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

Transcription

1 The Runoff Reduction Theory Created to deliver targeted training on new tools and practices to improve the quality of stormwater runoff.

2 Traditional Stormwater Approaches Focused primarily on managing stormwater quantity and, perhaps, quality, and relied heavily on traditional stormwater management practices to mitigate, rather than prevent the negative impacts of watershed development. Chesapeake Center Bay for Stormwater Watershed Training Protection Partnership 2

3 If we can use all of these Chesapeake Bay Stormwater Training Partnership 3

4 why do we always get these? Chesapeake Bay Stormwater Training Partnership 4

5 We ve been asking for it! We encourage large volume detention facilities with our Post-Construction Stormwater Ordinances, Criteria, and compliance tools Chesapeake Bay Stormwater Training Partnership 5

6 Traditional Regulatory Criteria: i Requires that stormwater quality and increases in volume, velocity, and peak rates of discharge be managed to protect downstream aquatic resources. Basic engineering principles establish a hierarchy for BMP selection within the site development process: Address the physical impacts to the site plan: larger to Smaller Flood Control Channel Protection Stormwater Quality Chesapeake Bay Stormwater Training Partnership 6

7 Traditional Regulatory Criteria: i The predominant practice ce used for compliance ce with traditional requirements has been ratecontrol detention and extended detention facilities; Water quality compliance typically met through the inclusion of micro-pools, extended drawdown, or other design enhancements; Experience and observations downstream have identified ed that these strategies es are not always ays effective in addressing stormwater quality or channel protection (increased volume and duration of peak flows). Chesapeake Bay Stormwater Training Partnership 7

8 Runoff Volume Reduction The solution: Minimize the increase in runoff volume through the use of Runoff Reduction (RR) defined as: the total annual runoff volume reduced through: Canopy Interception; Soil Infiltration; Evaporation; Transpiration; Rainwater Harvesting and Reuse; Engineered Infiltration; and Extended Filtration. Chesapeake Bay Stormwater Training Partnership 8

9 Runoff Volume Reduction Runoff Reduction is characterized by 3 design steps: 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; 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. Chesapeake Bay Stormwater Training Partnership 9

10 The transition from one large BMP to multiple small ones leads to the inevitable it question: How am I supposed to design and calculate all this stuff?? Chesapeake Bay Stormwater Training Partnership 10

11 A New Generation of Stormwater t Management BMPs Terminology: Maryland Stormwater program Requires implementation of a suite of 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 with the MD SWM Manual (Supplement 1). Chesapeake Bay Stormwater Training Partnership 11

12 A New Generation of Stormwater Management BMPs Terminology (Maryland) continued: Must Use ESD Practices to treat the runoff from 1 rainfall (P E = 1 ) Still Retain the Old Recharge Requirement for Infiltration Must Use ESD to the MEP to address the Channel Protection Volume (Cp v ) from the 1-yr 24-hr design storm Chesapeake Bay Stormwater Training Partnership 12

13 A New Generation of Stormwater Management BMPs Terminology Virginia Stormwater Program: 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 Chesapeake Bay Stormwater Training Partnership 13

14 Runoff Reduction Method Technical Memorandum April, 2008 The Maryland Environmental Site Design (ESD) requirements and the Virginia Runoff Reduction Method utilize the same principles p of runoff 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 presentation where possible. Viewers are encouraged to review the requirements of the State in which they are designing. Chesapeake Bay Stormwater Training Partnership 14

15 Different Terminology Same Goal Runoff Reduction/Environmental Site Design: The total annual runoff volume reduced through canopy interception, soil infiltration, evaporation, transpiration, rainwater harvesting, engineered infiltration, or extended filtration. Chesapeake Bay Stormwater Training Partnership 15

16 Runoff Reduction (RR) Method Codifies 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), with volume reduction metric; Proper accounting of BMPs in series Chesapeake Bay Stormwater Training Partnership 16

