6 STORMWATER

MANAGEMENT
POND REQUIREMENTS

6.1 BACKGROUNDINFORMATION
.................................................................................. 1

6.2 AIMS/PURPOSES
........................................................................................................ 2

6.3 GENERAL
PROVISIONS
FOR
APPROVALS
............................................................... 2

6.4 SWMMODELING
PARAMETERS ANDDESIGN CRITERIA ........................................ 4
6.4.1 Introduction
..............................................................................................................
4
6.4.2 SWMDesign
Criteria..............................................................................................
4
6.4.2.1 WaterQuality
Storage ............................................................................................
4
6.4.2.2 ErosionControl
Storage........................................................................................
5
6.4.3 Hydrologic
Modeling...............................................................................................
5
6.4.3.1 Imperviousness
.......................................................................................................
6
6.4.3.2 ModelsforDetermining SiteRunoff ..................................................................... 7
6.4.3.3 InitialAbstraction
....................................................................................................
"7
6.4.3.4 CurveNumber
.........................................................................................................
8
6.4.3.5 Design
StormSelection.........................................................................................
8

6.4.4 SWMF
INLET
PIPE
DESIGN
CRITERIA
............................................................................... 9

6.4.5 CONCLUSION
.................................................................................................................. 9

6.5 DESIGN
REQUIREMENTS
.......................................................................................... 10
6.5.1 General
...................................................................................................................
10
6.5.2 References
............................................................................................................10
6.5.3 Figures
....................................................................................................................
11
6.5.4 Parks,Open Spaces andPathways .................................................................. 11
6.5.5 Specific
Design Features ..................................................................................... 11

6.6 COMMISSIONING
CONSIDERATIONS
....................................................................... 15
6.6.1 Maintenance
andMonitoring Priorto Assumption ........................................... 15
6.6.2 Proportional
CostSharing...................................................................................
15
6.6.3 Operation
PriortoAssumption ........................................................................... 1
6.6,4 Maintenance
AfterAssumption ...........................................................................
6.6.5 Operation
AfterAssumption................................................................................1

6.7 REFERENCES
............................................................................................................. 16

LIST
OFFIGURES
....................................................................................................... 18

Environmental
andEngineering Services
Department
TheCorporation
of theCityof London
Updated:December 2005
6 STORMWATERMANAGEMENTPOND REQUIREMENTS

6.1 BACKGROUND
INFORMATION

Basedon the initiatives adaptedby the Provincerelated to Best ManagementPractice

(BMP) guidelines and the Stormwater Management Planning and Design Manual
developedby the Ministry of Environmentin 1994and completionand acceptanceof
the subwatershed studies by City Council, the City of Londondevelopedandadapted
the Stormwater Management(SWM)Pond Guidelines Requirements in 1994.
1998, this documentwasupdatedto incorporate the Community Plan Area approach
andagain in 2002to reflect changesthat havebeenapprovedby the City Council in
2002.

These requirements focus on water resources management and an ecologically

(holistic) basedapproachthat treats stormwateras a resourceto be used, not
waste to be shunted awaydownstreamas quickly as possible. Thesestormwater
management facilities must be basedon a systemapproachto achieve (regional
solutions) and need to be developed as an amenity and managewater resources
incorporatingan ecologicalapproach.

Prior to 1994, the City of Londonhad fencedvirtually all constructedstormwater

detention or retention facilities within its urbanarea, especially near residential
developments.Fencing, deemedan unsightly and unnecessaryrequirement, has
beenabandoned by the City of London,following the combined efforts of staff from
Environmental Services,Planning,Parks, LegalandInsuranceRisk Administration.

The SWMissuesevaluatedas part of this updateinclude:

Updated provincial andmunicipalrequirements;
Legalconsiderationsrelatedto public safetyandliability
Designconsiderationsrelated to safety performance, ecologyand
aesthetics;
Internal municipalcooperationas a basis for implementation; and
Thepotential for public stewardship.

The SWM facility requirements included in this documentwere developed in a

comprehensive manner by reviewing the SWMpractices in other Ontario
municipalities, including Cambridge, Oakville, Markham,Ottawa, Waterloo,
Scarborough,Mississauga,Kitchener, Guelphand past applications in the City of
London.Thefollowing sections discuss the goals/aims and purposesof these SWM
requirementsand identify specific design requirementsfor SWM facilities to be
constructedin the City of London.

Environmentaland EngineeringServices Department 6-1

TheCorporationof the City of London
Updated: December2005
 Wherestormwatermanagement pondsare to be implemented,they shall incorporate a
water resourcesmanagement approach,be consistent with the ecosystemand shall
be created in a mannerthat is the technically sound, aesthetically pleasing,
ecologically diverse and wherepassive and active pursuits maybe realized in a
generally safe environment.Creatively designedand developed,these pondswill
complywith andsatisfy the required stormwatermanagement functions while providing
as a significant assetto the openspacesystemwithin the community.

