EVOLVING CRITERIA FOR STORMWATER MANAGEMENT

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

2. 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?

3. Sewers historically pre-date both regulation and flow calculations

4. 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

5. 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.”

6. 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 waste • Later build larger street drainage • But what happened to the street muck?

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Milwaukee, WI criteria illustrate the evolution of sewer design criteria Court Case Wet Weather Control Plan Sewer Improvement, Deep tunnel storage Treatment expansion

15. Improved sewer design captured sanitary sewage, but left pollutants 1975 2000 source : SEWRPC and MMSD

16. Hart Park Flood Management

17. 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

18. 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.

19. 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

20. 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?

21. 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

22. 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)

23. 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.

24. 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.

25. 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?

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