HomeMy WebLinkAbout03.24.98 Special Planning Packet
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1. CALL TO ORDER
AGENDA
PLANNING COMMISSION
Special
March 24, 1998
7:00 P.M.
2. PUBLIC HEARINGS -7:00 P.M.
a) Farmington Middle School- 2nd Addition Preliminary Plat con't
3. ADJOURN
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City of Farmington
Community Development Department
Planning Division
325 Oak Street
Farmington, MN 55024
Community Dev.
Planning
Building Insp. .
(612) 463-1860
(612) 463-1820
(612) 463-1830
To:
Farmington Planning Commission
From:
Lee Smick, Planning Coordinator
Date:
March 24, 1998
RE:
Planning Commission Recommendation Summary
PUBLIC HEARINGS
a) Farmington Middle School- 2nd Addition Preliminary Plat con't
Recommendation
I. Recommend approval of the Farmington Middle School- 2nd Addition Preliminary Plat
contingent on Engineering and Planning comments and forward the preliminary plat to the
City Council after the plat has been reviewed and accepted as complete.
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TO: City Planning Commission
FR01\I:
Lee Smick, Planning Coordinator
DA TE:
March 2.4, 1998
RE:
Farmington Middle School
Preliminary Plat
Planning Department Review - March 20. 1998
Applicant:
Independent School District #192
510 Walnut St.
Farmington, :MN 55024
Meeting Summan' -March 17. 1998:
The following infonnation contains resolutions to issues discussed at the March 17th
Joint Planning Commission/City Council/School Board/City. Staff meeting. The agenda
outline will be follov.ed and the agenda to the meeting is attached.
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ISSUES:
a) 208th Street
Construct street to 56feet in width - The resolution of this topic consists of the proposed
24 foot roadway to remain as ShO\\l1 on the plan, however, the new right-of-way
alignment for the roadway must be reflected on a revised plan for the March 24th
meeting. Building half of the total 56 feet was discussed, however, a 28 foot roadway
would need to be reviewed by !\.fNDOT because this roadway width would constitute an
MSA route. Additional time would be required for :MNDOT to review a 28 foot roadway
and \\'ould delay the school project.
Final specifications for the 24 foot roadway will need to be approved by the City
Engineering staff. The proposed roadway will meet the 35 mph speed requirement. The
24 foot roadway \\ill be maintained by the City and must be built to City Standards. Any
streets constructed outside of the right-of.way will be privately maintained. When the 56
foot wide roadway is constructed, the school will rebuild it to MSA standards with a 10 .
ton road limit.
The school district v.ill be assessed 100% for the construction of the 56 foot wide
roadway to its property line and this requirement will be reflected in the developers
agreement.
Overpass or underpass for pedestrian traffic - It was determined that the overpass or
underpass would not be feasible or practical for access to the ballfields. However, a
crosswalk must be well marked on the pavement and should be ShO\\l1 on the plan.
Pedestrian crossing signs should be installed at the location with flashing lights, requiring
the need for an electrical conduit to be located near the crosswalk area.
Alignment of l08th Street needs to he shown correctly on the plat - The revised plat
must show the new alignment of 208th Street as provided by the City Engineers (see
attached alignment) and must be shown on the plat for the March 24th meeting.
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b) Circulation
Nortlt el1tral1ce - The north entrance to the school site will be a one-way, right-in, left-in
roadway. When the 56 foot wide roadway is constructed, the northerly entrance must
connect with 208th Street at a 900 angle.
The City recommended that the northerly entrance be combined with the proposed
entrance to the south to reduce the amount of access points onto the future 208th Street
collector. However, the school bus company claimed that one access would cause traffic
conflicts with automobiles using the drop-off areas in front of the school. The issue was
resolved by allowing the northerly entrance to remain as long as the entrance was a one-
way, right-in, left-in roadway. The roadway will be maintained privately.
School Bus Company's concerns - Concerns by the School Bus Company were met by
allowing the northerly entrance to remain as a one-way, right-in, left-in roadway,
allowing for buses to.drop students off at the proposed Middle School and the existing
Middle School in a continuous traffic !low pattern without conflicts from automobile
traffi c.