6.2 AIMS/PURPOSES

TheproposedSWM
PondRequirementshave six aims/purposes:

1) to minimize
the risk of threat to life andhealthof the public while avoidingfencing
of the stormwatermanagement ponds;

2) to ensurecompliancewith all applicable municipalrequirementsandprovincial

legislation;

3) to provide requirementsfor use by developerson newprojects (temporaryand/or

permanent)
andthe City for retrofit applications;

4) to expedite the developmentof safety-oriented and naturalized stormwater

management ponds that present an aesthetically pleasing amenity to the
community
and reducethe reliance on heavily engineereddesigns;

5) to provide opportunities to integrate stormwater management

systemsinto
neighborhoodopenspacesystems; and

6) to implementa stormwater management system that has due regard for the
Ecosystem andwatershedas dynamicandliving systemsand is integrated with
the urbanized humancommunitywhile managingconveyance,erosion, flooding
andquality of the stormwater
flows.

6.3 GENERALPROVISIONS FOR APPROVALS

Concurrencewith the requirements will be determinedby the City Engineer, in

consultation with ParksPlanning. TheSWM
Pondrequirementsare:

ensuring compliancewith all applicable municipal requirements,standards,

policies andprovincial legislation, thus ensuringthat the life andhealthof the
public will be adequatelyprotected;

Environmentaland EngineeringServices Department 6-2

TheCorporationof the City of London
Updated: December2005
 maintaining and promotingthe enhancement of urban ecosystems,including
integration of SWMfacilities within openspacein a mannerthat is consistent
with the City’s NaturalHeritageSystem;and

ensuring an implementationof safe, well engineered, and cost-effective

stormwatermanagementsites.

Basedon Ontario WaterResourcesAct (OWRA),Section 53, SWM works are subject

to Certificate of the Approvalsfor this workandmustbe in compliancewith the MOE
requirementsassociatedwith the Storm/drainageand SWM servicing works, as well
as be in compliancethe City’s SWM designrequirementsas follows:

a) i) Permanentstormwater management (SWM)facilities are required to

locatedon landsthat the proponent
shall dedicateto the City of London.

ii) TemporaryStormwatermanagement (SWM)facilities are required to

located on lands that are dedicated to the City as an easementby the
proponentto ensurethat the City will be able to complywith the emergency
requirementsin accordancewith the EnvironmentalProtection Act and
Ontario WaterResources Acts.

Temporary Stormwater management(SWM)facilities that become

permanentSWM facilities basedon the MasterPlan and/or MunicipalClass
EnvironmentalAssessment (EA) recommendations will be required to
locatedon landsthat the proponentshall dedicateto the City of London.

b) Designsshall havedueregard for appropriateprovincial legislation andthe

City’s designstandards,
policies, guidelinesincludingbut not limited to:

¯ relevant SWMcriteria and environmentaltargets of subwatershed

studies
adaptedby City Council on September
18, 1995;

¯ the Conceptualand/or Functional Storm/DrainageSWM

Master Plans for
Community
Areas, approvedby City Council; and,

¯ the City of LondonDesignSpecifications and consistencywith the London

Official Plan.

SWM Designsmayincorporate innovative approaches,providedthe intent of the SWM

Pondrequirements, goals, aims and purposes derived for public benefit are
achievable.

Cost effectiveness shall dependon capital, operational and maintenancecost

requirements, as well as a cost/benefit analysis of those factors which are more
difficult to define from an economicperspective(i.e. safety, environmental/social
benefits). Suchdeterminationandapprovalof intent will be at the discretion of the

Environmentaland EngineeringServices Department 6-3

TheCorporationof the City of London
Updated: December2005
 EnvironmentalServices Departmentin consultation with Parks Planning, the Legal
Department and Insurance/Risk Administration. The Environmental Services
Departmentis designated to coordinate commentsand approvals for SWM Ponds
fromall theseparties.

SWMMODELINGPARAMETERS
AND DESIGN CRITERIA

Introduction

TheLondonDevelopment Institute (LDI) has identified a numberof comments related

to the selection and implementationof the StormwaterManagement (SWM)criteria
hydrologic modeling,as well as the present City general practices, standardsand
requirements.

In responseto LDI’s request, the EnvironmentServices Department(ESD)undertook

a review of the SWM modelingcriteria and parameters.A numberof WaterResources
Engineers-Practitioners wereconsultedfor this reviewthat confirmed,similar to the
city of London,the majority of Ontario municipalities apply standard provincial
practicesfor the selectionof theseparameters.

A Professional Water ResourcesEngineer (Subdivider’s Consulting Engineer)

responsiblefor recommending all SWM modelingparametersto ensurethe application
of adequateengineeringknowledge is applied. At the sametime, ESDis required to
review the proposed SWMsystems and selection of the SWMmodeling
parameters/criteria to ensure compliancewith City and Provincial standards,
requirementsand practices, and also ensurethe adequateprotection of the people
andpropertiesof the City of London.

