POND RETROFITS

VWEA STORMWATER SEMINAR

Scott Smedley P.E.

Chesterfield County Dept. of Environmental Engineering

1

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!

3

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

5

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

10

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)

12

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

14

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:

17

18

19

20

21

22

Jurisdictional

23

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)

24

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)

25

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

27

28

Thank You