1. Subdivision Improvement District #345 - Willows SubdivisionThe Willows Stormwater
Drainage Work Session
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Overview of Presentation
Stormwater Design- "The Basics"
— Predevelopment Drainage Patterns
— Predevelopment Drainage Analysisr1
— Developed Site Layout "
— Developed Design and Calculation „
— Detention Facilities
— WQTreatment Facilitiesi;`
— Maintenance Responsibilities
The Willows Subdivision
— Area Historic Drainage;
— Current Drainage Layout
— Stormwater Development Progression
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SI❑ Improvements
Preliminary Plan and Cost y '
Completion to Date,,, .:
Next Steps
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Stormwater Design "The Basics"
Predevelopment
• Vacant
land
• Historic
drainage
patterns
• Assess
— ❑ffslte
run-off
— Down
stream
drainage
patterns
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Stormwater Scenario, subdivision development of Lot 2.
1. Start with vacant vegetated lot
2. Assess Current drainage patterns
• Site runoff
• Offsite runoff
• Downstream drainage conveyance and pattern
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5tormwater Design "The Basics"
Predevelopment Drainage Analysis
• Calculate
Predeveloped
flowrate and
volumes
— Typically for 10 yr
and 100 yr runoff
eve nts
— Offsite run off
flows
— Downstream
conveyance
capacity
o h, Hydrographs ors
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1. Calculated pre -developed flow rate and volume for proposed development
• 10 year and 100 year 24 hr storm events
2. Assess offsite runoff impacting proposed development
• 10 year and 100 year 24 hr storm events
3. Assess down stream
• Review current drainage status
• Flood issues
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Stormwater Design "The Basic"
Developed Site Layout
• Layout Design
Site Plan
• Stormwater
Management
Conveyance
and Facilities
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1. Develop proposed site layout
• Layout roads, sidewalks, lots
• Include Stormwater facilities
2. Use site layout to calculate:
• Stormwater conveyance locations and size
• Water quality treatment and size
• Flood control facilities volume and size
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Stormwater Design "The Basics"
Developed Design and Calculations
• Calculate Stormwater
— Developed flowrate- 10
year peak
— Flood control volumes —
10 and 100 year
— WQ volumes- V storm
event
• Size Stormwater Facility
• Perform Downstream
Analysis
— Ensure no effect on
downstream properties
ar&, Hydrographs own
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5tormwater Design "The Basics"
Detention Facilities
• The detention pond temporarily stores stormwater
runoff and slowly releases it through a specially
designed outlet or control structure.
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1. Detention facilities area designed to store water from the new development and
discharge at a flowrate matching the 10 pre -developed
2. Examples: DEQ facilities, Grandview Assisted Living Facility, MDT Ponds, Silverbrook,
Mountain Vista
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5tormwater Design "The Basics"
WQ Facilities
• New ❑evelopment
Water Quality
— Required to treat 1"
rainfall event
• Treatment Train
• Combination
— Treatment +
❑etention
Water quality treatment facilities are designed to remove pollutants contained in
stormwater runoff. The pollutants of concern include sand, silt, and other
suspended solids; metals such as copper, lead, and zinc; nutrients such as nitrogen
and phosphorus; certain bacteria and viruses; and organics such as petroleum
hydrocarbons and pesticides.
1. Water Quality Treatment is required for new development projects
• Required to treat 1" rainfall event
2. Treatment performed prior to detention facility or in conjunction with treatment facility
5tormwater Design "The Basics"
Maintenance Responsibilities
• stormwater Facilities Standard Maintenance Policies
— City Standards for Design and Construction, Nov 2009
— Resolution No. 5406, adopted Nov, 16, 2009
• The City of Kalispell maintains the stormwater system structures located
within the public road right of way
• The project owner shall provide for the perpetual maintenance of all elements
of the stormwater system located outside the public right of way.
— For privately maintained stormwater systems in residential
neighborhoods, a homeowner's association shall be formed to maintain
the facilities located outside of the public right of way.
— For commercial/industrial and multi -family residential developments with
joint stormwater systems and multiple owners, a property owners'
association (POA) or similar entity such as a business shall be formed, or a
reciprocal -use agreement executed.
Proper maintenance of stormwater facilities leads to better performance and
increases the life of the facility. Insufficient maintenance of stormwater control
facilities can lead to poor performance, shortened life, increased maintenance and
replacement costs, and property damage.
The City of Kalispell maintains the stormwater system structures located within the
public road right of way. Drainage parcels created by public projects will be
maintained by the City of Kalispell. The project owner shall provide for the
perpetual maintenance of all elements of the stormwater system located outside
the public right of way.
For privately maintained stormwater systems in residential neighborhoods, a
homeowner's association shall be formed to maintain the facilities located outside
of the public right of way.
For commercial/industrial and multi -family residential developments with joint
stormwater systems and multiple owners, a property owners' association (POA) or
similar entity such as a business shall be formed, or a reciprocal -use agreement
executed.
