1

Survey of MPEA Stormwater Outfalls

ByMPEF Subcommittee on Stormwater Outfalls

James PalmerJohn 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

# 15 Outfall and bank erosion below

Winter Long Way - #14, 15, 16

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 downspout

2010 2013 (tree is now undercut)

17

• # 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

18

#17 outfall and junction with #18 diversion

Outfalls #17 & 18 – Below townhouses

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