chapter 3 3 preventing and mitigating stormwater impacts 3.1 introduction .....3-2 3.2 guiding...

Chapter 3Preventing and Mitigating Stormwater Impacts

Chapter 3 Preventing and MitigatingStormwater Impacts

3.1 Introduction .................................................................................................3-2

3.2 Guiding Stormwater Management Principles .......................................3-2

3.3 Site Planning and Design............................................................................3-3

3.4 Source Control Practices and Pollution Prevention ...........................3-4

3.5 Construction Erosion and Sedimentation Control .............................3-4

3.6 Stormwater Treatment Practices .............................................................3-4

3.7 Stormwater Quantity Control .................................................................3-6

3.8 Watershed Management............................................................................3-7

Volume 1: Background

2004 Connecticut Stormwater Quality Manual 3-1

Effective site planning and design is the most critical and potentially beneficial element of a successful stormwater management program sinceit addresses the root causes of both stormwater quality and quantity prob-lems early in the development process. Source controls and pollutionprevention, as well as construction erosion and sedimentation controls, arealso key elements for preventing or mitigating stormwater quality prob-lems. These preventive measures can reduce the size and scope ofstormwater treatment and flood control facilities. However, it is also recognized that stormwater treatment and flood control measures are ofteneffective and necessary to achieve water quality and quantity control objec-tives. Figure 3-1 shows the relationship and recommended hierarchy ofthese stormwater management elements.

This manual primarily addresses water quality controls through site plan-ning and design, source controls and pollution prevention, and stormwatertreatment practices, which are highlighted in Figure 3-1. Constructionerosion and sediment control, and stormwater quantity control (i.e., floodcontrol and drainage design), are addressed as secondary topics as theyrelate to water quality. For instance, source controls and stormwatertreatment practices can also provide peak runoff attenuation and floodcontrol benefits. Other guidance documents, as well as local ordinancesand requirements, are recommended sources of information on these top-ics, as discussed later in this chapter.

3.2 Guiding Stormwater Management PrinciplesA comprehensive stormwater management strategy should prevent or mit-igate urban runoff problems and protect beneficial uses of receiving watersin a cost-effective manner. The stormwater management measuresdescribed in this manual are designed to accomplish this objective byadhering to the following guiding principles:

❍ Preserve pre-development site hydrology (i.e., runoff, infiltration,interception, evapotranspiration, groundwater recharge, and streambaseflow) to the extent possible

❍ After construction has been completed and the site is permanentlystabilized, reduce the average annual total suspended solids loadingsby 80 percent. For high quality receiving waters and sites with thehighest potential for significant pollutant loadings, reduce post-devel-opment pollutant loadings so that average annual post-developmentloadings do not exceed pre-development loadings (i.e., no netincrease)

Element Addresses Water Quality or Quantity?

Effective site planning and design Quality and quantity

Source control practices and pollution prevention Quality

Construction erosion and sedimentation controls Quality

Stormwater treatment practices Quality (primary), quantity (secondary)

Drainage design and flood control Quantity (primary), quality (secondary)

2004 Connecticut Stormwater Quality Manual3-2

3.1 IntroductionStormwater management

involves the selective use

of various management

measures to cost-effectively

address the adverse water

quality and quantity impacts

of urban stormwater runoff

described in Chapter Two.

Table 3-1 lists the major

elements and associated

objectives of a comprehensive

stormwater management

strategy.

Table 3-1 Elements of a Comprehensive StormwaterManagement Strategy


❍ Preserve and protect wetlands, stream buffers,natural drainage systems and other natural features that provide water quality and quantitybenefits

❍ Manage runoff velocity and volume in a mannerthat maintains or improves the physical and biological character of existing drainage systemsand prevents increases in downstreamflooding/streambank erosion

❍ Prevent pollutants from entering receivingwaters and wetlands in amounts that exceed thesystems’ natural ability to assimilate the pollu-tants and provide the desired functions

❍ Seek multi-objective benefits (i.e., flood control,water quality, recreation, aesthetics, habitat)from stormwater control measures

