Session 2 Strategies to Consider Prior to Retrofits

Session 2 Strategies to Consider Prior to Retrofits

1. Computing Retrofit Sediment and Nutrient Reductions a) Design Examples

2. Cost Effective Strategies that don’t involve traditional retrofits a) Specific Methods to document load

reductions from non-retrofit practices

Session 2 Agenda

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• Removal rates are a f(x) of runoff depth

captured and the amount of stormwater treatment (ST) or runoff reduction (RR) achieved by the retrofit

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 acres3. Input into Standard Retrofit

Equation: = ሺ𝑅𝑆 ሻ(12)𝐼𝐴

Classification of Retrofits Runoff Reduction Practices

(RR)Stormwater

Treatment Practices (ST)

All ESD credits in MD (2009) Constructed WetlandsAll ESD practices in MD (2009) Dry ED PondsBioretention Filtering Practices Dry Swale Wet Ponds Infiltration Wet Swale

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

= ሺ𝑅𝑆 ሻ(12)𝐼𝐴

ሺ1.67 ሻ(12)20 = 1.0 𝑖𝑛𝑐ℎ

TP TN TSS

52% 33%

66%


Pollutant Removal Efficienciesof the practice

Retrofit Example 1 – Constructed Wetland

TotalNitrogen

TotalPhosphorus

Suspended Sediment

Pounds/acre/year Tons/acre/yearIMPERV PERV IMPERV PERV IMPERV PERV

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/yrRemoval 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

= ሺ𝑅𝑆 ሻ(12)𝐼𝐴

ሺ1.3 ሻ(12)26 = 0.6 𝑖𝑛𝑐ℎ𝑒𝑠 TP TN TSS55% 55

%59%


Example 2 – BMP Conversion

TotalNitrogen

TotalPhosphorus

Suspended Sediment


MDE Loading Rates

10.85 9.43 2.04 0.57 0.46 0.07


Baseline Load 650 lbs/yr 75.3 lbs/yr 14.7 tons/yrRemoval Rate 55% 55% 59%

Load Removed 357.4 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”

Design Examples – BMP Enhancement

• 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 recovered 0.15” of lost

storage and created additional 0.3” of storage for a combined new storage of: 0.45” per impervious acre

Design Examples – BMP Enhancement• 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 capabilitiesTP TN TSS

Enhanced Rate 39% 25% 50%Original Rate 21% 14% 28%

Incremental Removal Rate

18% 11% 22%

Example 3 – BMP Enhancement

TotalNitrogen

TotalPhosphorus

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% 18% 22%Load Removed 11.94 lbs/yr 3.67 lbs/yr 1.01 tons/yr

Remember: the site is 100% impervious!!

Discussion

Cost Effective Strategies that don’t involve

traditional retrofits

The Changing Stormwater Landscape Under New MS4 Permits and the Chesapeake Bay

TMDL ~ 10 to 30% TP and TN load reductions needed from existing development

Massive Increase in Retrofitting

15 Years to Get It Done

This is inducing a phenomenon known as “ retrofit stress syndrome”

Infects local public works officials and their elected officials

Common Symptoms:• Fear of the unknown (what exactly is a retrofit?) • Excessive phobia about future costs and regulatory

liability

Look Beyond the Storage Retrofit…to other Restoration Tools

Inoculate Yourself From RSSUsing Other Nutrient Reduction Practices

1. P Bans and N Fertilizer Mgmt2. BMP Maintenance Upgrades3. Redevelopment Credits4. Watershed Reforestation5. Stream Restoration6. Street Cleaning7. Illicit Discharge Removal 8. On-site LID Incentives

1. Take credit for fertilizer reductions on urban turf

• Reflects recent MD and VA P- Ban in Fertilizer

• Modeled as an application reduction on urban pervious area

• Initial estimates of 12% TP reduction for urban sector in MARYLAND

• Automatic Credit for Localities

• Expert Panel to finalize this estimate in summer 2012

• 1.5 million acres of home lawn are fertilized

• Same acreage in State WIP Plans

• What is Urban Nitrogen Management ?

• What are the high risk situations where UNM is most effective?