Development examples: Silverbrook, Mountain View Estates, Spring Prairie Phase 3
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Overview Drainage Patterns
1. West of Willow Glen Drive- Drainage Area Pink
2. East of Willow Glen Drive —Drainage Area Red
3. The Willows Subdivision Location -Drainage Area Purple
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The Willows
Current drainage Layout
Current Drainage Layout- Full View
West of Willow Glen -Pink Drainage area, same as 1997
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Current Drainage Layout
1. East of Willow Glen Drive
• County Facilities-swale to detention facility to river
• Leisure Heights -detention facility - Willow Glen storm system - drainage swale
to detention facility to river
• North vegetated field- through the Willows storm system
• Willows -detention facility to river
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Drainage Swale
Willows
LM 5uIlivision
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Detention Facility
Varr[ Fanra
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uun[y
Detention Facility
Willows
Stermlwater Qutfall
OutfaI15 ure
eterrtion Faulity
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HiT:�►►MINTM
Storm►nrater Development Progression
• April 1. 1997: County Commissioners approved preliminary plat with condition that Phases I & II would be
annexed into the City of Kalispell before final plat.
• July 30, 1997: City of Kalispell Preliminary Review of Infrastructure Improvements
— Increase discharge pipe from 12" to 15" in anticipation of future flows from Greenacres Slough — City offered
to pay the difference in cost of materials
— Questioned haw wetland area would he accessed for dredging/maintenance
January 12, 1998: DER approval of development plans including the stormwater and wetland treatment system.
July 28, 1998: Schwartz Engineering redesigned the stormwater conveyance system by eliminating the proposed
wetlands stormwater treatment facility. original design by Billmayer included a constructed wetland treatment
10'x550', 2:1 slopes, designed for 2-year 6-hour storm. New design included a settling swale to allow sediment
removal, overflow into 15' pipe conveying water to the Stillwater River.
• August 26, 1998: Revised drainage plan proposed by Schwartz Engineering approved by City of Kalispell.
NovemberM. 1998: Phase 1 complete -Request for annexation and initial zoning.
April 27, 1999: 12" outfall was installed instead of the design size of 15,rLetter form City stating 12' will have to
be upsized to 15" prior to Final plat of Phase II.
• May S. 1999: Declaration of Condition, Covenants and Restrictions and signature, ❑utlining responsibility of
ownership and maintenance of stormwater drainage system.
• December 1, 20OD: Letter from Schwartz to City of Kalispell requesting approval to install 12" outfall pipe rather
than 15' outfall pipe.
April 1, 1997: County Commissioners approved preliminary plat with condition that Phases I & II would be
annexed iitCthe City uLi Kalispell
ueLiu_e Ln:ndI plat
July 30, 1997: City of Kalispell Preliminary Review of Infrastructure Improvements
• Increase discharge pipe from 12" to 15" in anticipation of future flows from Greenacres Slough —
City offered to pay the difference in cost of materials
• Questioned how wetland area would be accessed for dredging/maintenance
January 12, 1998: DEQ approval of development plans including the stormwater and wetland treatment
system
July 28, 1998: Schwartz Engineering redesigned the stormwater conveyance system by eliminating the
proposed wetlands stormwater treatment facility. Original design by Billmayer included a constructed
wetland treatment 10'X550', 2:1 slopes, designed for 2-year 6-hour storm. New design included a settling
Swale to allow sediment removal, overflow into 15" pipe conveying water to the Stillwater River
August 26, 1998: Revised drainage plan proposed by Schwartz Engineering approved by City of Kalispell
November 30, 1998: Phase 1 complete -Request for annexation and initial zoning
April 27, 1999: 12" outfall was installed instead of the design size of 15". Letter form City stating 12" will
have to be upsized to 15" prior to final plat of Phase II
May 5" 1999: Creation of CCR and signature, outlining responsibility of ownership and maintenance of
stormwater Drainage system
December 1, 2000: Letter from Schwartz to City of Kalispell requesting approval to install 12" outfall pipe
rather than 15" outfall pipe
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SID Improvements
• Improve Detention Facility and Discharge
System to Current City Standards
— Detention facility sizing
— Outlet structure
— Side slopes
— Offsite flow management
— Water quality treatment
— Maintenance access
1. Request from Willow Subdivision for the City to maintain operation of stormwater
syste m
2. City agrees based on stormwater facility improvements
3. Improve Detention facility and discharge system to current City Standards
•Detention facility sizing
•Outlet structure
•Side slopes
•Offsite flows
•Water quality treatment
•Maintenance access
