The Runoff Reduction Theory

The Runoff Reduction The Runoff Reduction The Runoff Reduction The Runoff Reduction TheoryTheory

Created to deliver targeted training on new tools and practices to improve the quality of stormwater runoff.

http://www.cwp.org/cbstp

Traditional Stormwater Traditional Stormwater ApproachesApproachesApproachesApproaches

Focused primarily on managingstormwater quantity and, perhaps, quality, and relied heavily on traditional and relied heavily on traditional stormwater management practices to mitigate, rather than prevent the negative

Chesapeake Bay Stormwater Training Partnership 2

g , p gimpacts of watershed development.

Center for Watershed Protection

If we can use all of these…

3Chesapeake Bay Stormwater Training Partnership

why do we always get these?

4Chesapeake Bay Stormwater Training Partnership

We’ve been asking for it!We’ve been asking for it!ggWe encourage large volume detention

facilities with our Post-Construction facilities with our Post Construction Stormwater Ordinances, Criteria, and compliance toolsp


T diti l R l t C it iT diti l R l t C it iTraditional Regulatory Criteria:Traditional Regulatory Criteria:

Requires that stormwater quality and increases in Requires that stormwater quality and increases in volume, velocity, and peak rates of discharge be managed to protect downstream aquatic resourcesresources.

Basic engineering principles establish a hierarchy for BMP selection within the site development pprocess: • Address the physical impacts to the site plan: larger to

Smaller– Flood Control

– Channel Protection

Stormwater Quality


– Stormwater Quality

T diti l R l t C it iT diti l R l t C it iTraditional Regulatory Criteria:Traditional Regulatory Criteria: The predominant practice used for compliance e p edo a t p act ce used o co p a ce

with traditional requirements has been rate-control detention and extended detention facilities;facilities;

Water quality compliance typically met through the inclusion of micro-pools, extended drawdown, o othe design enhancementsor other design enhancements;

Experience and observations downstream have identified that these strategies are not always de t ed t at t ese st ateg es a e ot a ayseffective in addressing stormwater quality or channel protection (increased volume and duration of peak flows).


duration of peak flows).

Runoff Volume ReductionRunoff Volume ReductionRunoff Volume ReductionRunoff Volume ReductionThe solution: Minimize the increase in runoff

volume through the use of Runoff Reduction volume through the use of Runoff Reduction (RR) defined as: the total annual runoff volume reduced through:• Canopy Interception;

• Soil Infiltration;

E i • Evaporation;

• Transpiration;

• Rainwater Harvesting and Reuse; • Rainwater Harvesting and Reuse;

• Engineered Infiltration; and

• Extended Filtration


Extended Filtration.

Runoff Volume ReductionRunoff Volume ReductionRunoff Volume ReductionRunoff Volume ReductionRunoff Reduction is characterized by 3 design steps:

1 Minimization and avoidance:1. Minimization and avoidance:

• generate less runoff through site design strategies that preserve hydrologically functional areas of the site and thereby maintain the pre-developed hydrologic response characteristics;the pre developed hydrologic response characteristics;

2. Runoff Reduction:

• Reduce the increase in runoff through the use of management practices that effectively reduce runoff volume through infiltration, extended filtration, soil amendments, rainwater harvesting and reuse, evapotranspiration, etc.; and

3. Additional controls as needed:

• Repeat Steps 1 and 2, or after steps 1 and 2 have been maximized, add additional structural controls to reduce the peak


rate of discharge or reduce the pollutant load concentrations in the runoff volume.

The transition from one large BMP to multiple small ones leads t th i it bl ti

How am I supposed to design and calculate all this stuff??

to the inevitable question:


A New Generation of A New Generation of St t M t St t M t BMPBMPStormwater Management Stormwater Management BMPsBMPs

Terminology:Maryland Stormwater program

Requires implementation of a suite of E i t l Sit D i ti t li t Environmental Site Design practices to replicate pre-development conditions to the Maximum Extent Practicable (MEP).( )

Environmental Site Design (ESD): Utilization of site planning techniques, non-structural practices, and small scale stormwater (structural) practices to mimic natural (pre-developed) hydrologic characteristics of the landscape, in accordance


p ,with the MD SWM Manual (Supplement 1).

A New Generation of A New Generation of Stormwater Management Stormwater Management BMPsBMPs

Terminology (Maryland) continued:Terminology (Maryland) continued:

Must Use ESD Practices to treat the runoff from 1” rainfall (P = 1”)rainfall (PE = 1 )

Still Retain the Old Recharge Requirement for InfiltrationInfiltration

Must Use ESD to the MEP to address the Channel Protection Volume (Cp ) from the 1-yr 24-hr design Protection Volume (Cpv) from the 1 yr 24 hr design storm


A New Generation of A New Generation of Stormwater Management Stormwater Management BMPsBMPs

Terminology gy

Virginia Stormwater Program:

Requires compliance with a site-based load limit Requires compliance with a site based load limit using a selection structural and non-structural management practices;

Incentives to utilize site planning techniques and non-structural Runoff Reduction practices practices


R ff R d ti M th dR ff R d ti M th dRunoff Reduction MethodRunoff Reduction MethodTechnical MemorandumTechnical Memorandum

April, 2008April, 2008

The Maryland Environmental Site Design (ESD) requirements and the Virginia Runoff Reduction Method utilize the same principles of g p prunoff volume reduction to achieve compliance with water quality, groundwater recharge, and channel protection (to the maximum extent practicable) criteria. The different design steps and hydrologic computations are referenced throughout this


hydrologic computations are referenced throughout this presentation where possible. Viewers are encouraged to review the requirements of the State in which they are designing.