SWM
DesiqnCriteria

WaterQuality Storage

Imperviouspercentageis describedby two parameters,Total ImperviousPercentage

(TIMP)and Directly ConnectedImperviousPercentage(XIMP) values. ESDaccepts
the application of the lesser value, XIMP,whendetermining required storage
according to Table 4.1 of the MOEStormwaterManagement Practices Planning and
Design Manual (MOEManual-1994). Wealso wish to acknowledge that the
recommendedselection of this parameter
is consistentwith current City practices.

Thewater quality storage volumesper hectare are established in Table 4.1 of the
MOEManualand consist of two components:40m3/haof extendeddetention quality
control storage (live storage) andthe remainingportion representspermanent pool
quality storage (deadstorage). Therequired 40 m3/haof quality extendeddetention

Environmental
andEngineering Services
Department 6-4
TheCorporation
of theCityof London
Updated:December2005
 storage is constant and required in all cases. The remaining permanentpool
componentof waterquality storageis dependent
uponthe three following factors:

i) imperviouspercentage
(discussedin Section6.4.3.1

ii) Protection Level of the ReceivingWatercourses

(the protection level of
receiving watercourseswill be revisited in the subwatershed studies
updates);and

iii) proposed type of SWM

facility ( i.e. wetpond,dry pond,wetland,
infiltration).

Additional extendeddetention storage maybe required for erosion/stream

morphologyand attenuation control to complywith the Council accepted
SubwatershedStudy requirements and/or to address lack of conveyance
capacity in the outlet system.Theseparametersare to be established by the
Subdivider’sConsultingEngineerall to the satisfactionof the City Engineer.

6.4.2.2 ErosionControlStora,qe

Erosion control storage volumesreflect the needto maintain existing fluvial

geomorphology,protect watercourses from further deterioration andensureprotection
of public safety and property. TheCity of Londoncompleted13 Subwatershed Studies
all of which were adopted by City Council in 1995. The SubwatershedStudies
identified SWM erosion control criteria on an individual basis, in somecaseseven
establishing requirementsfor each tributary of subwatershed(ie: Dingman Creek
Subwatershed)

All facilities require a minimum of 40 m3/haof extendeddetentionstorage. Additional

erosion control protection maybe required if the facility is to be located within a
subwatershed that identifies specific erosioncontrol requirementson top of the quality
control extendeddetention. Shouldthe consulting engineercompletea site specific
geomorphilogical/fluvial assessment, alteration to the erosion control requirements
maybe considered.

6.4.3 Hydrolo,qicModelin,q

This section provides guidancein the selection and implementationof hydrologic

modeling parameters. Consultants maymakeuse of available water resources
management manualsand texts as a reference to aid in the selection of hydrologic
modeling parameters. Any externally referenced material employedin parameter
selection shouldbe properly referencedin the FunctionalSWMReportandincludedin
the documentappendices.

Environmental
andEngineering Services
Department 6-5
TheCorporation
of theCityof London
Updated:December 2005
6.4.3.1 Imperviousness

Current City of Londonpractices for determiningsite runoff for Conceptualand/or

Preliminary SWM plans use the values for Total ImperviousPercentage(TIMP)and
Directly Connected ImperviousPercentage(XlMP).

TIMPrepresentsthe ratio of area coveredby an impervioussurface (e.g. asphalt,

concrete)to the entire area. XIMPrepresentsthe ratio of imperviousareasdirectly
connectedto the conveyance system.An exampleof a directly connectedimpervious
area wouldbe a parkinglot, a portion of roof areas, driveways,or roadsthat contain
catchbasins draining to the storm sewer. An exampleof a non-directly connected
imperviousarea is an outdoorbasketball court surroundedby park land or roof area
drainingto a rear yard.

In order to ensurethat the proposedSWM volumes,land requirementsand the

size of the SWMblock are estimatedcorrectly, imperviouspercentageselection
is extremelyimportant. If the SWM block is oversized, there maybe adverse
effects on the economicviability of the proposeddevelopment, and if
undersized,there couldbe negativeimpactson adjacentexisting propertiesand
homeowners.

Thetable belowlists current City of Londonpreferred TIMPandXlMPvaluesbasedon

land use. These allowable ranges for TIMPand XIMPshould be applied at the
conceptual/preliminarydesignstage to ensuresufficient land is allocated for the
proposed facility. Adjustment of Impervious Percentage values at the
functional/detailed design stage will always be consideredand accepted,
subjectto the consultingengineerprovidingengineeringcalculationsto justify
the revision of these parameters.