3.3 Site Planning and DesignEffective site planning and design (Chapter Four) con-sists of preventive measures that address the rootcauses of stormwater problems by maintaining pre-development hydrologic functions and pollutantremoval mechanisms to the extent practical. Site plan-ning that integrates comprehensive stormwatermanagement from the outset is the most effective wayto address the adverse water quality and quantityimpacts of stormwater runoff from new developmentand redevelopment projects. Often these site designtechniques can reduce or eliminate the need for costlypeak flow attenuation and stormwater treatment. Thismanual emphasizes the use of effective site planningand design techniques early on in the site develop-ment process to achieve the greatest stormwaterquantity and quality benefits. Site planning and designpractices described in this manual include:

❍ Alternative site design for streets and parking lotsand lot development

3. StormwaterTreatment

StormwaterQuality

Post-Construction

StormwaterQuantity

Construction

Erosion andSedimentation

Control

Drainage Designand Flood Control

2. Source Controlsand PollutionPrevention

1. Site Planning and Design

Addressed in this manual Addressed as secondary considerationsin this manual (refer to listed referencesfor detailed guidance)

Figure 3-1 Relationship of Stormwater Management Elements

2004 Connecticut Stormwater Quality Manual3- 4

❍ Low Impact Development (LID) managementpractices

❍ Watershed planning

3.4 Source Control Practices andPollution Prevention

Source control practices and pollution prevention(Chapter Five) are operational practices that canreduce the types and concentrations of pollutants instormwater runoff by limiting the generation of pollu-tants at their source. The guiding principle behindthese techniques is to minimize contact of stormwaterwith potential pollutants, thereby reducing pollutantloads and the size and cost of stormwater treatment.This manual emphasizes the use of source controlpractices and pollution prevention, in conjunctionwith effective site planning and design, to reduce theneed for and scope of stormwater treatment. Sourcecontrol practices commonly implemented at residen-tial, commercial, and industrial sites are discussed inthis manual, including:

❍ Street and Parking Lot Sweeping

❍ Roadway Deicing/Salt Storage

❍ Storm Drainage System Maintenance

❍ Other Road, Highway, and Bridge Maintenance

❍ Illicit Discharge Detection and Elimination

❍ Commercial and Industrial Pollution Prevention Plans

❍ Animal Waste Management

❍ Lawn Care and Landscaping Practices

❍ Model Stormwater Ordinances

❍ Public Education

3.5 Construction Erosion andSedimentation Control

As described in Chapter One, soil erosion and sedi-mentation control is addressed by the Soil Erosionand Sediment Control Act (CGS §§22a-325 through22a-335, inclusive). The primary goal of the Act is toreduce soil erosion from stormwater runoff and nonpoint sediment pollution from land being devel-oped. Controlling soil erosion and sedimentationduring construction is addressed through a combina-tion of measures that are described in a site-specificErosion and Sediment Control (E&SC) Plan. The basicprinciples of effective soil erosion and sediment control include:

❍ Use effective site planning to avoid sensitiveareas such as wetlands and watercourses

❍ Keep land disturbance to a minimum

❍ Stabilize disturbed areas

❍ Phase land disturbance on larger projects, start-ing subsequent phases after disturbed areas arestabilized

❍ Keep runoff velocities low

❍ Protect disturbed areas from stormwater runoff

❍ Properly install perimeter control practices

❍ Limit construction during months when runoffrates are higher due to decreased infiltration orextreme rainfall events

❍ Implement a thorough maintenance and follow-up program

❍ Assign responsibility for the maintenance program

As shown in Figure 3-1, soil erosion and sedi-ment control is a key component of any stormwatermanagement strategy in order to reduce the impactsof stormwater runoff during construction activities.Although many of the vegetative, filtration, and infil-tration stormwater management practices containedin this manual are based on the above principles, thismanual does not address construction soil erosionand sediment control practices. Municipal ordinancescontain specific soil erosion and sediment controlrequirements for developments disturbing more thanone-half acre. Additionally, the 2002 revision of theConnecticut Guidelines for Soil Erosion and SedimentControl, DEP Bulletin 34 (Connecticut Council on Soiland Water Conservation and the ConnecticutDepartment of Environmental Protection, 2002) con-tains detailed technical guidance on specific erosionand sediment control practices and recommendedprocedures for developing an effective E&SC Plan.Copies of this guidance manual have been issued toeach local Planning, Zoning, and Inland Wetlands andWatercourses Office.