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Preliminary Plan and Costs
TABLE 5-1
COST ESTIMATE - PHASE 1 TABLE S-1 (CONT)
TNMW LESCRIP}ION EST I'll w IT _ _ COST ESTIMATE - PHASE 1
16TG-+eai Rap.•e�eais Y1 L8 i10.300.00 Ef50.30000 En In tCangnMbn PeMWt % 555.a0000
10Z M1beitzeiio d,. I itzaiion 1 LS se,53.200.00 SIMM
rvwhne al 1oo0o 2mw .po s.m
'" Cnns1-11 Ea 55.000.00 35MM
,W Con 1—Dr 5tiucturc 1 LS 515,000.nt 415AWM Alt—Y. F.. 10% 327.703M
lm, 5n.m W.U.0
Conrir c51 15'o.Mbr, lm LF S .00 i+ZS00Aa Fneroel A6ie Fee lS) 31585 w
U.— $,ZAWuo
Ro1aa schamo 1 G 1:,00 w "A00Aa Prchm C/hcial ^Staiom�rtFx 5% it aasaoa
E•ca•eiln� io the eraa na.dome 1m0 cv 51o.w 51U. .N Sub-Tolal isvr,'mo.m
Pone shoo vvta.a . 3000 SF g.DO 315.WOA0
uct pra xccs rairp ilo,om.m SmkW40 OneWnllHe ONenuA .9% 310.m0.m
"5 ConeWcl 1 Ea — 32.000.00 Srmm
LS W.$21W40 C Y F fi.wlh N5 31Y.900.m
"5 Ri e ou M ilei my�ersly M TCH SOLO, AM,
RI a Mdeiane� �nl 50 TC W.% f9AW.00
MbnallWnaaaa'.'.k 1 LS 510.Om.00 510.OW.00 c Reserve ,rnvtYrt 515.s0mm
Comlr<tc[ian S1ih-Total U3170U0
��x.goe=r rm�1 f.16.,mW Total Cost r No
torcmvction Twat L1}0.10D:
Water Quality / Detention Facility
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12" Pipe
18" Pipe 18" Pipe
Bypass flows
The existin --water treatmentpond receiv— fl—, f �m a ditch that P.1ns a1� the —Ah— —1— �f the �11kdiwi�i� � tlPt t� M-1—t
Slough, and flow from the Leisure Heights Subdivision. Because this ditch may receive continuous flow from the slough and Leisure
Heights Subdivision, the pond cannot adequately treat stormwater runoff from the Willows. The solution is to bypass flow from the ditch
around the pond so that the pond can function properly. Therefore, a diversion structure and pipe should be constructed as shown in the
schematic below.
The bypass pipe would be 18" in diameter and run under the pond. It would then connect to a new 18" overflow pipe that would parallel
the existing 12" pipe to the river. In addition, the diversion structure would also allow flow to bypass over the top for larger storm events
and/or a pipe clog.
Construct a New Pond Outlet Structure
Construct new outlet structure. According to the as -built drawings, the existing outlet structure is 2.5' x 4.0' x 3.0 feet tall. As designed,
the inlet to the structure is at the same level as the bottom of the pond. This has led to clogging due to sediment and other debris in the
inlet. In addition, this structure does not regulate flows leaving the detention pond and does not meet current City standards.
The Stormwater Design Manual provides a number of standard details for outlet structures ranging from 5 to 6 feet in diameter. These
structures regulate the flows leaving the pond and would have an orifice sized to discharge the predevelopment flow rate. The structure
would also bypass the orifice to safely pass larger storms through the system.
Construct a New 18" Overflow Pipe
According to a note on the as -built drawings, the 12" storm drain pipe that remains in -place running from the pond outlet structure to the
Stillwater River was to be replaced with a 15" pipe during Phase H of the construction of the subdivision. However, the as-builts show a
12-inch pipe and field measurements confirm this.
To safely pass flows generated from the subdivision, a new 18" pipe running from the new outlet structure to the Stillwater River is
proposed to parallel the existing 12" pipe. The invert of this pipe would be located above the existing 12" pipe invert in the outlet structure
and would act as an emergency bypass for larger storm events and would accept off -site flow from the ditch.
Pond Slope Stabilization and Reconstruction
While the existing slopes of the pond do not meet current City Standards of 4H:IV, there is insufficient room at the site to meet this
requirement. Existing residential yards are immediately adjacent to the pond to the north, and the county property is immediately adjacent
to the pond on the south. Therefore, the existing slopes must be maintained.
In addition to the slope stabilization, the pond will have to be reconfigured with a second cell to meet water quality standards. This will
allow the pond to essentially function as a wetpond as outlined in Section 6.3 of the Stormwater Design Manual.
Construct an Access Gate and Maintenance Ramp
The most logical point of access would be to place a gate on the fence to the Flathead County Shop Complex and negotiate an access
easement with the County. In addition to the easement, an equipment ramp into the pond will have to be constructed to allow City crews
to remove accumulated sediment and other debris.
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Completed To Date
• Preliminary Engineering Design
— Provides baseline for cost to
perform design and construction
• SRF Funding
— Preliminary Application has been
submitted (need specific PER, EA
documentation, and procurement
of Engineer of Record)
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Next Steps
Council Action
— SID Approval or Denial
Approval
— Proceed with SRF Funding
— Engineer of record selection
— Design and construction
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