HomeMy WebLinkAboutCity Council Minutes 03-09-2004 SpecialAdopted Minutes
Special Council Meeting
March 9, 2004
Mayor Johnson called a Special Meeting of the Plymouth City Council to order at 5:10 p.m. in
the Public Safety Training Room, 3400 Plymouth Boulevard, on March 9, 2004.
COUNCIL PRESENT: Mayor Johnson, Councihnembers Hewitt, Slavik, Bildsoe, Neset, Black,
and Stein.
ABSENT: None.
STAFF PRESENT: City Manager Ahrens, Finance Director Hahn, Community Development
Director Hurlburt, Park and Recreation Director Blank, Public Works Director Faulkner, and
City Clerk Paulson.
Consider Remaining Flood Improvement Study Areas
Rocky Keehn, consultant with SEH, reported on their analysis for Site 2, which is in the
Greentree West neighborhood south of 30th Avenue and east of Alvarado Lane. He stated there
was flooding that occurred at Greentree Pond from the storm event on the evening of June 24,
2003, which produced over six inches of rainfall in less than a 12 -hour period. The City's design
storm for ponding areas is the 100 -year storm, which is a storm with a statistical chance of
happening in any one year of 1 %. He stated that they determined that two storm events should
be modeled for all alternatives. After a site visit and review of existing information collected, it
was determined that a detailed hydrologic model would be needed to complete the analysis of the
site. For the existing conditions model, the outlet was a 21" concrete storm sewer with a flared
end section and trash guard. Based on the as-builts, the normal elevation of the pond with this
outlet configuration was 994.30 feet. This was the outlet that was in place the night of June 24,
2003. He explained that due to the large amount of duck weed and other debris in the pond, the
trash guard had a history of partially plugging and thus could have contributed to higher than
normal water elevations in the pond. If the pond elevation is higher than the planned normal
elevation and has a partially plugged outlet, there is a higher probability of increased high water
elevations during extreme flood events. Because of these concerns, the outlet has since been
replaced with a skimmer outlet structure that will prevent the outlet from being plugged with
duckweed and other floatable materials. He stated that once the existing condition model was
created and calibrated, the model was revised, with the new skimmer outlet configuration, at an
outflow elevation of 994.22 feet.
He stated the second point of calibration was to compare the high water elevations observed by
the residents during the June storm event and those simulated in the model for this same event.
He explained during the June storm event, the resident at 2940 Alvarado Lane indicated that
there were three inches of water in the basement, and the pond elevation was about one foot over
the top of the culvert. The lower floor of the house is 996.78 feet, and when three inches of
water in the basement is added to this elevation, the pond high water would be near 997.0 feet.
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Special Meeting of January 31, 2004
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However, the final water elevation in the basement may not indicate the actual high water in the
pond since it could have built-up on the backdoor before it flowed into the basement.
He explained the observed information indicates that the pond reached an elevation between
997.0 and 997.5 feet during the June storm event. Since the purpose of the study is to compare
alternatives and the relative impact they have on reduction of assumed high water elevations, the
high water elevation calculated in the model for the June storm event would be the assumed base
existing high water elevation (997.6 feet). He stated since the home at 2940 Alvarado Lane had
a basement elevation that was below the assumed 100 -year event high water of 997.0 feet,
additional information was collected that included the Certificate of Survey on file with the City
dated June 1977. This document indicates the basement floor was to be constructed at an
elevation of 998.17 feet. Survey information collected by the City in December 2003 indicated
the home has a walkout elevation or lowest floor opening of 996.78 which is about 1.39 feet
below the elevation on the Certificate of Survey.
He stated the resident at 2940 Alvarado Lane inquired about the impact to the area north of
County Road 24, known as Churchill Farms, on Greentree Pond. He stated they analyzed
Greentree Pond with Churchill Farms in a developed and undeveloped condition to address this
question. The computer model indicated the elevation of Greentree Pond for the 100 -year event
is 997.1 feet with Churchill Farms in a developed condition as it is currently. With Churchill
Farms in an undeveloped condition, the pond elevation produced by the model is 997.0. He
stated they concluded that the development of Churchill Farms has little impact on the elevation
of Greentree Pond due to adequate detention storage in Churchill Farm's storm water ponds. He
discussed alternatives to correct outlet changes, pond excavation, home flood proofing, and
reducing or diverting flows upstream which varied in cost from $135,500 to $438,600.