Land Use TIMP XIMP

55% 45%
Residential 43%- 48%
51%- 60%
Mediumand High
Density 70% 55%
Residential 65%- 75% 45%- 55%
Commercial/Industrial 75%- 90% 70%- 80%

In 2001,ESDconducteda cursory evaluationof three newresidential subdivisionsto

assessimperviousness andthe results wereconsistent with the City’s acceptedTIMP
and XlMPvalues. This issue has beendiscussedwith a numberof the Consultants,
LDIandthe City will acceptthe following twostepapproach:

Environmental
andEngineering Services
Department 6-6
TheCorporation
of theCityof London
Updated:December 2005
 iv) At the Master Plan level, TIMP and XlMPshould be assigned the
MAXIMUM(not average)imperviousnessallowed by the City.

ii) At the detail designlevel, TIMPandXlMPcan be assignedthe "actual"

imperviousness.

6.4.3.2 Modelsfor Determinin.qSite Runoff

Site runoff for both pre-devel0pment and post-developmentconditions is determined

by subtractingthe predictedinfiltration volumefromthe estimatedtotal rainfall volume.
Thereare multiple modelsand methodsfor determininginfiltration and thus total
runoff. TheCity of Londonwill considerall acceptedmethods/models for determining
infiltration andrunoff providedthey are applicable andappropriateto the proposed
development.

Themajority of water resourcessubmissionsreceivedby the City of Londonapply the

SCSMethod, fewer submissions apply the Horton Methodand there have been no
submissionto date that apply the Green-Ampt Methodof quantifying runoff. SCS,
Horton or Green-Ampt methodsare all acceptable modelingtechniques. "The best
modelis the one you knowbest". This is true, the moreyou knowthe model, the
better you will able to modela particular phenomena."

The LDI recommendation

of applyingthe HortonMethodis already practiced by
the City of London,

6.4.3.3 Initial Abstraction

Initial abstraction(la) representsthe interception,infiltration, andsurfacedepression

storageof rainfall at the beginningof stormevents. CurrentCity of Londonmodeling
practices recommend the la values summarized below. Thesevalues are applied by
the majority of WaterResources Engineeringpractitioners acrossthe Province.

Land Cover la Typical Values

Impervious 2 mm
Pervious- lawns 5 mm
Pervious - meadows 8 mm
Pervious - woods 10 mm

In order to consider any deviation from these recommended

la values, the proposed
SWM modelingwill needto be tested in the field and technical data presentedto
confirmthe suitability of the calibratedparameters.

Withoutsuchconfirmation,this requestis not consistentwith either the general

SWM modelingselection practices, nor is it justifiable beyondan academic

Environmentaland EngineeringServices Department 6-7

TheCorporationof the City Of London
Updated: December2005
 exercise. The City’s mainconcernis that the facilities maypotentially be
undersizeddueto overestimationof initial abstractionsvalues, resulting in
overtoppingof the facility duringstormevents.

6.4.3.4 Curve Number

Thecurve number(CN) is a parameterusedto determinethe extent of rainfall that

infiltrates, rather than becomingsurface runoff. It is a measureof a watershed’s
hydrologic responsepotential and is usually selected from available government
documentsand handbooks(See attached). Currently, the City of Londondoes not
have a specific table of CNvalues to be usedin modelingpractices. However,CN
values mustbe consistent with provincial guidelines and standardwater resources
management practices and correspondwith the specific geotechnical conditions of
proposeddevelopments.

Selection of CNshould be correlated with the applied Initial Abstraction (la)

OTTHYMO model recommendsthe use of CN*. The CN* procedures account for
recalculating CNwhenan initial abstraction of less than 0.2"S is used. OTTHYMO
doesnot recommendthe use of 0.2"Sas initial abstraction,requiring the useof CN*."

6.4.3.5 Desi.qnStormSelection

In the designof individual SWMF’s a 3-hourChicagoRainfall Distribution shouldbe

applied as recommended in the subwatershed studies. TheCity of Londonpresently
applies this standard.Withrespect to larger subwatershed andcommunity area plans,
the applicationof AESstormsis standardandthe City is planningto continuein this
practice. TheChicagodistribution is widelyaccepted
as a syntheticdistribution to be
usedin the designof urbanareas.

Also, where:Rainfall intensity (mm/hr)= A/(t+B)^C

A 724.69 1330.31 1497.19 1455~00 1499.06 1499.53

B 5.500 7.938 7.188 5.000 4.188 3.297
C 0.800 0.855 0.850 0.820 0.809 0.794

Thesubdivider’s consultingengineeris requiredto evaluateall applicablestormsand

recommend the most appropriate on a case-by-casebasis. For each problem(i.e.
Flood Control, ErosionControl), a "critical" storm should be selected for design
purposes.

Environmental
andEngineering Services
Department 6-8
TheCorporation
of theCityof London
Updated:December 2005
6.4.4 SWMF
Inlet Pipe Desi.qnCriteria

According to the MOEGuidelines for the Design of Storm DrainageSystems,the

SWMfacility inlet pipeshouldrepresenta free outlet. Therefore,the inlet pipeinvert is
to be abovethe projected 2-year stormpondingelevation. Non-compliance with this
standard maycreate surcharge conditions within the newstorm sewer system
requiring additional maintenance associatedwith the potential sedimentaccumulation,
as well as create potential liabilities underthe OntarioHighways Act shouldsurface
pondingoccuron streets.

