4a. West Central Lotus Lake Improvement Project-Feasibility Report Update
CITY OF
CHANHASSEN
7700 Market Boulevard
PO Box 147
Chanhassen, MN 55317
Administration
Phone: 952.227.1100
Fax: 952.227.1110
Building Inspections
Phone: 952.227.1180
Fax: 952.227.1190
Engineering
Phone: 952.227.1160
Fax: 952.227.1170
Finance
Phone: 952.227.1140
Fax: 952.227.1110
Park & Recreation
Phone: 952.227.1120
Fax: 952.227.1110
Recreation Center
2310 Coulter Boulevard
Phone: 952.227.1400
Fax: 952.227.1404
Planning &
Natural Resources
Phone: 952.227.1130
Fax: 952.227.1110
Public Works
1591 Park Road
Phone: 952.227.1300
Fax: 952.227.1310
Senior Center
Phone: 952.227.1125
Fax: 952.227.1110
Web Site
www.ci.chanhassen.mn.us
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MEMORANDUM 0
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TO: Paul Oehme, Director of Public Works/City Engineer \j
Terry Jeffery, Water Resources Coordinator -,; i/t!'
November 10,2008 (
FROM:
DATE:
SUBJECT:
West-Central Lotus Lake Improvement Project No. 08-02:
a. Update Council on Feasibility Report
This is an informational presentation intended to apprise Council of the findings
and recommendations from the feasibility study performed by WSB &
Associates, Inc. Consulting engineers from WSB & Associates will be present to
discuss their procedure, findings and recommendations. Staff will also be present
to address additional questions which may arise.
BACKGROUND
The Meadow Green Park pond failed in 2005 during largeback-to-back rain
events. This resulted in flooding within the area, erosion of the outlet channel
with subsequent discharge of sediments into Lotus Lake and the exposure of a
sanitary sewer line. The pond was repaired but the deficiencies were not
addressed. Additional capacity issues are known to exist within the storm sewer
conveyance system resulting in erosion and sediment deposition on private
property and surcharging of manholes. This feasibility study is intended to
address volume, water quality and rate control issues within the overall Meadow
Green Park Watershed (aka West-Central Lotus Lake).
The consultant contract was awarded to WSB & Associates at the February 25,
2008 City Council meeting. WSB began their study of this area in the spring of
2008 after Council approved the bid award in February of2008 and has prepared
a feasibility report. The study returned a list of 17 potential components to
address the overall scope of the known issues and the estimated costs associated
with each option.
It is important to note that since approval of the feasibility report, Lotus Lake was
listed as an "impaired" water for nutrients. This development lends further merit
to this project.
FINDINGS
The findings are best viewed as three separate phases of a larger project. Phase 1
would be performed first -likely in January through May of2009. This phase
could potentially include Items A-4 through B-5 (see Table 1 on Page 4 of this
staff report). However, staff and the consultant are not convinced of the need for
Chanhassen is a Community for life - Providing for Today and Planning for Tomorrow
West-Central Lotus Lake Imp. Project
November 10, 2008
Page 2
Item A-5 (Installation of a redundant pipe under Kerber Boulevard) as the updated
survey data and revised model indicate that adequate capacity exists if the
downstream beml is lowered (Item A-6).
It is known that increased storage capacity will be needed within the watershed.
Originally, the intent was to get this additional storage within the Meadow Green
Park Pond located easterly in the project area. However, after evaluation, it has
been determined that this is a jurisdictional wetland and any impacts would need
to be mitigated for. This need for mitigation would add additional costs to the
project and the excavation costs would be greater for this area because of the need
for dewatering and the general muck soil conditions. Based upon these findings,
upstream areas were evaluated. In particular, the area southeast of the ball fields
within Meadow Green Park. This would be Phase 2 of the project. What is
indicated in the feasibility report as Items A-I through A-3 (see Table 2 on Page 5
of this staff report) is one possible scenario for increasing storage capacity,
providing rate control and offering additional water quality treatment within the
watershed. This may not necessarily be the only option or preferred option.
Phase 3 involves the stabilization of the outlet channel to Lotus Lake. This
would occur only after the causes of the erosion within the channel are addressed
through the actions of the first two phases. This phase would include Items C-l
through C-3 of the feasibility report as shown in Table 3 on Page 5 of this staff
report.
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West-Central Lotus Lake Imp. Project
November 10,2008
Page 3
Phase 1 would address many of the issues surrounding the culvert crossing under
Kerber Boulevard as well as the channel which flows to the Meadow Green Park
Wetland and the wetland itself. The storm sewer manhole east of Kerber
Boulevard surcharges. This is a condition where water pressure builds up at some
point within the system due to inadequate capacity. This has resulted in water
coming up through the manhole and, in at least one instance, removing the casting
from the structure. Additionally, the backyards adjacent to the stream have
experienced erosion and washing out of the backyards. Options A-4 through A-7
are intended to address these issues and provide rate control of runoff into the
Meadow Green Park Wetland.
The reduction of the height of the berm east of Kerber Boulevard should address
the backflow conditions under Kerber Boulevard. It is because of this finding that
Item A-5 is likely not necessary. In order to accomplish the same goal which the
berm was intended to fulfill, a series of rock veins (ditch checks) will be placed in
the channel. This will slow the water through the channel and provide an
opportunity for sediment to settle out of suspension providing downstream water
quality benefit.
Finally, Phase 1 will involve corrections within the pond which failed during the
2005 rain event. The current inadequate outlets will be replaced and the berm
will be reconstructed so that it is less likely to fail during large rain events.
