Survey of MPEA Stormwater Outfalls By MPEF Subcommittee on Stormwater Outfalls James Palmer John McCoy 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

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

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

# ’ erosion behind the head wall 15

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

# 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 Winter Long Way - #14, 15, 16 Survey Results

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

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

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 - #

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