Different Terminology Different Terminology –– Same GoalSame Goal

Runoff Reduction/Environmental Site Runoff Reduction/Environmental Site Design:

The total annual runoff volume reduced The total annual runoff volume reduced through canopy interception, soil infiltration, evaporation, transpiration, , p , p ,rainwater harvesting, engineered infiltration, or extended filtration.


R ff R d ti (RR) M th dR ff R d ti (RR) M th dRunoff Reduction (RR) MethodRunoff Reduction (RR) MethodCodifies avoidance and minimization;;

Goes beyond impervious cover as the sole water quality indicator;

Utilizes up to date science for nutrient reductions;

Credits total BMP performance;

Updates BMP specifications (Level 1 & 2) Updates BMP specifications (Level 1 & 2), with volume reduction metric;

Proper accounting of BMPs in series



Reduce Stormwater Runoff by Reduce Stormwater Runoff by Design (Minimization & Avoidance)Design (Minimization & Avoidance)

Better site planning & p gdesign techniques Preserve natural areas Conservation design Conservation design Reduce clearing &

grading limits Reduce roadway widths Reduce roadway widths Use alternative cul-de-

sacs

www.cwp.org > O li St

Promote redevelopment


Online Store > Better Site Design

R ff R d tiR ff R d tiRunoff Reduction:Runoff Reduction:

R ff R d ti (RR) E i t l Sit Runoff Reduction (RR), Environmental Site Design (ESD), and Low Impact Development (LID) all mean more than Development (LID), all mean more than simply minimizing impervious cover:• Minimize clearing and grading (soil Minimize clearing and grading (soil disturbance);

• Preserve hydrologically effective soils (HSG A & B)B);

• Preserve natural areas;• Maintain natural flow paths;


Maintain natural flow paths;

Environmental Resource Environmental Resource Inventory: Inventory:

Identify & Characterize:Identify & Characterize: Natural Areas Natural Areas

•• Woods (specimen trees) Woods (specimen trees)

•• Streams Streams

Wetlands Wetlands •• Wetlands Wetlands

•• Natural Flow paths Natural Flow paths

Steep (critical) Slopes Steep (critical) Slopes Steep (critical) Slopes Steep (critical) Slopes

Soils Soils

Stream or Other BuffersStream or Other Buffers


Stream or Other BuffersStream or Other Buffers

Environmental Site Design Environmental Site Design Example:Example:Example:Example:Requires the designer to demonstrate that the

layout and design of the development has y g pachieved the goals of ESD to the MEP.

ESD I l i Ch kli Y N N/A

The following is a simple checklist of the basic ESD elements: ESD Implementation Checklist Yes No N/A

1. Environmental site mapping was conducted prior to site layout

2. Natural areas were conserved (e.g., forests, wetlands, steep slopes)

3. Stream, wetland and shoreline buffers were reserved3 ,

4. Disturbance of permeable soils were minimized

5. Natural flow paths were maintained across the site

6. Building layout was fingerprinted to reduce site clearing/grading

7. Site grading promoted sheet flow from impervious areas to pervious ones

8. Better site design was used to reduce needless impervious cover

9. Site Design maximized disconnection of impervious cover

10. Future site operations evaluated to identify potential stormwater hotspot


10. Future site operations evaluated to identify potential stormwater hotspot

11. Installation of ESC and ESD Practices are integrated together

12. Tree planting was used at the site to convert turf areas into forest


Goes beyond impervious cover as the sole water quality indicator;



Updates BMP specifications (Level 1 & 2) Updates BMP specifications (Level 1 & 2), with volume reduction metric;




The impacts of impervious cover The impacts of impervious cover are well documented:are well documented: Significant increase in runoff volume, velocity and peak

ates of discha gerates of discharge; Traditional detention serves to extend the duration of peak

flows below the basin, resulting in increased erosive work on the channel

12

14

16

Post-developed

on the channel

6

8

10

Run

off

Post-developed (with

Pre-developed

0

2

4

0 5 10 15 20 25 30

(with Detention)


-2

0 5 10 15 20 25 30

Time

The impacts of impervious cover The impacts of impervious cover are well documented (contd.):are well documented (contd.):

Impervious cover generates significant Impervious cover generates significant increase in runoff volume by decreasing the infiltrative and attenuation capacity of the natural land cover;

Impervious cover also provides an efficient conveyance of pollutants from atmospheric conveyance of pollutants from atmospheric deposition (TN), adjacent pervious areas (TP), and direct deposits from land use activities (automobiles hotspots etc ) potentially (automobiles, hotspots, etc.), potentially increasing the concentration (mg/l) of pollutants;


Results in greater total pollutant load (lb/ac).