Richard Manthe, 3010 Alvarado Lane, stated the flooding didn't impact his property. He stated
David Goodrie, at 2940 Alvarado Lane, has discussed with him the increasing water elevations.
He stated one of the alternatives just mentioned was the flood proofing of the homes, and he's
unsure if any of the other homeowners were notified of this option.
Public Works Director Faulkner noted that as with all developments, the Churchill developer was
required to determine before and after runoff. He noted that there could be additional runoff
from County Road 24 due to it being a hard surfaced road that has been widened with paved
shoulders. He stated what they have discovered is that this area is conforming to the City's
overall drainage plan. Mr. Keehn agreed that the 100 -year high water elevation of 997 is correct.
From a modeling perspective, it matched existing and proposed conditions.
Councilmember Black asked if any of the neighbors observed any blockage of the structure
during the night of the June storm. Public Works Director Faulkner replied that no one observed
it during the event, but it had previously plugged rather easily. However, the structure has been
replaced, and there is significantly less likelihood that it would occur again. In addition, the
model assumed free flow conditions which was the more conservative approach and produced
results closely conforming to what was actually observed by the residents.
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Special Meeting of January 31, 2004
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Mayor Johnson stated there are no funds for any of the alternatives identified in the staff report.
She asked that with the replacement of the structure, is everything functioning correctly, other
than with the elevations of the home. She indicated that she would like to have a conversation
with Mr. Goodrie, whose home was flooded. She questioned if this situation is the responsibility
of the City when it could be the lower level elevations of homes that are the problem.
Councilmember Bildsoe stated it wasn't the Council's intention to make a decision this evening,
and it's difficult to determine the correct solution. He stated that the Churchill Farm
development has had some water issues with their main pond, but not to the extent of the
Greenhill neighborhood. He also stated drainage from County Road 24 could be contributing to
the problem.
Councilmember Black stated she doesn't recall a situation where the City has done such
extensive work to correct a flooding problem other than general maintenance items to keep the
system in operation. She too stated that she's unsure where the funds would come from for any
of the alternatives. She stated if the drainage system meets the design specifications, she doesn't
feel that the flooding issue is the City's problem.
Councilmember Bildsoe asked if there are any incremental improvements that could be made to
move water downstream over time? Public Works Director Faulkner replied the analysis
indicates that the drainage system is functioning correctly. Mr. Keehn added that Mr. Goodrie
indicated that the high water elevation was at 997.1 feet, and that is where it should be for the
design event. He then explained how a 100 -year flood is determined.
Councihnember Slavik stated the costs of the alternatives concern her. She stated many of the
homes weren't constructed to the Certificate of Survey elevation; therefore, she believes that the
liability belongs to the developer. She suggested that staff have further discussion with Mr.
Goodrie.
Councilmember Neset agreed with Councilmember Slavik.
Councilmember Bildsoe asked if staff changed the elevation of the outlet. Public Works
Director Faulkner replied only about 1/10. Due to the downstream storm sewer elevations, the
outlet cannot be lowered further without lowering the entire system.
Mr. Keehn stated that he doesn't believe that on the night of the flooding, that the blockage of
the trash guard at the pond was the issue.
Mayor Johnson stated that the Ward and At Large Councilmembers as well as herself would be
meeting with the property owners and directed staff to schedule this meeting.