Fire Alarsltal/'s recommendation - The Fire Marshall made a recommendation to
maintain the existing road\\"ay to the existing Middle School in order for fire vehicles to
reach the rear of the existing Middle School building in a shorter time period. If the
access was removed, the Fire :r"farshall stated that fire vehicles would have to enter the
school site at the northerly entrance to the proposed school or drive against the flow of
one-way traffic by entering the school site at the existing westerly entrance to the existing
Middle School.
School officials want to remove the existing entrance because a proposed pedestrian
sidewalk will connect the proposed Middle School with the existing Middle School in
this location, and the existing roadway \\ill cause safety hazards to pedestrians using the
sidewalk. The School's engineers also stated that if the roadway was required to remain,
it would have to be removed and reconstructed in order for the site drainage to flow
correctly.
The Fire Marshall has made a recommendation that the existing entrance remain,
however, by removing the entrance, it does not violate the Fire Code. The School
District may refuse the recommendation by the City, however, the School District
assumes any liability for the inability for fire vehicles to promptly arrive at the school site
in case of a fire.
c)
Utilities
Sanitary Sewer location, Storm Sewer location - Both locations were described in detail
by the School's engineer and were acceptable to all in attendance. FUrther review by the
City's Engineering staff will commence when construction drawings are submitted.
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d) Drainage and Water Quality
Drainage of aI/listed areas - The School's engineers have had on-going discussions with
Erik Peters of Bonestroo in order to meet his comments shown in his attached memo.
The ponds ShO\\l1 at the front of the school will be permanently filled with water, the
pond at the northeast comer of the site \\ill hold water for short periods of time during
hea\y rains, and the pond near the ballfield will dry out in between rain events. The
School will install wet prairie vegetation in these locations. The School will also work to
restore the existing wetland near the ballfields through wetland restoration management
techniques.
e) Landscaping, Ballfields and Fencing
Landscape buffer between Riverside Estates and ballfields - The revised landscape plan
shows the buffer area to consist of eight Black Hills Spruce, nine Autumn Blaze Maples,
and six Clump Hawthorns. The Black Hills Spruce will be installed at six feet in height .
and typically grows to a height of twenty-five feet and a spread of twenty feet. The
Autumn Blaze Maples will be installed at 2 1/2 inches in caliper and typically grows to a
height of fifty feet and a spread of forty feet 'with a rounded tree form and dense foliage.
The Clump Hawthorns will be installed at five feet in height and typically grows to a
height of twenty-five feet v'lith a spread of twenty-five feet and provides a dense foliage.
The vegetation ShO\\l1 will pro\'ide an adequate landscape screen for the residences to the
west.
The two stub roadways within the Riverside Estates subdivision were also discussed at
the meeting. Residents are concerned that cars will be parked in the stub area to access
the ballfields. The Community Development Department is reviewing the possibility of
vacating the stub roads with approval from the residents. The stub road could eventually
be removed and trail accesses to the ballfields could be installed to allow residents from
Riverside Estates to utilize the fields.
Recollfiguration of ballfields - At the March 10th Planning Commission meeting, a
resident living next to the proposed ballfields in Riverside Estates raised concerns about
the configuration of the backstops being located so close to the residential area. The
resident requested that the backstops be located towards the wetland area and the outfield
be located next to the residential area.
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To meet the resident's concerns. the ballfields ha\"e been relocated to the east by twenty-
six feet allowing for a fifty-three foot buffer area measured from the property line to the
backstop area. Landscaping has also been added to screen the fields as well.
Fence recommendation along railroad right-of-way - City staff has recommended a
fence be installed along the railroad right-of-way which \vould TUn the entire length of the
school property and continue to the south along City property and terminate at the trail
underpass below the railroad line. City staff recommended this fence be installed by the
School District in order to deter students from crossing the tracks to get to the Middle
School as they currently do now. There is no City Ordinance which requires the
installation of the fence, howeyer, because of safety concerns, City staff has
recommended the fence.
