Outfall Retrofit Feasibility Study

Transcription

1 Outfall Retrofit Feasibility Study 2013 APWA-NC SWM Conference September 16-17, 2013

2 Overview PWD history with outfall retrofits Project setting Review of screening factors and weighting scale QA/QC process Results Lessons Learned

3 Acknowledgments and Contributors Erik Haniman, PWD Rick Anthes, PWD Chris Brooks, MT Joe Knieriem, MT Joe Berg, Bio Jon Hathaway, Bio (UT) Hunt Engineering Rodriguez Consulting

4 Background: Stormwater Management in the City of Philadelphia The City has been long recognized for their work in reducing combined sewer overflows First stormwater wetland implemented: 2006 (Saylor s Grove) Overwhelmingly successful and well received Began new vision of MS4 system Two additional wetlands implemented: 2012 (Cathedral Run and Wises Mill) Hydrologic monitoring of sites to determine effectiveness Calibrated SWMM modeling performed Benefits of retrofits documented

5 PWD Saylor Grove Wetland Retrofit

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7 PWD Retrofits Initial projects successful and effective How can new retrofits be identified? 394 outfalls in City of Philadelphia Need simple, repeatable, desktop process for identifying and evaluating retrofit opportunities at stormwater outfalls

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9 Phase 3 Ideas Regenerative Stormwater Conveyance

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12 Stormwater Retrofits What influences retrofit cost and feasibility? Constraints can be numerous Utilities Topography Pipe depth Tree impacts Limited space

13 Screening Factor Reference 1 Hydraulics User judgment on where / how to connect to stormwater system, GIS Layers: Storm Water Gravity Mains, Storm Water Outfalls, Storm Water Structures 2 Utilities GIS Layers: Water, Waste Water, HPFS 3 Property Ownership GIS Layer: Philadelphia Parcels 4 Impervious Cover Treated GIS Layer: Philadelphia Impervious Surfaces 5 Earthwork (Excavation) User judgment on where to place inlet, where to divert flow, GIS Layers: Storm Water Outfalls, Storm Water Structures, Philadelphia 2 ft Contours, Digital Elevation Model (DEM) 6 Accessibility GIS Layers: Philadelphia DEM, Philadelphia Arterial Streets, Aerial Photography 7 Clearing& Grubbing Bing basemaps / aerials in ArcGIS 8 Historical / Cultural GIS Layer: Philadelphia Historic Sites, National Historic Register 9 Practice Area to Drainage Area Ratio User proposed BMP footprint, GIS Layer: OWS_GISDATA 10 Watershed Health GIS Layer: Pennsylvania DEP Total Maximum Daily Load

14 Screening Factors 1. Hydraulics How is stormwater routed to proposed practice? 2. Property Ownership Is practice on public land? 3. Utilities Degree of utility impacts 4. Impervious Cover Treated 5. Earthwork (Pipe Depth) How deep is the outfall being intercepted?

15 Screening Factors 6. Accessibility Ease of construction equipment accessing proposed practice 7. Clearing and Grubbing Degree to which forested areas will be impacted 8. Historical / Cultural 9. Practice Area to Drainage Area Ratio Is proposed practice large enough to accept runoff from catchment? 10. Watershed Health Is the receiving stream on the 303D list or under a TMDL

16 Example 1 Average elevation of existing ground surface inside proposed practice Elevation per GIS data is 320 ft Outfall daylights into proposed practice Invert elevation is ft per GIS data

17 Example 2 Average elevation of existing ground surface inside proposed practice Elevation per GIS data is 142 ft Intercept pipe, create inlet for proposed practice Based on pipe data, elevation at proposed inlet is ft Discharge from proposed practice through existing outfall

18 Example Score Calculation

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20 QA / QC Two layers of QA / QC Select subset of outfalls to test screening methodology (accuracy) Perform screening remotely Field visits to verify results Modify methods as needed Select subset to test methodology among team members (reproducibility) Select 10 outfalls for all team members to screen Review results together Modify methods as needed

21 Scoring Summary (All Watersheds) Watershed Outfalls Assessed Cobbs Creek 2 Darby Creek 1 Delaware Direct 0 Pennypack Creek 134 Poquessing Creek 151 Schuylkill River 9 Tacony Creek 33 Wissahickon Creek 64 Total 394 Score Range Count Dropped 139 Average 38 Median 50 Highest Score 88 Lowest Score (Non Zero) 28

22 Table 14. Screening Factor Summary of All Outfalls Assessed Screening Factor Summary (All Watersheds) Screening Factor Scoring (Percent of Total Possible) 0% 25% 50% 75% 100% Mean Std. Dev. SF SF2* 80 N/A N/A N/A SF3* 60 N/A 87 N/A SF4* 135 N/A 65 N/A SF5* 58 N/A 76 N/A SF6_Roadway* 67 N/A N/A N/A SF6_Slope* 150 N/A N/A N/A SF7* 180 N/A 21 N/A SF8* 4 N/A 9 N/A SF9* 64 N/A 96 N/A SF10* 53 N/A 180 N/A * Summary statistics for Screening Factors 2 10 include only outfalls that were not dropped from consideration. N/A not applicable

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24 Scoring Summary (Wissahickon Creek) Watershed Outfalls Assessed Wissahickon Creek 64 Score Range Count Dropped 28 Average 33 Median 43 Highest Score 88 Lowest Score (Non Zero) 40

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27 Observations More than 1/3 of outfalls deemed infeasible for stormwater wetland retrofits (SF1) Approximately 2/3 of outfalls were located on City land (SF2) Majority of sites have good access from roads but are on steep slopes (SF6) Sites typically scored poorly for clearing and grubbing (SF7) Often located near streams / rivers in forested areas Sites often scored well for historical / cultural (SF8) Not located near sensitive areas

28 Table 14. Screening Factor Summary of All Outfalls Assessed Screening Factor Summary (All Watersheds) Screening Factor Scoring (Percent of Total Possible) 0% 25% 50% 75% 100% Mean Std. Dev. SF SF2* 80 N/A N/A N/A SF3* 60 N/A 87 N/A SF4* 135 N/A 65 N/A SF5* 58 N/A 76 N/A SF6_Roadway* 67 N/A N/A N/A SF6_Slope* 150 N/A N/A N/A SF7* 180 N/A 21 N/A SF8* 4 N/A 9 N/A SF9* 64 N/A 96 N/A SF10* 53 N/A 180 N/A * Summary statistics for Screening Factors 2 10 include only outfalls that were not dropped from consideration. N/A not applicable

29 Lessons Learned QA/QC is critical Determined more specific guidance was needed during process Field verification of initial sites Make sure methodology is working / reasonable Many sites are infeasible for large, regional stormwater wetlands Floodplain wetlands were commonly identified Balance between stormwater treatment and riparian forests

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31 Questions?

32 RSC Opportunities in the Wissahickon (Example looking at 14 outfalls that initially scored 0) Criteria* Site_ID RSC > 30 width <36"pipe & >5%slope or <5%slope >100 ft long Comments W yes y y y move to side of sewerline W no y y n W no n y y W no y n n W no y n y W no n n n W no y n n W yes y y y in blue line stream, outfall is beginning of stream W yes y y y in blue line stream, outfall is beginning of stream W no y n y W yes y y y W no y y n W yes y y y W no y n y *to be considered for RSC, must have all three criteria Criteria: 1) Is there greater than 30 feet in width available for construction between infrastructure. 2) Is the pipe smaller than 36 in diameter and on a slope greater than 5% or on a slope flatter than 5%? 3) Is the length available for construction greater than 100 feet.