HomeMy WebLinkAbout5E, Drainage Report for Townhouse Villages North and Townhouse Villages at Arden Hills�
EN HILLS
Request for Council Action
Prepared By: Terry Maurer,
Public Works Director Council Meeting Date: November 14, 2011
Drainage Report for Townhouse Villages North and Townhouse Villages at Arden Hills
Budgeted Amount: Actual Amount: Funding Source:
N/A N/A N/A
Recommendation:
Accept the fnal Drainage Report prepared by Bolton and Menk dated October 21 for Townhouse
Villages North and Townhouse Villages at Arden Hills.
Supqoi'tinE Documents:
L Memorandum £rom Terry Maurer, Public Works Director, dated November 14, 2011.
2. Copy of final Drainage Report prepared by Bolton and Menk.
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EN HILLS
MEMORANDUM
DATE: November 14, 2011
TO: Honorable Mayor and City Councilmembers
Patrick Klaers, City Administrator �
� �
FROM: Terry Maurer, Public Works Director
SUBJECT: Drainage Report for Townhouse Villages North and
Townhouse Villages at Arden Hills
Background
On Saturday, July 16, 2011, the City experienced a large rainfall event which caused
flooding in several areas. The flooding caused property damage to some of the
townhomes on Arden View Court and Arden View Drive.
Because of the drainage issue in this area, staff engaged Bolton and Menk to review the
drainage system to attempt to identify potential improvements. Representatives of Bolton
and Menk were in attendance at the October 10, 2011, Work Session to present their
initial findings. At that meeting they were asked to look at one additional option; taking
a portion of the townhome drainage area to the north to the Old City Hall site in an effort
to increase storm water capacity in the southern townhome area.
Attached is a copy of the final Drainage Report prepared by Bolton and Menk. You will
note that when the additional option of taking drainage to the north was analyzed it did
not significantly reduce the flood' elevation in the southern townhome area. This
indicates that the limiting point in the existing storm sewer pipe system is downstream
where all the flow from both the north and south areas combine. Bolton and Menk did
analyze increasing this pipe (Option 6) and found 825 feet of existing 27 inch storm
sewer pipe would have to be increased to at least 33 inch capacity in order to have a
significant effect on the flood level in the southern townhome area. �
Staff believes the key finding in the study is stated in the last paragraph on the first page
under Existing Condition Analysis. That paragraph states:
City Council Meeting
P:�,Admin\Council�Agendas & Packet Information�2011\l 1-14-11 Regular�Packet
Information\TownhouseDrainageReport. doc
Page 1 of 2
"The model results show that the existing drainage system has the capacity to handle a
10-year storm event without surcharging. The results also showed that the existing
drainage system has the capacity to handle a 100-year storm event with minimal,
localized flooding at the low points, which would not cause flooding onto the adjacent
homes,"
Basically, this indicates that the existing storm sewer system has the appropriate capacity
- to handle up to a 100-year storm with no property damage. This is the same design
criteria that would be used today in reviewing a private development. As was discussed
at the October Work Session, what is missing is an overland emergency outlet should an
inlet be plugged or a greater than 100-year storm occur. An overland emergency outlet
would be difficult if not impossible to create at this time.
I will be attending the southerly townhome Association meeting on November 22, 2011,
to discuss the findings in the final report. After meeting with the Association Board I
will report back to the City Council. ,
Council Action Requested
At this time the only action requested from the City Council is to accept the final
Drainage Report for Townhouse Villages North and Townhouse Villages at Arden Hills
as prepared by Bolton and Menk and dated October 2011.
City Council Meeting
P:�Admin\Council�Agendas & Packet Information�2011\11-14-11 Regular�Packet
Information\TownhouseDrainageReport.doc
Page 2 of 2
DRAINAGE REPORT
for
Townhouse Villages North and Townhouse
Villages at Arden Hills
October 2011
City of
Arden Hills, MN
BMI PROJECT NO. T16.103924
a
CERTIFICATION SHEET
I hereby certifythat this report w.as prepared by me or under my direct supervision, arid�that; I; am
a dul"y Licerised:Professional Engineer under the laws of the State of Minnesota:
,
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� Lanol L. Leichty
Date: November 3, 201'1
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Lic. Na 20846
Introduchon
The Townhouse Villages North and Townhouse Villages at Arden Hills experienced a significant
flood event in July of 20l l. This incident caused street, yard and home flooding, along with
damage to some vehicles. Bolton & Menk, Inc. was retained to investigate the storm sewer
drainage system to determine if feasible solutions could be made to the system that would provide
relief from future heavy rainfall events.
BackSround
The watershed area that drains into the storm sewer system analyzed consists of approximately 46
acres (see Figure 1). Land use in the watershed consists of single family residential dwellings
(townhomes), open space and some natural woodland.