17 Reduce Stormwater Runoff by Design (Minimization & Avoidance) Better site planning & design techniques Preserve natural areas Conservation design Reduce clearing & grading limits Reduce roadway widths Use alternative cul-desacs Promote redevelopment > Online Store > Better Site Design Chesapeake Bay Stormwater Training Partnership 17

18 Runoff Reduction: Runoff Reduction (RR), Environmental Site Design (ESD), and Low Impact Development (LID), all mean more than simply minimizing impervious cover: Minimize clearing and grading (soil disturbance); Preserve hydrologically effective soils (HSG A & B); Preserve natural areas; Maintain natural flow paths; Chesapeake Bay Stormwater Training Partnership 18

19 Environmental Resource Inventory: Identify & Characterize: Natural Areas Woods (specimen trees) Streams Wetlands Natural Flow paths Steep (critical) Slopes Soils Stream or Other Buffers Chesapeake Bay Stormwater Training Partnership 19

20 Environmental Site Design Example: Requires the designer to demonstrate that the layout and design of the development has achieved the goals of ESD to the MEP. 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 reserved 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 11. Installation of ESC and ESD Practices are integrated together 12. Tree planting was used at the site to convert turf areas into forest Chesapeake Bay Stormwater Training Partnership 20

21 Runoff Reduction (RR) Method Codifies 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), with volume reduction metric; Proper accounting of BMPs in series Chesapeake Bay Stormwater Training Partnership 21

22 The impacts of impervious cover are well documented: Significant increase in runoff volume, velocity and peak rates of discharge; Traditional detention serves to extend the duration of peak flows below the basin, resulting in increased erosive work on the channel Runoff Postdeveloped Predeveloped Post-developed (with Detention) Time Chesapeake Bay Stormwater Training Partnership 22

23 The impacts of impervious cover are well documented (contd.): 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 deposition (TN), adjacent pervious areas (TP), and direct deposits from land use activities (automobiles, hotspots, etc.), potentially increasing the concentration (mg/l) of pollutants; Results in greater total pollutant load (lb/ac). Chesapeake Bay Stormwater Training Partnership 23

24 Impervious Cover and Stream Health Impervious Cover Model Chesapeake Bay Stormwater Training Partnership 24

25 ICM supported by over 200 Studies on 26 aquatic indicators (CWP,2003) Index of Biotic Int tegrity Copyright Chesapeake 2000, Bay Stormwater Center for Training Watershed Partnership Protection 25

26 Water Quality Implications Simple Method Pollutant Load Computations: Load (lb/yr) = P * P j * R v * C * A * 2.72/12 where: P = annual precipitation (inches) P j = rainfall correction factor = 0.9 R v = volumetric runoff coefficient = ( *I) 009*I) C = pollutant concentration (mg/l) A = drainage area (acres) 2.72; 12 = unit conversions The Simple Method computes the pollutant load based on The Volumetric Runoff Coefficient based on impervious cover. Chesapeake Bay Stormwater Training Partnership 26

27 Hydrologic Implications WQ v = (P * R v * A) / 12 Where: WQ v = water quality design volume (ac-ft) P = water quality design storm depth (inches) R v = volumetric runoff coefficient = ( *I) I= percent impervious cover) A = drainage area (acres) 12 = unit conversion Stormwater quality practices have traditionally been sized using a Volumetric Runoff Coefficient based on impervious cover Chesapeake Bay Stormwater Training Partnership 27

28 Impervious Cover Companion: Highly and moderately impervious sites often include pervious turf areas (required as green space) placed over compacted soils; Source: Schwartz, UMBC, 2010 Chesapeake Bay Stormwater Training Partnership 28

29 Impervious Cover Companion: Research is demonstrating that the runoff characteristics of these graded and compacted areas can be comparable to pavement (Schwartz, UMBC March 2010). Chesapeake Bay Stormwater Training Partnership 29

30 Managed Turf Documented impacts of grading and compaction of soils: Increased bulk density Decreased permeability Increased runoff coefficient Documented impacts from turf management activities: Fertilization; Pest management; Center for Watershed Protection Technical Memorandum: The Runoff Reduction Method; 4/18/08 Chesapeake Bay Stormwater Training Partnership 30

