CR 2003-093 Acept Feasibility Study And Order Preparation Of Plans,Shady Oak Pond
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June 12,2003
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HOPKINS
Council Report 2003-093
Accept Feasibility Study and Order Preparation of Plans, Shady Oak Pond
Proposed Action
Staff recommends approval of the following motion: Accept Feasibilty Study, Shady Oak Pond
and direct staff to prepare plans to install temporary outlet to basin 514 City Proiect 02-06.
Overview
Shady Oak Pond currently does not have an outlet and as a result the surrounding street and
residential properties flood if the pond is not pumped out \;Vhen it gets near flood levels. In the
1997 Hopkins Flood Response Action Plan adopted by the Hopkins City Council, the Shady Oak
Pond area was identified as Area No.1 and is the last remaining project area in the action plan.
The first step in proceeding with the Shady Oak Pond Storm Sewer Improvement Project is
ordering a feasibility report. The Council authorized preparation of the feasibility study in May
of 2002 to look at the various options associated with alleviating the flooding problem. At that
time it was anticipated that this work would ultimately be constructed at approximately the same
time as the Shady Oak Road reconstruction project and other planned area roadway
reconstructions. Because the schedule for reconstruction of Shady Oak Road has been pushed
back by Hennepin County the outlet work may need to be implemented in phases.
Primary Issues To Consider
. Project Background
. Conclusions detailed in Feasibility Study
. Schedule for County Road 61 reconstruction
. Temporary or permanent outlet to basin 514
Supportine Documents
. Feasibility Report, May 2003
Recommendation
. Accept Feasibility Study
. Prepare plans for a temporary outlet to basin 514
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Jo derson
Assistant City Engineer
Financial Impact: $ 20,000 Budgeted: Y/N
Related Documents (ClP, ERP, etc.):
Notes:
Source: Storm Sewer Fund
City Council Report 2003-093, June 12, 2003 - Page 2
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Primary Issues To Consider
. Project Back2round
The lack of an outlet to Shady Oak Pond has resulted in many problems for Hopkins
residents over the years. The pond has been and continues to necessitate high
maintenance costs in order to keep the pond somewhat functional without an outlet. The
problems associated with this area are vast and include areas such as the intersection of
18th Avenue North and 1st Street South, which floods upstream of the pond during heavy
rains because the water has no downstream outlet after it reaches the pond. In addition to
looking at the outlet options, the potential for gaining additional storage capacity through
dredging will be looked at as an option. One problem that exists with many of the
potential solutions to the flooding problems this area faces is the fact that the existing
pond contains purple lustrife which is an undesirable species of plant that the DNR does
not want spread to potential downstream ponds. Coordination of the proposed outlet has
been ongoing with the DNR, Hennepin County, the City of Minnetonka, and Nine Mile
Creek Watershed District during the feasibility preparation.
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. Conclusions detailed in Feasibility Study
The Feasibility Report recommends a phased approach to providing the outlet for Shady Oak
Pond. The first phase includes construction of an outlet from Shady oak Pond crossing shady
oak road to the wetland basin number 514. Basin 514 is located on west side of Shady Oak
Road and south of Oak Drive Lane. This basin is located in the City of Minnetonka and
currently does not have an outlet. Minnetonka's Water Resource Management Plan
(MWRMP) includes a future outlet for this basin in conjunction with Shady Oak Road
reconstruction. With no outlet for basin 514 in place currently, Hopkins Shady Oak Pond
outlet is required to be limited to 6 inches in diameter in order to restrict the flow. The first
phase of this work as suggested in the feasibility report would be to construct a 21-inch
concrete pipe connecting the two basins but limit the outlet capacity to a 6-inch orifice until
the outlet is installed for basin 514. This connection would provide additional infiltration
area for the storm water collected in Shady Oak pond to enter the ground water. This would
help maintain a lower level in shady Oak pond but would not prevent flooding in larger
rainfall events.
The second phase recommended in the Feasibility Report includes construction of an outlet
that ties into the storm sewer system constructed with Shady Oak Road. This phase is
obviously very dependent on the schedule for the reconstruction of Shady Oak Road. At this
point staff anticipates construction of Shady Oak Road in 2006. After completion of the first
two phases Shady Oak Pond would have a 6 inch outlet to basin 514 and an 18 inch outlet to
the Shady Oak Road system.
City Council Report 2003-093, June 12, 2003 - Page 3
The third phase involves removing the 6 inch orifice and allowing the full 21 inch outlet to
discharge to basin 514. This phase would be implemented after Minnetonka has installed
their outlet for basin 514 which would most likely be in conjunction with the Shady Oak
Road project. The outlet for basin 514 in MWRMP is shown to flow to basin 512 in
Minnetonka and not directly to the Shady Oak Road system and therefore would not
necessarily be included in the Shady Oak Road project.
. Schedule for Shady Oak Road Reconstruction
Hennepin County currently shows reconstruction of Shady Oak Road in their Capital
Improvement Plan to occur in 2005. City Staff has been in contact with Hennepin County
staff and based on those conversations we expect construction not would begin until 2006 at
the earliest. As this project will require Planning, public involvement, and right of way
acquisition it is likely that construction may be even further out that 2006.
. Temporary or permanent outlet to Basin 514
Until the reconstruction of Shady Oak Road takes place the largest outlet operating will be
limited to 6 inches in diameter and that outlet will flow to a land locked basin, staff has
considered a variation to the phasing suggested in the feasibility study. The alternate phasing
would be as follows:
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Phase 1
Phase 2
Installation of a temporary 6 inch pipe to outlet Shady Oak Pond to basin 514
construction of 21-inch outlet to basin 514 with 6 inch orifice
Removal of temporary 6 inch outlet
Construction of 18 inch outlet to new Shady Oak Road storm sewer system
removal of 6 inch orifice when outlet of basin 514 is constructed
Phase 3
This approach would provide a number of benefits. The initial cost of installing the 6 inch
temporary pipe would be much less than installing a permanent 21 inch pipe. A temporary
pipe would allow for more flexibility as the construction plans develop for Shady Oak Road.
