The document discusses modeling of wet weather flow for a sewer separation project in South Boston, Massachusetts. It summarizes modeling of existing conditions, calibration of the model, and modeling of future conditions under various levels of sewer separation. The modeling found that a 92% separation level is needed to meet long-term water quality goals of less than 3 combined sewer overflows per year with volumes under 1.5 million gallons each.

1. * The photo is cropped from MWRA’s Combined Sewer Overflow Control Plan-Annual Progress Report 2006. Reserved Channel Pleasure Bay North Dorchester Bay Modeling the Wet Weather Flow for Reserved Channel Sewer Separation in South Boston Dingfang Liu, PhD, P.E., Al Carrier, P.E., Rick Moore, P.E.

3. Project Area

5. MWRA’s Collection System Model in the Project Area Reserved Channel Pleasure Bay 83 sewer nodes, 78 conduits, and 39 subcatchments. No enough detail for design purpose Need update to reflect recent changes on the sewer and drain system

7. Imperviousness Ref: Mass GIS 1 meter imperviousness map Total % of impervious area: 87%

8. Existing Condition Model

9. Flow Metering Locations

10. Storm Events for Model Calibration 1 : return period is determined by comparison with IDF curves derived from NRCC data HIGH 6 months 2.41 0.96 14 10/28/06 HIGH < 3 months 1.32 1.48 10 10/11/06 Tidal conditions (high/low) Return Period 1 Total Rain (in) Peak Intensity (in/hr) Duration (hrs) Date

11. Sensitivity Analysis 40 14.039 2 40 14.039 3 Hortonian infiltration decay rate ( k , 1/hour) 40 14.039 0.1 40 14.039 0.2 Hortonian constant infiltration rate ( f C , inches/hour) 40 14.039 0.8 40 14.039 1 Hortonian initial infiltration rate ( f 0 , inches/hour) 40 14.106 0.013 40 13.201 0.017 50 11.531 0.021 Manning’s Roughness Coefficient ( n ) 40 10.43 0.52 25 13.507 0.7 50 15.436 0.88 Runoff Coefficient for impervious Surface 30 13.397 Observed FWHP (minutes) Peak flow (mgd) Values Parameter

13. Model Calibration RE is relative error; O i is observed data point at time ( t i ); S i is simulated data point at time ( t i ); N is the data population. 28.60% 21.70% 0.55 0.75 25.60% 21.60% 0.65 0.85 27.60% 35.90% 0.75 0.95 29.00% 44.90% 0.85 1 n/a n/a Observed 10/28/07 10/11/07 Pavement Surface Rooftop Surface Relative Error Runoff Coefficient

14. Model Calibration

17. Storm Drain System During 10 year Storm Coincide with Spring High Tide

18. Building Inspection

19. Levels of Separation 95% 17 89% 40.5 83% 63.1 365 Total 95% 2.3 88% 5.8 79% 9.8 47 BOS080 93% 2.7 81% 7.8 65% 14.3 41 BOS079 95% 5.4 88% 12.1 83% 17.8 102 BOS078 96% 6.7 92% 14.8 88% 21.3 175 BOS076 (%) (Acres) (%) (Acres) (%) (Acres) (Acres) Separation Remain Connected Separation Remain Connected Separation Remain Connected *Level C (Orange) *Level B (Yellow) *Level A (Green)

20. Future Condition < 1.5 < 3 Target 0.04 2 Level C (Orange) 0.65 5 Level B (Yellow) 2.57 9 Level A (Green) 29.26 36 Baseline Condition (2006) Volume (MG) Annual CSO Activation

21. Storm Drain System during Extreme Storm Events During 25 year storm During 50 year storm During 100 year storm

24. Depression Storage