Impervious Cover and Stream Impervious Cover and Stream HealthHealth

Impervious Cover Model


ICM supported by over 200 Studies on 26 aquatic indicators (CWP,2003)

tegrity

Bio

tic

Int

Index

of

I

25Chesapeake Bay Stormwater Training Partnership Copyright 2000, Center for Watershed Protection

Water Quality Implications Water Quality Implications Water Quality Implications Water Quality Implications Simple Method Pollutant Load Computations:

Load (lb/yr) = P * Pj * Rv * C * A * 2.72/12where: P = annual precipitation (inches)

Pj = rainfall correction factor = 0.9Rv = volumetric runoff coefficient

(0 05 + 0 009*I) = (0.05 + 0.009*I) C = pollutant concentration (mg/l)A = drainage area (acres)A drainage area (acres)

2.72; 12 = unit conversions

The Simple Method computes the pollutant load based on


p p pThe Volumetric Runoff Coefficient based on impervious cover.

Hydrologic Implications Hydrologic Implications Hydrologic Implications Hydrologic Implications WQv = (P * Rv * A) / 12

Where: WQv = water quality design volume (ac-ft)WQv water quality design volume (ac ft)

P = water quality design storm depth (inches)Rv = volumetric runoff coefficient

= (0.05 + 0.009*I) I= percent impervious cover)

A = drainage area (acres)A = drainage area (acres)12 = unit conversion

Stormwater quality practices have traditionally been sized


q y p yusing a Volumetric Runoff Coefficient based on impervious cover

Impe io s Co e CompanionImpe io s Co e CompanionImpervious Cover Companion:Impervious Cover Companion:Highly and moderately impervious sites

often include pervious turf areas (required often include pervious turf areas (required as green space) placed over compacted soils;soils;


Source: Schwartz, UMBC, 2010

Impervious Cover Companion:Impervious Cover Companion:Impervious Cover Companion:Impervious Cover Companion:

Research is demonstrating that the runoff gcharacteristics of these graded and compacted areas can be comparable to

t (S h t UMBC M h 2010)pavement (Schwartz, UMBC March 2010).


Managed TurfManaged TurfManaged TurfManaged TurfDocumented impacts of grading and

compaction of soils:compaction of soils:• Increased bulk density• Decreased permeabilityDecreased permeability• Increased runoff coefficient

Documented impacts from turf Documented impacts from turf management activities:• Fertilization;• Pest management;


Center for Watershed Protection –Technical Memorandum: The Runoff Reduction Method; 4/18/08

6.1 million “grass farmers in the Chesapeake Bay watershedT di t 2 4 t 3 8 illi (5 9% t 9 3%)Tending to 2.4 to 3.8 million acres (5.9% to 9.3%) Approx 75% of which is home lawnsInfluence turf management practices = influence water quality


W t Q lit I li tiW t Q lit I li tiWater Quality ImplicationsWater Quality Implications The National Stormwater Quality Database (NSQD) Q y ( Q )

version 1.1 reflects higher nutrient loads from sites with lower percent impervious cover;

Simple Method pollutant load computations should Simple Method pollutant load computations should reflect this potential for increased loads from non-impervious areas.

f References:

• Runoff Reduction Technical Memo, Appendix G: Derivation of EMCs for Virginia; g

• Virginia Nutrient Design System, Appendix A: Analysis of Virginia Event Mean Concentrations (EMCs) and Land Use Loading Rates from the National Stormwater Quality Database


(2007)

H d l i I li ti H d l i I li ti Hydrologic Implications Hydrologic Implications NRCS Methods assume non-impervious to be open NRCS Methods assume non impervious to be open

space in “good condition”; ignores the potential for increased runoff resulting from impacted and compacted conditions;compacted conditions;

Water quality volume is typically used in BMP sizing; BMPs should be sized to manage the runoff volume coming to them from the contributing drainage area;

Some jurisdictions have implemented the policy of down grading the hydrologic soil designation of down grading the hydrologic soil designation of disturbed soils:

A B; B C; etc


A B; B C; etc.

Treatment Volume: Treatment Volume: Beyond Impervious CoverBeyond Impervious CoverBeyond Impervious CoverBeyond Impervious Cover

Runoff Reduction theory utilizes a design parameter: Treatment Volume (Tv):

The Tv is computed using a “weighted” Volumetric Runoff Coefficient (Rv) that reflects the different runoff coefficientsCoefficient (Rv) that reflects the different runoff coefficients assigned to impervious, managed turf (or disturbed soils), and forest;

Weighted Rv creates incentives to conserve forests and open space, and reduce mass grading by providing a basis for computing runoff reduction volumes for these actions.computing runoff reduction volumes for these actions.

Tv provides an objective measure to gage the aggregate performance of environmental site design, LID and other


innovative practices, and conventional BMPs using a common currency (runoff volume)


Runoff Reduction theory utilizes a design parameter: Treatment Volume (Tv).

The Tv is used in the Simple Method to compute the load generated by the total “developed” site (not limited to thegenerated by the total developed site (not limited to the impervious cover);

The Tv is used to size the Runoff Reduction Practices so The Tv is used to size the Runoff Reduction Practices so they are more appropriately sized to manage the runoff coming to the practice.