The cost of designing, bidding and installing the permanent pipe would be much less if it is
part of the shady oak road project rather than contracted as a separate project. The down side
of the temporary pipe is the cost of installation and removal.
Recomendations
Based on our review of the report and consideration of the alternatives staffs recommendation is
as follows:
. City council move to accept the Feasibility Report
. Direct Staff to prepare plans for the installation of a temporary 6 inch outlet
. Direct staff to continue working with Hennepin County in the development-of reconstruction
plans for Shady Oak Road
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FEASIBiliTY REPORT
Shady Oak Pond Outlet
Hopkins, Minnesota
SEH No. A-HOPKN0202.00
-May, 2003
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SHORT ELLIOTT HENDRICKSON INC
Mu Itid isciplined.
Single Source.
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3535 Vadnais Center Drive, St. Paul, MN 55110-5196
651.490.2000
651.490.2150 FAX
environmental
transportation
architecture
engineering
May 21, 2003
RE: City of Hopkins
Shady Oak Pond Feasibility Report
SEH No. A-HOPKN0202.00 14.,00
Mr. Steve Stadler, PE
Public Works Director/City Engineer I
City of Hopkins
1010 First Street South
, Hopkins, MN. 55343
Dear Mr. Stadler:
On behalf of Short Elliott Hendrickson Inc. we are 'pleased to submit the final Feasibility Report .for
Shady Oak Pond. The report covers possible outlet options for the pond and a solution for the
flooding at the intersection of 18th Avenue N. and 1st Street N. '
The recommended outlet option is a phasep. solution due to the tentative schedule of the full
implementation of the Min1?-etonka Water Resource Management Plan (MWRMP) and the
reconstruction of Shady Oak Road.
A. Phase I - Install 21-inch culvert crossing under Shady Oak Road to Minnetonka, with
6-inch orifice restriction. This provides a 25-year/24-hour level of protection.
B. ~ Phase II - Install outlet along Shady Oak Road in conjunction with Shady Oak Road
improvements. This provides a 75-year/24-hour level of protection.
C. Remove orifice at full MWRMP implementation. This provides a 100-year/24-hour "
level of protection.
The wetland on the west side of Shady Oak Road, where the proposed culvert discharges to
Minnetonka subwatershed 514, does not currently have an outlet. According to the MWRMP an
18-inch culvert is proposed to outlet the wetland. There is an overflow berm that is higher than
Shady Oak Road that will drain the wetland to the south towards - Excelsior Boulevard. 'Until
Minnetonka fully implements their Water Resource Management Plan and constructs the specified
outlets, there is still the potential for Shady_Oak Pond and Shady Oak Road to flood.
To alleviate the flooding at the intersection of 18th A venue N. and 1 st ~treet N., the existing storm
sewer system between 18th Avenue N. and Shady Oak Pond should be replaced with a 36-inch RC~
or equivalent storm s~wer pipe. Also, the inlet capacities of the storm sewer at 18th Avenue N. and
1st Street N. should be upgraded to increase the intake of overland flow at the intersection. This~ will
provide a 25-year/24-hour level of p~otection for the intersection.
Short Elliott Hendrickson Inc.
Your Trusted Resource
Equal Opportunity Employer
Mr. Steve Stadler
May 21. 2003May 12,2003
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Please feel free to contact us if you have any further questions. It was our pleasure to complete this
project for you.
Sincerel y,
SHORT ELLIOTT HENDRICKSON INC.
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David C.Halter, PE
Project Manager
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Beth Peterson, PE
Project Engineer
bcp
c: John Anderson, PE, City of Hopkins
Paul Hegre, PE, SEH
x:\tj\hopk.n\020200\word\report translTllttalltr.doc
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Feasibility Report
Shady Oak Pong Outlet
City of Hopkins
SEH NO: A-HOPKN0202.00
May 2003
I hereby certify that this report was prepared by me ~r under my direct
supervision, and that I am a duly Licensed Professional Engineer under the
laws of the State of Minnesota.
fJ~ /) }/~(f0 t}.
Date:
s-- cJol- Os
Lic. No.:
ReVieweW--O C ~(~
Short Elliott Hendrickson Inc.
3535 Vadnais Center Drive
, St. Paul, MN 55110-5196
651.490.2000
YJ3JJ,
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Date
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Table of Contents
!
