Stormwater Retrofitting for Nutrient Reduction

Transcription

1 Stormwater Retrofitting for Nutrient Reduction

2 otos: Chesapeake Bay Program Welcome to the Webcast To Ask a Question Submit your question in the chat box located to the left of the slides. We will answer as many as possible during Q&A. To Answer a Poll Question Simply select the preferred option. For those viewing this session alongside several colleagues, respond in a manner that represents your organization as a whole. We ARE Recording this Session All comments and questions will be recorded and included in the archives. We Appreciate Your Feedback Fill out our evaluations our funders need to hear it! 2

3 otos: Chesapeake Bay Program Session Resources Today s presentation and resources will be posted at R1. Stormwater Retrofit Manual and Appendices R2. Technical Bulletin on Local Nutrient Accounting R3. Pdf of this presentation

4 Speaker Info Tom Schueler Chesapeake Stormwater Network and Chesapeake Bay Program Stormwater Coordinator Baltimore, MD

5 Chesapeake Bay Stormwater Training Partnership Visit: To learn how you can have access to: Discounted Webcasts Free One-day design workshops Intensive master stormwater design seminars Direct On-site technical assistance Self guided web-based learning modules

6 Webcast Agenda 1. Why Retrofit? 2. Basics of Retrofitting 3. The Stormwater Retrofit Process 4. Calculating Retrofit Nutrient Reduction 5. Effective Local Retrofit Delivery

7 Webcast Caveats & Ground Rules The opinions in this webcast are exclusively those of CSN Please ask questions or make comments using the chat box The proposed retrofit nutrient reduction protocols will be reviewed by a BMP expert panel later this fall

8 POLL Question 1 I am interested in retrofitting because I: work for a local government am a private consultant work for a state or federal regulatory agency work for a watershed or advocacy group other

9 POLL Question 2 I would characterize my retrofit experience as being: Not much Limited Decent Good, but have some technical questions Strong, I should be giving this webcast

10 Part 1. Why Retrofit? Local watershed restoration Meet IC Treatment Targets in MD Comply with Bay-wide TMDLs (and local ones too) Improve local stream habitat and diversity Fix old mistakes/drainage problems Improve performance of existing stormwater infrastructure 10

11 Part 2. Retrofit Basics 11

12 Why Retrofits Are Different Urban Retrofit Practices Construction costs are 1.5 to 4 times greater Assessment and design costs are higher Sized to meet watershed restoration objectives Typically installed on public land Urban soils often cannot support infiltration Fingerprinted around existing development New Stormwater Practices Designers seek least costly options Focus on low cost design and construction Sized to meet local stormwater design standards Installed at new development projects Soils may support infiltration More flexibility on where to locate practices 12

13 Why Retrofits Are Different Urban Retrofit Practices Must be acceptable to adjacent neighbors Most are publicly maintained Not all candidate sites are feasible Tied into existing conveyance system Integrated with other restoration practices Public investment in watershed infrastructure New Stormwater Practices Aesthetics are not always a major design factor Most require private maintenance Nearly all sites are made to work Usually creates new conveyance system Stand-alone practice Private investment in stormwater infrastructure 13

14 Five Retrofit Categories New retrofit facilities BMP conversions BMP enhancements Green street retrofits On-site LID retrofits 14

15 NEW RETROFITS Upstream of Roadway Crossings Source: CWP 15

16 NEW RETROFITS Downstream of Outfalls Source: CWP 16

17 NEW RETROFITS Within the Existing Conveyance System Source: CWP 17

18 NEW RETROFITS Within Transportation Rights-of-Way Source: CWP 18

19 NEW RETROFITS Adjacent to Large Parking Lots Source: CWP Wet Pond Bioretention 19

20 BMP CONVERSIONS Existing Stormwater Ponds Source: CWP 20

21 BMP CONVERSIONS Existing Stormwater Ponds 21

22 BMP CONVERSIONS Rehabilitating Failed Infiltration Practices 22

23 BMP CONVERSIONS Adding Bioretention/Filtering to Ponds 23

24 BMP ENHANCEMENTS Increase treatment volume, flow path, retention time or wetland elements 24

25 GREEN STREETS 25

26 GREEN STREETS 26

27 ON-SITE RETROFITS Hotspot Operations 27

28 ON-SITE RETROFITS Individual Rooftops 28

29 ONSITE RETROFITS Individual Rooftops Source: CWP Source: City of Portland, OR 29

30 ON-SITE RETROFITS Small Parking Lots Source: CWP 30

31 Part 3. The Stormwater Retrofit Process 31

32 Step 1 Retrofit Scoping Consult with stakeholders Understand watershed conditions and develop restoration objectives Select subwatersheds Identify targeted land use for treatment Identify typical locations where retrofitting may be most successful 32