The formula for computing the Treatment Volume (Tv):

Tv = P (RvI*%I + RvT*%T + RvF*%F)* A / 12 Tv = Runoff Reduction Treatment Volume (ac-ft) P = Water quality design storm rainfall depth (inches) RvI = runoff coefficient for impervious cover1

RvT = runoff coefficient for turf or disturbed soil cover1 RvT runoff coefficient for turf or disturbed soil cover RvF = runoff coefficient for forest cover1

%I = % of site in impervious cover (fraction) %T % f it i t f di t b d il (f ti ) %T = % of site in turf or disturbed soil cover (fraction) %F = % of site in forest cover (fraction) A = Drainage area (acres)


1 Rv values defined; Refer to next slide

Treatment Volume: Treatment Volume: Site Runoff Coefficients (Site Runoff Coefficients (RvRv))11Site Runoff Coefficients (Site Runoff Coefficients (RvRv))

Cover HSG A HSG B HSG C HSG DCover HSG A HSG B HSG C HSG D

Forest 0.02* 0.03* 0.04* 0.05*

Managed Turf / Disturbed Soil 0.15 0.20 0.22 0.25

Impervious Cover 0.95 0.95 0.95 0.95

1 Center for Watershed Protection – Technical Memorandum: The Runoff Reduction Method; 4/18/08*Forest coefficient adjusted for assessing compliance


Pitt et al (2005), Lichter and Lindsey (1994), Schueler (2001a, 2001b, 1987), Legg et al (1996), Pitt et al (1999), and Cappiella et al (2005)

Treatment VolumeTreatment VolumeTreatment VolumeTreatment Volume

Not all states utilize the Tv to compute loads or size BMPs;

Not all states include managed turf or impacted soils in determining the total impacted soils in determining the total load or sizing practices, or managed same turf in MD

Virginia DCR has established definitions for Forest (to include certain “open space” designations) managed turf and other designations), managed turf, and other land uses:• Runoff Reduction Method Technical Memo D C D i


Document C: Documentation

R ff R d ti (RR) M th dR ff R d ti (RR) M th dRunoff Reduction (RR) MethodRunoff Reduction (RR) Method

Codifies avoidance and minimization;Codifies avoidance and minimization;

Goes beyond impervious cover as a water quality indicator;quality indicator;

Utilizes up to date science for nutrient reductions;reductions;


Updated BMP specifications (Level 1 & 2), with volume reduction metric;



Nutrient Reduction (Pollutant Nutrient Reduction (Pollutant Removal)Removal)

The Runoff Reduction Method Technical The Runoff Reduction Method Technical MemorandumApril, 2008p , 008

www.cwp.org/cbstp


Nutrient Reduction (Pollutant Nutrient Reduction (Pollutant Removal Efficiency)Removal Efficiency)

P ll t t l ffi i ll Pollutant removal efficiency generally refers to the pollutant reduction from the inflow to the outflow of a systeminflow to the outflow of a system.

The two most common methods of calculating pollutant removal efficiency are calculating pollutant removal efficiency are Event Mean Concentration (EMC) Efficiency and Mass Load Efficiency.and Mass Load Efficiency.


Pollutant Removal (PR): Event Pollutant Removal (PR): Event Mean Concentration (EMC)Mean Concentration (EMC) EMC removal efficiency is derived by averaging

the influent and effluent pollutant concentrations for individual storm events, and then calculating the median change in concentration.the median change in concentration.

Concentrations measured as mg/l can be highly variable since the volume of runoff is not always addressed in the computation.• References:

Jones J Clary J Strecker E Quigley M 2008 15 Reasons You Should Think Jones, J., Clary, J., Strecker, E., Quigley, M. 2008. 15 Reasons You Should Think Twice Before Using Percent Removal To Assess BMP Performance. Stormwater Magazine.; Jan/Feb 2008.

Strecker, E., Quigley, M., Urbonas, B., and Jones, J. 2004. Stormwater management: State of the Art In Comprehensive Approaches To Stormwater The Water Report


State-of-the-Art In Comprehensive Approaches To Stormwater. The Water Report. Issue #6. Envirotech Publishers Inc., Eugene, OR.

Pollutant Removal (PR): Mass LoadPollutant Removal (PR): Mass Load

Mass load efficiency is calculated by determining the Mass load efficiency is calculated by determining the pollutant load reduction from the influent to effluent;

Efficiency calculation is directly influenced by the measured volume of water through the practice;volume of water through the practice;

Many early studies measured the volume (or flow) from a single location and assumed “flow in” = “flow out”; and

Most early studies generally reported performance in terms of pollutant removal (EMC reduction):

• possibly over-reporting EMC reduction performance where volume reduction has occurred; or

• Possibly under-reporting the total performance by ignoring the volume reduction


– (Refer to Crediting Total BMP Performance below)

Pollutant Removal (PR) Pollutant Removal (PR) Pollutant Removal (PR) Pollutant Removal (PR) EfficiencyEfficiency

EMC Removal Efficiency: EMCin vs. EMCout

(EMC = Event Mean Concentration (mg/l))

Mass Load Removal Efficiency: (Volin)(EMCin) vs. (Volout)(EMCout)

Mass load removal results from reductions in EMC, Volume, or both (however, as noted, many early studies did not isolate and evaluate volume reduction).


isolate and evaluate volume reduction).