31 6.1 million grass farmers in the Chesapeake Bay watershed Tending to to million acres (5.9% to 93%) 9.3%) Approx 75% of which is home lawns Influence turf management practices = influence water quality Chesapeake Bay Stormwater Training Partnership 31

32 Water Quality Implications The National Stormwater Quality Database (NSQD) version 1.1 reflects higher nutrient loads from sites with lower percent impervious cover; Simple Method pollutant load computations should reflect this potential for increased loads from nonimpervious areas. References: Runoff Reduction Technical Memo, Appendix G: Derivation of EMCs for Virginia; 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) Chesapeake Bay Stormwater Training Partnership 32

33 Hydrologic Implications NRCS Methods assume non-impervious to be open space in good condition ; ignores the potential for increased runoff resulting from impacted and 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 disturbed soils: A B; B C; etc. Chesapeake Bay Stormwater Training Partnership 33

34 Treatment Volume: Beyond 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 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. 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) Chesapeake Bay Stormwater Training Partnership 34

35 Treatment Volume: Beyond Impervious Cover 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 the impervious cover); The Tv is used to size the Runoff Reduction Practices so they are more appropriately sized to manage the runoff coming to the practice. Chesapeake Bay Stormwater Training Partnership 35

36 Treatment Volume: Beyond Impervious Cover The formula for computing the Treatment Volume (Tv): Tv = P (Rv I *%I + Rv T *%T + Rv F *%F)* A / 12 Tv = Runoff Reduction Treatment Volume (ac-ft) P = Water quality design storm rainfall depth (inches) Rv I = runoff coefficient for impervious cover 1 Rv = 1 T runoff coefficient for turf or disturbed soil cover Rv F = runoff coefficient for forest cover 1 %I = % of site in impervious cover (fraction) %T = % of site in turf or disturbed d soil cover (fraction) %F = % of site in forest cover (fraction) A = Drainage area (acres) 1 Rv values defined; Refer to next slide Chesapeake Bay Stormwater Training Partnership 36

37 Treatment Volume: Site Runoff Coefficients (Rv) 1 Cover HSG A HSG B HSG C HSG D Forest 0.02* 0.03* 0.04* 0.05* Managed Turf / Disturbed Soil Impervious Cover 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) Chesapeake Bay Stormwater Training Partnership 37

38 Treatment 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 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 land uses: Runoff Reduction Method Technical Memo Document C: Documentation Chesapeake Bay Stormwater Training Partnership 38

39 Runoff Reduction (RR) Method Codifies avoidance and minimization; Goes beyond impervious cover as a water quality indicator; Utilizes up to date science for nutrient reductions; Credits total BMP performance; Updated BMP specifications (Level 1 & 2), with volume reduction metric; Proper accounting of BMPs in series Chesapeake Bay Stormwater Training Partnership 39

40 Nutrient Reduction (Pollutant Removal) The Runoff Reduction Method Technical Memorandum April, Chesapeake Bay Stormwater Training Partnership 40

41 Nutrient Reduction (Pollutant Removal Efficiency) Pollutant P t t removal efficiency i generally refers to the pollutant reduction from the inflow to the outflow of a system. The two most common methods of calculating pollutant removal efficiency are Event Mean Concentration (EMC) Efficiency and Mass Load Efficiency. Chesapeake Bay Stormwater Training Partnership 41

42 Pollutant Removal (PR): Event 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. 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 Reasons You Should Think Twice Before Using Percent Removal To Assess BMP Performance. Stormwater Magazine.; Jan/Feb Strecker, E., Quigley, M., Urbonas, B., and Jones, J Stormwater management: State-of-the-Art the In Comprehensive Approaches To Stormwater. The Water Report. Issue #6. Envirotech Publishers Inc., Eugene, OR. Chesapeake Bay Stormwater Training Partnership 42

43 Pollutant Removal (PR): Mass Load 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; 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) Chesapeake Bay Stormwater Training Partnership 43