Cover Letter
Certification Page
Table of Contents
1.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................1
1.1 CONCLUSIONS..................... ..:.................................................................................. ................ 1
1.2 RECOMMENDATIONS.......... ...1................................................................................................. 2
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2.0 -" INTR 0 D U CTI ON ...... ............~............ ..... ............ .................... ....~.......................................... 3
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2.1 - AUTIiORIZA TION ............ .......'.. .................:....................... ............. ........... ...... ~....... ................ 3
2.2 BACKGROUND....... ......... ..... ..:. ..................................... ................................... ....... ................. 3
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3.0 EX IS TINGe 0 ND ITI 0 NS....i....... ............ .............................................................................. 4
3.1 DRAmAGE AREA DESCRIPTION................................................................................. .............4
3.2 SHADY OAK POND DESCRIPTiON ................................................... ................. ......... ..............4
3.3 STORM SEWER SYSTEM DESCRIPTION ................... ..... .................. ......... ..... ............ ................5
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4.0 D ESI G N ME TH 0 DO LOGy;:......................... .................................................................... !'~' 5
4.1 HyDROLOGy................. ..... ...:............................................................................ .'.................... 5
4.2 HYDRAULICS ...................... ..!......................................................................... ........................5
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5.0 CRITERIA FOR INVESTIGA TION ....................................................................................5
5 .1 MUNICIPAL CRITERIA....... ....J......:........................................... ............................ ...................5
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5.2 OTHER AGENCY CRITERIA ...~..... .................................... ........................................................6
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6.0 PR 0 POSED IMPROVEMENTS..................... ................... .............................................. ..... 6
6.1 OUTLET OPTIONS CONSIDERED.. .......... ................ ............ ......... .......... ........... ......... ............... 6
6.1.1 Option I-Storm Sewer' along Shady Oak Road (Figure 6).......................................... 7
6.1.2 Option II - Storm Sewer along 18th Ave to 1st St S (Figure 7) ......................................8
6.1.3 Option III - Entire Dis~harge through Culvert Crossing to Minnetonka (Figure 8) . 10
6.1.4 Option IV - Lift Station with gravity system from 1st St N to 1st St S (Figure 9).........10
6.2 18TH AVE N AND 1 ST ST N FLOOD MITIGATION OPTIONS CONSIDERED ........:.......................12
6.2.1 Replace Existing Systetft and Inlets................ ............. ...................... ........... .......:....... 12
6.2.2 Split Flow into Propos~d Pond Outlet ........................................................................13
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7. 0 FINAN CIN G.................... ......1................... ............................... ...................... ...... ................. 13
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7.1 OpmION OF PROBABLE COSTS ......... ........ ................ .............. .......................... ....... ....;........ 13
7.2 PROPOSED Fm ANCmG.... ......1....................................................................... ........................ 13
8.0 PRO JE CT S CHED ULE .......~.......................... ..... .......... ......... ..... .............................. .......... 14
8 .1 TENTATIVE SCHEDULE FOR RBLA TED PROJECTS: ........ ...................... ................ .................... 14
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8.2 SCHEDULE FOR POND OUTLET AND FLOOD MITIGATION .....................................................14
8.2.1 Option I ....................... ~............................................................................................... 14
9.0 PR OJECT CO NT A CTS .......f............................................................................................... 14
Shady Oak Pond Feasibility Report
Hopkins, Minnesota
A-HOPKN0202.00
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Table 1
Table 2
Appendix A
Appendix B
Appendix C
List of Tables
Peak Flows.............................................................................. 12
Opinion of Probable Cost ........................................................13
List of Appendices
Figures 1-10
Table of Results
Drainage Map
Calculations
Estimates
References
Shady Oak Pond Feasibility Report
Hopkins, Minnesota
A-HOPKN0202.00 .
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May 20, 2003
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Feasibility Report
Shady Oak Pond Outlet
Prepared for the City of Hopkins
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1.0 Conclusions and Recommendations
1.1 Conclusions
As a result of oui study and investigation we can conclude that:
1. The watershed boundary for Shady Oak Pond is currently labeled
CE in the City of Hopkins Water Resources Management Plan
(HWRMP). Property development and storm sewer modifications
within the watershed boundary have decreased the boundary area
from 114 to 82 acres. .
2. The City of ,Minnetonka's Water Resources Management Plan
(MWRMP) indicates. drainage north of Highway 7 will be routed
through Shady Oak Pond in the future. The MWRMP includes a
future 21-inch culvert from Shady Oak Pond crossing Shady Oak
Road discharging into Minnetonka's drainage system. The
hydrologic model data included in the MWRMP indicate a peak
flow of 11 cfs for the 100 year storm discharging from Shady Oak
Pond into Minnetonka's jurisdiction (Appendix C).
3. With the Shady Oak Pond Outlet constructed, 2nd St N
immediately north of Shady Oak Pond will have to be raised
approximately 1 foot to provide protection against overtopping
from the 100-yrI24-hr storm.
4. Dredging the islands will not provid~ eIJough additional storage to
prevent Shady Oak Pond from overtopping Shady Oak Road for
the 100-yr/24-hr storm. An outlet would still need to be
constructed.
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5. It is incumbent upon the City of Hopkins to justify to the
Minnesota Department of Natural Resources (DNR) the reason for
A-HOPN0202.00
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dredging the islands which contain Purple Loosestrife. The DNR
requires either same-site disposal or existing-site disposal of spoil
material containing Purple Loosestrife. The DNR requires a
permanent outlet structure be constructed as opposed to a flared
end section outlet, and the Ordinary High Water (OHW) to remain
at or near the established elevation.
6. The Nine ,Mile Creek Watershed District requires a mInimUm
manhole dimension of 60-inches for outlet structures. The
watershed district also requires design criteria regarding runoff rate
as established by the following guideline: Minnesota Pollution
Control Agency, 1989, "Protecting Water Quality in Urban Areas."
7. Hennepin County has future plans for the reconstruction of Shady
Oak Road between Excelsior Boulevard and Highway 7. The
county has a standard cost share program based on contributing
watershed area for sponsors that wish to be included in the project
scope. A formal request to Hennepin County is the first step of the
cost share program.
8. The storm sewer system along Shady Oak Road south of Excelsior
Boulevard is sized to accommodate the lOO-yrI24-hr > storm for
MWRMP's proposed improvements.
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9. The existing storm sewer system along Excelsior Boulevard
between 18th Ave S and Nine Mile Creek surcharges at 17th Ave S
from the 1O-yr/24-hr storm.
10. To alleviate flooding at the intersection of 18th Ave N and 1st St N,
the existing 18 and 24-inch storm sewer system between 18th Ave
Nand S,hady Oak Pond should be replaced with a 36-inch RCP
storm sewer pipe. Also, the inlet capacities at 18th Ave Nand 1st
St N should be upgraded to increase the intake of overland flow at
the intersection.