Pollutant Removal (PR) EfficienciesPollutant Removal (PR) EfficienciesPractice Total Phosphorus (%) Total Nitrogen (%)p ( ) g ( )

Green Roof1 0 0Disconnection 0 0Rainwater Harvesting1 0 0Permeable Pavement 25 25Grass Channel 15 20Bioretention 25 to 502 40 to 602

D S l 20 40 25 35Dry Swale 20 to 40 25 to 35Wet Swale 20 to 40 25 to 35Infiltration 25 15ED Pond 15 10 ED Pond 15 10 Soil Amendments 0 0Sheetflow to Open Space

0 0p

Filtering Practice 60 to 65 30 to 45Constructed Wetland 50 to 75 25 to 55Wet Pond 50 to 75 30 to 401 l h h f h l d f l b l l k d


1EMC removal is occurs through a variety of mechanisms, including filtering, biological uptake, adsorption, and settling. Green roof, rainwater harvesting, and other practices have demonstrated performance through volume reduction, not EMC reduction2Range of values is for the median and 75th percentile pollutant removal rates (Level 1 and Level 2)



Goes beyond impervious cover as a water quality indicator;q y ;

Utilizes up to date science for nutrient reductions;;


U d d BMP ifi i (L l 1 & 2) Updated BMP specifications (Level 1 & 2), with volume reduction metric;

P ti f BMP i i



BMP PerformanceBMP PerformanceBMP PerformanceBMP Performance

BMP f i t f P ll t t BMP performance in terms of Pollutant Removal (PR) or EMC reduction is limited by the existence of an “irreducible concentration” the existence of an irreducible concentration that represents the lowest concentration achievable through stormwater BMPs( dl f h d EMC l (regardless of the reported EMC removal efficiency).

Th f lti l BMP i i ill Therefore, even multiple BMPs in series will not serve to provide pollutant removal any further than the irreducible concentration.


further than the irreducible concentration.

Total BMP Performance = Total BMP Performance = Pollutant Removal (PR) Pollutant Removal (PR) andandPollutant Removal (PR) Pollutant Removal (PR) andandRunoff Reduction (RR)Runoff Reduction (RR)Traditional Performance Goals: Pollutant Traditional Performance Goals: Pollutant

Removal (PR) Performance is measured and reported as a function of reducing the p gpollutant load in terms of the percentage of EMC reduction

Runoff Reduction Performance Goals: Runoff Reduction (RR) performance is measured and

t d f ti f d i th l reported as a function of reducing the annual volume of runoff associated with the 90th

percentile rain event.


percentile rain event.Volin vs Volout (Vol = volume of runoff)

Total BMP Performance:Total BMP Performance:

Total BMP Performance:

Pollutant Removal Reported Performance:• EMCin vs EMCout

AND

Runoff Reduction Reported Performance:• Vol vs Vol• Volin vs Volout

Equals Total BMP Performance reported as Load Equals Total BMP Performance reported as Load Reduction:• (Volin)*(EMCin) vs (Volout )*(EMCout)


=Total BMP Performance

Total BMP PerformanceTotal BMP PerformanceTotal BMP PerformanceTotal BMP PerformanceRunoff Reduction (RR) and pollutant removal

(PR):(PR):

Allows for additional reductions beyond the irreducible concentration by reducing the y gvolume;

Provides for maximum performance through a “T t t T i ” h“Treatment Train” approach:• Reduction of pollutants generated on the site using

non-structural site design practices;• Volume reduction using one or multiple runoff

reduction practices;• Pollutant removal achieved by runoff reduction


ypractices and additional pollutant removal practices as needed.

Stormwater Practices Differ Stormwater Practices Differ Sharply in Ability to Reduce Runoff Sharply in Ability to Reduce Runoff p y yp y yVolumeVolume

Bioretention, Infiltration, Wet Ponds, ED Ponds Dry Swales, Soil Amendments, disconnection, and Related

and Constructed Wetlands and Filters Reduce Runoff Volumes


,Practices Reduce Runoff Volumes by 50 to 90%

by zero to 10%

Inflow V. Outflow RatesInflow V. Outflow Rates

0 1 1 2

Sample Bioretention flow monitoring

0.08

0.09

0.1

1

1.2

Inflow

0.05

0.06

0.07

arge

(cfs

)

0.6

0.8

pth

(in)

Inflow

Outflow

0.02

0.03

0.04

Dis

ch

0.4

Dep

CumulativeRainfall

0

0.01

1/13/2005 12:00 1/14/2005 0:00 1/14/2005 12:00 1/15/2005 0:00 1/15/2005 12:00 1/16/2005 0:000

0.2


Source: Dr. Bill Hunt; NC State

Reported Reductions in Runoff Reported Reductions in Runoff VolumeVolumeLosses Due to Exfiltration,

Evapotranspiration and Evapotranspiration and Post Storm Delivery;

Sampling of reductions d b hreported by research:

CT: 99%UK: 58% UK: 58% FL: 98%NC: 30 to 65% (4)PA 80%PA: 80%Aus: 73%WA: 96%

Key Factors: Infiltration Rate, Media Depth, Hydraulic


MD: 46 to 54% Gradient, and Absence of Underdrain

Runoff Reduction ProcessesRunoff Reduction ProcessesRunoff Reduction ProcessesRunoff Reduction Processes

Runoff Reduction is not just infiltration!j

InfiltrationCanopy Interception EvaporationTranspiration Transpiration Rainwater Harvesting Extended Filtration


Runoff Reduction ValuesRunoff Reduction ValuesPractice RR (%)Green Roof 45 to 60Rooftop Disconnection 25 to 50pRaintanks and Cisterns 40Permeable Pavement 45 to 75Grass Channel 10 to 20Bi t ti 40 t 80Bioretention 40 to 80Dry Swale 40 to 60Wet Swale 0 Infiltration 50 to 90Infiltration 50 to 90ED Pond 0 to 15Soil Amendments 50 to 75Sheetflow to Open 50 to 75SpaceFiltering Practice 0 Constructed Wetland 0Wet Pond 0


