HomeMy WebLinkAbout12_Cannery District Stormwater Report_3-31-2020Engineer’s Stormwater Report
Cannery District PUD
Stormwater Update for BLDG J
March 2020
For Submittal to:
City of Bozeman Engineering Department
20 East Olive
Bozeman, MT 59715
Prepared by:
Stahly Engineering and Associates
851 Bridger Drive, Suite 1
Bozeman, MT 59715
(406) 522-9526
Stormwater Plan 1 3/27/2020
Cannery District Stormwater Report
Contents
General Information
Existing Conditions and General Design Criteria
Storm Drainage Plan
Estimation of Retention Volume
Retention Facilities
Curb and Gutter Capacity
Inlet and Area Drain Capacity
Pipe Conveyance Capacities
Facilities Maintenance
Figures and Tables
Figure 1. Vicinity Map
Figure 2. Storm Water Plan
Table 1. Required Retention Volume
Table 2. Retention Capacity by Phase Area
Table 3 Curb and Gutter Capacity Calculations
Table 4 Inlet Drainage Basin and Pipe Capacity Calculations
Appendices
Cannery District Covenants
Stormwater Plan 2 3/27/2020
General Information
The Cannery District is a multi-phase development located in the City of
Bozeman on the north side of East Oak Street, west of Rouse Avenue, and north
of the Gallatin County Fairgrounds. The existing property consisted of several
parcels of land totaling approximately 12.22 acres. The existing Northside PUD
area was included in the stormwater mitigation volume and added 2.99 additional
acres to the drainage basin area.
The development consists of four phases. The first phase, which was completed
in 2016, consisted of remodeling four existing buildings originally constructed as
part of the original pea canning operation in 1917. Phases 2, 3 and 4 include the
construction of six new buildings on the Cannery District property. Phases 2 and
4 improvements were completed in 2018, Phase 3 improvements are currently
under construction and are anticipated to be completed in the summer of 2020.
The final site building, Building J, will begin construction in the fall of 2020 after
plan approval from the City. In addition to Building J, the Northwest Parking Lot
will be constructed as a site plan modification to Building O. Construction of the
Northwest Parking Lot is planned to happen in the summer of 2020.
With most of the Cannery District PUD site and buildings constructed or nearing
final construction, the final stormwater facilities will be constructed. The final
stormwater management facilities will be built in conjunction with the Northwest
Parking Lot as a part of the Building O site plan modification. The final facility will
consist of reconfiguring the existing stormwater retention basin in the northwest
corner of the site, into two connected vegetated retention basins and an
infiltration chamber system under the proposed new Northwest Parking lot.
No stormwater infrastructure is required for the Building J pad-site development.
All required conveyance and retention facilities have been constructed in
previous phases as described in this report. Proposed site stormwater
improvements for Building J pad-site development are:
• Site grading to divert stormwater away from the building
• Connection of building roof drains to the existing storm drain service stub
• Install area drains in the plaza space to provide sidewalk drainage
• Install a landscape area drain to prevent ponding on the site.
The developer’s contact information is:
Cannery District Partners, LLC
1006 W Main Street
Bozeman, MT 95715
Stormwater Plan 3 3/27/2020
Figure 1: Vicinity Map
Existing Conditions and General Design Criteria
The site’s existing conditions consisted of four remodeled buildings within the
previously completed Phase 1 area. Most of the land was undeveloped, but has
since been improved with buildings, hardscape, landscape and utilities. The site’s
topography allows for drainage to the northwest corner. During Phase 1, a storm
water retention basin was constructed in this location, as was a network of storm
pipes and bioswales. Combined, these facilities provide the storm drainage
infrastructure needed for the site’s stormwater management.
In accordance with the City of Bozeman Design Standards and Specifications
Policy (COB Standards) an engineering analysis was conducted evaluating all
four phases within the Cannery District property. Including the final
reconfiguration of the retention basin in the northwest corner of the site.
All proposed storm sewers have been sized to convey the 25-year storm event
with no surcharging. The reconfigured stormwater retention pond has been sized
to handle the volume generated from a 10-year, 2-hour event per COB
Standards. Stormwater bioswales have been installed throughout the site to
further reduce surface runoff, reduce peak flows, and enhance stormwater
infiltration.