11. Option I for the Shady Oak Pond outlet should be constructed to
provide protection against the pond overtopping Shady Oak Road
during a lOO-yrI24-hr storril. This option includes 'installing a 21-
inch culvert crossing Shady Oak Road to Minnetonka's jurisdiction
and routing an 18-inch storm sewer along Shady Oak Road to
Excelsior Boulevard. Option I includes 3 phases of constructibn.
See section 6.1.1 for details on phases.
12. The improvements are necessary, cost-effective, and feasible.
1.2 Recommendations
It is recommended that:
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1. This report be reviewed by the City Council, staff, and financial
advisors.
, Shady Oak Pond Feasibility Report
Hopkins. Minnesota
A-HOPKN0202.00
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2. The City Council accepts this report and proceeds with
implementation of Option I for the Shady Oak Pond outlet. The
opinion of probable project cost (Appendix C) for Option I IS
$876,500. The City proceeds with replacing the storm sewer inlet
system along 1st St N from 18th Ave N to Shady Oak Pond. The
opinion of probable project cost is $436,100. The City investigates
the cost of raising 2nd 5t N to protect it from inundation during
large storms. The City should implement these improvements for
the following reasons:
a. They address the immediate needs.
b. They can be integrated into future construction projects.
c. They implement an item of the capital improvement plan.
3. The pond outlet and 1st 5t N storm sewer improvement should be
constructed as proposed in this report.
2.0 Introduction
2.1
Authorization
This feasibility report has been - prepared and is submitted in
accordance with an agreement between the City of Hopkins and Short
Elliott Hendrickson Inc. (SEH). The report, authorized by the City of
Hopkins on June 26, 2002, is a result of city resolution 97-104. The
resolution outlines an Action Plan to alleviate flooding problems
within the city. This report addresses Problem Areas 1 and, 3
described in the Action Plan (Appendix C).
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2.2
Background
Shady Oak Pond and the intersection of 18th Ave Nand 1 st St N are
defined as Problem Area 1 and 3, respectively, in the "Action Plan".
The areas are shown in Figure 1 (Appendix A). A feasibility study
titled Storm Water Flooding Analysis was performed for the city in
1998 that, in part, addressed the flooding problems of Problem Area 3.
The 1998 study recommended a 3-phase solution to the flooding
problems.
Phase 1
a. Increase the inlet capacity in the alley between 18th Ave Nand
17th Ave N.
b. Reconstruct the entrance apron of the alley.
c. Bulkhead the 12-inch storm sewer connection between the
alley and 18th Ave N.
Phase 2
Shady Oak Pond Feasibility Report
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A-HOPKN0202.00
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a. Increase the stor:r:p sewer size from the alley to 17th Ave N.
b. Lower the intersection of 18th Ave N and 1st St N.
c. Increase the storm sewer size from 18th Ave N to Shady Oak
Pond. .
Phase 3
a. Construct an outlet for Shady Oak Pond.
To date, all items of Phase 1 have been constructed. Item a of
Phase 2 has not been constructed. Items band c of Phase 2 and
i tern a of Phase 3 are addressed in this report.
3.0 Existing Conditions
3.1 Drainage Area Description
The drainage area boundary for Shady Oak Pond produced from this
study is shown in Figure 2 (Appendix A). The boundary covers 82
acres. The HWRMP identifies this area as watershed CE and lists it at
114 acres. The 32 acre difference is primarily a result of two changes;
property development north of the pond that routes runoff to the north
eliminating 11 acres and runoff collection that stays west of Shady
Oak Road eliminating 21 acres of watershed from the HWRMP
boundary. The area is predominantly single family residential homes
roughly bordered by Highway 7 to the north, Mainstreet to the south,
Shady Oak Road to the west, and 17th Ave N to the east. The natural
overland overflow of the drainage area is across Shady Oak Road into
Minnetonka. Once across Shady Oak Road drainage generally flows
south to Shady Oak Lake. .
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3.2
Shady Oak Pond Description
The DNR lists Shady Oak Pond as protected water number 27-777P
and established the Ordinary High Water level (OHW) at 921.0 feet.
The pond is bounded by 2nd St N to the north, 1st St N to the south,
Shady Oak Road to' the west, and residential property to the east.
Presently, there is not a functioning manmade outlet from the pond.
There is an existing 18-inch corrugated metal pipe (CMP) crossing
Shady Oak' Road at the south end of the pond. Field. survey
information from April of 1988 indicates the west end of this pipe is
buried in the shoulder of the road and is not functioning. There is an
18-inch eMP crossing Shady Oak Road at the north end of the pond
that discharges runoff into the pond from approximately 5 acres of
residential property on the west side of Shady Oak Road.
Total area of the pond at OHW level is approximately 6 acres. The
pond has 2 islands that have a total surface area of 1.92 acres shown in
Figure 3 (Appendix A). The islands are infested with a perennial
Shady Oak Pond Feasibility Report
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A-HOPKN0202.00
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noxious weed called Purple Loosestrife. Biological control methods
have been implemented over the past few years in an effort to control
the weed.
Based on the numerical model, the pond overtops Shady Oak Road at
approximately the 25-yr/24-hr storm. The pond frequently overtops
20 St N causing extended street closures that block traffic' on West
Park Road and 2nd St N.
3.3 Storm Sewer System Description
Shady Oak Pond's storm sewer system consists of lateral 12-inch
Reinforced Concrete Pipes (RCP) discharging to a trunkline located on
1 st St N. The existing storm sewer system is shown in Figure 4
(Appendix A). The trunkline ultimately discharges into the southeast
corner of Shady Oak Pond. The storm sewer system collects runoff
from approximately 55 of the pond's 82 acre drainage area. System
modifications constructed in recent years have added catchbasin inlets
to the system on 2nd St N. There are 4 catchbasin inlets and 1 manhole
grate at the intersection of 18th Ave N and 1st St N.