Wet Pond 0Range of values is for median and 75th percentile

reported performance; (Level 1 and Level 2 designs )

Total Performance:Total Performance:Total Performance:Total Performance:Runoff Reduction and Nutrient Runoff Reduction and Nutrient Removal TableRemoval TableRemoval TableRemoval Table


Practice DesignLevel

Runoff Reduction

TN EMCRemoval3

TN LoadRemoval

TP EMCRemoval

TP LoadRemoval 6

Rooftop Disconnect 1 2 25 to 50 1 0 25 to 50 1 0 25 to 50 1No Level 2 Design

Sheet Flow to Veg Filter 1 25 to 50 1 0 25 to 50 1 0 25 to 50 1Sheet Flow to Veg. Filter or Conserv. Open Space

1 25 to 50 1 0 25 to 50 1 0 25 to 50 12 5 50 to 75 1 0 50 to 75 1 0 50 to 75 1

Grass Channels 1 10 to 20 1 20 28 15 23No Level 2 Design

Soil Compost A d t

Can be used to Decrease Runoff Coefficient for Turf Cover at Site. See the design specs for Rooftop Disconnection, Sheet Flow to Vegetated Filter or Conserved Open Space, and GrassAmendment Rooftop Disconnection, Sheet Flow to Vegetated Filter or Conserved Open Space, and Grass Channel

VegetatedRoof

1 45 0 45 0 452 60 0 60 0 60

RainwaterHarvesting 1 Up to 90 3, 5 0 Up to 90 3, 5 0 Up to 90 3, 5

No Level 2 Design

Permeable Pavement 1 45 25 59 25 592 75 25 81 25 81

Infiltration Practices 1 50 15 57 25 632 90 15 92 25 93

Bioretention Practices 1 40 40 64 25 552 80 60 90 50 902 80 60 90 50 90

Urban Bioretention 1 40 40 64 25 55No Level 2 Design

Dry Swales 1 40 25 55 20 522 60 35 74 40 76

W t S l 1 0 25 25 20 20Wet Swales 1 0 25 25 20 202 0 35 35 40 40

Filtering Practices 1 0 30 30 60 602 0 45 45 65 65

Constructed Wetlands 1 0 25 25 50 502 0 55 55 75 75


Wet Ponds 1 0 30 (20) 4 30 (20) 4 50 (45) 4 50 (45) 42 0 40 (30) 4 40 (30) 4 75 (65) 4 75 (65) 4

Ext. Det. Ponds 1 0 10 10 15 152 15 10 24 15 31

Refer To Next Slide for Footnotes

Notes for Total Performance: Notes for Total Performance: Runoff Reduction and Nutrient Runoff Reduction and Nutrient Runoff Reduction and Nutrient Runoff Reduction and Nutrient Removal TableRemoval TableNotes1 Lower rate is for HSG soils C and D, Higher rate is for HSG soils A

and B.2 Th l b i d t 50% f C d D il b ddi il2 The removal can be increased to 50% for C and D soils by adding soil

compost amendments, and may be higher yet if combined with secondary runoff reduction practices.

3 Credit up to 90% is possible if all water from storms of 1 inch or less3 Credit up to 90% is possible if all water from storms of 1-inch or less is used through demand, and the tank is sized such that no overflow occurs. The total credit may not exceed 90%.

4 Lower nutrient removal in parentheses apply to wet ponds in coastalLower nutrient removal in parentheses apply to wet ponds in coastal plain terrain.

5 See BMP design specification for an explanation of how additional pollutant removal can be achieved.


6 Total mass load removed is the product of annual runoff reduction rate and change in nutrient EMC.

C dit T t l BMP P fC dit T t l BMP P fCredits Total BMP PerformanceCredits Total BMP Performance

T t l BMP P f i l d l Total BMP Performance includes volume reduction credited for large storm control:

Channel Protection Channel Protection Flooding Protection


Increasing the Retention Storage Increasing the Retention Storage Increasing the Retention Storage Increasing the Retention Storage of Runoff Reduction Practicesof Runoff Reduction Practices

Additional Surface Storage

Additional Sub-Surface Storage in Soil Layer, Stone


Stone, Chambers, or Other Means

Volume Reduction: Hydrograph Volume Reduction: Hydrograph y g py g pModificationModification

Objective: Account for hydrologic effect of distributed retention storage;

Simplifying Assumptions:• Assume retention is uniformly distributed if

considering multiple features or sub-areas;

• Assume negligible discharge from under-drains (if any)drains (if any)


Volume Reduction: Hydrograph Volume Reduction: Hydrograph ModificationModification

Methods Considered:1. Hydrograph Truncation2. Hydrograph Scalar Multiplication3. Precipitation Adjustment4 R ff Adj t t4. Runoff Adjustment5. Curve Number Adjustment

Excerpted from work by Paul R. Koch, Ph.D., P.E.


Runoff Hydrograph ModificationRunoff Hydrograph Modification

Curve Number Adjustment:


NRCS Runoff Equation is used to derive a curve number that reflects the reduced volume of

ff l i f h di ib d i runoff resulting from the distributed retention storage.

A simplified derivation of the computational procedure starts with the combined Runoff procedure starts with the combined Runoff Equations in order to express the runoff depth (Q, inches) in terms of rainfall (P, inches) and


(Q, ) ( , )potential maximum retention (S).