The storm sewer system contains reinforced concrete pipe (RCP) ranging in size between 12" and
27" in diameter. The drainage outlet for the main storm sewer system empties into in a pond
located near the southwest corner of the property.
On July 18, 2011 the Minnesota Climatology Working Group website reports that the general
area near the Townhouse Villages received 5.45 inches of rainfall. Localized flooding was
reported at two road low points and adjacent townhouses. These two areas are identified as the
"northern low point" and the "southern low point" on Figure 1. To complicate matters, the 2011
Pavement Management Program was being constructed in this area when the storm event
occurred. As a result, the Contractor had inlet protection installed in the street catch basins to
catch sediment from discharging into the downstream water bodies, but also restrict the flow of
water into the storm sewer castings.
Methodology
Using Autodesk� Storm and Sanitary Analysis (SSA), the drainage area was modeled to evaluate
impacts associated with various storm events.
A theoretical 100-year, 24-hour storm event was used to model the options that were looked at.
The 100-year event is defined as that event likely to occur on the average once every 100-years.
This is not to say that a 100-year rainfall cannot occur in consecutive years or even twice in the
same year. In reality, the 100-year storm has a 1% chance of occurring in any given year. The
100-year, 24-hour storm is defined as a 5.9-inch event for Ramsey County.
Existing Condition Analysis
In July of 2011, the two land-locked low points in the development flooded due to what appeared
to be an approximately 100-year storm event.
The model results show that the existing drainage system has the capacity to handle a 10-year
storm event without surcharging. The results also showed that the e�sting drainage system has
the capacity to handle a 100-year storm event with minimal, localized flooding at the low points,
which would not cause flooding into adjacent homes.
When the model was run with a 100-year, 24-hour storm event, assuming no restrictions on the
inlets or pipes, the high water levels (HWL) for each low point was:
Northern Low Point (on Arden View Drive) = 973.35
Southern Low Point (rectangular grate east of Arden View Ct) = 965.15
There is no emergency overflow to provide relief if the catch basin grates or storm sewers are
plugged, or if water in the pipe surcharges and comes back out of the catch basins. When either
one of these conditions occur there is potential that the low points will flood.
Water will not escape overland from the north low point until it reaches an elevation of
approximately 980. At this point, from reviewing the survey, if water reached an elevation of
976.7, it would' enter the window sills of the adjacent townhomes.
Water will not escape overland from the south low point until it reaches an elevation of
appro�cimately 968.5. At this point, the survey shows that if water reached an elevation of 966.6,
it would enter the garages/doors of the adjacent townhomes adjacent to this low point.
Outions
An on-site field visit and a review of eacisting topography maps and construction drawings were
used to develop several drainage options. The storm sewer map for this project area shows that
appro�mately 75% of the northern portion of the site flows into the southwest corner pond
through a single run of storm sewer pipe.
The improvement options considered were based upon the location of where the flooding
occurred and what appeared to be potential solutions based upon a visual review of the existing
drainage system.
The following options were modeled and their associated costs calculated:
Option 1 (Figure 2)
This option involves building a berm azound the southwest corner of Pond 2 and reconstructing
the.pond outlet. The existing overflow for Pond 2 is in the southwest corner where an existing
pathway is located: Building a berm in this area and replacing the existing outlet with a new
structure would allow additional storage (pond bounce) to occur, while at the same time limit
the pond discharge to the downstream storm sewer system.
Modeling Option 1 showed that this action would have an insigni�cant impact on either the
nortt� or south low points. The north low point HWL was 973.26 (-0.09') and the south low
point HWL was 965.08 (-0.07') with this altemative. The estimated cost for Option 1 is shown
in Table 1.
Option 2 (Figure 3)
This option would provide an emergency overflow pipe for the low point area located to the
southeast of the Arden View Court cul-de-sac. A pipe or a swale could be constructed that
would extend from the driveways between units 4364 Arden View Court and 4324 Arden View
Court. For the purposes of this analysis a pipe was considered, since this would likely be less
disruptive to the existing landscape. To maximize pipe flow potential and limit head build up, a
24" arch pipe was assumed for cost estimating purposes.
This was an option originally considered in the preliminary analysis. Once the survey data was
obtained it was recognized that this may not be a viable alternative. Because the invert of an
emergency overflow pipe at the east end of the driveway would be higher than the low garage
floor elevations, this option would not serve a practical purpose. Modeling Option 2 showed
that this action would have no impact on either the north or south low points. The estimated
cost for Option 2 is shown in Table 1.
Option 3 (Figure 4)
Option 3 would serve to isolate the low point catch basins on Arden View Court from the main
line storm sewer. A 24" storm sewer bypass pipe would be constructed between units 4401 and
4402 of Arden View Court.