3.6 Stormwater Treatment PracticesStormwater treatment practices, which are the focusof the second half of this Manual, are primarilydesigned to remove pollutants from stormwaterrunoff. In addition to water quality treatment, thesepractices can also provide groundwater recharge,stream channel protection, and peak runoff attenua-tion. As described above, stormwater treatmentpractices should be selected and designed only afterconsideration of effective site planning/design and

Mechanism

Gravity settling of particulate pollutants

Filtration and physical straining of pollutants through a filter media or vegetation

Infiltration of particulate and dissolved pollutants

Adsorption on particulates and sediments

Photodegradation

Gas exchange and volatilization

Biological uptake and biodegradation

Chemical precipitation

Ion exchange

Oxidation

Nitrification and denitrification

Density separation and removal of floatables

Pollutants Affected

Solids, BOD, pathogens, particulate COD, phosphorus, nitrogen,synthetic organics, particulate metals



Dissolved phosphorus, metals, synthetic organics

COD, petroleum hydrocarbons, synthetic organics, pathogens

Volatile organics, synthetic organics

BOD, COD, petroleum hydrocarbons, synthetic organics,phosphorus, nitrogen, metals

Dissolved phosphorus, metals

Dissolved metals

COD, petroleum hydrocarbons, synthetic organics

Ammonia, nitrate, nitrite

Petroleum hydrocarbons


source controls, which can reduce the volume ofrunoff and the size and cost of stormwater treatment.

Stormwater treatment practices are designed forsmall storms to achieve water quality objectives (i.e.,smaller than a one-year return frequency storm), incontrast to drainage and flood control facilities, whichare typically designed for the two-year and largerstorms. However, many stormwater treatment practices can also be designed for flood control pur-poses and vice versa. Stormwater treatment practicescan be integrated into the landscape, drainage or flood control system, and other spaces of develop-ment projects. When properly located, designed, and maintained, stormwater treatment practices can be amenities for, rather than detractions from, devel-opment projects.

Pollutant Removal MechanismsStormwater treatment practices remove pollutantsfrom stormwater through various physical, chemical,and biological mechanisms. Table 3-2 lists the majorstormwater pollutant removal mechanisms and theaffected stormwater pollutants.

Since many pollutants in urban stormwater runoffare attached to solid particles, treatment practicesdesigned to remove suspended solids from runoff willremove other pollutants as well. Exceptions to thisrule include nutrients, which are often in a dissolvedform, soluble metals and organics, and extremely fine

particulates (i.e., diameter smaller than 10 microns),which can only be removed by treatment practicesother than traditional separation methods.

Primary and Secondary Stormwater Treatment PracticesStormwater treatment practices described in thisManual include both primary treatment practices,which provide demonstrated, acceptable levels ofwater quality treatment, and secondary treatment prac-tices which are not suitable as stand-alone treatmentfacilities but can be used for pretreatment or as sup-plemental practices. This Manual includes five majorcategories of primary stormwater treatment practices:

❍ Stormwater ponds

❍ Stormwater wetlands

❍ Infiltration practices

❍ Filtering practices

❍ Water quality swales

Examples of secondary stormwater treatmentpractices described in the Manual include traditionalpractices such as dry detention ponds, vegetated filterstrips and level spreaders, oil/particle separators, anddeep sump catch basins. The Manual also includes

Table 3-2 Stormwater Pollutant Removal Mechanisms


innovative and emerging technologies as secondarytreatment practices. These technologies are designedto remove a variety of stormwater pollutants, but havenot been evaluated in sufficient detail to demonstratethe capability to meet established performance stan-dards. Sizing and selection criteria for stormwatertreatment practices are addressed in Chapter Sevenand Chapter Eight, respectively.

New Development Versus RetrofitsStormwater treatment practices can be implementedfor new development projects as well as existing,developed sites. Retrofitting existing developmentscan improve water quality mitigation functions ofolder, poorly designed, or poorly maintainedstormwater management systems. Incorporatingstormwater retrofits into developed sites is typicallymore difficult than implementing treatment practicesfor new development due to the numerous site con-straints associated with developed areas such assubsurface utilities, buildings, conflicting land uses,and maintenance access. Chapter Ten describes com-mon stormwater retrofit options for existingdevelopment and redevelopment projects, including:

❍ Stormwater collection system retrofits

❍ Stormwater management facility retrofits

❍ New stormwater controls at storm drain outfalls

❍ In-stream practices in existing drainage channels

❍ Parking lot stormwater retrofits

❍ Wetland creation and restoration

3.7 Stormwater Quantity ControlStormwater quantity controls include drainage andflood control. As shown in Figure 3-1, stormwaterquantity and quality controls are related and com-plementary elements of an effective stormwatermanagement strategy. Stormwater drainage systemscan be designed to reduce the potential erosive velocity of stormwater runoff and maintain pre-devel-opment hydrology through infiltration and the use ofvegetated conveyances, thereby preserving the waterquality mitigation functions of a site. Similarly,stormwater treatment practices such as stormwaterponds and wetlands can provide dual flood controland water quality treatment benefits.

This Manual addresses the topics of drainagedesign and flood control as they relate to stormwaterquality management. The Manual identifies storm-water treatment practices that also provide peakrunoff attenuation and channel protection functions.However, this document is not intended to serve as a

drainage or flood control design manual. Other rec-ommended guidance documents and manuals onthese topics include:

❍ 2000 Connecticut Department of TransportationDrainage Manual, October 2000

❍ Connecticut Department of EnvironmentalProtection, Model Hydraulic Analysis, revisedFebruary 13, 2002

❍ Urban Hydrology for Small Watersheds, TR-55,Natural Resource Conservation Service (formerlySoil Conservation Service), June 1986

In addition, municipal ordinances, as well assome DEP regulatory programs, contain specificstormwater quantity control requirements for landdevelopment projects, as described in Chapter One.

Drainage Design and Flood Control Principles for Water QualityThe traditional approach to drainage design has beento collect and remove runoff from the site as quicklyas possible through the use of curbs, gutters, catchbasins, and storm sewers, often resulting in the dis-charge of polluted runoff directly to receiving waters.While this approach effectively removes runoff from asite, it does not address water quality or downstreamflooding and erosion issues. Similarly, the traditionalapproach to flood control has been to attenuate peakrunoff to pre-development levels through the use ofdetention and retention ponds. While stormwaterdetention or retention facilities can effectively reducepeak discharge rates, they also typically prolong the duration of elevated flows and do not reducerunoff volumes unless infiltration is incorporated into their design. Historically, these facilities have not adequately addressed problems associated with water quality, runoff volume, and downstreamchannel erosion.

Drainage and flood control facilities should bedesigned according to the following principles toaddress water quality objectives:

❍ Identify and assess existing stormwater runoffrates and volumes at the site, as well as down-stream flooding and erosion concerns.

❍ Preserve pre-development hydrologic conditions,including peak discharge, runoff volume,groundwater recharge, and natural drainage paths.

❍ Reduce the potential for increases in runoffquantity by minimizing impervious surfacesand maximizing infiltration of stormwaterrunoff. Eliminate curbs where possible andencourage sheet flow from paved areas. If


curbing is required, use Cape Cod curbing orother similar curbing, which allows amphibiansto climb.

❍ Encourage infiltration of stormwater through theuse of vegetated depressions, swales, rain gar-dens and bioretention, and other vegetateddrainageways to convey and hold stormwaterand provide for a slow recharge to groundwater,where soils permit. Special care must be taken inareas of sensitive groundwater resources such asaquifer protection areas and groundwater sup-ply wells in order to prevent their contamination.In addition, in areas with soil or groundwatercontamination, the potential for infiltratedstormwater to mobilize contaminants must alsobe considered.

❍ Control increases in stormwater runoff volumeand peak flows through properly designed andlocated stormwater management facilities.Manage stormwater so that both the volume andpeak rate of runoff from the site after develop-ment does not exceed the volume and peak rateof runoff from the site prior to development.

❍ Encourage the development of watershed-based stormwater management strategies toeffectively control the cumulative effects ofincreases in runoff volume and peak flows atcritical locations throughout the watershed.Coordinate the timing of detention basin outflows to avoid increases in peak flows indownstream watercourses.

❍ Use adequate outlet protection at drainage out-falls to reduce discharge velocities, disperse flow,and prevent or reduce downstream erosion.

❍ Coordinate construction erosion and sedimentcontrol measures with post-constructionstormwater management measures. For example,a sediment basin designed to trap sediment dur-ing the construction phase of a project maysometimes be converted to a detention basin orstormwater treatment facility to meet peakrunoff attenuation or water quality mitigationobjectives following construction.