The School District will assume liability for any accidents if the fence is not installed. At
the meeting, the School District stated that they would install the fence along the railroad
right-of-way within their property boundaries. The School District will not install a fence
on City property.
f) Other staff comments
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Plat requirements - Requirements to be sho\\TI on the plat are the alignment of 208th,
Street located correctly on the document, identification of metes and bounds for the
property lines, a surveyor's registration number and the dedication of right-of-way to be
shov.'ll on the plat. The plat must be finalized with these changes for the March 24th
Planning Commission meeting.
Additional Comments:
I, The preliminary plat was submined after working hours on Friday, March 20th and
will require a formal reyiew by the Planning Division. Review comments will be
distributed at the March 24th Planning Commission meeting.
2. The School's Engineers and the City Engineer met on March 20th to discuss the
drainage issues and wetland vegetation. Final acceptance of these issues is
contingent on the City Engineer's approval.
3. City staff will work with the Rherside Estates developer to determine if a
vacation for the two stub roads is feasible. Regrading of the Riverside lots
adjacent to the ballfields will also be discussed to allow for grades to match
sufficiently at the property line and allow the grades on the ballfields to be raised
for adequate drainage on the school site property.
4.
The plan shows the connection of the bituminous trail at the south end of the
Middle School property to be connected with a trail which will run along the west
side of the proposed bus road. This recommendation was made by City staff to
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. provide a safer route for pedestrians and bicycle traffic to access both school
facilities.
5.
A six foot high fence is shov.n on the plan and will be located along the railroad
right-of-way line. The fence v.ill be installed at the riprtheast comer of the
property and will tenninate at the southeast corner of the property, requiring 1,345
linear feet of fencing.
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6. The Police Department and traffic engineers will review the placement of the
crosswalk and any signage required to be installed for the safety of pedestrians
crossing 2081h Street.
7. The landscape plan needs to be revised to reflect the correct road alignment.
8. The gas line sho\\n along the existing 208th Street right-of-way needs to be
relocated and .~ phasing plan for moving the gas line needs to be shown on the
plan. .
Requested Action:
Recommend approval of the Fannington ~1iddle School- 2nd Addition Preliminary Plat
contingent on Engineering and Planning comments and forward the preliminary plat to
the City Council after the plat has been reviewed and accepted as complete.
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PROPOSED 8!.JIlOINC
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Memo
n-BO."Iestrtlo
Rosene
-=- Ander1ik'&
1\1 Associiltes
t"9'......"" ArCft',.nl
Project Name: Farmington 6-7 Middle School
Client: City of Farmington
.
To: Lee Mann
File No: 141-1061
From: Erik Peters
Date: March 5, 1998
Re: Drainage review of plans (submitted on 2"'2/98) and summary of meeting on 2/3/98 with the school
board and A TS&R engineering staff regarding crainage issues.
Remarks: The site to be developed poses some design challenges with regards to drainage and water
quality protection. The first challenge :s the ex:sting on-site topsoil which is predominantly a lean clay. The
runoff from clay soils is very high, es;:>ecially ;;'} developed areas where the surface is graded to drain
quickly and turf grass is the predominant vege:ation. The high runoff from the site will result in larger than
typical water quality ponds.
The second challenge is the high groundwater table (approximately 894'). To protect groundwater from
direct inputs of surface water runoff, the bottom of the ponds are recommended to be at least 3' above the
groundwater table. The pond bottom may be lowered a couple of feet if the bottom is sealed with an
impervious liner. The limited depth of excavation allowable on this site affects the construction of water_
quality ponds. Unless a substantial amount of fill is brought onto the site, the wet storage volume andW
minimum average depth requirements for water quality ponds will not be able to be met.