Should, in rare cases, we needto consider deviation on the abovenoted design

criteria, the subdivider’s consulting engineer will be required to undertakean
engineeringanalysis to demonstratethat the proposeddeviation will havea minimum
effect on the proposedsewerHydraulic GradeLine and will not create an adverse
effect on the system.

The storm sewer systemsare generally designed for minor system and basedon
current City of Londonpracticesit is a 2-yearstormevent.Specialcasesrepresenting
exceptionsto this generalrule will be evaluatedon a site-by-site basis. Surcharging
is
expectedduring larger storm events and a major flow route mustbe established to
accommodate these flows.

6.4.5 Conclusion

TheSWM modelingcriteria and parametersshould be established at the conceptual

design stage and approval of this plan mustbe received prior to proceedingwith
functional detailed design,as is current City practice. Theintention of the Functional
SWM Plan is to refine the findings of the conceptual SWM Master Plan. Major
deviationsfrompreliminarydesigninformationoccuronly in rare events,whendetailed
design has proceededwithout final acceptanceof the ConceptualSWM MasterPlan
(ie: StoneyCreek).

A draft revision of the MOEStormwaterManagement Practices Planning and Design

Manualis presently underpublic review with an intendedrelease by the endof this
year, Thecurrent edition of the Manualis a widely accepteddocumentuponwhichthe
City of Londonhas chosento base its standards, as have manyother Ontario
MunicipalitiesandConservation Authorities.

Flooding has not beenobservedat any of the recently constructed SWMF’s, all of
which applied MOEand MTOstandard parameter values. However, the City of
Londonhas received a numberof homeowner inquiries regarding SWMF water levels
andexpressingconcernthat the facilities be adequatelydesignedto protect their
properties. This illustrates a growingpublic awareness of SWMissues related to the
location and size of proposedfacilities anddemonstratesthe importancecarefully
selecting parameters andmethods to protect public safety andproperties.

Environmental
andEngineering Services
Department 6-9
TheCorporation
of theCityof London
Updated:December 2005
 Wetrust the information clarifies the SWM modelingdesignpractices usedby the City
of London.Researchof modelingparametersand SWM practices employedby other
Municipalities, Conservation Authorities and water resources management
practitioners reveal that City of London
practicesare consistentwith current province-
widepractices. TheCity metwith LDIrepresentativesandconsultantson
January27, 2003to discuss a draft of this documentand which wasmodified to
incorporatecomments providedat this meeting.

Shouldthe parametersof the SWM ModelingParametersand DesignCriteria Section

require further clarification, pleasecontact the Stormwater
Management
andDrainage
Engineering Division.

DESIGN REQUIREMENTS

General

SWM facilities (temporaryand/or permanent)shall meetengineering, maintenance,

safety, planning, environmental,aesthetic andeconomic
requirements,while avoiding
fencing.

SWM Pondrequirementsfor private property/developments are to generally conform

to thedesigncriteria in this manual,
all to the satisfactionof the City Engineer.

6.5.2 References

Thebasis for implementing these requirements

will be a designcriteria that includes
but is not limited to the following and subject to approval by the applicable SWM
agencies:

¯ the City’s Subdivision and Development Guide Manual: (Guidelines

Procedures);

¯ the SWM criteria and environmentaltargets identified in the Subwatershed

studies acceptedby City Council;

¯ Conceptual/Functional Storm/Drainage SWM

Master Plans for Community
Areaplans;

¯ City of London
Official Plan

¯ UTRCA
Guidelinesfor the SWMFs
located in Flood Plains;

Environmentaland EngineeringServices Department 6-10

TheCorporationof the City of London
Updated: December2005
 the Stormwater ManagementPractices Planning and Design Manual
developedby the Ministry of the Environment
in 1994,draft updaterevised in
1999;

Best Management Practices (BMPs)issued by the MOEand successfully

implemented
by variousmunicipalities in southernOntario; and

¯ all applicablemunicipalrequirements
andprovincial legislation.

6.5.3 F_~ures

Attached, Figures 1, 2 and 3, which accompany these requirements, incorporate

generalized design features as published in recognizedmanualsor guidelines, as
adaptedandmodifiedto reflect acceptedpractice in southernOntariomunicipalities.

The figures are basedon the use of Attenuation/ExtendedDetention and/or Wet

/Hybrid WetFacilities. However,they canalso applyto dry facilities if the wet pond
and sedimentforebay componentsare removed.Thesefigures provide conceptual
sketchesthat maybe used to guide design, while respecting individual SWM needs
andsite features, andshould not be viewedas criteria for designpurposes.Design
criteria informationshouldbe obtainedfromthe source(s)identified above.