❍ Retain on-site the volume of runoff generated bythe first inch of rainfall from areas adjacent toor within 500 feet of tidal salt marshes and estuarine waters. Excessive quantities of freshwater can be a pollutant to tidal wetlands andcause a decrease in vegetative diversity and wetland productivity.

❍ Protect wetland and watercourse resources fromstormwater discharges. Do not drain stormwaterdirectly to a wetland or watercourse or to a

municipal storm drainage system that drainsdirectly to a wetland or watercourse withoutadequate stormwater treatment. Protect wetlands, watercourses, and submerged aquatic vegetation from scour.

3.8 Watershed ManagementStormwater management is most effectively under-taken in the context of a watershed managementplan. A watershed management plan is a comprehen-sive framework for applying management tools in amanner that achieves the water resources goals forthe watershed as a whole (CWP, 1998). Typically,watershed management plans are developed fromwatershed studies undertaken by one or more munic-ipalities located within the watershed. The watershedapproach has emerged over the past decade as therecommended approach for addressing nonpointsource pollution problems, including pollutedstormwater runoff. Watershed planning offers the bestmeans to:

❍ Address cumulative impacts derived from anumber of new land development projects

❍ Plan for mitigation to address cumulativeimpacts from existing developments

❍ Focus efforts and resources on identified priority water bodies and pollutant sources in a watershed

❍ Achieve noticeable improvements to impairedwaters or waters threatened with impairment

The watershed approach is built on three mainprinciples. First, the target watersheds should be thosewhere stormwater impacts pose the greatest risk tohuman health, ecological resources, desirable uses ofthe water, or a combination of these. Second, partieswith a stake in the specific local situation (i.e., stake-holders) should participate in the analysis of problemsand the creation of solutions. Third, the actions under-taken should draw on the full range of methods andtools available, integrating them into a coordinated,multi-organization attack on the problems. The water-shed approach has the following significant advantagesover traditional piecemeal approaches to stormwatermanagement that require individual land developmentsto provide on-site stormwater management facilities(adapted from Aldrich, 1988):

Lower capital and O&M cost: Typically, water-shed management plans yield fewer and largerstormwater management facilities. Economies ofscale are achievable in capital costs and especially


in O&M. Strategic placement of regional facilities per-mits concentrating funds on areas where potentialbenefits are greatest. Cost sharing arrangements signifi-cantly reduce the net cost of stormwater management tothe community as a whole.

Increased effectiveness on a watershed-widebasis: Often different portions of watersheds requiredifferent types of stormwater controls. Watershedplanning permits the siting of a variety of on-site andregional facilities in locations where the greatest benefits are achieved.

Greater use of nonstructural measures: Often themost practical stormwater controls involve nonstructuralmeasures such as land acquisition, floodplain zoning,subdivision drainage ordinances, and land use controls.Watershed planning provides a coordinated, compre-hensive framework and decision-making process toallow the effective implementation of these measures.

Less risk of negative “spillover” effects: The piece-meal approach may adequately solve localizeddrainage problems, but seldom addresses downstreamimpacts. Thus, dynamic interactions betweenupstream drainage improvements may actuallyincrease downstream flooding. An objective of water-shed planning is to account for these upstreaminteractions and achieve solutions to both localizedand regional stormwater management concerns.

Watershed management plans should include rec-ommended criteria for stormwater source controls andtreatment practices in the watershed. These criteria arebased on watershed-specific factors such as physicalattributes, land use, pollution sources, and sensitivereceptors, and are the basis for selecting and locatingstormwater controls in the watershed. At a minimum, awatershed management plan should contain the elements listed in Table 3-3 to address stormwater-related issues.

The watershed management plan should addressintegrating flood control and stormwater managementcontrols with community needs, including openspace, aesthetics, and other environmental objectivessuch as habitat or river restoration. This synchroniza-tion with other programs can create better fundingopportunities and enhance the overall benefit of thestormwater management practices in the watershed.

On-Site Versus Regional ApproachesWatershed management plans can identify conditionsand locations in the watershed where regionalstormwater management facilities may be moreappropriate or effective than on-site controls. On-siteand regional stormwater management approaches areillustrated schematically in Figure 3-2. Theseapproaches apply to both stormwater quality andquantity controls.