The design challenges posed by this site necessitate the investigation into alternative best management
practices to improve runoff leaving the site and water quality protection for the Vermillion River and the
adjacent wetlands. The River is a protected waterbody and the wetlands are some of the highest quality
identified on the Wetland and Waterbody Classification Map in the Cities' Surface Water Management Plan
(SWMP). The recommended alternative best management practices are the promotion of infiltration and
vegetative buffers/filters. To help ensure the long term success of the alternative best management
practices, the Farmington School District is encouraged to develop a vegetation management plan for the
Farmington Middle School. The vegetation management plan is recommended to contain provisions
limiting the use of fertilizers (phosphorus-free only), pesticides, and herbicides, and guidance on their
application and storage,
The site contains a significant amount of sanej soil underneath the clay topsoil. These sandy soils are
suitable for use in the construction of infiltration basins for water quality purposes. The infiltration basins, if
designed and constructed properly, hold the pc:ential to meet and possibly surpass the benefits of typical
wet detention ponds. To ensure the infiltration basin will function properly, the site must be kept clean of
on-site soils (specifically the lean clay topsoil) during the entire construction period. The pore spaces
Bonestroo, Rosene, Anderlik and Associates, Inc. Page 1 of 5 .
St Paul Office:
2:3::5 ','lest HIghway 36
Sl. ;:,,~l. lioN 55113
P~:.~,e 6'2-636-'600
Fex 5'2-636.1311
Milwaukee Office:
, 5'6 VIes! r'''e::i~:':'I ;=:.,,0
t.l,equon. Wi 53C'~2
Pho'>e: <'<.2<1-'':55
F"x, <',<.2<1-'~:1
Rochester Office:
2222 Hwy 52 'J:.~h
::;ochesler. t/.~ 55~01
;:~o:;e, 507.a2.2'OO
~"x: 5~7.n2.::',OO
Willmar Office:
205 51h Sl'eet SW
V/tllrr.ar. '/.N 55201
Phone ::20.2'<.~557
F"x: ::20.2'<.~<58
St Cloud Office:
2:o:e eln Sl. No~h
51 ClOud. 1m 55303
;:~,:.~.e ::20.251-'553
r,x ::20.251.5252
",Memo
'(' Boncstroo
L...I Ro~ene
Ie:. Andcr1ik &
1\1 Associ.-tcs
ln9tft""" I. ArC".'",,_
between the sand grains must be kept as clean as possible to maintain the infiltration capacity of the sandy
. soils. To prevent degradation of the infiltration basin due to construction activities. it is recommended that
the infiltration basin(s) be constructed after vegetation has been established on the site. If that cannot be
accomplished over the whole site, at least the field areas where runoff will directly enter the infiltration
basin should be established before construction of the infiltration basin. Otherwise, extensive erosion
control measures will need to be taken to trap sediment before it enters the infiltration basin. To improve
the aesthetics of the infiltration basir:s, wet r,-;eadow grasses and shrubs tolerant of wet conditions are
recommended to be planted within the :asins. Using a diversity of plant materials is recommended.
Vegetative buffers and infiltration trenches are proposed to treat runoff flowing directly to the Vermillion
River. The design of plant densities and type will be imponant so the vegetation will slow the rate of runoff,
provide effective filtration and trappin~ of pollutants present in overland flow and encourage seepage of
runoff into the underlying soils. Required maintenance will include periodic mowing and harvesting.
Below are recommendations regardi...,~ drair,age and runoff water quality improvements to guide the
development of plans for the Farmingtc:1 6-7 Middle School.
Drainage Between the Proposed School and 20ath Street:
Generally the two ponding basins east of 20a~'" Street are envisioned to act as preliminary sedimentation
ponds prior to draining into the wetlandlinfiltration basin west of 20Sth Street. The two ponding basins east
.of 20ath Street are significantly undersized to meet the requirements for water quality protection by
themselves.
1. Both ponding basins east of 20aUl street are recommended to be designed with an aquatic/safety bench
at the outlet elevation or normal water level (NWL) with skimmer structures at the outlets of both ponds.
The grading of a maintenance bench above the NWL as recommended in the SWMP may be
unnecessary due to the easy access to the ponds from surrounding paved areas. Using a submerged
outlet as a skimmer as currently proposed on the plans submitted is not recommended due to
maintenance issues and plugging concerns due to sedimentation and ice.
2. Ponds with a bottom contour elevation within 3 feet of the assumed groundwater elevation of 894'
should be sealed with clay (minimum 1 foot in depth) as is currently proposed.