6.5.4 Parks, OpenSpacesand Pathways

Wherestormwatermanagementfacilities are designedto be integrated with adjacent

parks, openspacesor pathwaysystems,greater setbacksmaybe required subject to
PlanningDivision Requirements
and Approvals.

Since SWM Facility land is claimable under the rules of the UrbanWorksReserve
Fund(UWRF),whereadditional land, beyondthe technical requirementsof the SWM
Facility is required, theselandswill be identified as a separatePark/Open
Space
Block
on the registeredplan.

6.5.5 Specific DesignFeatures

Fifteen key SWM Ponddesignfeatures havebeenidentified to reducethe risk of injury

to children (aged1 to 8 years), while maintainingfacility function. Thesebiophysical
safety featuresare intendedto restrain accessto deepstandingwaterthrougha series
of spatial, physical, natural andaesthetic barriers or throughalternatives to direct
access.This is not intendedas a replacementfor adult supervisionbut rather as a
deterrent to casual accidents. Theintent is to replace fencing with an appropriate
alternative, while maintainingSWM function and public safety. The15 key SWM Pond
designfeaturesinclude:

Environmentaland EngineeringServices Department 6-11

TheCorporationof the City of London
Updated: December2005
 1) A sediment forebayis incorporatedto induce treatmentand improvepollutant
removalby trappinglarger particles nearthe outlet of the pond:

a) the sedimentforebaymustbe 1.0-1.5mdeepto minimizea potential re-

suspension
andecologicalconditionsfor WestNile Virus,

b) the sedimentforebaysizing must be donein accordancewith the MOE’s

SWM Practices Planning and Design Manual,and

c) the sedimentforebayshould be constructed with a maintenanceaccess

route to permit future monitoring and maintenanceas well as provide
accessin the event of an emergency;

2) A ponddepth of 1.5-2.0mis preferred. Shallowpondsof less than 1.0mare

likely to be ineffective, and should be discourageddue to the possible re-
suspension of sediment and greater land requirements. The maximum SWM
facility depth shall not exceed3.0m with a maximum 0.3m freeboard. A
positive overlandflow path mustbe providedat the 3.3mwater level. The
permanent pool depthin wet SWM facilities mustbe 1.0-1.5mdeep.

3) A naturalized low flow channelwith a shallowchanneldepth(0.3 to 0.6m

preferred)leading to the area of ponddrawdown;SWM facility inlet sewers
mustbe designedto enter the facility as free outlet systemsduring 1:2-year
storm events, This standard is in accordancewith the Ministry of the
EnvironmentGuidelinesfor the Designof StormSewerSystems.

4) For extendeddetention/hybridandwet faclities 5:1 side slopesmaximum

or flatter, for dry facilities 4:1 side slopesmaximum must be applied
aroundthe perimeterof the sedimentforebayandupperandlower cell; slopes
mayvary arounda facility to create a natural appearance
with the preferred
slopes being maximums;

5) Steeper slopes (maximum

3:1) maybe allowed to be used when these
slopesare:

representingonly 15-20%fromthe total area;

combinedwith a minimumbuffer of 5.0mfrom 0.3 m abovethe 100

year stormeventelevationto the propertyline; and

¯ combinedwith unfriendly vegetation.

6) The 2 year stormevent extendeddetention and storage component of wet

facilities shoulddischargeover a 24 to 48 hourperiod and the quality
controlpondsare not allowedto be locatedin line. Dry facilities shouldbe
used mostly as an attenuation/flood control systemand pondingwill be of

Environmentaland EngineeringServices Department 6-12

TheCorporationof the City of London
Updated: December2005
 relatively short duration and infrequent in occurrence; the permissible
discharge for all facilities is basedondetailedengineering
analysis;

All maintenanceholes located within stormwatermanagement pondsrequire

hard surfaceaccess.Accessroadsbelowthe 100year flood line will require a
turfstone surfaceon a granularbase. Theturfstone voids shall be filled with
granularA. For all other requirements,
refer to Section3.15.15for hardsurface
details.

7) Stormwaterfrom the forebay shall be held in a permanent wetretentionpond

andshouldbe located in the facilities lower cell (assumingthe generalmain
cell designreflects an overall safety criteria of gentleslopesandaquaticsafety
benchesor suitable barriers);

8) AnySWM
facility proposedto be located withinFloodPlain lands are subject
to:

a) UTRCA
guidelines and approvals;

b) forebays being located abovethe 50 year storm line with any

deviationfromthis requirement beingsubjectto specific technical
justifications approved
by the City;

c) mainfacilities beinglocatedabovethe 25 year flood line;

9) A naturalized landscapeplan, approvedby Parks Planning in consultation

with the City Engineer,is requiredfor all stormwaterretention anddetention
facilities. Landscapeplans shall conformto the "Planting Guidelines for
StormwaterManagement Ponds"available from the PlanningDivision.

Seedingof exposed soil surfacesshouldbe doneas soonas possibleafter fine

grading is complete.All landscapetreatmentsspecified in the approvedplan
should be installed after seed has established, but within two years of
registration of a subdivisionplan or development
agreement.

