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POND RETROFITS
VWEA STORMWATER SEMINAR
Scott Smedley P.E.
Chesterfield County Dept. of Environmental Engineering
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April 27, 2016
Why do BMP pond retrofits?2
Disputed Reedy Creek restoration project
set to begin this summer
…has drawn the ire of neighbors
who see the project as an
unnecessarily risky move…
Richmond Times-DispatchApril 17, 2016
A nice little BMP pond retrofit…..
On County Property!
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Outline4
Planning
Why retrofit BMP ponds
Types of pond retrofits
Guidance and site selection
Implementation
Maximizing pollutant removal credits
Lessons learned
Conclusions
Failing BMPs – Age and/or Maintenance
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Pond Retrofits:
Enhance pollutant removal
Prevent downstream erosion
Flood protection
Why retrofit BMP ponds
Impoundment Structure Vegetation6
Congested Principle Spillway7
Impoundment Structure Failure8
Deteriorated Primary Spillway9
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Types of retrofits
Old wet pond Wet pond 1 or 2
Old dry pond Constructed wetland
Old constructed wetland Wet pond 1 or 2
Old dry pond Filtering Practice 1 or 2
Flood control SWM Wet pond 1 or 2
Smorgasbord of conversion choices
Selection of retrofit site based
on:
Site constraints
Funding
Retrofit goals
DEQ Crediting Methodologies
Published Guidance:
Expert Panel Report to Define
Removal Rates for Urban
Stormwater Retrofit Projects
Guidance Memo 15-2005:
Chesapeake Bay TMDL Action
Plan Guidance Memo (May 2015)
DEQ BMP Design Guidance
(BMP Clearinghouse)
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BMP Enhancements and Conversions
• DEQ Guidance Memo 15-2005
Restored
BMP
Efficiency
Existing BMP
Efficiency
Incremental
Efficiency
Improvement for
Credit
Calculations
Sources of Efficiencies:
• BMP Clearinghouse
• CB Retrofit Curves
• CB Established BMP Efficiencies
Downward Modification:
• Missing design elements
• Undersized BMPs
Efficiency downward modification
Only with Bay program efficiencies
10% for each design criteria that is missing
Short circuiting
No forebay
Significantly undersized
Impoundment structure repair?
Not to exceed 50%
DEQ approval
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BMP downward modification
example15
Nitrogen Phosphorus TSS
Type 1 wet pond 30% 50% 66%**
Current efficiency 14% 31.5% 42%
Final efficiency for
TMDL credit
16% 18.5% 24%
Nitrogen Phosphorus TSS
Bay Program eff 20% 45% 60%
No forebay -2 -4.5 -6
Short circuiting -2 -4.5 -6
Undersized -2 -4.5 -6
Current efficiency 14 31.5 42
** Retrofit Curve
BMP Retrofit - Cost effective?16
Yes. Great!
No
Selection of retrofit type (pond levels, wetlands)
Drainage area controlled by retrofit
Topography, soils, and groundwater in the retrofit site
Conveyance and pre-treatment (e.g., forebays) devices
Implementation:
Maximizing Pollutant Removal Credits
Different options/guidance: pollutant load calculations
Factors impacting efficiency:
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Jurisdictional
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Design Lessons Learned
James River High School Pond Retrofits
Impacts on school activities that
resulted in design changes –
pond layout and volume
Drainage areas/designs in old
plans were not representative of
existing conditions – changes in
pollutant reduction from
planning (SLAF) to final design
Outlet structure information not
on plans – additional survey Dry pond to wet pond (added forebay)
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Permitting:
start as early as possible
inconsistent interpretations
resulted in delays
Design Lessons Learned
James River High School Pond Retrofits
Dry pond to constructed wetland (added forebay)
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Lessons Learned26
Gather as much information as possible during
planning stage
Consider impacts on surrounding properties
Select innovative techniques as needed
Look at entire drainage area
Run calculations with several methods
Conclusions
Available design guidance is adequate
New design requirements typically require a larger
footprint that may not meet site constraints
Many options exist to retrofit existing systems for
water quality/nutrient reductions
Finding the right mix to make the project cost
effective can be a challenge
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Thank You
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