3. The school district representative expressed a desire to design one pond as a dry pond. The bottom of
the dry pond would be constructed with sandy soils to promote infiltration. The concept is to allow
runoff to collect and pond without d:scharge for storms up to the 5 year event. This is an acceptable
concept. However, sediment washed off the pavement surfaces and into the pond will likely seal the
bottom of the pond in time. The pond will likely take on characteristics of a wet pond design unless
periodic maintenance is done to re~ove accumulated sediments and restore the transport capacity of
. Bonestroo, Rosene, Anderlik and Associates, Inc. Page 2 of 5
St Paul Office:
2;:35 ...,'est Highway 36
St. PaJ!. I.'.N 551' 3
?~o~,e 6' 2-635..:500
Fax ~'2.~35,'3'1
Milwaukee Office:
~~~6 West Mr::;Jor. :::ad
1.1r:::;:Jon. \'Jl :::~~2
Frlo~.e: ~ ~ (.~, ~ ~":S
Fax: <~~.~"""';:'1
Rochester Office:
2222 Hwy :2 NO:1h
",ochesler. 1.1', ::~81
Phone: 507.2E2.2iOO
Fa: 5G7.2E2.::iOO
Willmar Office:
205 Slh SI~ee1 SW
\'.r,lJrr,ar. I.IN 5:201
Phone. ::20-2~'.~:57
Fc:x: 320-2~"~'58
st. Cloud Office:
2C'CiE Blh St. North
51. Cloud. 1m 563C3
Pnone: 320-25' '" :53
Fax 320.2:'-62:2
IVlemo
n ;;:ee~~roo-
-=- Ander1ill'& ' <
1\.1 AsSOcii'ltes
(nv",.." , ArChflPC'U
the underlying soils. Therefore, the pond outlet and downstream infiltration basin is recommended to
be designed assuming the "dry pond" discharges for all storm events and little infiltration is taking place.
, .
4. If the "dry pond" is designed as proposed in the current plans, it, would be preferable to have the street
drainage carried into the pond by storm se....'er rather than overland flow down the slope of the pond.
This would eliminate a potential erosion concern.
5. Storm sewer discharge into the proposed "dry pond" is recommended be designed with the ability to
outfall at two elevations. The storm sewer should be designed to discharge at the bottom of the dry
pond and at the invert elevation of the outlet of the dry pond. To prevent freezing of the storm sewer
pipes discharging into the pond (assuming the pond bottom seals) the invert elevations should be at or
above the outlet elevation of the "dry pond". Erosion control and energy dissipation measures will be
required at the discharge point(s).
6. The discharge from the p~nds will reed to :.e designed with appropriate energy dissipation measures.
The slower the discharge velocity exiting tr.e ponds and the greater the flow is dissipated over a broad
area after exiting the outlet culverts the better. Little rate control is intended to occur with the two ponds,
so priority should be given to maximizing t:-:e wet storage volume of each pond, reducing discharge
velocities. and promoting sheet flow after discharging from the outlet culverts.
Drainage Between the 8all Fields and 20Sth Street:
The area around the wetland is recommended to become an infiltration basin into which a Significant.
amount of school property will drain. Infiltration of runoff is the principal best management practice to
improve the quality of runoff leaving the school property.
1. The native topsoil will need to be removed to expose the underlying sandy soils. Clean sand will need
to brought from off-site or on-site as directed by the City Engineer to replace the excavated material up
to the 897' contour elevation.
2. The minimum recommended storage volume within the infiltration basin is the runoff volume from the 5-
year event (2.9 acre-feet based on grading plan reviewed). The storage volume provided will need to
take into account the future extension of 208t'\ Street and its encroachment toward the wetland. For
rainfall events greater than the design storm event, an overflow structure should be constructed to
discharge into an extension of the existing 42" storm sewer.
3. It is recommended that the surface area of the infiltration basin should be as large as feasible to
provide a large area for infiltration and slow the rate at which the basin will begin to see a reduction in
infiltration capacity.
Bonestroo, Rosene. Anderlik and Associates, Inc.