• CBP-approved rates of 17% TN for Urban Nutrient Management is NOT likely to be continued

Urban NitrogenFertilizer Reduction

CURRENT EXPERT PANEL

Update on Expert Panel on Urban Nitrogen Fertilizer Management

Current CBP-approved rates of 17% TN for Urban Nutrient Management is NOT likely to be continued

N Removal Rate will be Tied to Effectiveness of Local Outreach Efforts to Change Behavior for:

• Established Low Risk Lawns• Established High Risk Lawns• New Start Up Lawns

For both lawn care companies and do it yourselfers

• Public land can comprise 10-15% of total turf cover in a community

• Fertilizer application rates already tend to be lower

• Assess hi risk public lands and change landscaping practices

Urban Nitrogen Management on Public Land

Urban Fertilizer Management ExamplePanel not ready to

define method yet, and the per acre rate may not be very high (2 to 10%), but given that turf comprises 50 to 70% of urban areas, even a small credit could yield impressive benefits

More than just brochures!

2. Transform Your Stormwater Maintenance Program

Use your stormwater maintenance inspection, tracking and repair budgets to fix problems and boost performance through major maintenance upgrades

Potential for both public and private stormwater facilities

Design Example – BMP Restoration

• Captures 0.5” of runoff from the impervious cover of contributing watershed: 40 acres @ 50% impervious

• Sedimentation and invasive plant growth decreased storage volume by 60%

• Only actually provides 0.2” of runoff capture• City conducts major sediment dredging, invasive

plant removal, replants pond with natives• Recovers 0.2” of storage for a total storage of

0.4”

Design Examples – BMP RestorationIf the BMP has previously reported to the state

(and already included in CBWM input deck), then the removal rates is determined from the curves as an incremental rateincremental removal rate = restored rates –

original rates.

TP TN TSSRestored Rate 37% 24% 47%Original Rate 26% 16% 33%Incremental Removal Rate 11% 8% 14%

BMP Restoration Example

TotalNitrogen

TotalPhosphorus

Suspended Sediment


MDELoading Rates

10.85 9.43 2.04 0.57 0.46 0.07


Baseline Load 405.6 lbs/yr 52.3 lbs/yr 10.6 tons/yrRemoval Rate 8% 11% 14%

Load Removed 32.5 lbs/yr 5.74 lbs/yr 1.49 tons/yr

3. Take nutrient reduction credits for more stringent stormwater requirements at redevelopment projects

• Calculate the expected acres of impervious cover slated for redevelopment

• New CBP tool calculates the aggregate nutrient reduction credit


Design Examples – Redevelopment

A MD developer is redeveloping a 2-acre facility to build a new warehouse.

• The pre-development and post-development conditions are 50% impervious and 50% turf land cover.

• The BMPs treat the entire site • There are 100% D soils at the site and

the site will be developed using RR practices.

Design Examples – Redevelopment

Using the prescribed method, we calculate the target runoff depth we are controlling for to be: 0.24 inches.

Again, we go to the curves!

TP TN TSS36% 37% 40%


Calculating Pollutant Reduction for Redevelopment Site

TotalNitrogen

TotalPhosphorus

Suspended Sediment


MDELoading Rates

10.85 9.43 2.04 0.57 0.46 0.07

Area (acres) 1.0 ac 1.0 ac 1.0 ac 1.0 ac 1.0 ac 1.0 ac

Baseline Load 20.3 lbs/yr 2.61 lbs/yr 0.53 tons/yrRemoval Rate 37% 36% 40% Load Removed 7.5 lbs/yr 0.94lbs/yr 0.21 tons/yr

Individually, load reductions from redevelopment are not impressive

But across a community and over 15 years, they can really add up !

• Many MD have existing reforestation programs and urban tree canopy goals

• Decent load reduction credits are possible for these:

• Get extra reduction credit for• Runoff from adjacent IC • Stream buffer upgrades

4. Watershed Reforestation

PENDING EXPERT PANEL

Qualifying Conditions for Reforestation Credit in MD

• Survival rate of 100 trees/acre• 50% of the trees need to be >2” in diameter

at 4.5’ above ground• Aggregate of smaller sites is okay• Reforestation done for mitigation purposes is

not eligible• Credits are determined as land cover

conversion based on the following efficiencies:

Load Impacts of ReforestationConversion from (lbs/ac/yr)

TN (lbs/ac/yr)

TP (lbs/ac/yr)

TSS (tons/ac/yr)