The trunkline from 18th Ave N to 19th Ave N is I8-inch RCP, and from
19th Ave N to Shady Oak Pond it is 24-inch RCP. The 18th Ave Nand
1 st 5t N intersection floods during large storms.
4.0 Design Methodology
4.1 Hydrology
HydroCAD version 6.0 was used to develop the storm water runoff
models for the options explored. The 100-yr/24-hr storm (5.9 inches)
was input as the design storm for all options. The SCS TR-20 method.
was used to generate flows from the watersheds. Simultaneous pond
routing which allows for dynamic tail water elevations was used for
. routing hydro graphs through the ponds.
4.2 Hydraulics
Eagle Point 2002 design software was used to develop the storm sewer
hydraulic models of the existing and proposed storm sewer systems.
The Rational Method was used to develop the peak flows and the
Standard Step Method to develop the hydraulic grade lines of the
storm sewer systems.
5.0 Criteria for Investigation
5.1 Municipal Criteria
The Water Resources Management. Plans from Hopkins and
Minnetonka were used as a basis for analysis and preliminary design
of the pond outlet.
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Shady Oak Pond Feasibility Report
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Both plans were referenced because various options allow flow to
enter both jurisdictions. The plans include guidelines and data to help
meet municipal requirements.
5.2
Other Agency Criteria
This drainage system is within the Nine Mile Creek Watershed
District. It is anticipated that compliance with municipal requirements
for the design of the drainage system will comply with the
requirements of the Watershed District. -
Because Shady Oak Pond is a designated DNR protected water,
agency permits will be required before construction of an outlet
begins.
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6.0 Proposed Improvements
6.1 Outlet Options Considered
Four main options were considered for the proposed outlet. A
schematic of each option is shown in Appendix A. Each option was
analyzed with and without dredging the existing islands. Three of the
four options were analyzed with and without the culvert crossing to
Minnetonka's jurisdiction with the remaining option being the culvert
crossing as the only outlet. A summary 'of the results is tabulated in
Appendix B.
Dredging the islands would provide additional storage for runoff
during large storms.'. However, because the islands are infested with'
Purple Loosestrife the spoil material must be disposed of in one of two
ways. Either the spoil material stays on site or is hauled to a site
where Purple Loosestrife already exists. Considering .the project
location, keeping the material on site IS not a feasible option. Hauling
the spoil increases the cost associated with dredging the islands. The
opinion of probable cost for dredging the islands is $400,000.
Dredging the islands will not provide enough additional storage to
prevent Shady Oak Pond from overtopping Shady Oak Road for the
100-yr/24-hr storm. Therefore, an outlet must be constructed.
The culvert crossing to Minnetonka's jurisdiction is part of the
. MWRMP. The MWRMP states that high flows will enter their
jurisdiction from Shady Oak Pond. The MWRMP designates
Subcatchment 514 as the point of entry. In accordance with the
MWRMP, the maximum flow rate allowed intoSubcatchment 514 for
the 100-yrI24-hr storm is 11 cfs. All options considered were designed ,
to accommodate MWRMP's stated flow rate (+/- 1 cfs).
To comply with DNR requirements, the crossing is designed with an
outlet structure as opposed to a precast open flared end section. The
invert elevation exiting the outlet structure is 921.0 feet. Full design
implementation of any option using the crossing as an integral element
Shady Oak Pond Feasibility Report
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A-HOPKN0202.00
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of the solution is based on when Minnetonka implements their
MWRMP. Construction of Option I or III would include installing a
6-inch orifice on the crossing to restrict flow into Minnetonka pending
full implementation of the MWRMP. The opinion of probable cost for
installation of the crossing is $75,000.
Under all options 2nd St N bordering the north side of Shady Oak Pond
must be raised to prevent flooding fr9m larger storms. The low point
on 2nd St N is approximately 922.2 feet. Inundation of 2nd St N from
larger storms after full implementation of recommended. option I or III
is shown in Figure 5. ,The critical elevation used in analyzing the
outlet options was the low point of Shady Oak Road at 923.5 feet
using NGVD 1929 as the datum. Circumstances surrounding the pond
such as available pipe cover for larger outlets, agency restrictions for
maximum flow entering Minnetonka, and minimum elevation
maintaining OHW preclude the option of lowering the pond's flood
elevation to protect 2nd St N from inundation at the street's present
elevation. The City should consider including the cost of raising 2nd St
N in this project or include it in the Capital Improvement Plan (CIP)
for street improvements. It is listed in section 8.0 as a future phase for
construction.
A preliminary investigation of existing utilities was done to ensure the
proposed alignments considered were constructible.
. The criterion for recommendation is based on protecting Shady Oak
Road from the 100-yr/24-hr storm.
6.1.1 Option I - Storm Sewer along Shady Oak Road (Figure 6)
This option includes a proposed outlet structure that sets Shady Oak
Pond's NWL at 920.5 feet. . The outlet discharges to a proposed storm
sewer system that' flows south 1970 feet to Excelsior, Boulevard. At
Excelsior Boulevard the system connects to an existing 30-inch storm
sewer system that ultimately discharges to Shady Oak Lake. The
existing storm sewer system south of Excelsior Boulevard is designed
for the 100-yrI24-hr storm.,
l:Iennepin County has future plans to reconstruct Shady Oak Road
from Excelsior Boulevard. to Highway 7. Therefore, this option would
be best constructed under a cost sharing agreement with Hennepin
County. The county has a standard cost share program based on
contributing watershed area with interested sponsors. Coordination
with the City of Minnetonka will be required as portions of Shady Oak
Road either border or are within Minnetonka's jurisdiction.
Because of the tentative schedule for reconstruction of Shady Oak
Road and full implementation of the MWRMP, the construction
. process would follow a 3 phase scenario:
Shady Oak Pond Feasibility Report A-HOPKN0202.00
Hopkins. Minnesota Page 7
.