Page 3 of 5
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St Paul Office:
2335 ,'Jest Highway 36
51. Pe:.rI.II.N 55"3
?hor,e 5'2-636--'500
Fex 5~2.535.'3'1
Milwaukee Office:
i5'6 West lJ.equoro =:c:l
t.~equon. W1 53C':2
?horoe: ~'~.2~'~~S
Fex. ~'~.2~1-:E:'
Rochester Office:
2222 Hwy 52 NO:'ih
~o::hester, W.: 5:E~1
?~one: 507.2E2.2iOO
reX: 507.2E2.3 i DO
Wil/mar Office:
205 5:h Slfe-et SW
\'/,llrr,ar. I.~N 5520'
Phone: 320.2'~.E5:7
Fell.: 320.2i~'&':5B
Sl Cloud Office:
2D:'e e:h 51. Nonh
5t Clou:l. lIoN 56303
P~,:ne: 320.25'--'553
Fcx: 320.25'.6252
.' Memo
J[J BonestrtlO
Rosene
-=- Anderlik &
1\1 AssociMes
("0'''.'''' " Af'ch'1H11
4. The fields adjacent to the infiltration basin should be raised 2' to 3' higher in order to lessen the extent
and frequency of inundation by runoff into the infiltration basin. Raising the fields may introduce
drainage issues with the properties adjace:lt to the proposed' fields that will need resolution.
.
Vermillion River Floodpiain and Water Quality Improvements for Runoff:
After reviewing the submitted plans a:id considering water quality issues, the proposal to create a small
ponding basin within the 'floodplain is not reco:nmended. The pond would not be large enough to meet the
protection requirements required for tr.e Vermillion River and its wetlands. Instead it is recommended that
vegetative buffers and infiltration trenches be used to improve the quality of runoff leaving the site. The
result would be that the entire area be~\"'een the bus road and the wetlands associated with the Vermillion
River are recommended to become c~e large vegetative buffer area. The use of this area for overland
water quality improvement of runoff pr;Jr to ir.~ow into the Vermillion River could meet the requirements of
the SWMP. The area could -also beco:7:e a nat'Jral science area for the schools
1.
It is recommended the grading p',an incl'Jde the future bus corral planned for the south side of the
school. This will assist in the prc~er design and placement of the proposed vegetative buffers and
infiltration trenches so they will be effective now and in the future. The grading of the 'floodplain is
recommended to be changed to look more natural with some undulation. As currently designed, it
looks unnaturally uniform. The grading plan should show compensation for fill within the 'floodplain at
the school building and the future bus corral.
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2. The proposed berm and outlet along the southern edge of the property would no be longer necessary.
3. A variety of plant types will need to be planted beginning with wetland vegetation in the lower elevations
and native prairie grasses along the edge of the road. The type of native vegetation selected and their
placement on the site will need to match their preferred environment. The slope of the site and
resulting changes in the local environment ..."ill result in some differentiation in vegetation types.
4. Multiple vegetative infiltration trenches are recommended to be constructed along and parallel to the
bus road. The intent of the trenches is to slow the rate of runoff, promote sheet flow rather than
concentrated flow of runoff, encourage infiltration of runoff into the soil and filtration through the use of
vegetation (grasses). The trenches are recommended to be constructed using earthen berms,
approximately l' in height. The to;:> of the earth berms should be fairly level to promote dispersion of
'flow over the berms. To further the promotion of infiltration, the bottoms of the trenches may be lined
with pervious soils/gravel under a t1-::n layer of topsoil. Further recommendations to their design can be
provided.