Stormwater Plan 4 3/27/2020
Storm Drainage Plan
Storm drainage improvements for the Cannery District will be constructed in
accordance with City of Bozeman Requirements. The existing stormwater
retention pond in the northwest corner of the site has provide the “backbone”
drainage infrastructure for the previous phases. With most of the Cannery District
buildings and site fully constructed, the final site stormwater facilities can be built
without the added risk of contamination from construction site runoff.
The overall storm drainage plan for the Cannery District is depicted on Figure 2
at the end of this report. Overall drainage direction on the proposed development
is indicated by arrows showing the proposed drainage patterns and inlet
locations.
Estimation of Retention Volume
Runoff estimates for the full site were obtained using a 10-year 2-hour design
storm. The Bozeman IDF curves show a rainfall intensity of 0.41 in/hour for the
10-year 2-hour design storm. Using a runoff coefficient of 0.8 (Commercial
Downtown), and the sites total area, it was determined that a 10-year, 2-hour
retention volume of 35,842 cf will be required. The actual site runoff coefficient
was calculated to be 0.76. The more conservative 0.8 runoff coefficient was
maintained with the final calculations to allow for future minor increased to the
impervious area.
Table 1 – Required Retention Volume
weighted C Factor C - 0.8
Intensity I in/hr 0.41
Area A acres 15.18
Runoff Q cfs 4.98
Retention Volume V cf 35,842
The original temporary stormwater basin was sized based on a total drainage
area of 16.45 Acres. Due to the Oak Street widening project and the associated
Oak Street Basin, 1.27 Acres of drainage was diverted from the site. The sites
final stormwater design is based on a drainage basin area of 15.18 acres.
Retention Facilities
The majority of the Cannery District’s site runoff will be treated by the
reconfigured stormwater retention basins located on the northwest corner of the
site, bordering the interstate right-of-way. The existing temporary retention basin
will be reconfigured into three separate stormwater features consisting of two
vegetated retention basins and an infiltration chamber system under the
proposed new parking lot. The two retention basins will have slopes of 4:1, and a
maximum water depth of 1.5 ft, and will be revegetated with grass. The chamber
Stormwater Plan 5 3/27/2020
system under the proposed parking lot will consist of 5 rows of Stormtech
Infiltration chambers that will evenly distribute stormwater though a crushed
gravel reservoir. The three retention basins will be built as part of the Northwest
Parking Lot construction and will have a total storage volume of 19,599 cubic
feet.
To supplement the retention volume, and achieve Low Impact Design (LID),
bioswales and area inlets dry wells were installed throughout the site within the
landscaped parking islands and near the buildings. Bioswales and area inlet dry
wells add retention volume and slow the peak flows thus increasing efficiency of
stormwater infiltration by dispersing it throughout the site.
The site bioswales have a 12” ponding depth below the overflow outlet and
consist of 24” of bioswale media (mixture of sand, topsoil, and compost) topped
with vegetation per the landscape plans. Bioswale capacities were calculated
using a 0.35 void ratio. Throughout the Cannery District site, 10,575 sf of
bioswales and subsurface gravel infiltration systems were constructed, adding an
additional 17,179 cf of storage.
The volume provided by each phase of construction shown in Table 2, totaling
36,778 cf of retention storage, and exceeding the required storage amount for
the calculated 10-year, 2-hour storm event by approximately 936 cf.
Table 2 – Retention Capacity by Phase Area
Phase Volume (cf) Notes
1 6,878 Bio-swales
2 4,614 Bio-swales
3 5,340 Bio-swales / Sub Surface Gravel Infiltration Systems
4 348 Sub Surface Gravel Infiltration Systems
5 19,599 Reconfigured Retention Basins / Chamber System
TOTAL 36,778 Total Site Stormwater Retention Capacity
Required 35,842 Total Required Site Stormwater Retention Volume
Curb and Gutter Capacity
Storm runoff will be collected in street and parking lot curb and gutters. The
hydraulic capacity of the curb and gutter was analyzed to determine if the curb
would be overtopped in a 25-year event. The maximum 25-year event for any of
the new curb inlet basins produces approximately 3.77 cfs of runoff. The capacity
of the curb at the lowest designed slope of 0.5% is 5.03 cfs at a depth of 0.15
feet below the top of the curb. Therefore, curb capacities in the development are
adequate to carry the 25-year event.
