Session 1E Non-Structural BMPs. Chesapeake Bay Stormwater Training Partnership 1

Transcription

1 Session 1E Non-Structural BMPs Chesapeake Bay Stormwater Training Partnership 1

2 Agenda Site Design (self-crediting) Soil Amendments Simple l Disconnection Sheet Flow Grass Channels Site Plan courtesy of Chesapeake Bay Stormwater Training Partnership 2

3 Key Resources on your disk October 2010 Webcast Archive: Disconnection Disconnection Webcast Resource Sheet Filter Strip and Soil Amendments Presentation Relevant DCR Design Specifications: No. 1 Disconnection No. 2 Sheet Flow No. 4 Soil Amendments Urban Watershed Forestry Manual Part 2: Conserving and Planting Trees at Development SitesThe Runoff Reduction Theory Presentation ti Urban Watershed Forestry Manual Part 3: Urban Tree Planting Guide Chesapeake Bay Stormwater Training Partnership 3

4 Non-Structural Practices Direct growth to identified areas; Protect sensitive aquatic resources; Establish buffers along stream and other waters; Maintain existing runoff patterns and Tc; Minimize impervious surfaces; Minimize disturbance of soils and vegetation; Encourage infill and re- development; Educate on material disposal/recycling Spill Prevention/Clean Up Identify and eliminate illicit discharges Promote street sweeping Develop Public Education/Participation programs Chesapeake Bay Stormwater Training Partnership 4

5 Non-Structural Practices as part of the Runoff Reduction Method 1. Minimization & Avoidance 2. Reforestation 3. Soil compost amendments 4. Impervious disconnection 5. Sheet flow to Vegetated Filter and Conservation Areas (or Buffers) 6. Grass channels Chesapeake Bay Stormwater Training Partnership 5

6 Site Design - Things to Consider Riparian Buffers Wetlands and Stream Channels Existing Woods Transitional buffers Soils Constraints (Texture, HSG, Perm., Bedrock, Water Table) Slope Constraints Access to Site Chesapeake Bay Stormwater Training Partnership 6

7 Example acre development Chesapeake Bay Stormwater Training Partnership 7

8 Identify Water Resources and Forested Areas Chesapeake Bay Stormwater Training Partnership 8

9 Soils Investigation Chesapeake Bay Stormwater Training Partnership 9

10 Determine Limits of Constraints Chesapeake Bay Stormwater Training Partnership 10

11 Typical Development Patterns Acres 25 ½ acre lots Chesapeake Bay Stormwater Training Partnership 11

12 Alternative Development Acres 25 ¼ acre lots Chesapeake Bay Stormwater Training Partnership 12

13 Effects of Typical Layout Clearing i of entire site (disturbed d soils) Significant Impacts to Wetlands and Stream Channels Impacts to designated Riparian corridors Increase in Impervious Cover Directly Connected Drainage Systems Chesapeake Bay Stormwater Training Partnership 13

14 LID Resources Report: Performance Standards Chesapeake Bay Stormwater Training Partnership 14

15 Minimization & Avoidance Design Goal: Allow/Promote Simple, Low Maintenance Practices As Part of a Treatment Train Approach to Help Meet the Runoff Reduction Goal. Chesapeake Bay Stormwater Training Partnership 15

16 Start By Reducing Clearing and Preserving Highly Permeable Soils Reduces runoff coefficient from pervious areas Increases runoff reduction rates for non-structural practices in poor soils Chesapeake Bay Stormwater Training Partnership 16

17 Disturbed Soils Increase the Runoff Coefficient For Urban Turf Runoff Coefficients for Use for Different Pervious Areas 1 Hydrologic Undisturbed Disturbed Restored Restored Soil Group Soils 2 Soils 3 Soils 4 and Reforested A B C D Source: CWP and CSN (2008) and Pitt (2004) 2 Portions of a new development site, outside the limits of disturbance 3 Previously developed sites, and any site area inside the limits of disturbance 4 Pervious areas on the site restored per DCR BMP Spec 4 Chesapeake Bay Stormwater Training Partnership 17

18 We Have A Lot of Managed Turf in the Bay! For every acre of new impervious cover, we create 3 acres of managed turf cover. What are the hydrologic implications? Chesapeake Bay Stormwater Training Partnership 18

19 Chesapeake Bay Stormwater Training Partnership 19

20 Disturbed Soils Are Not Very Pervious and Ineffective in Treating Stormwater Top Soil is Stripped Soil Structure is Lost Subsoils are Compacted Reduced Water Holding Capacity Low Infiltration Rate High Nutrient Concentrations Erosion & Runon to Impervious Cover Chesapeake Bay Stormwater Training Partnership 20

21 Soil Amendments Chesapeake Bay Stormwater Training Partnership 21

22 Soil Amendments Reduce the Runoff Coefficient For Urban Turf Runoff Coefficients for Use for Different Pervious Areas 1 Hydrologic Undisturbed Disturbed Restored Restored Soil Group Soils 2 Soils 3 Soils 4 and Reforested A B C D Source: CWP and CSN (2008) and Pitt (2004) 2 Portions of a new development site, outside the limits of disturbance 3 Previously developed sites, and any site area inside the limits of disturbance 4 Pervious areas on the site restored per DCR BMP Spec 4 Chesapeake Bay Stormwater Training Partnership 22

