EVOLVING CRITERIA FOR STORMWATER MANAGEMENTby Nancy U Schultz, PE, D.Wre. CH2M HILL with many thanks to Jon Schladweiler at www.sewerhistory.org

One logically asks the underlying questions: What is the design objective? What affects the achievement of the objective?

Peak flows? Average Velocity? Volume? Are the design criteria specified by regulation? Are the specified design criteria appropriate for

the design objective? How should the design flow be selected? How were the original sewers designed?

Sewers historically pre-date both regulation and flow calculations

What were the original sewer objectives?

Get the muck out of the street road gutters, deep enough

to convey rain if streets flooded too often,

deepen the gutters or build elevated walkways

Get the urban effluvium out of the streets

Metcalf and Eddy, 1914

“Public latrines were doubtless used by most of the people and it is probable that the gutters were the chief receptacle of the ordure of the city, which washed thence into the sewers.”

Cholera outbreaks in the middle of the 19th century changed that Early proponents of dual sewers

suggested Initially build smaller, less expensive sewers

or household wasteLater build larger street drainage

But what happened to the street muck?

Early sewer criteria were conflicting

Sewers were to be large enough (> 2 meters) for easy cleaning

Sewers were to be small enough for economical construction

And early sewer sizing was not based on science

“American sewerage practice is noteworthy among the branches of engineering for the preponderating influence of experience rather than experiment upon the development of many of its features”

Metcalf and Eddy, 1914

20th Century engineering texts and training called for First, calculate the expected flow

size and nature of the area collected times a design flow per unit area of a given nature

(gpd/residential acre) times a peaking factor

Second, size to convey with adequate velocity (Manning's) V=ƒ(P,Qp,S) where

V= velocity P = wetted perimeter Qp = flow S = slope

Since 1972 the Clean Water Act Construction Grants Program added an I/I allowance CWA Grants demanded low I/I allowances Many thought the peaking factor adequate

allowance for I/I Grant applicants

measured flows, calculated I/I searched for I/I sources planned for I/I reduction

Storm sewers apparently unregulated

Stormwater management (BMPs) are necessary for water quality Stormwater is NOT clean water

National Urban Runoff Pollution Study (NURPS) Urbanization concentrates pollutants while

eliminating natural filters “Green” urban drainage encourages natural

processes: mimics the flow frequency (sustained low flows) infiltration rather than impervious filter rather than hard channels shade rather than open, or closed, channels

National Urban Runoff Pollution Study (NURPS), 1983 National Urban Runoff Pollution Study (NURPS), 1983

Demonstrated that urban runoff is polluted Demonstrated statistical limitations to comparing discrete

samples Focused on Event Mean Concentration

1990-1992 ASCE sponsored Stormwater BMP Workshops Disseminated reference materials Suggested guidance Recognized site specificity

National Policies set performance goals CSO

capture 85% of wet weather flow allow only 4-6 untreated discharges per year

SSO No overflows

Bypass Only if no feasible alternatives

Stormwater Best Management Practices ‘capture’ the first inch, release at 1 yr storm rate

Milwaukee, WI criteria illustrate the evolution of sewer design criteria

Flow Allowances

0

100

200

300

400

500

600

700

800

900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

2010

Year

Gal

/ca

pit

a/d

ay

Court CaseWet Weather Control

Plan

Sewer Improvement, Deep tunnel storageTreatment expansion

Improved sewer design captured sanitary sewage, but left pollutants

source : SEWRPC and MMSD

1975

2000

Hart Park Flood Management

Stormwater Reduction BMP Volume Reduction 1. Downspout Disconnection (dd) 12% 2. Rain Barrel (w/ dd) 14% 3. Rain Garden (w/ dd) 36% 4. Rain barrel and Rain Garden (w/ dd) 38% 5. Green Roof 22% 6. Bioretention 70% 7. Green Parking Lot 76% 8. Stormwater Trees 10%

Source: CDM

Policies, and guidance, are silent on how to select the design flows1 Engineering science implies conveyance

(sewers) should be designed for peak instantaneous flow

Peak flow is a combination of peak stormwater runoff peak sanitary contribution peaking factor peak infiltration peak inflow

1 Except CSO policy advocates continuous simulation modeling.

Flood Planning sets a useful precedent for storm related flows Design conveyance for the probability of peak flow

estimate probability from river flow records, or estimate probability by relating to rainfall probability

Relate peak flows to peak rainfall intensity Qp = ƒ(CIA) where Qp = peak flow C = a judgment factor, related to land type I = rainfall intensity for appropriate duration A = area contributing

Select design flow probability from consequences 100 year probability to protect rail transportation 25 year probability for secondary roadways

Sewer network design flows, however, are not simply related to rain Peak sewer flows

Does the peak rain coincide with peak dry weather flow?

What is the effective tributary area? Is it changing?

What is the effective time of concentration?Does the design event come in the dry or wet

season?

Sewer flow design events consider (or simplify) Inter-event periods

(when pollutants accumulate on the land surface) Antecedent conditions Storm volume Storm duration Frequency of volume in critical time period Shape of the storm Spatial distribution of the event

Sewer flow design events also consider What values are at risk2? How do sewer design flows affect those

risks and values? What target level of risk will protect the

values? What design event will achieve best

protect the values?

2 WEF Guide to Managing Peak Wet Weather Flows (Nov. 2006)

MSD of Greater Cincinnati experience with ‘risk to values’ design storm selection Community values and measures were imputed from

stakeholder meetings Four test storms were defined

all had similar distribution, duration, antecedent conditions and shape

frequency was associated with storm frequency for a specific duration

Sewer relief projects were defined for each test storm Values achieve with the “relieved” sewers were defined

3 Johnson, R. et. al., Design-Storm Analysis Extrapolated to Estimate Long-Term Performance. WEFTEC06.

MSD of Greater Cincinnati experience with ‘risk to values’ design storm selection MSDGC demonstrated that values were

maximized with a 2-year test storm3

Engineering judgment selected a 10-year design storm, to be applied to new sewers and to sewers required to relieve existing problems during the 2-year test storm

3 Johnson, R. et. al., Design-Storm Analysis Extrapolated to Estimate Long-Term Performance. WEFTEC06.

One logically asks the underlying questions: What is the design objective? What affects the achievement of the objective?

Peak flows? Average Velocity? Volume? Are the design criteria specified by regulation? Are the specified design criteria appropriate for

the design objective? How should the design flow be selected? How were the original sewers designed?

Pollutant Removal Estimates for Stormwater BMPs Pollutant

Infiltration Practices

BioretentionPorous Pavement

Constructed Wetland

Total phosphorus 70 34 85 49

Soluble phosphorous

85 38 -- 35

Total nitrogen 51 84 -- 30

Nitrate 82 31 30 67

Copper -- 51 -- 40

Zinc 99 71 -- 44

TSS 95 81 85 76

Sources: National Pollutant Removal Performance Database for Stormwater Treatment Practices, Center for Watershed Protection, June 2000Pennsylvania Stormwater Manual (draft, 2004)