Stormwater Plan 6 3/27/2020
Table 3 – Curb and Gutter Capacity Calculations
Right-side slope X:1 0.05
Left-side slope X:1 33.00
Channel bottom width (ft) 0
Flow Depth (ft) 0.350
Flow Area (ft^2) 2.024
Wetted Perimeter (ft) 11.906
Width 0.035
Hydraulic Radius (ft) 0.170
Manning's Roughness 0.013
Slope (ft/ft) 0.005
Average Velocity (ft/sec) 2.49
Flow (ft^3/sec) 5.03
Inlet and Area Drain Capacity
The curb inlets and area drains on the roadways in the development were
analyzed to ensure they have capacity to accept flows conveyed to them without
reaching a point 0.15 feet from the top of the curb. The COB approved Neenah
Foundry R-3067-L inlet structure has a capacity of 5.98 cfs at 0.35 feet of head
(0.15 below top of curb). The COB approved Neenah Foundry R-2533 area drain
has a capacity of 3.2 cfs at 0.45 feet of head (0.15 feet below top of lowest
adjacent curb). These capacities will convey the estimated flows.
Pipe Conveyance Capacities
The reconfiguration of the northwest retention basin will include a 24-inch culvert
connecting the two detention basins. A 24” storm drain will connect the
reconfigured east basin to the chamber system under the new parking area.
Giving the layout of the three basins and the maximum basin ponding depth of
1.5 feet, the pipes will act as equalizer pipes between the basins rather than
traditional conveyance pipes. Attached Table 4 shows that the capacity of the
Cannery District storm sewer remains near 75% during peak runoff. Calculations
predict surcharging of the 15” outlet pipe coming from the Northwest PUD
development; however, the Cannery District storm mains are able to
accommodate the full-flow from the Northwest PUD’s 15” outlet—as well as the
anticipated runoff from the Cannery District development. Pipe 1B calculations
indicate the 30” main will be running at 86% capacity at peak flows; however the
infiltration and retention from the bioswales throughout the site have not been
included in the calculations of the runoff—thereby inflating the flows that should
actually be seen within the collection network.
Facilities Maintenance
The proposed storm drainage facilities will be operated and maintained by the
property owners’ association (POA), as required by the existing Cannery District
Covenants. The POA is responsible for quarterly maintenance of all storm water
Stormwater Plan 7 3/27/2020
facilities. The maintenance includes, but is not limited to, visual inspection and
removal of sediment from curbs, inlets, retention basins, and outlet structures as
well as any necessary repairs to the facilities. Relevant portions of the Cannery
District covenants’ have been included in the appendix of this report which
outline the prescribed maintenance activities.
Figure 2
Stormwater Site Plan
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BUILDING JCANNERY DISTRICT PH 2211 EAST OAK STREET
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Appendix
reasonabletime from the commencement of such work.Delays in commencement of
constructionmay be grantedforgood causeatthesolediscretionoftheAssociation.
(d)Repairs;Replacements.Allrepairs,alterations,replacementsor additionstoImprovements
shallbe at leastequal to the originalwork in classand quality.The adequacy of such
repairsshallbe measured by the same standardsrequiredforthe originalconstruction.
Maintenancework shallbe incompliancewiththeDesign Guidelinesand,dependingupon
thescope of work,may be subjectto priorreview and approvalof the Cannery District
Review Committee.