Curve Number Adjustment:


2Equation 2-1 (TR-55):

SIaP

IaPQ

2

Equation 2-2 (TR-55):

SIaP

SIa 20Equation 2 2 (TR 55):

Initial abstraction (Ia) is expressed in terms

SIa 2.0

of the potential maximum retention (S) after runoff begins (Eq. 2-2), and substituted into the runoff depth equation


substituted into the runoff depth equation (Eq.2-1).

Runoff Hydrograph ModificationRunoff Hydrograph ModificationRunoff Hydrograph ModificationRunoff Hydrograph Modification

SP 20 2Equation 2-3 (TR-55):

SPSPQ

8.02.0

The retention storage provided on site is expressed in terms of watershed inches expressed in terms of watershed inches (R) and subtracted from the runoff depth as expressed by the following: p y g

Equation 2-3 (modified)

SPSPRQ

802.0 2


SPQ

8.0

Runoff Hydrograph ModificationRunoff Hydrograph ModificationRunoff Hydrograph ModificationRunoff Hydrograph Modification

SP 2.0 2Equation 2-3 (modified)

SPSPRQ

8.02.0

Modified Eq. 2-3 can be solved for a new value of S.

The potential maximum retention (S), is related to soil and cover conditions and

b d f hcan be expressed in terms of the curve number:

101000SChesapeake Bay Stormwater Training Partnership 66

Equation 2-4 (TR-55) 10CN

S


A new runoff curve number (CN) that reflects


the reduced volume of runoff resulting from the distributed retention storage can be calculated for each design storm using Eq 2 4 calculated for each design storm using Eq. 2-4 (TR-55):

Or: A new curve number can be read off of Figure A new curve number can be read off of Figure

2-1 in TR-55 using the new runoff depth (Q, inches) and the corresponding rainfall depth


(P, inches) for the selected design storms.

Runoff Hydrograph ModificationRunoff Hydrograph ModificationNo delay in the Tc is reflected, and the

reduction is distributed across the entire


reduction is distributed across the entire storm, resulting in a conservative estimate of the peak discharge.


Runoff Hydrograph ModificationRunoff Hydrograph ModificationThe two predominant methodologies use

different methods to achieve the goals of


different methods to achieve the goals of accounting for runoff reduction during large storms:

VA (and other states) use the direct method of applying retention storage and Hydrograph applying retention storage and Hydrograph Modification to achieve the required small storm (P=1”) pollutant load reduction to the extent practicable and hydrograph extent practicable and hydrograph modification to measure the reduction in peak flow during channel protection and flooding d i t t


design storm events.

Runoff Hydrograph ModificationRunoff Hydrograph ModificationMD requires retention storage to manage


the entire small storm runoff volume (P=1”) and to the maximum extent practicable to achieve a curve number practicable to achieve a curve number reduction to that of woods in good condition for the channel protection event condition for the channel protection event (1-year design storm).




Goes beyond impervious cover as a water quality indicator;q y ;

Utilizes up to date science for nutrient reductions;;


U d d BMP ifi i (L l 1 Updated BMP specifications (Level 1 & 2), with volume reduction metric;

P ti f BMP i i



Level 1 and Level 2 DesignsLevel 1 and Level 2 Designsgg

BMP Performance studies indicate variability in performance based on a variety of factors:performance based on a variety of factors:• design features, • influent concentration, • particle size distribution in runoff, • rainfall depth and intensity, • peak flow rates • peak flow rates, • soils, and • other site factors.

Regulatory performance goals commonly assign the median pollutant removal ffi i i i th l f t i d i


efficiency, ignoring the role of certain design factors in reducing or enhancing performance.

Level 1 and Level 2 DesignsLevel 1 and Level 2 Designsgg

Extensive process to assign specific design criteria to Level 1 and Level 2 designs: criteria to Level 1 and Level 2 designs:

Standard design features that ensure proper function of the BMP assigned to all designs:function of the BMP assigned to all designs:• safety, • appearance, • safe conveyance, • longevity, • standard feasibility constraints and • standard feasibility constraints, and • maintenance needs.


Level 1 and Level 2 DesignsLevel 1 and Level 2 DesignsggExtensive process to assign specific design

criteria to Level 1 and Level 2 designs (continued):

Design Point Tables in Stormwater Retrofit Manual (Appendices Manual 3)• Retrofit Manual Design Point Tables were used to

assign design features and corresponding assign design features and corresponding performance values to designs that could not incorporate full design features due to the limiting factors associated with retrofit situationsfactors associated with retrofit situations.

• Modifications to these Tables were made to account for additional practices and the new and re-


development condition (Runoff Reduction Technical Memorandum).

Level 1 and Level 2 DesignsLevel 1 and Level 2 DesignsLevel 1 and Level 2 DesignsLevel 1 and Level 2 Designs

Level 1 and Level 2 design factors based gon:

Updated National Pollutant Removal Updated National Pollutant Removal Database

Extensive literature search and review of Extensive literature search and review of more than 100 BMP performance studies

Professional Judgment Professional Judgment


Level 1 and Level 2 DesignsLevel 1 and Level 2 DesignsLevel 1 and Level 2 DesignsLevel 1 and Level 2 DesignsLevel 1 designs may allow for practices in

challenging locations (i e where one or challenging locations (i.e., where one or more design features such as geometry or larger storage volumes are not practical due to space, topography, etc.);

Level 2 designs may allow for improved f b h i performance by enhancing one or more

design features.N t ll t t t d i ti f Not all states accept deviation from specific design standards (Level 1, Level 2); refer to the State Manual or


Level 2); refer to the State Manual or local plan reviewer.