1o) In lieu of fencing, unmowedvegetatedbuffers will be required aroundthe

perimeter. This buffer shouldbe comprisedof tall grassesand wild flowers,
followed by trees and denselyplanted shrubs. A denselyvegetatedmarginon
the aquaticsafety benchwouldserveas an aesthetic amenityandan additional
naturalbarrier.

This denseunfriendly vegetationshouldact as a naturalbarrier to all but the

mostdeterminedindividuals. Openings can be providedif warningsare posted
advisingthosewhoapproach the facility of its purpose,operationandpotential
safety hazards;postedwarningsshouldbe visible as emergency accesspoints
in the eventthat the barrier is penetrated;

Environmental
andEngineering Services
Department 6-13
TheCorporation
of theCityof London
Updated:December 2005
 Therequirementsfor fencingstipulated in Section11.5, Parks& OpenSpaces,
Fencing,are not applicableto SWM
Facilities.

11) An aquatic safety benchmust be constructed around the forebay and the
maintreatmentcells with the lower edgeto be located 0.9 mabovethe pond
bottomwith a minimum2 mwidth and incorporate a minimum slope of 10:1 or
flatter.

12) Pedestrianand cycle paths mustalwaysbe located no lower than the 5

year stormeventwaterelevationand usedin conjunctionwith the preferred
slopesdiscussedin item (4) to further maximizerecreational user safety and
minimizepublic risk andliability. Pathsbelowthis point andleading to the
lower portions of a facility shouldbe postedto warnthe public of potential
safety hazardsduring pondoperation; and

13) Restrictedareasignagewill be necessaryto warnthe adult public to avoid

areasor activities undercertain conditionsif a number
of thesefeatures are
modifiedextensivelyand/ornot included.

14) The minimum buffer width (separation area betweenthe SWMF and land
features suchas ESA,mainwatercourses,significant ecological features and
openspace,designation,etc.), is subject to City Official Plan requirements,
policies, Provincial andFederalActs, Policies andRequirements.

15) A Sediment andErosionControlPlan during the construction activities must

be developedand included in the Functional SWM Report for the proposed
SWM Facility, to be reviewedandacceptedby the City. Specific requirements
for the protection of adjacentnatural areasmaybe requiredas outlined in the
relevant Environmental ImpactStatementfor the development.

Theserequirementsmustbe applied to all SWM applications. It is recognizedthat in

someinstances, unique circumstancesmayarise wheresomerequirementscannot be
accommodated. In these cases, the onusis on the proponentto demonstratehowthe
proposed designdeviatesfromthe requirements,yet still meetsthe spirit andintent of
this overall document.Deviations must be approvedby City Council. Additional
design guidelines for inlet structures, outlet structures, maintenanceaccess,
pathways,etc. is available in the City of LondonSubdivisionandDevelopment Guide
Manual,Section E: Guidelines:andProcedures.All SWM Facility design standards
identified in this document,EnvironmentalServices Design Specifications and
Requirements,are basedon the revised standardsapprovedby City Council in July
2002 and supercede any similar requirements listed in the Subdivision and
DevelopmentGuide Manual.

Environmental
andEngineering Services
Department 6-14
TheCorporation
of theCityof London
Updated:December2005
6.5.6 MaintenanceHole Access

All maintenanceholes located within stormwater management pondsrequire hard

surface access.Accessroads belowthe 100year flood line will require a turfstone
surfaceon a granularbase.Theturfstone voids shall be filled with Granular’A’. For
all other requirements,
refer to Section3.15.15for hardsurfacedetails.

6.5.7 Emer.qency Sanitary SewerOverflow

Emergency sanitary seweroverflow (SSO)outletting upstreamof the SWM
facility
directly to SWMfacilities is not permitted.

COMMISSIONINGCONSIDERATIONS

Maintenance
and MonitoringPrior to Assumption

Maintenance
andmonitoringof the SWM
facility prior to City’s assumption,
mustbe:

carried out by the Subdivider/Developer

to demonstrate
the effectivenessof the
performanceof these facilities in accordancewith the approved design
constructionpractices;

in compliancewith the City and MOE’s"Monitoring and Operational Procedure

for the SWM
Facilities Prior to the City’s Assumption",
October1996;

¯ carried out by the Subdivider,prior to the City’s assumption,at no cost to the

City;

In compliancewith PlanningDivision’s Landscape MonitoringGuidelines. There

maybe other site specific monitoringrequirements
as a result of an EAor EISfor
the facility or development;
and

All landscapematerials are to be maintainedin a healthy state in accordance

with the approvedlandscapeplan until the time of assumption.A final inspection
is requiredbythe PlanningDivision, prior to assumption
of the facility.

6.6.2 ProportionalCostSharinq

Proportional Cost Sharing for maintenanceand monitoring of the SWM

facility
amongstbenefiting developerswill be required.