Phase I - Install 21-inch Minnetonka culvert crossing with orifice
restriction. This provides a 25-yr/24-hr level of
protection.
Phase II ...: Install outlet along Shady Oak Road in conjunction with
Shady Oak Road improvements. This provides a 75-
yr/24-hr level of protection.
Phase III - Remove orifice at full MWRMP jmplementation. This
provides a 100-yr/24-hr level of protection. '
Some of the variations described below do not correlate with the 3
phase construction scenario because cost precludes them from being a
feasible option. The values stated for each variation below assume full
design implementation.
6,1.1.1 Shady Oak Road Outlet
Considering the proposed outlet as the only outlet, the system size is a
24-inch RCP. Peak discharge for the outlet is approximately 17 cfs.
Peak elevation of Shady Oak Pond is 923.36 feet.
.
6.1.1.2 Shady Oak Road Outlet and Island Dredging
Considering the proposed outlet as the only outlet and dredging the
islands, the system size is an IS-inch RCP. Peak discharge for the
system is approximately 9 cfs. Peak elevation of Shady Oak Pond is
923.31 feet.
6.1.1.3 Shady Oak Road Outlet and Minnetonka Culvert Crossing
Considering the proposed outlet and a 21-inch RCP culvert crossing to
Minnetonka's jurisdiction the proposed storm sewer system size is an
IS-inch RCP. Peak discharge for the proposed outlet and crossing is 9
cfs and 12 cfs, respectively. Peak elevation of Shady Oak Pond is
923.34 feet.
.
6.1.1.4 Shady Oak Road Outlet, Minnetonka Crossing, and Island Dredging
Considering the proposed outlet, a 21-inch RCP culvert crossing to
Minnetonka's jurisdiction, and dredging the islands, the proposed
storm sewer system size is a 12-inch RCP. Peak discharge for the
proposed outlet and crossing is 3 cfs and 10 cfs, respectively. Peak
elevation of Shady Oak Pond is 923.15 feet.
6.1.2 Option II - Storm Sewer along 18th Ave to 1st St S (Figure 7)
This option includes a proposed outlet structure that sets Shady Oak
Pond's NWL at 920.5 feet. The outlet discharges to a proposed storm
sewer system that flows east 900 feet along 1 st St N to ISth Ave, then
south 1690 feet along lSth Ave to 1st St S, then east 330 feet to 17th
Ave S. At 17th Ave S the storm sewer system connects to an existing
27-inch storm sewer system that ultimately discharges to Nine Mile
Shady Oak Pond Feasibility Report
HopkinS, Minnesota
A-HOPKN0202.00
Page 8
.
Creek at Excelsior Boulevard and 13th Ave S. The existing 27-inch
system is designed for the 1O-yrI24-hr storm that surcharges during
larger storms.
This option will add surcharge to the existing storm sewer system
possibly causing nuisance flooding in streets during larger storms.
However, this option will alleviate flooding at Shady Oak Pond.
Because the existing storm sewer system surcharges during larger
storms, this option cannot ensure a 100-yr/24-hr level of protection.
Variations of this option were considered including replacing the
existing 27-inch storm sewer system on 17th Ave S from 1st StS to
Excelsior Boulevard with a 36-inch storm sewer system, to improve the
capacity of the system. However, the magnitude of improvement isn't
large enough, as the existing storm sewer system would still surcharge
at larger storms even with the improved capacity. '
Another variation considered was routing the proposed storm sewer
system south on 18th Ave across Excelsior Boulevard then east to Nine
Mile Creek. However, the additional construction cost for this
variation would preclude Option II froni being feasible.
Hopkins has future plans to reconstruct 18th Ave N. Therefore, the
street reconstruction could be constructed in conjunction with this
option. The average depth of the outlet along 18th Ave is 15 feet.
Therefore, when developing the opinion of probable cost for this
option, removal and replacement of the street from curb to curb was
included in the construc~ion quantities.
Option II has construction scheduling advantages over other options.
The entire outlet is within Hopkins jurisdiction allowing the City to
control when construction and full implementation of the Pond outlet
takes place. Also, coordinating County projects and cost sharing
agreements is not required with Option II. However, because a 100-
yr/24-hr level of protection cannot be ensured, it is not the
recommended option.
The values stated for each variation below aSSUme full design
implementation.
6.1.2.1 18th Ave Outlet
Considering the proposed outlet as the only outlet, the system size is a
24-inch RCP. Peak discharge for the outlet is approximately 17 cfs.
Peak elevation of Shady Oak Pond is 923.38 feet.
6.1.2.2 18th Ave Outlet and Island Dredging
Considering the proposed outlet as the only outlet and dredging the
islands, the system size is an 18-inch RCP. Peak discharge for the
system is approximately 8 cfs. Peak elevation of Shady Oak Pond is
923.35 feet.
Shady Oak Pond Feasibility Report A-HOPKN0202.00
Hopkins, Minnesota Page 9
.
6.1.2.3 18th Ave Outlet and Minnetonka Culvert Crossing
Considering the proposed outlet and a 21-inch RCP culvert crossing to
Minnetonka's jurisdiction the proposed system size is an 18-inch RCP.
Peak discharge for the proposed outlet and crossing is 8 cfs and 12 cfs
respectively. Peak elevation of Shady Oak Pond is 923.34 feet.
6.1.2.4 18th Ave Outlet, Minnetonka Culvert Crossing, and Island Dredging
Considering the proposed outlet, a 21-inch RCP culvert crossing to
Minnetonka's jurisdiction, and dredging the islands, the proposed
system size is a 12-inch RCP. Peak discharge for the proposed outlet
and crossing is 3 cfs and 10 cfs, respectively. Peak elevation of Shady
Oak Pond is 923.15 feet.
