Survey of MPEA Stormwater Outfalls By MPEF Subcommittee on
Stormwater Outfalls James Palmer John 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 2
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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
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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
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Exposed outfall pipe behind #6 head wall 5
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Bank erosion above #6 (from outfalls 7 & 8) 6
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Bank erosion below #6 7
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Yellow Rush Pass #10 8
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Series of small head cuts below #10 9
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Gold Needle Way - #11 Merges with #10 above deep head cuts
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Deep head cut and erosion ~200 below # 10 & 11 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 12
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Outfall # 14 and adjacent house 11813 driveway erosion 13
Winter Long Way - #14, 15, 16
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# 15 Outfall and bank erosion below 14 Winter Long Way - #14,
15, 16
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#16 - 5.5 erosion behind the head wall 15
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Winter Long Way - #16, 15, & 14 200 below #16 - left bank
erosion from 3 outfalls + pipe from residents downspout 2010 2013
(tree is now undercut) 16
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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 17 Winter
Long Way - #14, 15, 16 Survey Results
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#17 outfall and junction with #18 diversion 18 Outfalls #17
& 18 Below townhouses
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#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
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Combined #17 and 18 channel is now ~8 deep 20
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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 21
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Bright Passage 2 - #22 Deep head cut ~30 below #22 outfall
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Erosion below #22 continues for 200+ 23
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Below Bright Passage 2 - #22 2013 24 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 25
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Bright Passage 3 - #24 Slight erosion within 35 of outfall
Increasing erosion 6 deep below bridge 26
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#24 Increasing erosion below bridge 27
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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
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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
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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