Next, Mr. Keehn discussed the flood analysis report for Site 8, which is the lot at the southwest
corner of Teakwood Lane and Sunset Trail (145 Teakwood Lane). He stated they determined
that two storm events should be modeled for all alternatives. He stated they modeled both the
100 -year, 24-hour storm event and the storm that occurred in June 2003 to determine high water
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Special Meeting of January 31, 2004
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elevations. He stated the back or west side of 145 Teakwood Lane includes a lower level door
from the basement that includes a walkout well. He noted there is also a full sized window north
of the walkout well. The walkout well is a block or concrete structure with a bottom elevation
near the door opening of 972.72 feet. He stated the model was set up with a direct drainage area
of 8.20 acres plus the 0.36 acres on the south side of Sunset Trail that was assumed to be the
contributing area to the low area. The result for the June storm event indicated the flood
elevation would be 971.5 feet. This is considerable less than what was observed for the June
storm event (975.6 feet). The 100 -year storm high water elevation was predicted to be 975.8 feet
which would be closer to the elevations observed during the June storm event and would support
the potential for flooding given the right hydrologic circumstances.
He stated since the model did not produce the observed flood elevation of 975.6 feet for the June
storm event, three variations of the model were considered to create a calibrated model that
reproduces the 975.6 feet elevation. Because of the location of the outlet in a low area and the
potential for debris collecting in the area, it was first assumed that the outlet was partially
blocked during the storm and thus the flow from the site was greatly reduced. The blocked outlet
concept would also reproduce an event in which the downstream outlet pipe capacity is limited
and thus again the flow restricted from the ponding. If the outlet is 74% blocked, the high water
elevation in the model is 975.6 feet.
He stated the second scenario assumed that more than six inches of rain fell in this location
which would increase the possibility of flooding. City staff did have some reports from private
residents in the southern portion of the City who indicated that 10 inches of rain fell during this
event. The model was modified and with 9.2 inches of rainfall and with a direct drainage area of
8.56 acres, the high water would be 975.6 feet.
He discussed the final scenario to consider which could be that all the potential area contributes
runoff to the low area. Since the event in June 2003 which was of very high intensity, it is
possible, as the resident has indicated, that all the drainage area described (8.56 acres of direct
area plus 1.89 acres of area on the north side of sunset Trail and 3.8 acres of area to the south)
would flow the low area When all these areas are added into the model, the high water was
estimated to be 975.6 feet, which matches the observed elevation for the June storm event.
He stated there are two critical elevations that need to be checked. The first is the elevation of
the walkout well around the back entrance stairs to the house at 145 Teakwood Lane. It was
assumed that the critical elevation would be 975 feet with no changes to the house. The other
critical elevation is the house to the south of 145 Teakwood Lane located at 125 Teakwood Lane.
Based on the survey, the elevation at the northwest corner of 125 Teakwood Lane is 976.57 feet
and at the northeast corner 976.71 feet. The actual elevation of the garage was not included in
the survey; however, based on a site visit, it appears that the elevation of the northeast corner is
very close to the elevation of the garage opening since the driveway pavement wraps around the
northeast corner of the house. The assumed critical elevation for 125 Teakwood Lane would be
the lower and thus more conservative elevation of 976.6 feet. He then discussed alternatives.
The first alternative would be the construction of a new storm sewer pipe across Teakwood Lane
northeast to Cavanaugh Lake. It's assiuned that the existing storm sewer would continue to
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Special Meeting of January 31, 2004
Page 5 of 7
operate, and the new pipe would be sized for only the additional flow needed to prevent flooding.
The cost for this alternative would range between $42,300 and $48,800 depending on which size
of storm sewer pipe is used.
He stated the second alternative would be to flood proof the home. The estimated cost for this
alternative would be $30,000.
He stated the third alternative would be a derivation of the flood proofing alternative in which
the yard would be filed except for a smaller low area on the south side of the lot. This option
would cost $94,800, which would include $30,000 for flood proofing around the house.
He discussed the fourth alternative which is identical to the third alternative except the street is
lowered to provide more protection for the house to the south and to also eliminate the outlet
from the low area by draining the area directly to the street once it is lowered. The cost of this
option would be $110,700, which would include $30,000 for flood proofing around the house.