(e)Enforcement;Remedies.Ifan Owner failsto performproperlysuch Owner's maintenance
responsibilities,the Associationmay perform them and assessallcostsincurredby the
AssociationagainsttheLot and theOwner.However,exceptwhere entryisrequireddue to
an emergency situation,the AssociationshallaffordtheOwner reasonablenoticeand an
opportunityto curethe problem priorto entry.The noticeshallsetforthwith reasonable
particularitythe maintenance,repairsorreplacementsdeemed necessary.The Owner shall
have 10 days withinwhich tocomplete such maintenance,repairsor replacements.Ifthe
maintenance,repairsor replacementsarenotreasonablycapableof completionwithinthe
10-dayperiod,the Owner shallcommence such work promptly and diligentlyproceedto
completeitwithina reasonabletime.Ifthe Owner does not comply withtheprovisions
hereof,theAssociationshallhave the rightand power to enterontotheLot and perform
such maintenance,repairsorreplacementswithoutany liabilitytoany Personfordamages
forwrongful entry,trespassor otherwise.The Owner of the Lot on which the work is
performedshallbe liable(jointlyand severally,ifmore thanone)forthecostof suchwork
and shallpromptlyreimbursethe Associationforsuch cost,togetherwith interestatthe
same rateas providedfordelinquentassessments,costsand expensesof collection,court
costs,and reasonableattorney'sfees(collectively"charges").The chargesshallalsobe a
continuinglienagainstthe Lot on which thework was performed.Ifthe Owner failsto
reimbursetheAssociationwithin30 days afterreceiptof a statementforsuch work from
the Association,the Associationmay record a notice of lien againstthe Lot.The
Associationmay foreclosethe lienin the same manner as alien forAssessments,and it
shallbe entitledto recover,in additionto the charges,allcostsand expenses of lien
enforcement,includingcourtcostsand reasonableattorney'sfees.
6.3 Noxious Weeds.The controlof noxiousweeds by theAssociationon thoseareasforwhich the
Associationisresponsibleand thecontrolofnoxiousweeds by each Owner on itsrespectiveLot
shallbe assetforthand specifiedundertheMontana Noxious Weed ControlAct (Montana Code
AnnotatedSections7-22-2101through7-22-2153)and therulesand regulationsof theGallatin
County Weed ControlDistrict.The Associationisresponsibleforcontrolof stateand county
declarednoxious weeds in and on the Common Open Space Lots.Each Owner shallbe
responsibleforthe controlof the stateand county declarednoxious weeds on itsLot.Both
unimproved and improved Lotsshallbe managed fornoxiousweeds.Ifan Owner failsto control
the noxious weeds after10 days noticefrom the Association(givenin accordancewith the
provisionsof Subsection6.2(e)above),the Associationmay cause the noxious weeds to be
controlled,and the Associationshallhave the enforcementauthorityand remediesprovidedin
Subsection6.2(e)ofthisDeclaration.
6.4 Storm Water Maintenance Plan.
(a)GeneralInformation.The Cannery DistrictutilizesLow Impact Development ("LID")to
mitigatestormwaterimpacts.The stormwaterfacilitiesconsistofcurband gutter,drainage
swales,bio-retentionareas(bio-swales),conveyancepiping,and a finalretentionbasin.The
systemisdesignedtoaccommodate allstormwaterfrom theProperty.A document entitled
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Storm Water Management Plan and Record Drawings ison fileatthe City of Bozeman,
EngineeringDepartment.
The principalstormwater conveyanceand treatmentfacilitiesarelocatedon theCommon
Open Space Lots,and are owned by Association.Privatelyowned LID storm water
facilitiesservingindividualLot development are encouraged and privatefacilitiesare
allowedtoconnecttotheAssociationfacilitiesthroughtheCommon Open Space Lots.
The AssociationisresponsibleformaintainingstormwaterfacilitiesontheCommon Open
Space Lotsexceptforprivateundergroundservicelines,which aretobe maintainedby the
Lot Owner.Allstorm waterfacilitieslocatedon privatelyowned Lotsshallbe maintained
by theLot Owner inaccordancewiththeapplicablemaintenancescheduleprovidedinthis
Paragraph6.4(b).
The storm water facilitiesare designed to operate without excessivemaintenance.
However,likeallinfrastructure,periodicmaintenance willprevent costlyrepairand
replacement.The maintenanceplan setforthin thisSection6.4 has been preparedin
accordancewith City of Bozeman guidelines.Over time,recommended maintenance
guidelinesmay evolve.PleasecontacttheCityof Bozeman Storm Water Divisionifany
questionsarise.
(b)Storm WaterFacilitiesMaintenanceSchedule.
(i)SiteHousekeeping.Sitehousekeepingisto be conductedcontinuouslyas needed.