In the on-site approach, land developers have responsi-bility for deploying treatment practices and runoffcontrols at individual development sites. Developers areresponsible for constructing on-site stormwater manage-ment facilities to control stormwater pollutant loadingsand runoff from the site. The local government is respon-sible for reviewing the design of stormwater managementfacilities relative to specified design criteria, for inspectingthe constructed facilities to ensure conformance with thedesign, and for ensuring that operation and maintenanceplans are implemented for the facilities (Novotny, 1995).

The regional approach involves strategically sitingstormwater management facilities to control stormwaterrunoff from multiple development projects or largedrainage areas. Local or regional governments assumethe capital costs for constructing the regional facilities.Capital costs are typically recovered from upstreamdevelopers as development occurs. Individual regionalfacilities are often sited and phased in as developmentoccurs according to a comprehensive watershed man-agement plan. Municipalities generally assumeresponsibility for operation and maintenance of regionalstormwater facilities (Novotny, 1995).

Both approaches have a number of advantages anddisadvantages, which are summarized in Table 3-4. Mostof the advantages of the regional approach can be attrib-uted to the need for fewer stormwater managementfacilities that are strategically located throughout the water-shed (Novotny, 1995). However, the on-site approachaddresses stormwater pollution close to its source, offersgreater opportunities to preserve pre-development hydro-logic conditions, and reduces the overall volume ofstormwater runoff. Historically the on-site approach tostormwater management has been more common inConnecticut. The major drawbacks that have limited thewidespread use of the regional approach include signifi-cant required advanced planning, financing, and landacquisition. Local governments must finance, design, andconstruct regional stormwater facilities before the majorityof the watershed is developed, with reimbursement bydevelopers over build-out periods of many years (WEFand ASCE, 1992). Due to these limitations, the regionalapproach generally is more appropriate for:

❍ Highly developed watersheds with severe waterquality and flooding impacts, where stormwatercontrols for new development alone cannot ade-quately address the impacts in these areas

❍ Watersheds where the timing of peak runoff mayincrease downstream flooding if on-site peakrunoff attenuation criteria are applied uniformlythroughout the watershed

(Pennsylvania Association of Conservation Districts et al.,1998). In most watersheds, a mix of regional and on-sitecontrols is desirable and has the greatest potential forsuccess when implemented as part of a comprehensivewatershed management plan. (DEP, 1995).


Plan Elements

Table 3-3 Elements of a Watershed Management Plan

Watershed delineation and identification of watershed characteris-

tics such as topography, soils, surficial geology, impervious cover,

and land use (current and projected)

Inventory of flood hazard areas as identified by Flood Insurance

Studies or DEP, plus historic floods and damages

An evaluation of watercourses, including areas of limited flow

capacity, bank or bed erosion, sediment deposition, water quality,

principle water uses and users, recreation areas, morphology classi-

fication, and channel stability

An inventory and evaluation of hydraulic structures, including cul-

verts, bridges, dams and dikes with information on their flow

capacity and physical condition

An inventory of significant water storage areas, including principal

impoundments, floodplains, and wetlands

Identification of sensitive and impaired wetlands and waterbodies

Evaluation of functional value of wetlands to identify sensitive and

high quality wetland resources

Sensitive groundwater recharge or aquifer protection areas

Identification of existing problem land uses and impacts on

water quality

Land use restrictions in sensitive areas

Inventory of local wetlands, conservation, planning and zoning, and

subdivision regulations of the watershed municipalities to identify

potential regulatory changes for addressing stormwater impacts

A runoff hydrograph analysis of the watershed for floods of an

appropriate duration, including a 24 hour event, with average

return frequencies of 2, 10, 25, and 100 years for existing and

future land uses

The relationship between the computed peak flow rates and

gauging station data, with modification or calibration of the hydro-

graphs to obtain a reasonable fit where necessary

Identification of the peak rate of runoff at various key points in the

watershed, and the relative timing of the peak flows

Identification of points in the watershed where hydraulic struc-

tures or watercourses are inadequate under existing or anticipated

future conditions

Recommendations on how the subwatershed’s runoff can be man-

aged to minimize any harmful downstream (flooding) impacts

Existing and projected future pollutant loads, impacts of these

loads, and pollution reduction goals

Existing and projected aquatic habitat disturbances and goals for

habitat restoration

Recommendations for watershed-specific stormwater treatment

controls, conceptual design, and operation and maintenance

(O&M) needs and responsibilities

Water quality monitoring program

Prioritized implementation plan for recommendations

Identification of public water supply watershed areas and DEP-

delineated aquifer recharge areas.