44 Pollutant Removal (PR) Efficiency EMC Removal Efficiency: EMC in vs. EMC out (EMC = Event Mean Concentration (mg/l)) Mass Load Removal Efficiency: (Vol in )(EMC in ) vs. (Vol out )(EMC out ) Mass load removal results from reductions in EMC, Volume, or both (however, as noted, many early studies did not isolate and evaluate volume reduction). Chesapeake Bay Stormwater Training Partnership 44

45 Pollutant Removal (PR) Efficiencies Practice Total Phosphorus (%) Total Nitrogen (%) Green Roof Disconnection 0 0 Rainwater Harvesting Permeable Pavement Grass Channel Bioretention 25 to to 60 2 Dry Swale 20 to to 35 Wet Swale 20 to to 35 Infiltration ED Pond Soil Amendments 0 0 Sheetflow to Open 0 0 Space Filtering Practice 60 to to 45 Constructed Wetland 50 to to 55 Wet Pond 50 to to 40 1 EMC removal is occurs through h a variety of mechanisms, including filtering, biological luptake, adsorption, and settling. Green roof, rainwater harvesting, and other practices have demonstrated performance through volume reduction, not EMC reduction 2 Range of values is for the median and 75 th percentile pollutant removal rates (Level 1 and Level 2) Chesapeake Bay Stormwater Training Partnership 45

46 Runoff Reduction (RR) Method Codifies avoidance and minimization; Goes beyond impervious cover as a water quality indicator; Utilizes up to date science for nutrient reductions; Credits total BMP performance; Updated d d BMP specifications i (Level 1 & 2), with volume reduction metric; Proper accounting of BMPs in series Chesapeake Bay Stormwater Training Partnership 46

47 BMP Performance BMP performance in terms of Pollutant t Removal (PR) or EMC reduction is limited by the existence of an irreducible concentration that represents the lowest concentration achievable through stormwater BMPs (regardless of the reported EMC removal efficiency). Therefore, even multiple l BMPs in series will not serve to provide pollutant removal any further than the irreducible concentration. Chesapeake Bay Stormwater Training Partnership 47

48 Total BMP Performance = Pollutant Removal (PR) and Runoff Reduction (RR) Traditional Performance Goals: Pollutant Removal (PR) Performance is measured and reported as a function of reducing the pollutant load in terms of the percentage of EMC reduction Runoff Reduction Performance Goals: Runoff Reduction (RR) performance is measured and reported as a function of reducing the annual volume of runoff associated with the 90 th percentile rain event. Vol in vs Vol out (Vol = volume of runoff) Chesapeake Bay Stormwater Training Partnership 48

49 Total BMP Performance: Total BMP Performance: Pollutant Removal Reported Performance: AND EMC in vs EMC out Runoff Reduction Reported Performance: Vol in vs Vol out Equals Total BMP Performance reported as Load Reduction: (Vol in )*(EMC in ) vs (Vol out )*(EMC out ) =Total BMP Performance Chesapeake Bay Stormwater Training Partnership 49

50 Total BMP Performance Runoff Reduction (RR) and pollutant removal (PR): Allows for additional reductions beyond the irreducible concentration by reducing the volume; Provides for maximum performance through a Treatment t 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 practices and additional pollutant removal practices as needed. Chesapeake Bay Stormwater Training Partnership 50

51 Stormwater Practices Differ Sharply py in Ability to Reduce Runoff Volume Wet Ponds, ED Ponds and Constructed Wetlands and Filters Reduce Runoff Volumes by zero to 10% Bioretention, Infiltration, Dry Swales, Soil Amendments, disconnection, and Related Practices Reduce Runoff Volumes by 50 to 90% Chesapeake Bay Stormwater Training Partnership 51

52 Inflow V. Outflow Rates Sample Bioretention flow monitoring Inflow arge (cfs) Disch Outflow Cumulative Rainfall Dep pth (in) /13/ :00 1/14/2005 0:00 1/14/ :00 1/15/2005 0:00 1/15/ :00 1/16/2005 0:00 0 Source: Dr. Bill Hunt; NC State Chesapeake Bay Stormwater Training Partnership 52

