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Stormwater Retrofitting

Demystified!

A training for local governments to cost effectively implement retrofits to meet MS-4 permit and Chesapeake

Bay TMDL requirements.

Workshop Agenda

9:30 – 9:40 Welcome and Learning Objectives for the Day

9:40 – 10:00 State Perspectives on Stormwater Retrofitting

10:00 – 10:45 Session 1. Basics of Stormwater Retrofitting

10:45 – 11:15 BREAK

11:15 – 12:30 Session 2. Strategies to Consider Prior to Retrofits

12:30 – 1:30 LUNCH

1:30 – 2:30 Session 3. The Retrofit Discovery Process

2:30 – 3:45 Session 4. Retrofit Costs, Delivery and Maintenance

3:45 – 4:00 Concluding Remarks

4:00 Evaluations!

To learn how you can have access to: Discounted Webcasts

Free One-day design workshops Intensive master stormwater design seminars

Direct On-site technical assistance Self guided web-based learning modules

Visit: www.chesapeakestormwater.net

Chesapeake Bay Stormwater Training Partnership

http://www.chesapeakestormwater.netrg/

Session 1 Basics of Stormwater

Retrofitting

1. Where do Nutrients and Sediment Come From?

2. Retrofit Categories 3. Envisioning Retrofits 4. Computing Retrofit Sediment and

Nutrient Reductions a) Design Examples

Session 1 Agenda

Where do Nutrients and Sediment Come From?

There are many sources of N and P in the urban environment

The main sources of nutrients to the Bay Watershed are:

•Runoff from Forests •Wastewater •Atmospheric Deposition to Open Water •Urban and Suburban Runoff •Agricultural Runoff •Septic Systems (N only)

Relationship of Atmospheric Deposition to Urban Runoff Quality

Nutrient

Atmospheric Deposition 1

Stormwater Runoff Load 2

Pounds per impervious acre per year Total Phosphorus 0.7 2.0 Total Nitrogen 13 to 17.0 3 15.4 1 measured rates during Washington NURP Study (MWCOG, 1983) 2 Simple Method annual stormwater runoff loads for one acre of impervious cover (Schueler, 1987) 3 About 40% of nitrogen deposition occurs through wetfall, which would presumably be quickly converted into runoff. 60% of nitrogen deposition occurs via dryfall, which is available for washoff in future storms, or may be blown over to pervious areas

Much of the nitrogen in urban runoff is derived from atmospheric deposition, either in the form of dryfall or wetfall

Other sources of nitrogen in urban runoff include: •Washoff of fertilizers •Nitrogen attached to eroded soils and streambanks •Organic matter and pet wastes on IC

Nitrogen EMCs for different urban land covers

Urban Land Cover Total N (mg/l)

Lawns 9.70

Highway 2.95

Streets (Variable) 1.40

Parking Lots 1.94

Rooftops 1.50

Source; CWP, 2003

Runoff sampling shows that lawn runoff is very high in nitrogen. Also, rooftop runoff concentration shows effect of atmospheric deposition

Many sources of TP in urban runoff

• Blow in of organic matter onto impervious surfaces (leaves, pollen, clippings, flowers, etc.)

• Phosphorus attached to eroded soils and streambanks

• Fertilizer washoff

• Atmospheric deposition

Phosphorus EMCs for different urban land uses

Urban Land Use Total P (mg/l)

Residential 0.30

Commercial 0.22

Industrial 0.26

Freeway 0.25 Source: Pitt et al 2004

Residential runoff is slightly higher in TP concentration, which reflects the effect of vegetation and fertilization

Phosphorus EMCs for different urban land covers

Urban Land Cover Total P (mg/l)

Lawns 1.90

Highway 0.60

Streets (Variable) 0.50

Parking Lots 0.16

Rooftops 0.12

Source; CWP, 2003

The sources of phosphorus are more complex. While lawn runoff is high in nitrogen, atmospheric deposition is less important as a source of TP

Total Phosphorus Loads By Sector in Maryland Portion of Bay Watershed

Sector 2009 Load

Target Load % Reduction Needed to Meet Target

Million pounds per year

Forest 0.34 0.34 0

Atm. Deposition 0.04 0.04 0

Wastewater 0.67 0.70 0

Urban and Suburban 0.68 (22%) 0.39 43%

Agricultural 1.37 1.25 9%

Septics -0- -0- 0

TOTAL 3.10 2.72 12%

Source: US EPA Chesapeake Bay Program, 2010

Sources of Urban Sediment

• Urban stream channel erosion

• Wash-off from impervious areas

• Erosion from pervious areas

• Construction sites

Edge of Stream Unit Loading Rates for MD

Using CBWM v. 5.3.2

Pounds/acre/year

Total N Total P TSS

IMPERV PERV IMPERV PERV IMPERV PERV

Urban 15.3 10.8 1.69 0.43 1116 175

Forest 3.16 0.13 60

Source: CBPO, 1/4/2012

Discussion

1964

My Early Retrofitting Years

Wiggle-tail

Why Retrofit ?