Where are Where are pollutantspollutants removed?removed?

TSSTSS

TP

Temp

TNTN

Pathogens

Metals

Oil & Grease


Oil & Grease

Where is Where is VolumeVolume removed?removed?

Volume


Primary Design Factors That Primary Design Factors That Primary Design Factors That Primary Design Factors That Segregate Level 1 and Level 2 Segregate Level 1 and Level 2 Designs Designs Designs Designs

Increased Treatment Volume (Tv) SizingIncreased Treatment Volume (Tv) SizingIncreased Runoff Reduction VolumeEnhanced Design GeometryEnhanced Design GeometryMultiple CellsVegetative ConditionVegetative ConditionMultiple Treatment PathwaysOther “bells and whistles”


Other bells and whistles

Level 1 and Level 2 Design Level 1 and Level 2 Design S ifi tiS ifi tiSpecificationsSpecificationsPrimary Design Factors for defining Level 1 and

Level 2:

Increased Treatment Volume• Since the Standard Design accommodates the 90th

l h ll l dpercentile rain event, increases in the Tv will only provide for a modest performance upgrade unless the increase creates increased residence time for nutrient uptake (as noted in ponds and wetlands);noted in ponds and wetlands);

• Therefore, 3 incremental increases in Tv (110%, 125%, and 150% of base Tv) are assigned to specific BMPs for L l 2 f dit Level 2 performance credit.

Increased Runoff Reduction Volume• The storage capacity of some RR practices can be


The storage capacity of some RR practices can be increased to provide a corresponding increase in runoff reduction credit.

Samples of Additional RR Storage Samples of Additional RR Storage p gp g


Level 1 and Level 2 Design Level 1 and Level 2 Design S ifi tiS ifi tiSpecificationsSpecifications

Primary Design Factors for defining Level 1 y g gand Level 2 (continued):

Enhanced Design Geometry & HydraulicsEnhanced Design Geometry & Hydraulics

• Geometry factors that are known to influence hydraulic performance or treatment conditions hydraulic performance or treatment conditions can be isolated and enhanced, including:

– flow path lengthflow path length–depth of filter media–multiple cells

f t d i ti


–surface area to drainage area ratio–extended detention time and ponding depth

Flow Path GeometryFlow Path Geometry


Level 1 and Level 2 Design Level 1 and Level 2 Design S ifi tiS ifi tiSpecificationsSpecificationsPrimary Design Factors for defining Level 1 and

Level 2 (continued):Level 2 (continued):

Vegetative Condition

• Specifying the type and cover of vegetation insofar that it • Specifying the type and cover of vegetation insofar that it influences:– Nutrient uptake– Increases evapotranspiration pumpIncreases evapotranspiration pump– Stabilizes trapped sediments– Enhances filter bed performance and longevity

Multiple Treatment MethodsMultiple Treatment Methods

• Combining several pollutant removal mechanisms to increase reliability of performance


• Includes one or more combinations of settling, filtering, soil adsorption, and biological uptake


Goes beyond impervious cover as a water quality indicator;



Updated BMP specifications (Level 1 & 2) Updated BMP specifications (Level 1 & 2), with volume reduction metric;




Practices in SeriesPractices in SeriesPractices in SeriesPractices in SeriesIrreducible Load?

Volume Reduction?


First: Reduce Stormwater Runoff by First: Reduce Stormwater Runoff by Design (Minimization & Avoidance)Design (Minimization & Avoidance)

Better site planning & p gdesign techniques Preserve natural areas Conservation design Conservation design Reduce clearing &

grading limits Reduce roadway widths Reduce roadway widths Use alternative cul-de-

sacs

www.cwp.org > O li St

Promote redevelopment


Online Store > Better Site Design

Second: Reduce Volume of PostSecond: Reduce Volume of Post--Construction Stormwater RunoffConstruction Stormwater Runoff

Runoff Reduction Runoff Reduction PracticesSoil AmendmentsRooftop disconnectionRain gardens/

bioretention areasRainwater harvestingPermeable pavementGreen RoofsNatural DrainagewaysVegetated ChannelsSite Reforestation


Buffers

Third: Capture & Treat Third: Capture & Treat Remaining Stormwater RunoffRemaining Stormwater Runoff

Larger-scale, g ,engineered practicesStormwater pondspStormwater wetlandsLarger bioretentionInfiltrationM di filMedia filtersSwalesAnd more…

Post-Construction Tool 5: Manual Builderwww.cwp.org > Resources >C t lli R ff & Di h


Controlling Runoff & Discharges >Stormwater Management

Next: Iterate or MitigateNext: Iterate or Mitigategg

When stormwater criteria has not been met:Back to Step 1 (Iterative site design process);

Increase the volume component of RR practices

Additional BMPs;

Offsite options:

• Comprehensive watershed plan

• Pro-rata fee program

• Off-site compensatory treatment

• Payment: Nutrient Offset or Buy-down fee


Refer to Individual State Refer to Individual State Refer to Individual State Refer to Individual State Manuals for details on Runoff Manuals for details on Runoff Reduction design specifications Reduction design specifications Reduction design specifications Reduction design specifications and computational methodsand computational methods


The Runoff Reduction Theory

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