53 Reported Reductions in Runoff Volume Losses Due to Exfiltration, Evapotranspiration and Post Storm Delivery; Sampling of reductions reported dby research: CT: 99% UK: 58% FL: 98% NC: 30 to 65% (4) PA: 80% Aus: 73% Key Factors: Infiltration Rate, WA: 96% Media Depth, Hydraulic MD: 46 to 54% Gradient, and Absence of Underdrain Chesapeake Bay Stormwater Training Partnership 53

54 Runoff Reduction Processes Runoff Reduction is not just infiltration! Infiltration Canopy Interception Evaporation Transpiration Rainwater Harvesting Extended Filtration Chesapeake Bay Stormwater Training Partnership 54

55 Runoff Reduction Values Practice RR (%) Green Roof 45 to 60 Rooftop Disconnection 25 to 50 Raintanks and Cisterns 40 Permeable Pavement 45 to 75 Grass Channel 10 to 20 Bioretention ti 40 to 80 Dry Swale 40 to 60 Wet Swale 0 Infiltration 50 to 90 ED Pond 0 to 15 Soil Amendments 50 to 75 Sheetflow to Open 50 to 75 Space Filtering Practice 0 Constructed Wetland 0 Wet Pond 0 Range of values is for median and 75 th percentile reported performance; (Level 1 and Level 2 designs ) Chesapeake Bay Stormwater Training Partnership 55

56 Total Performance: Runoff Reduction and Nutrient Removal Table Chesapeake Bay Stormwater Training Partnership 56

57 Practice Design Runoff TN EMC TN Load TP EMC TP Load Level Reduction Removal 3 Removal Removal Removal 6 Rooftop Disconnect to to to 50 1 No Level 2 Design Sheet Flow to Veg. Filter 1 25 to to to 50 1 or Conserv. Open Space to to to 75 1 Grass Channels 1 10 to No Level 2 Design Can be used to Decrease Runoff Coefficient for Turf Cover at Site. See the design specs for Soil Compost Rooftop Disconnection, Sheet Flow to Vegetated Filter or Conserved Open Space, and Grass Amendment Channel Vegetated Roof 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 Infiltration Practices Bioretention Practices Urban Bioretention No Level 2 Design Dry Swales Wet Swales Filtering Practices Constructed Wetlands Wet Ponds (20) 4 30 (20) 4 50 (45) 4 50 (45) (30) 4 40 (30) 4 75 (65) 4 75 (65) 4 Ext. Det. Ponds Chesapeake Bay Stormwater Training Partnership Refer To Next Slide for Footnotes 57

58 Notes for Total Performance: Runoff Reduction and Nutrient Removal Table Notes 1 Lower rate is for HSG soils C and D, Higher rate is for HSG soils A and B. 2 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 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 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. Chesapeake Bay Stormwater Training Partnership 58

59 Credits Total BMP Performance Total BMP Performance includes volume reduction credited for large storm control: Channel Protection Flooding Protection Chesapeake Bay Stormwater Training Partnership 59

60 Increasing the Retention Storage of Runoff Reduction Practices Additional Surface Storage Additional Sub- Surface Storage in Soil Layer, Stone, Chambers, or Other Means Chesapeake Bay Stormwater Training Partnership 60

61 Volume Reduction: Hydrograph Modification 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) Chesapeake Bay Stormwater Training Partnership 61

62 Volume Reduction: Hydrograph Modification Methods Considered: 1. Hydrograph Truncation 2. Hydrograph Scalar Multiplication 3. Precipitation Adjustment 4. Runoff Adjustment t 5. Curve Number Adjustment Excerpted from work by Paul R. Koch, Ph.D., P.E. Chesapeake Bay Stormwater Training Partnership 62