• Local Watershed restoration

• Meet IC Treatment Targets in MD

• Comply with Bay-wide TMDLs (and local ones too)

• Improve local stream habitat and diversity

• Fix old mistakes/drainage problems

• Improve performance of existing stormwater infrastructure

Why Retrofits Are Different

Urban Retrofit Practices New Stormwater Practices

Construction costs are 1.5 to 4

times greater Designers seek least costly options

Assessment and design costs are

higher

Focus on low cost design and

construction

Sized to meet watershed

restoration objectives

Sized to meet local stormwater

design standards

Typically installed on public land Installed at new development

projects

Urban soils often cannot support

infiltration Soils may support infiltration

Fingerprinted around existing

development

More flexibility on where to locate

practices

Why Retrofits Are Different

Urban Retrofit Practices New Stormwater Practices

Must be acceptable to adjacent

neighbors

Aesthetics are not always a major

design factor

Most are publicly maintained Most require private maintenance

Not all candidate sites are

feasible Nearly all sites are made to work

Tied into existing conveyance

system

Usually creates new conveyance

system

Integrated with other

restoration practices Stand-alone practice

Public investment in watershed

infrastructure

Private investment in stormwater

infrastructure

Caution: The “rules” are in flux

• MDE 2011 Guidance • CSN Technical Bulletin 9 • Roll out of New MS4 Permits • 6 New Urban BMP Expert Panels • New BMP Verification Protocols • Updated editions of MAST Bad news: the numbers will change Good news: the numbers will improve Advice: use them for general planning and evaluation of alternatives

Dual BMP Reporting in MD

• For MS4 Permits: • Report BMPs implemented ** • Report “Treated Acres” of Existing Impervious Cover *

For Bay TMDL/Local WIPs: • Report BMPs implemented ** • Report TSS, TN, and TP reductions

** both are done using Appendix A of MS4 BMP Reporting * ESD to MEP for existing IC defined as minimum site WQv

Best Opportunities for Retrofitting in the Urban

Landscape

Retrofit Categories

1. Near Existing Stormwater Outfalls 2. Within the Conveyance System 3. Adjacent to Large Parking Lots 4. Green street retrofits 5. On-site LID retrofits

1. BMP Conversions 2. BMP Enhancements 3. BMP Restoration

NEW RETROFITS

Near Existing Stormwater Outfalls

Source: CWP

NEW RETROFITS

Within the Existing Conveyance System

Source: CWP

Wet Pond

Bioretention

NEW RETROFITS

Adjacent to Large Parking Lots

Source: CWP

NEW RETROFITS

Green Street Retrofits

NEW RETROFITS

On-Site LID Retrofits

Retrofit Categories

1. BMP Conversions

2. BMP Enhancements

3. BMP Restoration

EXISTING RETROFITS

BMP CONVERSION

DRY POND CONSTRUCTED

WETLAND

BMP CONVERSIONS

Rehabilitating Failed Infiltration Practices

BMP CONVERSIONS Adding Bioretention/Filtering to Ponds

EXISTING RETROFITS

BMP ENHANCEMENT

INCREASE IN HYDRAULIC RETENTION TIME

EXISTING RETROFITS

BMP RESTORATION

DREDGING AN UNDERPERFORMING POND TO RESTORE FULL PERFORMANCE

Computation of Sediment and Nutrient Reductions associated with Retrofits

Retrofit Removal Adjustor Curves

• Method Developed by CBP Retrofit Expert Panel

• In the final stages of adoption by Chesapeake Bay Program (June, 2012)

• Tech memo provides technical documentation


• Each retrofit has its own unique removal rate based on the amount of runoff it treats and the degree of runoff reduction it provides

• Determined composite “Anchor Rates” of pollutant removal values at 1.0 inch of Runoff Depth Captured for each category of practices

Classification of Retrofits Runoff Reduction Practices

(RR) Stormwater Treatment

Practices (ST)

All ESD credits in MD (2009) Constructed Wetlands

All ESD practices in MD (2009) Dry ED Ponds

Bioretention Sand Filters

Dry Swale Wet Swale

Infiltration Wet Ponds

All practices sorted into 2 categories: Runoff Reduction (RR) and Stormwater Treatment (ST)

Achieve at least 25% reduction of annual runoff volume

Traditional Practices

Table A-3 Composite Approach to Derive Nutrient Mass Load Reductions for RR ad ST Runoff Reduction Practices 1,

PRACTICE TP Mass

Reduction (%)

TN Mass Reduction

(%) Bioretention 73 77 Dry Swale 66 63 Infiltration 75 78 Permeable Pavers 70 70 Green Roof/Rain Tank 55 55

Average RR 70 702 Wet Ponds 63 35 Const. Wetlands 63 40 Filtering Practice 63 38 Wet Swale 30 30

Average ST 55 35 1 Source: Table A-5, nutrient rates computed using the average mass reduction for both Design Level 1 and Level 2. 2 This value was subsequently discounted by 18% to reflect the impact of nitrate migration from runoff reduction practices described later in this appendix.