Currently, the entire flow upstream of the low point catch basins in Arden View Court must
make a 90-degree turn at the catch basin to continue downstream. Given the momentum and
velocity of a 24" pipe flowing full, it is likely that the storm sewer pipe and grate in the
driveway to the east of this low point is surcharged and backs up during heavy rainfall events,
as occurred in July of 2011. It was thought that a proposed bypass pipe would help to relieve
flow pressure from the low point in Arden View Court and likely reduce the flooding potential
in this area. To minimize disruption to the existing landscape in this area, a new pipe was
considered to be jacked.
Modeling Option 3 showed insigniiicant improvement to either the north or south low points.
The HWL at the north low point was 973.33 (-0.02') and the HWL at the south low point was
964.98 (-0.17' ). The estimated cost for this improvement is shown in Table 1.
Option 4 (Figure 5)
This option involves constructing a new outlet from Pond 2 towards the west, which would
outlet near Pond 5. A new outlet pipe from Pond 2 would reduce the outflow from the
downstream storm sewer conveyance system. This would provide extra carrying capacity of
the existing system, thus allowing for greater storm events to be handled before surcharging
occurred. A directionally drilled pipe from Pond 2 towards Pond 5 would limit the overland
disruption.
Modeling Option 4 showed no significant improvements at either low point. The HWL at the
north point was 973.29 (-0.06') and the HWL at the south low point was 965.09 (-0.06'). The
estimated cost for this improvement is shown in Table 1.
Option 5 (Fieure �
This option focused on reducing the flooding in the southern low point and is similar to Option
2. The difference is that a new storm sewer outlet was simulated being constructed near the
existing rectangular driveway grate, which is east of the south low point on Arden View Court.
This pipe system was routed toward the southeast pond (Pond 3), where it would outlet. This
scenario placed a new grate near the e�cisUng low point, which could start taking water
immediately once runoff from the catchment occurred and divert some of this flow to the
southeast.
This scenario was modeled simulating the July, 2011 storm, with the existing low point catch
basins in this area essentially plugged. Constructing a new catch basin structure and a 24"
outlet pipe resulted in a HWL of 966.4 at the southern low point. The existing low garage
elevation is 966.63. This option would protect the low townhomes adjacent to the low point
and prevent flooding from a 100-year storm event, or an event similar to that experienced in
July, 2011. The estimated cost for Option 6 is shown in Table 1.
An alternative that was investigated briefly involved routing the northern low point toward the
nor[h of this development, into the old City Hall site property. The object was to see what
impact this would have on the southern low point, by completely removing the northem
catchment from the existing system. The result was a negligible lowering of the southern low
point HWI,. Since this alternative would require watershed approval to reroute drainage
patterns and that it would not provide an emergency overflow for the southern low point, no
further action was taken.
Option 6 (Figure 7)
This option looked at what improvements to the existing storm sewer system would be
necessary to keep the pipes downstream of the southem low point from surcharging during a
100-year event.
Approximately 865-feet of storm sewer pipe would need to be removed and replaced from the
southern low point to the last manhole, prior to outletting into Pond 4. Extensive removal of
existing driveways would be required, along with removal of recent improvements that
occurred in 2011 Pavement Management Program. Most of the existing storm sewer pipe
would need to be increased several pipe sizes in diameter. Two of the largest stretches of new
pipe would need to be upsized from 27" to 33" diameter pipe. The estimated cost for Option 6
is shown in Table 1.
Estimated Costs of Options
Proposed construction costs for each option are summarized in the table below. These cost
estimates are based upon public construction cost information. Estimated construction costs
presented in the following table include a 20 percent contingency factor.
Table 1- Cost Summar .
O tion 1- Berm & New Outlet from Pond 2 $17,400
O tion 2— Overflow Pi e from SE Low Point $20,300
O tion 3— Diversion Pi e $75,100
O tion 4— New Outlet Pi e from Pond 2 $59,000
O tion 5— New Outlet Pi from SE Low Point $68,900
O tion 6— Increased Storm Sewer Pi e Size $100,800
Recommendations
The most cost effective solution would be Option 5. This option would provide an alternative
route for storm water to discharge from the southern isolated low point, without relying on the
existing storm sewer pipe system to carry runoff away from this area of concern.
The 'existing�, storm drainage`;system in this' townhome development, should be inspected on a
re'gular bas.is to ensure :that there is no' blockage of the grates or in the storm . sewer pipe that
would cause water fo backup during storm events. '
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There is_' no emergency� overflow° adjacent to the two ;low� points atudied: Storin events greater
than the 100-year design storm'will continue to pose a flooding threat `to adjacent homes;:.even if
the.proposed recommendation is.constructed. ' �
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