6.1.3 Option III - Entire Discharge through Culvert Crossing to
Minnetonka (Figure 8)
The description and details of the culvert crossing can be read in
Section 6.1. To achieve the flow restriction of 11 cfs to Minnetonka's
jurisdiction, the islands must be dredged. This option is wholly
dependent on the implementation of theMWRMP. As stated in
Section 6.1 a6-inch orifice would be installed on the culvert crossing.
pending full implementation of the MWRNlP. The construction
process would follow a 3 phase scenario:
Phase I - Install culvert crossing with orifice restriction. This
. provides a 25-yrI24-hr level of protection.
Phase II - Dredge Shady Oak Pond islands. This provides a 50-
yrl24-hr level of protection.
Phase III - Remove orifice at full MWRMP implementation. This
provides a 100-yr/24-hr level of protection.
Because of the complications related to the presence of purple
loosestrife on the islands (permitting and disposal), this is not the
recommended option.
The values below assume full design implementation.
6.1.3.1 Miimetonka Culvert Crossing and Island Dredging
Considering the proposed culvert as the only outlet and dredging the
islands, the culvert size is a 21-inch RCP. Peak discharge for the
system is approximately 11 cfs.Peak elevation of Shady Oak Pond is
923.21 feet.
6.1.4 Option IV - Lift Station with gravity system from 1st St N to 1st St
S (Figure 9)
This option includes a proposed outlet structure that sets Shady Oak
Pond's NWL at 920.5 feet. The outlet discharges to a lift station that
pumps water through 1000 feet of forcemain along 1st St N to a gravity
Shady Oak Pond Feasibility Report A-HOPKN0202.00
Hopkins, Minnesota Page 10
.
.
6.1.4.1
6.1.4.2
storm sewer system at 18th Ave N. From there the gravity storm sewer
system flows south 1690 feet to 15t St S, then east 330 feet to 1 ih Ave
S. At 17th Ave S the storm sewer system connects to an existing 27-
inch system that ultimately discharges to Nine Mile Creek at Excelsior
Boulevard and 13th Ave S. The existing storm sewer system is
designed for the lOyr/24hr storm that surcharges during larger storms.
This option will add surcharge to the existing storm sewer system
possibly causing nuisance flooding in streets during larger storms.
However, this. option will alleviate flooding at Shady Oak Pond.
Because the existing storm sewer system surcharges during larger ,
storms, this option cannot be considered a true 100-yr/24-hr outlet.
Land acquisition and annual maintenance are additional costs
associated only with this option. The lift station construction costs,
along with utility costs are also exclusive to this option. However, due
to the lift station and force main, a shallower gravity system can be
constructed on 18th Ave. The average depth of the gravity system is 8
feet. This allows one side of the street to remain in-place during
construction. Savings in street construction offset the increased
construction cost of the lift station. However, because future street
improvements are planned for 18th Ave, the advantage of cost savings
due to street replacement cannot be fully realized. Because of this, and
operation and maintenance costs, this is not the recommended option.
The values stated for each variation below assume full design
implementation.
Lift Station Outlet
Considering the proposed lift station as the only outlet, the pump size
is 5000 gpm, the force main size is an 18-inch DIP, and the gravity
system size is an 18-inch RCP. Peak discharge for the outlet is
approximately 11 cfs. Peak elevation of Shady Oak Pond is 923.45
feet.
Lift Station Outlet and Island Dredging
Considering the proposed lift station as the only~outlet and dredging
the islands, the pump size is 3000 gpm, the force main size is an 18-
inch DIP, and the gravity system size is an 18-inch RCP. Peak
discharge for the outlet is approximately 7 cfs. Peak elevation of
Shady Oak Pond is 923.40 feet.
6.1.4.3 Lift Station Outlet and Minnetonka Culvert Crossing
Considering the proposed lift station and a 21-inch RCP culvert
crossing to Minnetonka's jurisdiction, the pump size is 2000 gpm, the
force main size is a 16-inch DIP, and the gravity system size is an 15-
inch RCP. Peak discharge for the proposed lift station and crossing is
5 cfs and 12 cfs respectively. Peak elevation of Shady Oak Pond is
923.42 feet. '
Shady Oak Pond Feasibility Report
Hopkins. Minnesota c
A-HOPKN0202.00
Page 11
.
.
.
6.1.4.4 Lift Station, Minnetonka Culvert Crossing, and Island Dredging ,
Considering the proposed lift station, a 21-inch RCP culvert crossing
to Minnetonka's jurisdiction, and dredging the islands, the pump size
is 1200 gpm, the force main size is a 16-inch DIP, and the gravity
system size is an 15-inch RCP. Peak discharge for the proposed lift
station and crossing is 3 cfs and 10 cfs, respectively. Peak elevation of
Shady Oak Pond is 923.08 feet.
6.2 18th Ave Nand 15t St N Flood Mitigation Options Considered
6.2.1 Replace Existing System and Inlets
The intersection of 18th Ave Nand 1st St N floods during large storms
due to inadequate capacity of the existing storm sewer system. The
capacity of the existing storm sewer system is limited by an I8-inch
RCP that allows 14 cfs to flow west towards Shady Oak Pond. The
total inlet capacity of the inlets at the intersection is approximately 8
cfs. The intersection collects runoff from approximately 35 acres...
The boundary is shown in Figure 10 (Appendix A). The peak runoff
for the 35-acre catchment area is summarized in Table 1.
Table 1
Peak Flows
Event Peak Flow (cfs)
2-yr/24-hr 28
5-yrI24-hr 37
10- yr/24-hr 42
Upstream of .the intersection the total inlet capacity is approximately
20 cfs. To alleviate flooding the inlet capacity at the intersection and
the storm sewer system capacity to Shady Oak Pond must be
improved.