33 Step 2 Desktop Analysis Analyze mapping and metrics to find prospective sites Aerial photos Topography Hydrology Ownership Land cover Storm sewers Existing BMPs Outfalls Road Crossings 33

34 Step 3 Field Investigation Characterize drainage patterns and retrofit potential at each parcel 34

35 Develop Conceptual Design Develop conceptual designs 35

36 Step 4 Compile Retrofit Inventory 36

37 Step 5 Evaluation and Ranking Assign weights to ranking factors Nutrient reduction Community Benefits Permitting Environmental Site Constraints Costs Category Ranking Factor Potential Score Potential for Treatment 40 Total drainage area treated 15 Impervious area treated 15 Land use treated 10 Potential for Greater Ecological and Community Benefits 25 Potential education / demonstration project AND/OR public accessibility Treatment of area(s) of concern 5 Potential to support other planned or on-going watershed restoration projects and activities Site Constraints 35 Access for construction and maintenance 15 Potential for conflict with existing utilities 5 Potential ecological conflicts 10 Project for further consideration (yes/maybe) 5 Total Potential Score

38 Comparative Retrofit Construction Costs * (per impervious acre treated) New Retrofits: 12 to 30 K BMP Conversions: 5 to 15 K BMP Enhancements: 5 to 10 K Green Streets: 100 to 150 K Residential LID: * 80 to 120 K Non-residential LID: * 40 to 80 K * Demonstration project phase, should drop 38

39 Step 6 Subwatershed Analysis 39

40 Step 7: Retrofit Design Convert concept to a construction drawing Requires site analysis and construction plan More realistic construction costs come to light Copyright 2000, CWP 40

41 Manual 3: Urban Stormwater Retrofit Practices Contains extensive guidance on different retrofits and the process for finding them in the urban landscape Free download at 41

42 Questions and Answers

43 POLL Question 3 What is the major driver for stormwater retrofits in your community? Local watershed restoration plans Bay-wide TMDL (or local TMDL) MS4 Stormwater permit requirements Demonstration grants Not really doing much retrofitting yet

44 Part 4. Calculating Nutrient Reduction for individual retrofit projects Depends on: Baseline Load Class of Retrofit Rainfall Volume Captured Runoff Reduction Achieved 44

45 Technical Issues with Retrofit Nutrient Reduction Every retrofit is unique, depending on the drainage area treated, BMP treatment mechanisms, volume or sizing and the antecedent degree of stormwater treatment, if any. Many retrofits are under-sized compared to BMPs designed to new development standards, which can reduce pollutant removal capability Many retrofits may not be classified according to an existing CBP approved BMP removal rate Need to evaluate dozens or even hundreds of candidate projects during WIP retrofit investigation to find the best ones. 45

46 Overall Method Step 1: Compute the baseline load for the drainage area to the proposed retrofit using the Simple Method (Schueler, 1987), the Virginia spreadsheet (CWP, 2009) or the unit nutrient load method MDE (2011). Step 2: Define the projected removal rate for the retrofit project using the method appropriate to its classification. Step 3: Adjust removal rates using the runoff capture method if retrofit is under-sized Step 4: Multiply the adjusted retrofit removal rate by the preretrofit baseline load to get pounds reduced 46

47 Three Ways to Project Removal Rates CBP-approved removal rate Retrofit adjustor table Level 1 and 2 Design rates 47

48 CBP Approved Urban BMP Rates URBAN BMP Total Nitrogen Total Phosphorus MASS LOAD REDUCTION (%) Wet Ponds and Constructed Wetlands Dry Detention Ponds Dry Extended Detention Ponds Infiltration Filtering Practices (sand Filters) Bioretention C & D w/ud A & B w/ UD A & B w/o UD Permeable Pavement TSS C & D w/ud A & B w/ UD A & B w/o UD Grass Channels C & D w/o UD A & B w/o UD Bioswale aka dry swale

49 Stormwater Retrofit Removal Rate Adjustor Volumetric Criteria Mass Removal Rate % Rainfall depth Degree of runoff Total Phosphorus Total Nitrogen controlled reduction 0.25 LO HI LO HI LO HI LO HI LO HI LO HI LO HI LO HI Source Technical Bulletin No. 9 49

