Survey of MPEA Stormwater Outfalls By MPEF Subcommittee

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

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 2

Survey of MPEA Stormwater Outfalls Approach • 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 3

Survey of MPEA Stormwater Outfalls Survey 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 – – 4

Exposed outfall pipe behind #6 head wall 5

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

Bank erosion below #6 7

Yellow Rush Pass #10 8

Series of small head cuts below #10 9

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

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

#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 12

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

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

Winter Long Way - #14, 15, 16 #16 - 5. 5’ erosion behind the head wall 15

Winter Long Way - #16, 15, & 14 200’ below #16 - left bank erosion from 3 outfalls + pipe from resident’s downspout 2010 2013 (tree is now undercut) 16

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 17

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

#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 19

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

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 21

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

Erosion below #22 continues for 200’+ 23

Below Bright Passage 2 - #22 2013 2009 24

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 25

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

#24 Increasing erosion below bridge 27

Survey of MPEA Stormwater Outfalls Comparison 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 28

Survey of MPEA Stormwater Outfalls Considerations 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. 29

Survey of MPEA Stormwater Outfalls Next 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 30