The Subdivider/Developer constructing a SWMPond which services other

subdivisions and that carry out maintenance,operations and monitoring of SWMF’s

Environmental
andEngineering Services
Department 6-15
TheCorporation
of theCityof London
Updated:December 2005
 prior to the City’s assumption,shouldbe allowedproportional cost sharing by others
serviced by these SWMF’s. Theabovenotedproportional cost sharing shall be based
on contributing storage volume of a SWM facility. Contributing Subdivider
/Developer’spayments to third parties shall:

commenceuponcompletionof the subdivider’s service work connections

to the existing unassumed
SWM services; and

continueuntil the timeof assumption

of the affectedservicesby the City.

6.6.3 OperationPrior to Assumption

Operationof the SWM

facility, prior to the City’s assumption,shall be in compliance
with the Operational and MaintenanceManual developed by the Subdivider’s
ConsultingEngineerand approvedby the City’s EnvironmentalServicesDepartment.

6.6.4 MaintenanceAfter Assumption

Maintenance of the SWM facility site, replacement

of biophysical components
suchas
deadtrees and shrubs or soil erosion, after the City’s assumption,will be the
responsibility of the City’s Environmental
ServicesDepartment.

6.6.5 OperationAfter Assumption

Operationof the SWM facility after the City’s assumption,will be carried out by the
City’s EnvironmentalServicesDepartment andwill include periodic dredgingof silt
deposits from the sediment forebay of the SWM pond. Removalof potentially
contaminated sediments mayrequire compliance with regulations under the
EnvironmentalProtection Act. Lawnmowing,litter removal, trail maintenanceand
vegetation inspection (especially wherea SWM facility is part of an openspace
scenario) will be subject to the EnvironmentalServicesDepartment’s maintenance and
operationsbudget.

6.7 REFERENCES

It shouldbe noted that in creating the SWM Pondrequirements,available literature

andcasestudy information wasreviewed,incorporating the intent andobjectives of
the City of London’sOfficial Plan subwatershedstudies, Natural Heritage System
environmentaltargets andcriteria, as well as provincial guidelinesin regardsto Best
Management Practices and the MOE’sSWM Practices Planning and DesignManual.

Therequirementspromotingthe integration of SWMpondsinto a community as part of

openspacesystems,rely on designandrisk reducingsafety measures andecological
features as replacements
for fencing. Theadvantages
that they provideto the City of

Environmental
andEngineering Services
Department 6-16
TheCorporation
of theCityof London
Updated:December 2005
 Londoninclude an enhancedurban environment,water resourcesmanagement, a
greatersenseof social well being, community
pride, increasedopportunitiesto enjoy
recreation close to homeandpotential neighbourhood"stewardship"of a facility when
implemented in an ecologically soundmanner.

Environmentaland EngineeringServices Department 6-17

TheCorporationof the City of London
Updated: December2005
 LIST OF FIGURES

FIGURE 1 PLAN VIEW

TYPICAL CONCEPTUALPOND DESIGN
WET/DRY/HYBRID

FIGURE 2 SECTIONAL VIEWS OF TYPICAL CONCEPTUALPOND DESIGN

WET/DRY/HYBRID

FIGURE 3 BASIN CROSS-SECTIONBASED ON MODIFICATION OF THE

PROPOSED SWM REQUIREMENTS

Environmental
andEngineering ServicesDepartment 6-18
TheCorporation
of the City of London
Updated:December 2005
 Typical ConceptualPondDesign
Wet/Dry/Hybrid

’located above the high

water level)

of bank
Maximumwater level
Maximum depth 3.0m)

of bank "manent Pool wet, \ Minor system outlet

~um 1.5m depth \ control structure
/,Storm Inh \

/ g s°ePwe~r
~’rnm channel
/ ! Restricted Outlet~-~

Sediment
Forebey Permanent Pool
(Herd Surface) or
Wet Meadow for
Dry Forebay t ~’\
Flow Dissapator

Top of ~ Safety sign wor~

/ / hazard during e~

/
-3 ~ ~ ~
. ~
/
/~
/
Maintenance (hard su~ace) "~ Major system overflow
"/--~emergency spillway to
open channel and/or roadway
--" / Natural,zeal w,th nat,ve trees, shrubs
/ ~ and grasses to create wildlife-
~ habitat/neighborhood amenity // /

Transition zone minimum 0.3m obove~

~
the 100 year storm event elevation.
Transition zone to be a minimum 5.0m wide.

CANADA
BASIN CROSS-SECTION BASED ON MODIFICATIONS OF
THE PROPOSED SWM REQUIREMENTS

5m

FREEBOARDLEVEL

100 YR WATERLEVEL 0.3m

ATTENUATION ZONE 0.5m

1 EXTENDED DETENTION ZONE 1.0m

5

~
PERMANENT POOL
MAX 1.5m

SAFETY BENCH
10:1 SLOPE 5
2 m Wide
NOT TO SCALE