Phase 2 is intended to look for additional water treatment opportunities upstream
as well as additional storage and opportunities to decrease the rate at which runoff
enters the system. The current feasibility report looks at expanding the existing
wetland area immediately west of Kerber Boulevard. While Phase 1 is relatively
static, there is flexibility in how Phase 2 may be accomplished. Staff will
continue to evaluate this area for other opportunities which may be more cost
effective. This may involve construction of a pond south of the wetland, the
installation of alternative stormwater management techniques along the southern
park boundary, increasing the bounce within the wetland or some combination of
these techniques. The goal will be to achieve the same net benefit but with less
excavation - the largest expense associated with Phase 2.
Phase 3 involves the stabilization of the channel which outlets to Lotus Lake. The
presence of erosion throughout the channel speaks to the importance of this phase.
However, it would not be prudent to mitigate the channel erosion without
addressing the upstream flow conditions which have led to the erosive conditions.
Other activities appurtenant to the overall project include inflow and infiltration
issues. Currently, sanitary sewer flows under or immediately adjacent to the
Meadow Green Park Wetland. Further, as alluded to earlier, the forcemain that
runs within the outlet channel has been exposed in the past. This area is not
scheduled for 1&1 inspection in the immediate future but given the scope of work,
it may be beneficial to investigate the sanitary sewer at this time.
g:\eng\public"-2008 projects\08-02 west-central lotus lake improvements\staffreporta 111008.doc
West-Central Lotus Lake Imp. Project
November 10, 2008
Page 4
SUMMARY OF COSTS
The following tables indicate a breakdown of the opinion of cost for each option
and each phase. The overall project is anticipated to occurin three phases with
each phase occurring at one- to two-year intervals depending upon the funding
available. Given the recent report by Ehlers, it may be necessary to push the time
frame out even further into the future unless outside funding sources can be
procured. It is important to note that cost associated with each option is estimated
as though it is a standalone item. It is reasonable to assume that when bid as a
whole project, costs will be reduced 10% to 15%.
2008 I 2009 IMPROVEMENT PROJECT
DEBRIS CATCHER STRUCTURE
UPSTREAM OF KERBER BOULEVARD
LOWER EXISTING BERM EAST OF KERBER
CONSTRUCT CHECK DAMS (3) EAST OF
KERBER
RECONSTRUCT POND OUTLET TO NORTH
RAISE AND STABILIZE EXISTING MEADOW
GREEN PARK WETLAND BERM;
RECONSTRUCT EMERGENCY OVERFLOW
REMOVE EXISTING WETLAND OUTLET
RECONSTRUCT OUTLET CONTROL
STRUCTURE
EXTEND STORM SEWER OUTFALL TO
LOW POINT
TOTAL ESTIMATED COST
Table 1. Phase 1 Opinion of Cost
A-4
A-6
A-7
B-1
B-2
B-3
B-4
B-5
$29,093
$17,658
$97,335
$44,186
$46,467
$10,800
$129,128
$23,321
$397,987
The table below indicates construction in 2010 carrying over into 2011. This is
based upon the consulting engineer's feasibility report which would indicate that
this is the third phase proposed. This is one option which has been proposed.
Staff would like to explore other options which may result in less excavation and
less cost to see if it would be feasible from a cost-benefit perspective. Staff is of
the opinion that this would be Phase 2 of the construction given that stabilization
of the channel would best be left until after the source has been remedied.
However, the years are representative of when this project might occur.
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West-Central Lotus Lake Imp. Project
November 10, 2008
Page 5
2010 I 2011 IMPROVEMENT PROJECT
RECONSTRUCT POND 10.11 OUTLET
EXPAND POND 10.11
PROVIDE WATER QUALITY VOLUME IN
POND 10.11
TOTAL ESTIMATED COST
Table 2. Phase 2 Opinion of Cost
$75,128
$533,318
$157,950
$766,395
A-1
A-2
A-3
Once upstream conditions are addressed, the symptom of the volume and
discharge issue should be addressed. This is primarily the stabilization of the
outlet channel to Lotus Lake. It was erosion within this channel from upstream
discharge which resulted in the force main being exposed and the sediment
deposition into Lotus Lake.
2012 I 2013 IMPROVEMENT PROJECT
STABILIZE STREAM BANK & ARMOUR TOE
OF STREAM EMBANKMENT; CONSTRUCT
ROCK VANES/CHECK DAMS
SUSTAINABLE LANDSCAPING NEAR
LOTUS LAKE
TOTAL ESTIMATED COST
Table 3. Phase 3 Opinion of Cost
C-1 & C-2
C-3
$496,800
$37,800
$507,600
CONCLUSIONS AND RECOMMENDATIONS
It is staff s goal to finalize the details of Phase 1 and begin to prepare plans to
submit for bid. Even if the City does not proceed to Phase 2, staff feels that the
benefits of Phase 1 are immediate and warrant the expense. Previous attempts to
gamer financial cooperation from partner agencies have not met with success.
This appears to be a result of having priorities that were not coincidental with one
another. The City will again request that the Riley-Purgatory-Bluff Creek
Watershed District partner with the City on this project. It is reasonable, based
upon past discussions, to conclude that this partnership may not occur. As such,
the City must consider whether to proceed with this project alone and how to
make the most efficient use of available Surface Water Management funds.
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West-Central Lotus Lake Imp. Project
November 10,2008
Page 6
If the subsequent phases are postponed into the future, it is conceivable that the
Total Maximum Daily Load (TMDL) study for Lotus Lake may be performed. If
a TMDL study is performed, this project may qualify for Clean Water Legacy Act
grant funds which may help defray future costs.
Attachment
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