Paul Josephson, 145 Teakwood Lane, stated three of the alternatives don't address the flooding
on their property. He stated their backyard isn't meant to be a retention pond. He stated if the
current pipe would've been designed to handle the 100 -year, 24-hour storm, the majority of
debris in June wouldn't have been there. He stated the first alternative would solve their
problems. He noted that the home was built around 1961, and previous owner to the property
adjacent to theirs indicated that the walkout was original.
Mayor Johnson asked about the drainage around the home. Public Works Director stated there is
a drain at the walkout level, which outlets near the low point of the existing storm sewer pipe on
the west side of Teakwood Lane.
Jan Josephson, 145 Teakwood Lane, stated they are looking at a one-way check valve solution to
prevent water from backing up into their drain pipe when storm water ponds in their yard. She
explained in order for any backpressure to occur, there would need to be two feet above the tile.
Councilmember Black stated that it seems to her that their backyard has always been a low area.
Ms. Josephson stated that the street was constructed after the house was built, which created that
condition.
Public Works Director Faulkner stated that hasn't been confirmed. He stated according to the
original aerial photographs, there was no home or road in April 1960. Mr. Keehn noted that
there was a bump out which appears to be some type of a road where Teakwood now exists.
Councihnember Black asked what the elevation is of the existing pipe. Mr. Keehn replied 969.5
feet is the flow line elevation.
Ms. Josephson stated that their woodpile was washed over there, and that was at a 970 foot
elevation.
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Special Meeting of January 31, 2004
Page 6 of 7
Mayor Johnson stated it appears that the first alternative is the solution.
Mr. Keehn stated that typically you don't design for a 100 -year event, but rather a 10 -year event.
He stated this isn't a normal situation, and it wouldn't be unusual for some type of ponding.
Public Works Director Faulkner noted there is no ponding easement in the Josephson's yard.
Mayor Johnson suggested that the Council tries to find a solution, which may be that the
Josephsons may have to tolerate some type of ponding. She stated there should be mechanisms
in place with the City's system to assure that there wouldn't be any flooding.
Councilmember Black stated perhaps there should be some kind of easement in place, especially
with new property owners.
Public Works Director Faulkner asked the Josephsons if they would be willing to allow the City
to obtain some kind of easement from them. Ms. Josephson replied that they would need some
time to consider it. She added that they need a way to drain out the property at the walkout. Mr.
Josephson stated they could add a check valve.
Public Works Director Faulkner stated that staff would work with the property owners to
determine a solution along with the ward Councilmember.
The Dorseys, 2615 Jewel Lane, who were in attendance, asked about the status of the flood
analysis for their property (Site 9).
Mr. Keehn gave a brief update on Site 9. He stated the property should be surveyed later this
week, and they would survey all backyards and check elevations of the drainage system. They
would also be looking at size of pipe and additional storage in the rear yards. It appears that a
pipe solution would be the best alternative.
Public Works Director Faulkner stated the Surface Water Task Force would be providing a
recommendation to the Council regarding funding at the proposed Joint Meeting with the
Council on May 18.
Set Future Study Sessions
Motion was made by Councilmember Neset, and seconded by Councilmember Hewitt, to
schedule a Special Meeting on March 23 at 5:00 P.m. for the Purpose of reviewing the Public
Safety Building/City Hall expansion and the Ice Center expansion, and to add the water
treatment Plants' renovation & expansion project to the agenda for the Special Meeting on March
16 at 6:30 p.m. With all members voting in favor, the motion carried.
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Special Meeting of January 31, 2004
Page 7 of 7
Motion was made by Councilmember Black, and seconded by Councilmember Hewitt, to
schedule a Joint Meeting with the Surface Water Task Force on May 18 at 6:30 p.m. With all
members voting in favor, the motion carried.
Councilmember Hewitt suggested that the Council meet with the area County Commissioners in
the near future to discuss the County roads in the City.
Motion was made by Councilmember Hewitt, and seconded by Councilmember Bildsoe, to
schedule a meeting with the area County Commissioners on May 17 at 6:00 p.m. With all
members voting in favor, the motion carried.
Adjournment
Mayor Johnson adjourned the meeting at 6:40 p.
andra R. Paulson, City Clerk