Anchor Rates @ 1”


• Use of Rainfall Frequency Analysis to determine the amount of bypass above and below 1.0”

• Same approach to determine mass pollutant removal for runoff depths above and below 1.0”

• Converted to series of Retrofit Removal Adjustor Curves


To determine the amount of runoff depth treated at a site:

1. Estimate the Runoff Storage volume (RS) available at the site in acre-feet.

2. Impervious Area (IA) in acres

3. Input into Standard Retrofit Equation:

= 𝑅𝑆 (12)

𝐼𝐴

Design Examples

Design Examples – New Retrofit Facility Constructed Wetland

• A constructed wetland is built in parkland as a retrofit, classified as a ST practice

• The retrofit storage is estimated to be 1.67 acre-feet

• Treats runoff from 50 acre residential neighborhood with 40% impervious cover


• Using the Standard Retrofit Equation:

• RS = Retrofit Storage ≈ 1.67 ac-ft

• IA = Impervious Area = 20 acres

= 𝑅𝑆 (12)

𝐼𝐴

1.67 (12)

20= 1.0 𝑖𝑛𝑐ℎ

TP TN TSS

52% 33% 66%


Pollutant Removal Efficiencies of the practice

Determining the Baseline Load Can calculate the baseline load using the

generic state-wide CBWM (version 5.3.0) urban unit loading rates from MDE guidance document:

• Calculate the number of pervious and impervious acres in the DA,

• Multiply by the unit loading rates:

* Note: these rates may be updated to reflect the most recent version of the CBWM

Retrofit Example 1 – Constructed Wetland

Total

Nitrogen

Total

Phosphorus

Suspended

Sediment

Pounds/acre/year Tons/acre/year


MDE

Loading

Rates

10.85 9.43 2.04 0.57 0.46 0.07

Area

(acres) 20 ac 30 ac 20 ac 30 ac 20 ac 30 ac

Baseline

Load 500 lbs/yr 57.9 lbs/yr 11.3 tons/yr

Removal

Rate 33% 52% 66%

Load

Removed 164 lbs/yr 30.1 lbs/yr 7.46 tons/yr

BMP Conversion Example

• Dry pond conversion

• Create new water quality storage using a combination of a forebay with a permanent pool, a submerged gravel wetland cell and a final bioretention polishing cell

• New facility now provides a runoff storage volume of 1.3 acre-feet

• Treats a site area of 65 acres @ 40% impervious

• Classified as a RR practice

Design Examples – BMP Conversion

• Using the Standard Retrofit Equation:

• RS = Retrofit Storage ≈ 1.3 ac-ft

• IA = Impervious Area = 26 acres

= 𝑅𝑆 (12)

𝐼𝐴

1.3 (12)

26 = 0.6 𝑖𝑛𝑐ℎ𝑒𝑠

TP TN TSS

55% 47% 59%


Design Examples – BMP Conversion

TP TN TSS

55% 47% 59%


Example 2 – BMP Conversion

Total

Nitrogen

Total

Phosphorus

Suspended

Sediment



MDE

Loading

Rates

10.85 9.43 2.04 0.57 0.46 0.07

Area

(acres) 26 ac 39 ac 26 ac 39 ac 26 ac 39 ac

Baseline

Load 650 lbs/yr 75.3 lbs/yr 14.7 tons/yr

Removal

Rate 47% 55% 59%

Load

Removed 305.5 lbs/yr 41.4 lbs/yr 8.67 tons/yr

Design Examples – BMP Enhancement

• Dry Extended Detention pond sized to capture 0.3” of runoff

• 10 acre commercial drainage area @ 100% impervious

• Short-circuiting of pond led to half of original storage volume ≈ 0.15”


• Pond enhanced to: – Increase hydraulic retention time (prevent short-

circuiting)

– Provide pretreatment

– Wetland cells added to bottom of pond in order to provide better treatment

• Enhancements created additional 0.3” of storage for a combined new storage of: 0.6” per impervious acre


• Enhancements are slightly different • New removal rates found as the difference

between the original rates and the enhanced rates

• Original and enhanced rates from the curves • Increase in both runoff volume captured AND

runoff reduction capabilities

TP TN TSS

Enhanced Rate 44% 28% 55%

Original Rate 22% 14% 28%

Incremental Removal Rate 22% 14% 27%

Example 3 – BMP Enhancement

Total

Nitrogen

Total

Phosphorus

Suspended

Sediment



MDE

Loading

Rates 10.85 9.43 2.04 0.57 0.46 0.07

Area

(acres) 10 ac 0 10 ac 0- 10 ac 0-

Baseline

Load 108.5 lbs/yr 20.4 lbs/yr 4.6 tons/yr

Removal

Rate 11% 22% 27%

Load

Removed 15.19 lbs/yr 4.49 lbs/yr 1.24 tons/yr

Remember: the site is 100% impervious!!

Discussion

What’s unique for stormwater retrofitting in your community?

Discussion

Retrofitting requires: Sleuthing skills to determine what can

work at highly constrained sites Simultaneously envisioning restoration possibilities and anticipating potential

problems

Activity: Envisioning Restoration

Activity

Activity

Questions?

Stormwater Retrofitting Demystified!chesapeakestormwater.net/wp-content/uploads/... · • Create new water quality storage using a combination of a forebay with a permanent pool,

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