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Survey of MPEA Stormwater Outfalls

Survey of MPEA Stormwater Outfalls. By MPEF Subcommittee on Stormwater Outfalls James Palmer John McCoy Brian England 5/15/2013. Survey of MPEA Stormwater Outfalls. Goals Determine the extent of erosion in the MPEA watershed due to stormwater runoff

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Survey of MPEA Stormwater Outfalls

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  1. Survey of MPEA Stormwater Outfalls By MPEF Subcommittee on Stormwater Outfalls James Palmer John McCoy Brian England 5/15/2013

  2. Survey of MPEA Stormwater Outfalls • Goals • Determine the extent of erosion in the MPEA watershed due to stormwater runoff • Recommend actions to minimize future erosion • Ultimately to reduce damage to the Patuxent River and the Bay • Background • Continued erosion of stormwater outfall channels degrades Middle Patuxent River and the Bay • January 2013 meeting with HC DPW rep, Mark Richmond: • Based on 2010 survey, FY 2014 plan for storm water repairs includes no outfalls in MPEA • If new surveys reveal any substantial changes, DPW will reconsider priorities of repairs

  3. Survey of MPEA Stormwater OutfallsApproach • Re-survey the top severity outfalls to compare with 2010 surveys • MPEA Outfalls Subcommittee survey training by C Farfaras - 4/10/2013 • Surveys of all Severity 4 and 3 outfalls (per 2010 documentation by C Farfaras) completed in 4/2013 • Severity 4: #6, 10, 11, 16, 22 • Severity 3: #4, 5, 17, 19, 24 • New Country Lane #18, a severity 5 site repaired in 2009 • Provide survey data and photos to HC (C Farfaras) • Compare 2010 results with 2013 survey • Recommend any sites for reconsideration for future repairs

  4. Survey of MPEA Stormwater OutfallsSurvey Results • Blue Flag Way, #6 – Severity 4 • Outfall pipe is exposed behind the concrete head wall • Apron rip-rap dispersed over 30’ with geotec fabric exposed • Severe bank erosion above #6 from outfalls # 7 & 8 • Flow from #6 combines with # 7 & 8 upstream to cause severe erosion of banks from 25’ to 100’ below #6 outfall • Terrain below outfall is V shaped, so all flow is channeled; cannot be dispersed over a plain • Consider revising the combined #6, 7, & 8 channel to severity 5

  5. Exposed outfall pipe behind #6 head wall

  6. Bank erosion above #6 (from outfalls 7 & 8)

  7. Bank erosion below #6

  8. Yellow Rush Pass #10

  9. Series of small head cuts below #10

  10. Gold Needle Way - #11Merges with #10 above deep head cuts

  11. Deep head cut and erosion ~200’ below # 10 & 11

  12. #10 & 11 Survey Results • Yellow Rush Pass, #10 – Severity 4 • Apron rip-rap dispersed to 45’ below apron • Series of small head cuts (each ~ 1’ deep) at approx. 55’, 90’, 140’, 160’ below outfall • Some temporarily arrested by tree roots • Gold Needle Way, #11 flow combines with #10 • Deep (5.5’) head cut ~200’ below outfall • Consider revising the combined #10 and 11 channel to severity 5

  13. Outfall # 14 and adjacent house 11813 driveway erosion Winter Long Way - #14, 15, 16

  14. Winter Long Way - #14, 15, 16 # 15 Outfall and bank erosion below

  15. Winter Long Way - #14, 15, 16 #16 - 5.5’ erosion behind the head wall

  16. Winter Long Way - #16, 15, & 14200’ below #16 - left bank erosion from 3 outfalls + pipe from resident’s downspout2010 2013 (tree is now undercut)

  17. Winter Long Way - #14, 15, 16 Survey Results • # 14, 15, &16 all feed the same channel • Erosion 5.5’ behind #16 head wall • Severe left bank erosion 200’ below #16 • Consider revising the combined #14, 15, 16 channel to severity 5

  18. Outfalls #17 & 18 – Below townhouses #17 outfall and junction with #18 diversion

  19. #18 outfall, repair, and diversion wall • Repair of erosion near head wall completed in 2009 • Note that no trees have been restored in the repair zone

  20. Combined #17 and 18 channel is now ~8’ deep

  21. Outfalls #17 & 18 – Below townhouses • Together these outfalls handle runoff from townhouses and parking lots – all impermeable • #17 was rated severity 3 in 2010 – little erosion at the outfall site • #18 was repaired in 2009, filling deep cuts near the outfall, but directing all runoff into outfall #17 channel • Runoff previously flowed partially away from #17 channel • ~40’ below the #17/18 junction, erosion is now ~8’ deep • The combined channel should now be rated severity 5

  22. Bright Passage 2 - #22Deep head cut ~30’ below #22 outfall

  23. Erosion below #22 continues for 200’+

  24. Below Bright Passage 2 - #22 2013 2009

  25. Bright Passage 2 - #22 • Minor issues at the outfall • From ~30’ to 200’+ below outfall, severe erosion – 8’ deep • Consider revising the #22 channel to severity 5

  26. Bright Passage 3 - #24 • Slight erosion within 35’ of outfall • Increasing erosion – 6’ deep below bridge

  27. #24 Increasing erosion below bridge

  28. Survey of MPEA Stormwater OutfallsComparison with 2010 survey • Most outfalls have little change near the outfalls • Measures on survey form are primarily near the outfalls • Little quantitative comparison downstream from outfalls • Photo comparison reveals some changes, but most of the downstream erosion cuts are not included in 2009 photos

  29. Survey of MPEA Stormwater OutfallsConsiderations for future repairs • Past funding availability and priorities of other HC storm water sites have resulted in only 2 repairs at MPEA since 2009 • If we can't find affordable solutions, the current HC approach will never keep up with the erosion rate at MPEA • We've got head cuts that are 6' deep and growing • Repairs can severely impact the wooded environment due to tree cutting for large equipment access • See #18 site photo 3 years after 2009 repair • Consider use of pumped concrete with artificial rock forms or products such as the Hydrotex® system to avoid hauling rock to sites • Consider repair of multiple sites as a single project vice one outfall at a time • Generally, severe channel erosion has multiple outfalls above • Efficiencies due to integrated design, single bid/award, transport of equipment to site, etc.

  30. Survey of MPEA Stormwater OutfallsNext Steps • Compare 2010 photos and identify significant changes • Assign severity level values based on the channels; not individual outfalls • Typically 2 or 3 outfalls merge to create mega head cuts • Add measuring methods to track the changes in channel erosion • Consider methods to explore new design approaches • Consider MPEA for experimental/improved methods of repair • Offer results to DPW for future consideration • Meet with DPW to recommend candidate sites for reconsideration • Keep MPEA sites under consideration for new fee revenue

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