Replacing the existing storm sewer system from the intersection to the
pond with a 36-inch RCP at 0.45 % will increase the capacity to 45
cfs. Replacing existing inlets with more efficient grates will
approximately double the inlet capacity. With the proposed
. improvements in-place the intersection will be protected from flooding
during a 1O-yr/24-hr storm. The average depth of the proposed storm
sewer system is 10 feet. Because of the depth of the proposed storm
sewer system, 1st St N would need to be reconstructed from curb to
curb.
The City of Hopkins' Capital Improvement Plan (CIP) indicates street
improvements for 1st St N are scheduled for 2004. Constructing this
storm sewer improvement in conjunction with the CIP street
Shady Oak Pond Feasibility Report
Hopkins, Minnesota
A-HOPKN0202.00
Page 12
.
improvements would reduce the construction costs of replacing the
storm sewer. The cost breakdown between street replacement and
storm sewer replacement is in Appendix C.
6.2.2
Split Flow into Proposed Pond Outlet
This option considered splitting flow from the intersection into the
proposed pond outlet (option II or IV) flowing south on 18th Ave.
However, because the proposed outlet connects to an existing storm
sewer system that surcharges from large storms, flood mitigation at the
intersection would be minimal.
7.0
7.1
Financing
Opinion of Probable Costs
Detailed opinions of probable costs were developed for all options and
improvements discussed in this report. They can be found in
Appendix C.
The totals in Table 2 show the opinion of probable cost assuming all
improvements recommended in this report are constructed under the
same contract.
.
Option I will benefit from a cost sharing program with Hennepin
County based on contributing drainage area of the shared storm sewer.
Table 2
Opinion of Probable Cost
Improvements Option I
Pond Outlet $ 876,500*
1 st St N Storm Sewer $ 268,500
1 st St N Street $ 167,600
Total $ 1,312,600*
* Figure does not reflect County cost sharing benefit
7.2 Proposed Financing
The 1 st St N storm sewer improvements to mitigate flooding at the
intersection of 18th Ave Nand 1 st St N would be funded through the
City's storm sewer utility fund.
,The Shady Oak Pond outlet would be funded through the City's storm
sewer utility fund.
The cost of street reconstruction for 1st St N from Shady Oak Pond to
18th Ave N would be funded through the City's Capital Improvement
Plan for street improvements.
Shady Oak Pond Feasibility Report
Hopkins, Minnesota
A-HOPKN0202.00
Page 13
.
If Option I for the Shady Oak Pond outlet is implemented, it is
possible that the City would enter into a cost sharing program with
Hennepin County to defray a portion of the cost to the County.
8.0
Project Schedule
This project would benefit if combined with other projects by taking _
advantage of lower unit prices on larger projects. Coordinating
construction with Hennepin County and the City's CIP plan is
important in reducing the cost of this project.
8.1 Tentative schedule for related projects:
Shady Oak Road Reconstruction (County)
1st St N/lSlh Ave N CIP Improvements (City)
2004-2005
,2004
8.2 Schedule for Pond Outlet and Flood Mitigation
8.2.1 Option I
Culvert Crossing (with orifice restriction) 2003
1st St N/1Slh Ave N Flood Mitigation
Shady Oak Pond Outlet
2004
2005
2005- ?
Remove restriction from culvert crossing
Raise 2nd St N
Future Phase
9.0 Project Contacts
Steve Stadler, PE
Public Works Director
City of Hopkins
1010 ISIStS
Hopkins, MN 55343
Beth Peterson, PE
Project Engineer
Short Elliott Hendrickson
3535 Vadnais Center Drive
200 SEH Center
St. Paul, MN 55110
(651) 765-2901
.
Shady Oak Pond Feasibility Report
Hopkins, Minnesota
A-HOPKN0202.00
Page 14
.
Appendix A
Figures 1-10
.
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HOPKINS, MN
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HOPKINS, MN
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AHOPKN0202.00
DATE:
10/15/02
FIGURE
NO. 10
PROPOSED OUTLET
FLOOD MITIGATION AREA
HOPKINS, MN
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Appendix B
Table of Results
Drainage Map'
Calculations
.
.
. SHADY OAK POND OUTLET SUMMARY
.
.
Peak
Pipe Dia. Elev. Outflow
Option Storm (in.) (msl) (cfs) Comments
1970 ft to Excelsior
Shady Oak Rd I 100yr/24hr 24 923.36 17.12 Blvd.
Shady Oak 1970 ft to Excelsior
Road\Dredging I 100yr/24hr 18 923.31 8.44 Blvd.
Shady Oak
Rd\Mtka I 100yr/24hr 18 923.34 8.44 11.93 cfs to Mtka
Shady Oak Rd .
Mtka\Dredging I 100yr/24hr 12 .923.15 2.87 9.77 cfs to Mtka
2590 ft down 18th
Hopkins Outlet II 100vr/24hr 24 923.38 16.58 Ave.
Hopkins Outlet 2590 ft down 18th
Dredging II fOOvr/24hr 18 923.35 8.17 Ave.
Hopkins Outlet
Mtka II 100yr/24hr 18 923.34 8.16 12.27 cfs to Mtka
Hopkins Outlet
Mtka\Dredging II 100yr/24hr 12 923.15 2.78 9.87 cfs to Mtka
9.6 ac-ft additional
Mtka\Dredging III 100yr/24hr 21 923.21 10.47 storage
Lift Station IV 100vr/24hr 5000 Qpm 923.45 11.14
Lift Station
Dredging IV 100yr/24hr 3000 Qpm 923.40 6.68
Lift Station\Mtka IV 1 00yr/24h r 2000 gpm 923.42 4.45 12.27 cfs to Mtka
Lift Station\Mtka
Dredging IV 100yr/24hr 1200 Qpm 923.08 2.7 9.66 cfs to Mtka
Shady Oak Summary.xls
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