50 LO and HI Runoff Reduction HI is assigned to retrofit projects that achieve at least 50% reduction of the annual runoff volume through canopy interception, soil amendments, evaporation, rainfall harvesting, engineered infiltration, extended filtration or evapotranspiration. LO should be used if the retrofit employs a permanent pool, constructed wetlands or filtering as the primary runoff treatment mechanism. 50

51 Mass Nutrient Removal Rates for Stormwater Practices Practice Design Level 1 TN Load Removal 4 TP Load Removal 4 Permeable Pavers Infiltration Practices Bioretention Dry Swales Wet Swales Filtering Practices Constructed Wetlands Wet Ponds ED Ponds

52 New Retrofit Facilities CBP Rate Option: If the new retrofit project can be classified into one of the existing CBP urban BMP categories and has enough treatment volume to treat the runoff from at least one inch of rainfall, then use appropriate CBP approved rate Stormwater Retrofit Removal Rate Adjustor. If the retrofit is over or under-sized, or utilizes treatment mechanisms or design enhancements that cannot be classified under current CBP BMP categories, then use the adjustor table. 52

53 Example for New Retrofit Contributing Drainage Area: 50 acres (residential) 40% IC, Runoff coefficient = Rv = 0.38 Off-line Wetland Retrofit: captures 1.0 inch of rainfall provides no runoff reduction Modern design criteria Stormwater Retrofit Removal Rate Adjustor Volumetric Criteria Mass Removal Rate % Rainfall depth Degree of RR Total Phosphorus Total Nitrogen 1.0 LO HI

54 Compute Baseline Load w/ Simple Method L = [ P * Pj* Rv/12 ] [ C * A *2.72] Where: L = Annual load (lbs) P = Annual rainfall (in) (40) Pj = Fraction of storms producing runoff (0.9) Rv = runoff coefficient (0.41) C = Pollutant concentration EMC (mg/l) for TP= 0.3 for TN = 2.0 A = Site Area (acres) (50) 2.72 = Unit conversion factor 54

55 Example for New Retrofit Simple Method Baseline Load (L) For TP: 50 Lbs/year TN: 335 lbs/yr Retrofit Removal Rate (55% for both) Nutrient Load Reduction Achieved For TP: 50 * 0.55 = 27.6 pounds For TN: 335 * 0.55 = 184 pounds 55

56 BMP Conversions Option 1: Incremental Improvement Method. Compute an incremental rate based on the difference between the old and new CBP BMP removal rate. Option 2: Incremental Improvement for Maryland Design by Era Method. Derive a incremental rate based on the age of the BMP being converted MDE (2011). Calculate the incremental difference in removal rates from a more recent design era to an older one. Option 3: Incremental Rate Using Stormwater Retrofit Adjustor. Define a project-specific mass removal rate for the original BMP and the proposed conversion based on the net change in rainfall depth controlled and degree of runoff reduction achieved. 56

57 Example for BMP Conversion Retrofit Contributing Drainage Area: Same as Previous Example Existing BMP Dry Detention Pond Conversion to Sand Filter Forebay and five sand filter cells Treats an inch of rainfall Traditional design criteria CBP Approved BMP Removal Rates: Incremental Method BMP TP Removal TN Removal Dry Detention Pond 10 5 Sand Filter Incremental Rate

58 Example for BMP Conversion Retrofit Removal Rate For TP: 50 For TN: 35 Nutrient Load Reduction Achieved For TP: 50 * 0.50 = 25 pounds For TN: 335 * 0.35 = 117 pounds 58

59 Existing BMP Enhancements Step 1: The base nutrient removal rate for the existing BMP (prior to enhancement) should be the conservative CBP-approved rates Step 2: The designer should then evaluate the range of enhancements to see if they qualify for the higher Level 1 or Level 2 rates 1. Step 3: The nutrient removal rate for the retrofit the difference from the Level 1 or 2 rates and the existing CBPapproved rate. 59

60 Example for BMP Enhancement Contributing Drainage Area: Same as Previous Example Existing BMP 20 year old Wet Pond, designed to treat half inch of rainfall Enhanced Wet Pond Design Level 2 Design (RR:0; TP:75; TN:40) TV = 1.5(Rv) (A) Multiple Cell Design Pool of 4 to 8 feet Flow path 1.5:1 or more Adequate water balance CBP Approved BMP Removal Rates: Incremental Method BMP TP Removal TN Removal Existing Wet Pond * Level 2 Wet Pond Design Incremental Rate * CBP-approved rate 60