Urban Pervious 9.43 0.57 0.07Forest 3.16 0.13 0.03

Load Reduction

6.27 0.44 0.04

Urban Impervious

10.85 2.04 0.44

Forest 3.16 0.13 0.03

Load Reduction

7.7 1.91 0.41

Adapted from MDE DRAFT Guidance Document 2011

Watershed Reforestation ExampleBay County reforests 10 acres of turf (pervious land)

in a subwatershedMDE Method for Converting Acres Reforested to Equivalent Impervious Cover Treated (acres) Pollutant Acres Converted

from Turf to Forest 2Load Reduction (lbs/acre/year) 1

Total Load Reduced

Total N 10 6.27 62.7Total P 10 0.44 4.4Sediment 10 80 8001 Adapted from Table 7 in MDE (2011)2 Additional credit provided when impervious cover is converted to forest

Although MDE does not specify how reforestation is converted to equivalent IC Treated, CSN recommends a 10:1 Rule (10 acres reforestation = 1 acre of IC treated, based on runoff relationships)

Other Watershed Reforestation Options

1. Forest Filter Strip*2. Urban Stream

Buffer*3. Urban Tree

Canopy**

* Existing EPA-approved rates, new panel to re-evaluate in Fall of 2012 ** Rate currently being evaluated by expert panel

Panel Issues on Tree Planting Credit

•Double counting•Is credit immediate or phased in?•Tree mortality•Canopy over Impervious cover•Verification•Tracking•Planting by public and watershed groups

5. Take credit for current and future stream restoration projects

• High nutrient reduction rates for qualifying projects• Provides both a local benefit and a Bay benefit• Generally popular with the public• Cost competitive with pond retrofits


Stream Restoration Removal Rate

Removal rate per Linear foot of Qualifying Stream Restoration Source TN TP TSSCBP 2005 N=1

0.02 lbs 0.0035 2.55 lbs

New Interim Rate

0.20 lbs 0.068 lbs 310 lbs

Expert Panel Project-Specific Method to

Define Rate

Stream Restoration

Qualifying Conditions• Stream reach > 100 ft that is still actively degrading

(usually 1st – 3rd order streams)• Comprehensive restoration design: involving

channel, banks and floodplain• Special consideration given to projects with

floodplain reconnection or in-stream habitat creation

Removal Rate per Linear Foot:

TN = 0.20 lbsTP = 0.068 lbsTSS = 310 lbs

Stream Restoration ExampleMD City opts to restore a 1000’

segment of stream located in a highly urbanized section of the city. Using the removal rates, the City determines the pollutant reduction credits it can take for the project:

Removal Rate

(lbs/ft)

Linear Feet of Restored Stream

Total Removal

(lbs)TN 0.20 lbs 1000’ 200 lbsTP 0.068 lbs 1000’ 68 lbsTSS 310 lbs 1000’ 155 TONS

Converting to Impervious Acres Treated

In addition to the above pollutant removal values, the MD City is required to report the impervious acres treated. Which can be determined by the following equation:

Impervious acreage treated =

1 acre/100 linear feet of restored stream

= 10 acres of impervious cover treated

A few expert panel notes • 3 part method rather than rate per linear feet• Not a license to use urban steam corridor for

stormwater treatment

6. Take the Mass Credit for Intensive Street Sweeping• Qualifying Frequency and Technology• Incentive for Sweeping Crews to Maximize Pickup

EXISTING EXPERT PANEL

Street SweepingTwo Methods 1. Mass Loading Approach* (PREFERRED)2. Qualifying Street Lanes Method

Qualifying Conditions:• Urban street with high average daily traffic

volume located in commercial, industrial, central business, high intensity residential

• Minimum frequency of 26 times a year (every 2 weeks)– Can be grouped for specific times (i.e., Spring and

Fall)• Reductions based on sweeping technology:

Mechanical << Regenerative/Vacuum

Mass Loading ApproachMass of collection measured (tons) at point of

disposalStep 1. Determine capacity of sweeperStep 2. Weigh solids collected (in tons)Step 3. Record annual mass collected (tons)Step 4. Convert from tons to pounds of solids

(multiply by 2000) and convert to dry weight (factor of 0.7)

Step 5. Multiply dry weight by established factors:Lbs of TN = 0.0025 lbs of dry weight solids

Lbs of TP = 0.001 lbs of dry weight solidsStep 6. Compute TSS reduction credit by multiplying the annual mass of dry weight by a factor of 0.3

Qualifying Street Lanes MethodConvert qualifying lane miles into total

impervious cover (acres):Miles swept feet swept multiplied by lane width (feet) divide by 43,560 = acres of street swept

Multiply acres swept by pre-sweeping annual load (simple method):

TP = 2.0 lbs/impervious acre/yearTN = 15.4 lbs/impervious acre/year

Qualifying Street Lanes MethodMultiple pre-sweep baseline load by pick-

up factors:Multipliers to Reflect Effect

of Street Sweeping on the Baseline Load 1

Technology TP TNMechanical .04 .04Regenerative/Vacuum

.06 .051 CSN 2011

Street Sweeping Example Qualifying Street Lanes Method

Over the past year, local MD community has swept the streets 26 times, with a regenerative street sweeper.