63 Runoff Hydrograph Modification Curve Number Adjustment: NRCS Runoff Equation is used to derive a curve number that reflects the reduced volume of runoff resulting from the distributed ib d retention storage. A simplified derivation of the computational procedure starts with the combined Runoff Equations in order to express the runoff depth (Q, inches) in terms of rainfall (P, inches) and potential maximum retention (S). Chesapeake Bay Stormwater Training Partnership 63

64 Runoff Hydrograph Modification Curve Number Adjustment: Equation 2-1 (TR-55): Q 2 P Ia P Ia S Equation (TR-55): Ia 0. 2S Initial abstraction (Ia) is expressed in terms of the potential maximum retention (S) after runoff begins (Eq. 2-2), and substituted into the runoff depth equation (Eq.2-1). Chesapeake Bay Stormwater Training Partnership 64

65 Runoff Hydrograph Modification P 0.2SS Equation 2-3 (TR-55): Q P 0.8S The retention storage provided on site is expressed in terms of watershed inches (R) and subtracted from the runoff depth as expressed by the following: Equation 2-3 (modified) Q R 2 P 0.2S P 0.8 S 2 Chesapeake Bay Stormwater Training Partnership 65

66 Runoff Hydrograph Modification Equation 2-3 (modified) Q R P 0.2SS P 0.8S 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 can be expressed in terms of the curve number: 1000 Equation 2-4 (TR-55) S 10 CN 2 Chesapeake Bay Stormwater Training Partnership 66

67 Runoff Hydrograph Modification 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 (TR-55): Or: 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. Chesapeake Bay Stormwater Training Partnership 67

68 Runoff Hydrograph Modification No delay in the Tc is reflected, and the reduction is distributed across the entire storm, resulting in a conservative estimate of the peak discharge. Chesapeake Bay Stormwater Training Partnership 68

69 Runoff Hydrograph Modification The two predominant methodologies use 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 Modification to achieve the required small storm (P=1 ) pollutant load reduction to the extent practicable and hydrograph modification to measure the reduction in peak flow during channel protection and flooding design storm events. Chesapeake Bay Stormwater Training Partnership 69

70 Runoff Hydrograph Modification MD requires retention storage to manage the entire small storm runoff volume (P=1 ) and to the maximum extent practicable to achieve a curve number reduction to that of woods in good condition for the channel protection event (1-year design storm). Chesapeake Bay Stormwater Training Partnership 70

71 Runoff Reduction (RR) Method Codifies avoidance and minimization; Goes beyond impervious cover as a water quality indicator; Utilizes up to date science for nutrient reductions; Credits total BMP performance; Updated d d BMP specifications i (Level 1 & 2), with volume reduction metric; Proper accounting of BMPs in series Chesapeake Bay Stormwater Training Partnership 71

72 Level 1 and Level 2 Designs BMP Performance studies indicate variability in performance based on a variety of factors: design features, influent concentration, particle size distribution in runoff, rainfall depth and intensity, peak flow rates, soils, and other site factors. Regulatory performance goals commonly assign the median pollutant removal efficiency, i ignoring i the role of certain design factors in reducing or enhancing performance. Chesapeake Bay Stormwater Training Partnership 72

73 Level 1 and Level 2 Designs Extensive process to assign specific design criteria to Level 1 and Level 2 designs: Standard design features that ensure proper function of the BMP assigned to all designs: safety, appearance, safe conveyance, longevity, standard feasibility constraints, and maintenance needs. Chesapeake Bay Stormwater Training Partnership 73

74 Level 1 and Level 2 Designs Extensive 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 performance values to designs that could not incorporate full design features due to the limiting factors associated with retrofit situations. Modifications to these Tables were made to account for additional practices and the new and redevelopment condition (Runoff Reduction Technical Memorandum). Chesapeake Bay Stormwater Training Partnership 74

75 Level 1 and Level 2 Designs Level 1 and Level 2 design factors based on: Updated National Pollutant Removal Database Extensive literature search and review of more than 100 BMP performance studies Professional Judgment Chesapeake Bay Stormwater Training Partnership 75

