plumber to determine a course of action to prevent future sewer
backups.
Background: The City Council requested that the City Engineer and Public Works
Director review the feasibility of lowering the wet well cover of the 9 Addition sewer
lift station and divert lift station failure overflow directly to the pond south of the lift
station. The overflow was to be low enough so that the sewage does not backup in
14245 53 Street residence. Currently, during a lift station failure the sewage backs
up within the system. A backflow preventor in the basement of 14245 53 Street is
partially effective and sewage enters their basement. The sewage continues to rise in
the sewer system until the manhole located in the intersection of 53 Street and
Obrien Avenue overflows. From the sewer manhole the sewage flows into the storm
sewer and then into the pond.
To reduce the wet well manhole to below the basement elevation of the home the
cone section would have to be replaced and the bubbler system relocated. A Tess
costly option would be to lower manhole 1A located to the north of the wet well to 4
inches below the basement level by removing all of the adjusting rings. A ditch built
from this manhole to the pond would be up to 3 feet deep and almost 20 feet wide
(assuming a 3:1 side slope) where it crosses the berm around the pond. During heavy
rain events the pond could then backup and cover sewer manhole 1A. The City
Engineer estimates the cost for this modification to be $5,500.
The homes at 14201 and 14085 53 Street may have similar problems if in the future
they connect to City sewer. These were existing homes when the 9 Addition was
built. If they were to connect to the sewer system the houses would need to be
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equipped with a residential sewer lift station (a.k.a. sewage ejector) that lifts their
sewage to a level higher than the lift station overflow elevation. Staff will address a
letter to the present homeowners informing them of this future requirement, place a
copy of the letter in their property files and review our building permitting system to
determine if we can place a note within the system that will flag this as a requirement
when they come in for a permit.
The City council has received a report from our City Engineer which, addresses the
options for upgrading the lift station. Therefore, this memo only addresses the request
to evaluate the possibility of lowering the manhole.
If you have any questions please contact me.
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Jay Johnson
From: jjohnson [ jjohnson @cityofoakparkheights.com]
Sent: Thursday, September 19, 2002 12:09 AM
To: 'David Beaudet; 'Dennis Postler; 'Kim Kamper'
Subject: Quick elevation Circuit
Mark and I ran a quick elevation circuit this evening. We could not get into their house since they left just after we got
there. Their back porch is close to the same elevation as their basement floor. If we want to get any closer we will have to
get inside.The maximine difference is probably around 0.1 to 0.2 feet.
I ran a three station circuit starting and ending at the same spot on the porch. The closing error was 0.04 feet. We did
rush the work since it was starting to rain and one of the level shots was over 500 feet on the return leg which is a long
shot for the City's level. A closing error of 0.04 is acceptable for our purposes.
assigned the back porch at the center of the door as an elevation of 100.00 Feet.
Porch at center of back door 100'
Manhole 10 in front of the house was at 102.62'
Manhole 1 at the intersection of 53 and Obrien was 101.40' (The house starts to backup 1.4 feet before the manhole.
My previous estimate was 1.2 feet.)
Manhole 1A was at 101.54'. This manhole has 22 1/2 inches of rings.
The wet well was at 102.01' The wet well has 13 inches of rings.
The high spot between the wet well and the pond was at 102.26'
If the objective is to keep the sewage 6 inches below the basement level (99.5 feet) to provide a safety margin:
The wet well would need to be lowered 30 inches but only has 13 inches of rings.
Man hole 1A would be required to be lowered 24.5 inches but it only has 22.5 inches of rings. We could accomplish 4
inches below the patio at MH 1A if all the rings are removed and the ditch dug to the pond.
Assuming the ditch starts at 99.67' (4 inches below the patio) at the high spot the ditch would be 2.59 feet deep plus the
slope of the ditch from manhole 1A to the high spot. The slope could add between 0.2 and 0.4 feet. Worst case the ditch
is 3 feet deep. To make the ditch appearance acceptable the side slopes would need to be around 5 to 1 (20 %). Each
side would be 15 feet wide. The ditch would be 31 feet wide (1 foot flat bottom). At 3 to 1 the ditch would be 19 feet wide. I
have sketched a possible ditch that is only 20 feet wide and will fax it to Dennis tomorrow.