Source: Adapted from Novotny, 1995; DEP¸ 1995; Pennsylvania Association of Conservation Districts et al., 1998; WEF and ASCE, 1992.

Table 3-4Comparison of On-Site and Regional Stormwater Management Approaches

Approach

On-Site

Regional

Advantages

❍ Requires little or no advanced planning❍ Addresses stormwater pollution close to its source,

thereby reducing the volume of stormwater runoff

and the need for treatment controls❍ Provides greater groundwater recharge benefits

❍ Reduced capital costs through economies of scale in

designing and constructing regional facilities❍ Reduced maintenance costs because there are fewer

facilities to maintain❍ Greater reliability because regional facilities are more

likely to receive long-term maintenance❍ Nonpoint pollutant loadings from existing developed

areas can be affordably controlled at the same

regional facilities that are sited to control future

development❍ Regional facilities provide greater opportunities for

multipurpose uses such as recreational and aesthetic

benefits, flood control, and wildlife❍ Can be used to treat runoff from public streets

which is often missed by on-site facilities❍ Identifies opportunities to reduce regional stormwater

pollutant loadings and provides a schedule for imple-

menting appropriate controls

Disadvantages

❍ Results in a large number of facilities that may not

be adequately maintained by developers or home-

owners❍ Consumes on-site land that could be used for other

purposes❍ May increase downstream flooding and quantity

control problems

❍ Significant advanced watershed planning required❍ Requires up-front financing❍ Requires land availability and acquisition❍ May promote “end-of-pipe” treatment mentality

rather than the use of on-site controls to reduce

stormwater runoff volume and the need for

stormwater treatment❍ Greater administrative responsibility for municipali-

ties and local governments❍ Some treatment practices are not appropriate for

large drainage areas (swales, filter strips, media fil-

ters, and oil/particle separators)

References

Center for Watershed Protection (CWP). 1998. RapidWatershed Planning Handbook. Ellicott City,Maryland.

Connecticut Council on Soil and Water Conservationand the Connecticut Department of EnvironmentalProtection. 2002. 2002 Connecticut Guidelines for SoilErosion and Sediment Control, DEP Bulletin 34.

Connecticut Department of Environmental Protection(DEP). 1995. Connecticut Stormwater QualityPlanning: A Guide for Municipal Officials andRegional Planners (draft). Bureau of WaterManagement, Planning and Standards Division,Hartford, Connecticut.

Connecticut Department of Environmental Protection(DEP). 1997. (Model Hydraulic Analysis), revisedFebruary 13, 2002.

Connecticut Department of Transportation (DOT).2000. Connecticut Department of TransportationDrainage Manual.

Natural Resource Conservation Service (formerly SoilConservation Service). 1986. Urban Hydrology forSmall Watersheds, TR-55.

Novotny, V. 1995. Nonpoint Pollution and UrbanStormwater Management. Technomic PublishingCompany, Inc., Lancaster, Pennsylvania.

Pennsylvania Association of Conservation Districts,Keystone Chapter Soil and Water ConservationSociety, Pennsylvania Department of EnvironmentalProtection, and Natural Resources ConservationService. 1998. Pennsylvania Handbook of BestManagement Practices for Developing Areas, preparedby CH2MHILL.

Water Environment Federation (WEF) and AmericanSociety of Civil Engineers (ASCE). 1992. Design andConstruction of Urban Stormwater ManagementSystems (Urban Runoff Quality Management (WEFManual of Practice FD-20 and ASCE Manual andReport on Engineering Practice No. 77).


Figure 3-2 On-site and Regional Stormwater Treatment Approaches

On-Site

Developers provide treatmentpractices on individual developments sites

Municipalities providestrategically located regionaltreatment facilities

Regional

Source: Adapted from Novotny, 1995.

chapter 3 3 preventing and mitigating stormwater impacts 3.1 introduction .....3-2 3.2 guiding...

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