5. The storm sewer carrying runoff from the roof is recommended to discharge into the first vegetative
filtration trench farthest from the river. The outlet should be constructed to direct flow along the filtration
. Bonestroo, Rosene, Anderlik and Associates, Inc. Page 4 of 5
St Paul Office:
2335 West Highway 35
SI. Pa;)l. lIoN 55" 3
Phone: 6~2-636~600
Fax 6'2.635., 3"
Milwaukee Office:
~5~6 West f/,e~;)o:': =:ad
f,leQuon. VJl 53:'E-2
?"'lo~le: <~'a2"~~~;
Fcx: .ci4.2""""S:'1
Rochester Office:
2222 Hwy 52 ',or.h
ROChester. !/', 55~O'
Phone: 5ui.~E2.2~OO
Fax: 507.~E2.::~OO
Willmar Office:
205 5:h Street SW
Vllllrr.ar. I.IN 5620'
Phone: 320.2'4.~557
Fax: 320-214.9458
. St Cloud Office:
2~08 8:h Sl. North
St. Clo;;d. fJ.N 56303
;:hor,e: 320.25'~553
;:..x: 320.251.E252
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trench to promote sheet flow by using the berms to disperse flow. Energy dissipation at the outlet will
likely be necessary.
Northwest Corner Pond/Seepage Area:
.
As proposed, the pond basin may be designed to function as an infiltration basin for water quality
purposes. The clay topsoil within the basin will be removed and replaced with clean sand up to the 897'
contour. However, because runoff into the pond will not have gone through a sedimentation basin, the
pond bottom may seal and lose its infiltration capacity over time. How long it will take for the pond bottom
to seal is unknown. The behavior of th:s pond should be observed over time and it may provide useful data
that could be used in future pond design in the City.
1. The 100-year peak discharge from the pond is recommended to be restricted to match existing
conditions or the capacity. of the dc'\'mstream ditch whichever is more restrictive.
2. Assuming the pond will be desig;-;ed as an infiltration basin, the required storage volume below the
outlet elevation is the runoff from a 5-year storm event (1 acre-foot). To maintain the existing discharge
to existing conditions (approximately 5 cfs) the recommended outlet is a drop inlet structure (elevation
899') with a 12" culvert (invert at 898') exiting the structure. Assuming the bottom of the pond is no
longer infiltrating and the starting water surface is at 899', the 1 DO-year HWL will be approximately 900'
with a peak discharge of 5 cfs. It is assumed that there is an additional 0.9 acre-feet of storage
between the 899' and 900' contours in estimating the HWL elevation. The peak discharge is a.
somewhat high, however, since the pond should, for the most, part function as designed, there won't be
many occurrences where the outlet is used.
3. The ponding basin could be designed as a wet pond. Using the PONDNET model, the wet storage
volume required to meet water quality criteria is 1.3 acre-feet. The drawback to this option is that the
minimum average depth criterion of 3' - 4' depending on the pond size is not likely to be met. The
purpose of the minimum average depth requirement is to avoid the re-suspension of sediment from the
pond bottom due to inflow into the pond. An infiltration basin would provide better treatment of runoff
water, assuming the basin works as designed. The benefit to this option is slightly better rate control
(estimated peak discharge of 3 cfs). An aquatic/safety bench and outlet skimmer structure will need to
be provided should the pond be designed as a wet pond.
This pond could be designed using either method described above. However, it would be of interest to
construct the pond as an infiltration basin and observe the behavior of the pond over time. The use of
infiltration ponds may become more prevalent within the City, especially along the designated trout
streams. This appears to be a good location for an infiltration pond and it could provide useful data for
future designs in the City.
Bonestroo, Rosene, Anderlik and Associates, Inc.
Page 5 of 5
.
Sl Paul Office:
2335 V.'fSt Highway 36
Sl. P2...1. lIoN 55113
Pho~,e. S'2-€36-1.600
Fa 5'2.5:;5,'3'1
Milwaukee Office:
1 516 \'Jesll,\e~u~~ ;::Ce~
I.\equo:'l, \'.~ 53:';2
P~:''''e: ~1~-2~1~~S
reX: "~-2'1~;:1
Rochester Office:
2222 Hwy 52 ~or.h
Rochester. II." 55901
P~one: 5G7.2E.2-21 00
rax: 507.2C2.3100
Willmar Office:
205 5th Street SW
Wlflmar. rl.N 56201
Phone: 320.214.9557
Fax: 320-2'4-9~58
5l Cloud Office:
2c,:,e ~:h 51. Nonh
s: Clo;Jd. I.IN 56303
?ho~.e 320.251-1.553
reX: 320.251.6252