61 Example for BMP Enhancement Retrofit Removal Rate For TP: 30 For TN: 20 Nutrient Load Reduction Achieved For TP: 50 * 0.30 = 15 pounds For TN: 335 * 0.20 = 67 pounds 61

62 Green street retrofits Delineate the contributing drainage area and IC Determine rainfall volume captured and degree of runoff reduction achieved by project Use the retrofit removal rate adjustor to define nutrient removal rate 62

63 Example for Green Street Contributing Drainage Area: 20 acres of Street Impervious Cover 20 Blocks of Green Street Retrofits Captures 0.50 inch of rainfall provides 60% runoff reduction (HI) Part of City downtown revitalization effort Stormwater Retrofit Removal Rate Adjustor Volumetric Criteria Mass Removal Rate % Rainfall depth Degree of RR Total Phosphorus Total Nitrogen 0.5 LO HI

64 Example for Green Street Retrofit Same baseline load as previous examples Retrofit Removal Rate For TP: 30 For TN: 20 Nutrient Load Reduction Achieved For TP: 50 * 0.45 = 23 pounds For TN: 335 * 0.45 = 151 pounds 64

65 On-site LID Retrofits Each LID retrofit serves a small drainage area Dozens or hundreds are needed to make a meaningful difference Track aggregate IC treated by LID retrofits Compute baseline load and use a removal rate based on the retrofit adjustor table 65

66 Example for On-Site LID Retrofits Contributing Drainage Area: Treatment Goal 20 acres of rooftop IC treated by rain gardens 880 individual rooftop disconnections and rain gardens 220 homes Captures 0.25 inch of rainfall provides 60% runoff reduction (HI) Stormwater Retrofit Removal Rate Adjustor Volumetric Criteria Mass Removal Rate % Rainfall depth Degree of RR Total Phosphorus Total Nitrogen 0.25 LO HI

67 Example for On-site LID Retrofit Same baseline load as previous examples Retrofit Removal Rate For TP: 30 For TN: 20 Nutrient Load Reduction Achieved For TP: 50 * 0.30 = 20 pounds For TN: 335 * 0.30 = 101 pounds 67

68 The retrofit options can then be compared based on cost basis Summary of the Retrofit Examples Retrofit Type TN Total Cost Cost/lb of N Removed removed New Retrofit ,000 $2,175 BMP Conversion ,000 1,709 BMP Enhancement ,000 2,089 Green Streets 151 2,000,000 13,245 On-Site LID Practices 101 2,000,000 * 19,801 68

69 Part 5 Tips on Cost-Effective Retrofit Delivery 69

70 Tip 1. Maximize Other Nutrient Reduction Practices Reforestation Stream restoration Fertilizer restrictions Septic upgrades Maintenance upgrades Stream buffer upgrades Redevelopment credits Street sweeping Illicit discharge elimination 70

71 Tip 2. Develop multiple revenue streams and delivery mechanisms Capital Improvement Budget Stormwater Offset Fees Stormwater Utility Discounts Stormwater Maintenance Budgets Maintenance Enforcement Street Reconstruction Piggyback on Municipal Construction Projects Public/Private Partnerships Trading 71

72 A slice of restoration Another slice of restoration 72

73 Tip 3 Maximize Drainage Area Treated by Individual Retrofits Large storage retrofits are usually the most cost effective solution They do require more permitting, easements and neighborhood consultation Experience has shown that storage retrofits can treat up to 20 to 30% of subwatershed area in suburban areas, much less in highly urban ones Green street and on-site LID retrofits are needed for the next chuck of treatment 73

74 Tip 4 Residential LID Retrofit Incentives Subsidies, technical assistance, stormwater utility credits and other incentives to build LID retrofits on private land Topic of September 15 webcast 74

75 Tip No. 5: Transform Your Stormwater Maintenance Program Use your stormwater maintenance inspection, tracking and enforcement authority to identify potential retrofits and/or major maintenance upgrades Potential for both public and private stormwater facilities 75

76 Tip 6: Streamline local government contracting process Design, engineering, permitting and contracting costs can be 30 to 50% of retrofit construction cost Project bundling, design/build, call contracts, bid incentives and other project management tools can reduce costs and improve quality 76

77 otos: Chesapeake Bay Program Thanks! Thanks to our funders, we have provided this webcast for free. Please take 3-5 minutes to let us know how we did! Our funders need to hear! Or your comments directly to

78 Upcoming Webcasts for 2011 September 15 Increasing the Delivery of Residential Stewardship Practices

79 Questions and Answers