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Survey of MPEA Stormwater Outfalls
ByMPEF Subcommittee on Stormwater Outfalls
James PalmerJohn McCoy
Brian England
5/15/2013
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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
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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
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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
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Exposed outfall pipe behind #6 head wall
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Bank erosion above #6 (from outfalls 7 & 8)
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Bank erosion below #6
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Yellow Rush Pass #10
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Series of small head cuts below #10
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Gold Needle Way - #11Merges with #10 above deep head cuts
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Deep head cut and erosion ~200’ below
# 10 & 11
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#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
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Outfall # 14 and adjacent house 11813
driveway erosion
Winter Long Way - #14, 15, 16
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# 15 Outfall and bank erosion below
Winter Long Way - #14, 15, 16
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Winter Long Way - #14, 15, 16#16 - 5.5’ erosion behind the head wall
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Winter Long Way - #16, 15, & 14200’ below #16 - left bank erosion from 3 outfalls + pipe from resident’s downspout
2010 2013 (tree is now undercut)
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• # 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
Winter Long Way - #14, 15, 16 Survey Results
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#17 outfall and junction with #18 diversion
Outfalls #17 & 18 – Below townhouses
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#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
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Combined #17 and 18 channel is now ~8’ deep
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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
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Bright Passage 2 - #22Deep head cut ~30’ below #22 outfall
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Erosion below #22 continues for 200’+
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Below Bright Passage 2 - #22
2013
2009
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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
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Bright Passage 3 - #24• Slight erosion within 35’ of outfall• Increasing erosion – 6’ deep below bridge
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#24 Increasing erosion
below bridge
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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
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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.
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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