Since the level survey that we performed did not start from a known elevation and the difference in elevations between
man holes that we measured do not match those on the record drawing from 1988 I am going to assume that MH 1 is the
closest to the record elevation from 1988 (14 years of freeze thaw on the adjusting rings may have cause some changes
and MH1A and the wet well may have been adjusted.). Also I am going to assume that the record elevation of the flared
end entering the pond is still correct. With these two assumptions I get the following:
Back porch elevation at center of the back door of 905.29 (previously assumed to be 905.48 (2 1/4 inches difference)
A Lowered MH1A elevation 904.96
Top of berm elevation of 907.55
Bottom of 3 foot deep ditch at top of berm 904.55.
Current water elevation of 903.66 (measured 40 inches of water to the flare)
This is 1.86 feet above normal water level and has been at these levels most of the summer.
Current freeboard if the ditch was installed today would be 0.89 feet (10.7 inches)
A large rain now could flow out the ditch. Since the bottom of the curb is reported on the record plans as 906.09 the back
of the curb is going to be higher than 906.42. Water will start coming out of the storm water catch basin in the curb before
it would overflow from the ditch to the street. MH1A would be underwater but could be fitted with a water proof casting.
To modify the wet well cover to 904.96 would require the removal of the cone. Installing a flat cover. Re plumbing the
bubbler system and having a steeper ditch beside the dry well. The 2.5+ foot deep ditch adjacent to the dry well would
become a safety concern and may restrict access to the dry well during an emergency.
Before the regulators will allow this solution I suspect that we will be required to have made as many engineering changes
as feasible. An argument that could be used is that this is merely an improvement to an emergency overflow that currently
makes the sewage flow into a public street.
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3 other options that could be consiWd. •
Install a sewer pump at the house:
We (the City and its former employees) allowed a home to be built which was below the lift station emergency overflow
without installing a sewer pump. The house should have been equipped with a residential sewer lift station that all it had to
do is raise the sewage up 5 feet and then discharge to the sewer. The cost would have been small when building the
home. It may be considerably more now. The advantage is that the only overflow that would occur would be their own
waste water during power outages and they used the toilet. /with the 5/8 water meter they have they probably could not
get more than 25 gallons per minute of water flow. This would require a $400- to $500 pump. Busting a hole in their floor
to install a large sump. And some piping and electrical.
Backflow valves. There is a $2000 backflow valve that is low maintenance and highly reliable. The sewage flows through
a rubber pipe that is surrounded by a steel pipe. It uses a sensor to detect the back -up then starts a small air pump that
pumps air into the steel pipe and pinches off the rubber pipe. When the pump operates it can also trigger an alarm so the
residents know not to use any water. It comes with a battery back -up. The more expensive side of the Peterson Valve is
the manhole that would need to be constructed to install the valve if it cannot be installed in their basement.
Another option that has not been discussed is to install a generator operated or even a battery operated submersible
(generator recharged) pump in MH1A that is operated by floats and connected to the pressure main. When the float
detects sewage backing up at 895 ( just above the high level alarm setting) in MH1A the float turns on the generator after a
short time delay the pump turns on and pumps the manhole down a few feet then shuts down. The generator would
continue to operate for a set period in case it is needed again. The emergency backup pump in MH1A would only need to
pump 150 gpm (just a little over the max flow). For this application the generator could be smaller than one needed to
operate the duplex lift station. It would operate independently of the lift station. We have had problems where a pump fails
to turn off and the lift station pumps become air locked and problems where someone flushes something that plugs the
pumps. A whole station generator cannot cure air locked pumps or cloth diapers wrapped up in the impellers. A whole
station generator would need to be powerful enough to operate both of the 200 gpm pumps and the control systems,
bubbler compressors, lights, blower, and dehumdifier. The hardest part is connecting to the pressure main that is buried.
It would not protect against a plugged pressure main which is one of the least likely failures.
Between this and a few other things it is now midnight and I hear thunder and heavy rain so I am now going to get out of
here while the getting is good.
Jay
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