The community swept 25 lane miles which included both sides of the street.

This is converted to an area: Both sides of the street were swept, so an average width of 20 feet could be used.

The lane miles were converted to feet and multiplied the 20’ width, and then divided by 43,560 to get the total acres of street swept in the past year = 60.61 acres.

Qualifying Street Lanes Example cont.Multiplying the impervious acreage

swept (60.61 acres) by the pre-sweeping annual pollutant load, the community was able to determine their baseline load:

Baseline Load

Phosphorus

(60.61 ac)(2.0 lbs/ac/yr) = 121.21 lbs/yr

Nitrogen (60.61 ac)(15.4 lbs/ac/yr)

= 933.39 lbs/yr

Qualifying Street Lanes Example cont.

The MD Community then multiplied the baseline load by the Regenerative Technology factors to get load reductions based on their program:

Pollutant Removal Loads

TP 121.21 lbs/yr*0.06 = 7.27 lbs/yrTN 933.39 lbs/yr*0.05 = 46.66 lbs/yr

Converting Acres Swept vs. Impervious Area Treated

MDE Method for Converting Acres Swept to Equivalent Impervious Cover Treated (acres) Pollutant Adjustment

Factor Acres Swept Equivalent IC

Acres TreatedTP 0.04 60.61 2.42TN 0.06 60.61 3.64TSS 0.12 60.61 7.27Average for all three 4.44Source: Table 12 in MDE (2011)

7. Take Credit for Eliminating Illicit Discharges

• For chronic and episodic sewage discharges that are physically eliminated from storm drain

• Reduction based on rate of dry weather flow and outfall concentration above background levels

• Outfall screening is big part of MS4 permits, so gives credit when screening is nutrient based PENDING EXPERT PANEL

Illicit Discharge Detection and Elimination

Currently there is no official process for calculating pollutant reduction credits. IDDE program.

Expert Panel planned for 2012 for this topic.

In the meantime, CSN’s Technical Bulletin #9 has a recommended process that we can use as a guide…

Illicit Discharge Detection and Elimination

Step 1. Dry weather flow rate and nutrient concentrations measured at outfalls

Step 2. Discharge source trackingStep 3. Monitor flow rate and nutrient

concentrations at discharge source prior to and after discharge elimination

Step 4. Follow-up monitoring to confirm nutrient concentrations have returned to background levels

Step 5. Compute the nutrient credit:(daily flow rate)(nutrient conc.)(# of days of discharge) = load reduced

8. Residential LID Retrofits

Subsidies, technical assistance, stormwater utility credits and other incentives to build LID retrofits on private land CURRENT EXPERT PANEL

Design Examples – Residential Stewardship Incentives

A MD County creates an incentive program for residential homeowners to install rain gardens on their property and would like to determine the pollutant removal rates associated with such a program.


Each homeowner installs a rain garden to treat 500 ft2 of rooftop

If 100 homeowners participate in the program, treatment can occur for a combined drainage area of 1.15 acres, at 100% impervious.

The runoff storage volume associated with the combined retrofits is estimated to be 0.05 acre-feet.

Rain gardens are an RR practice


The amount of runoff volume treated by the rain gardens is calculated using standard retrofit equation:

The township engineer uses the curves to estimate the projected removal rates associated with the rain garden incentive program:

TP TN TSS52% 52% 55%

ሺ0.05 ሻ(12)1.15 = 0.5 𝑖𝑛𝑐ℎ𝑒𝑠

Calculating Pollutant Load Reduction of On-site LID Incentives

TotalNitrogen

TotalPhosphorus

Suspended Sediment


MDELoading Rates

10.85 9.43 2.04 0.57 0.46 0.07

Area (acres) 1.15 ac 0 1.15 ac 0 1.15 ac 0

Baseline Load 12.48 lbs/yr 2.35 lbs/yr 0.53 tons/yr

Removal Rate 52% 52% 55%Load Removed 6.5 lbs/yr 1.22 lbs/yr 0.30 tons/yr

Other On-site Options and more on LID Incentives in Session 4

Q & A

Documents

Session 2 Strategies to Consider Prior to Retrofits