76 Level 1 and Level 2 Designs Level 1 designs may allow for practices in 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 performance by enhancing one or more design features. Not N t all states t accept deviation from specific design standards (Level 1, Level 2); refer to the State Manual or local plan reviewer. Chesapeake Bay Stormwater Training Partnership 76

77 Where are pollutants removed? TSS TP Temp TN Pathogens Metals Oil & Grease Chesapeake Bay Stormwater Training Partnership 77

78 Where is Volume removed? Volume Chesapeake Bay Stormwater Training Partnership 78

79 Primary Design Factors That Segregate Level 1 and Level 2 Designs Increased Treatment Volume (Tv) Sizing Increased Runoff Reduction Volume Enhanced Design Geometry Multiple Cells Vegetative Condition Multiple Treatment Pathways Other bells and whistles Chesapeake Bay Stormwater Training Partnership 79

80 Level 1 and Level 2 Design Specifications Primary Design Factors for defining Level 1 and Level 2: Increased Treatment Volume Since the Standard Design accommodates the 90 th percentile 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); Therefore, 3 incremental increases in Tv (110%, 125%, and 150% of base Tv) are assigned to specific BMPs for Level 2 performance credit. Increased Runoff Reduction Volume The storage capacity of some RR practices can be increased to provide a corresponding increase in runoff reduction credit. Chesapeake Bay Stormwater Training Partnership 80

81 Samples of Additional RR Storage Chesapeake Bay Stormwater Training Partnership 81

82 Level 1 and Level 2 Design Specifications Primary Design Factors for defining Level 1 and Level 2 (continued): Enhanced Design Geometry & Hydraulics Geometry factors that are known to influence hydraulic performance or treatment conditions can be isolated and enhanced, including: flow path length depth of filter media multiple cells surface area to drainage area ratio extended detention time and ponding depth Chesapeake Bay Stormwater Training Partnership 82

83 Flow Path Geometry Chesapeake Bay Stormwater Training Partnership 83

84 Level 1 and Level 2 Design Specifications Primary Design Factors for defining Level 1 and Level 2 (continued): Vegetative Condition Specifying the type and cover of vegetation insofar that it influences: Nutrient uptake Increases evapotranspiration pump Stabilizes trapped sediments Enhances filter bed performance and longevity Multiple 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 Chesapeake Bay Stormwater Training Partnership 84

85 Runoff Reduction (RR) Method Codifies avoidance and minimization; Goes beyond impervious cover as a water quality indicator; Utilizes up to date science for nutrient reductions; Credits total BMP performance; Updated BMP specifications (Level 1 & 2), with volume reduction metric; Proper accounting of BMPs in series Chesapeake Bay Stormwater Training Partnership 85

86 Practices in Series Irreducible Load? Volume Reduction? Chesapeake Bay Stormwater Training Partnership 86

87 First: Reduce Stormwater Runoff by Design (Minimization & Avoidance) Better site planning & design techniques Preserve natural areas Conservation design Reduce clearing & grading limits Reduce roadway widths Use alternative cul-desacs Promote redevelopment > Online Store > Better Site Design Chesapeake Bay Stormwater Training Partnership 87

88 Second: Reduce Volume of Post- Construction Stormwater Runoff Runoff Reduction Practices Soil Amendments Rooftop disconnection Rain gardens/ bioretention areas Rainwater harvesting Permeable pavement Green Roofs Natural Drainageways Vegetated Channels Site Reforestation Buffers Chesapeake Bay Stormwater Training Partnership 88

89 Third: Capture & Treat Remaining Stormwater Runoff Larger-scale, engineered practices Stormwater ponds Stormwater wetlands Larger bioretention Infiltration Media filters Swales And more Post-Construction Tool 5: Manual Builder > Resources > Controlling Runoff & Discharges > Stormwater Management Chesapeake Bay Stormwater Training Partnership 89

90 Next: Iterate or Mitigate 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 Chesapeake Bay Stormwater Training Partnership 90

91 Refer to Individual State Manuals for details on Runoff Reduction design specifications and computational methods Chesapeake Bay Stormwater Training Partnership 91