The main causeof storm waterfacilitydamage ispoor sitehousekeeping.Sediment
trackedonto pavement can be washed intostorm water bio-retentionbasinsand
conveyance piping and damage these facilities.Trash can clog pipes and inlet
structurescausingpropertydamage.Sitehousekeepingconsistsofthefollowing:
*Keep sidewalkand pavement areasclean
*Pickup trash
*Restoredamaged landscapinginordertopreventsedimentrunoff
(ii)System Monitoring.System monitoringis to be conducted quarterly,except in
winter.The storm water facilitiesshallbe inspectedquarterlyto quicklyidentify
smallissuesbeforeexpensivedamage can occur.In additionto regularmonitoring,
thebesttime to inspectthe performanceof storm water facilitiesisduringrunoff
events.System monitoringconsistsofthefollowing:
*Observe system duringrunoff.Look forponding outsideofretentionareas.This
can indicatea cloggedinletorpipe.
*Inspectbio-retentionand retentionbasins
*Inspectinlets,manholes and pipes
(iii)Bio-retentionand RetentionBasin Maintenance.Bio-retentionand retentionbasin
maintenanceisto be conductedquarterly.The bio-retentionand retentionbasinsare
designedto provide long-term,sustainabletreatmentof storm water.With poor
housekeeping,however,sedimentcan clog thesefacilitiesand reduce infiltration
capacity.Proper and timelycleaningof incidentalsediment in these basinscan
preventthesefacilitiesfrom beingdamaged.The bio-retentionand retentionbasins
are intended to be vegetated,since the vegetationprovides microbiological
communitiesthatcan treatcommon pollutantsin stormwater.Unhealthyvegetation
can diminishthe performanceof thesebasins.The landscapingof thebio-retention
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basinsshallfollowtypicallandscapemaintenanceguidelines.The vegetationon the
bottom of the retentionbasinshould not be regularlymowed,sincemowing can
cause plant debristo build up in the basin.Bio-retentionand retentionbasin
maintenanceconsistsofthefollowing:
*Remove sediment,trash,and debris
*Inspectforhealthyvegetation
*Inspectforuniformpondingand waterdisappearsin3 days
(iv)Bio-retentionand RetentionBasin Maintenance.Bio-retentionand retentionbasin
maintenance is to be conducted annually.The vegetationshould be maintained
annuallytoreduceplantdebrisbuild-up.The retentionbasinvegetationshouldbe cut
to a heightof6"inthefallwiththeclippingsremoved from thebasin.Bio-retention
and retentionbasinmaintenanceconsistsofthefollowing:
*Remove dead plantmaterialsfrom bio-retentionbasins.
*Cut retentionbasinvegetationand remove clippings.
(v)Bio-retentionand RetentionBasinMaintenance.Thismaintenanceistobe conducted
long-termifand when necessary.Ifregularhousekeepingand maintenanceisnot
performedadequately,sedimentand debriscan accumulateinthebasinand reduce
the requiredstoragecapacity.Ifthisoccursthe basinsshould be excavatedback
down to originalplan grade.Long-term bio-retentionand retentionbasin
maintenanceconsistsofthefollowing:
*Hire a contractortoinspectand returnbasinconditionto initialdesignfound on
Cityengineeringplans.
*Dredge basinifsedimentbuild-upisgreaterthan6"
(vi)Inlets.Manholes,and PipingMaintenance.Inlets,manholes,and pipingmaintenance
isto be conductedquarterly.All storm water inletsand manholes have a sump to
capture sediment.If this sediment is not periodicallyremoved it can wash
downstream and clog infiltrationfacilities.The sump istypicallyvisiblefrom the
surfacethroughtheinletgrate.Inlets,manholes,and pipingmaintenanceconsistsof
thefollowing:
*Remove visiblesedimentfrom sump.
*Inspectpipeinletsand outletsforsedimentinpipe,and remove ifpresent.
(vii)Inlets,Manholes,and PipingMaintenance.Thismaintenanceistobe conductedlong-
term if and when necessary.If regularhousekeeping and maintenance is not
performed adequately,sediment and debriscan accumulate in the storm water
conveyancepiping,and pipesmay become clogged.Long-term inlets,manholes,and
pipingmaintenanceconsistsofthefollowing:
*Hirea contractortocleanpipesand remove allsedimentfrom manhole sumps.
SECTION 7 COMMON PROPERTY
7.1 Descriptionof Common Property Declarantintendsto convey to the Associationthe tracts
delineatedas the "Common Open Space Lots"on the Final Plat of the Cannery District
subdivision,togetherwith theImprovements locatedthereon.Upon Declarant'sconveyance of
such propertytotheAssociation,suchpropertyshallbe Common Propertyto be maintainedby
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