23 Compost and Incorporation Depths Contributing Impervious Cover to Soil Amendment Area Ratio 1 IC/SA = 0 2 IC/SA = 0.5 IC/SA = 0.75 IC/SA = Compost depth (in) 4 2 to 4 5 3to6 5 4 to 8 5 6to 10 5 Incorporation Depth (in) 6 to to to to 24 5 Incorporation Method Rototiller Rototiller Subsoiler Subsoiler Notes: 1 IC = contrib. impervious cover (sq. ft.) and SA = surface area of compost amendment (sq. ft.) 2 For amendment of compacted lawns that do not receive off site runoff 3 In general, IC/SA ratios greater than 1 should be avoided 4 Average depth of compost added 5 Lower end for B soils, higher end for C/D soils Chesapeake Bay Stormwater Training Partnership 23

24 Impervious Disconnection Simple Disconnection Simple disconnection Compost amended flow path Rain garden (Urban Planter) Infiltration Rainwater Harvesting Chesapeake Bay Stormwater Training Partnership 24

25 Two Types of Disconnection Simple Disconnection: rooftops and/or on-lot residential impervious surfaces are directed to pervious areas; Alternative disconnection: the use of compensatory practices where the space required for simple disconnection may not be available, or enhanced performance is desired: Chesapeake Bay Stormwater Training Partnership 25

26 Simple Disconnection Chesapeake Bay Stormwater Training Partnership 26

27 Simple Disconnection Filter corridors from the downspout to the street Level spreader to distribute runoff over a ten foot width Minimum and maximum Length; Width; and Slope criteria Chesapeake Bay Stormwater Training Partnership 27

28 Summary of Stormwater Function provided d by Simple Disconnection Chesapeake Bay Stormwater Training Partnership 28

29 Simple Disconnection Design Criteria i Chesapeake Bay Stormwater Training Partnership 29

30 Simple Disconnection Applications: Minimum lot sizes (e.g. 6,000 ft 2 ) Minimum (and maximum) disconnection length Maximum disconnection slope Chesapeake Bay Stormwater Training Partnership 30

31 Feedback Question #1: What t are the best ways to document NSPs during Plan review? How should they be handled in ESC plans during construction? Chesapeake Bay Stormwater Training Partnership 31

32 Simple Disconnection Abuse Application of a maximum impervious area per disconnection; Commercial or mixed land-use applications can be challenging, and should consider utilizing i vegetated filter strips or conservation open space. Chesapeake Bay Stormwater Training Partnership 32

33 Alternative Disconnection Utilized where the space required for simple disconnection may not be available, or enhanced performance is desired or required. Rainwater i t Harvesting & Cisternss Micro-Infiltration (dry wells) Rain R i Gardens Urban Planter Photo courtesy S. Schwartz & Cuyahoga Sustainability Network Chesapeake Bay Stormwater Training Partnership 33

34 Alternative Disconnection Chesapeake Bay Stormwater Training Partnership 34

35 Large Scale Disconnection: Vegetated Filter Strips Re-Engineering i the Filter Strip: Two Filter Designs Sheet Flow to Conserved Open Space Vegetated Filter Strip Chesapeake Bay Stormwater Training Partnership 35

36 Design Considerations for Vegetated Filter Strips Slope; Length (parallel and perpendicular p to flow) Contributing DA Sheet flow only Flow depth and velocity Vegetation Soils Reference: National Cooperative Highway Research Program (NCHRP) Project 25-25: Stormwater Treatment With Vegetated Buffers Chesapeake Bay Stormwater Training Partnership 36

37 Conserved Open Space Chesapeake Bay Stormwater Training Partnership 37

38 Design Considerations for Conserved Open Space Sheet Flow Boundary Spreaders Level Spreaders for concentrated flow Maximum slopes Chesapeake Bay Stormwater Training Partnership 38

39 Stream Buffers Designated during initial lot recordation/subdivision; Protected with legal instrument to prevent alteration of the land or vegetation; Width varies (35. ft to 150 ft.); Must establish sheet flow into buffer Chesapeake Bay Stormwater Training Partnership 39

40 Summary of Stormwater Function Provided by Conserved Open Space and Filter Strips Chesapeake Bay Stormwater Training Partnership 40

41 DESIGN FILTER STRIP CONSERVED OPEN CONSIDERATION SPACE Ranges from 1% to 8% 1 Ranges from.5% to Slope 6% 1 Sheet Flow Concentrated Flow First 10 feet <2% Maximum flow length of 150 ft. from adjacent pervious areas; Maximum flow length of 75 ft. from adjacent impervious areas Engineered Level Spreader Boundary Spreader Gravel Diaphragm at Gravel Diaphragm at Top Top Permeable Berm at Bottom 1: Allowable slope dependent on filter width. Chesapeake Bay Stormwater Training Partnership 41

42 Level Spreader Applicable to filter strips, buffers, and conserved open space with concentrated inflow. Henrico County, VA Level Spreader 30 to 40 ft in length; 50 ft width at level lip Chesapeake Bay Stormwater Training Partnership 42

43 Feedback No. 2 What is the maintenance model to ensure NSP long term hydrologic function? What would an inspection be like? Chesapeake Bay Stormwater Training Partnership 43

44 Grass Channels Chesapeake Bay Stormwater Training Partnership 44

45 Grass Channel Critical Design Parameter: Maximum Design Storm or Retention Volume flow velocity of 1 ft/s* Edge of pavement drop w / flat pre-treat area Prefer trapezoidal channel geometry *Accepted methodology for computing small storm peak discharge Chesapeake Bay Stormwater Training Partnership 45

46 Summary of Stormwater Functions Provided by Grass Channels Chesapeake Bay Stormwater Training Partnership 46

47 Key Design Requirements Bottom width 4 to 8 feet. Side-slopes 3H:1V or flatter. The maximum total contributing drainage area 5 acres. The longitudinal slope less than 4%. The maximum flow velocity of the channel must be less than 1 foot per second during a 1-inch storm event. Flows non-erosive during the 2-year and 10-year design storm events 10-year design flow is contained within the channel (minimum of 6 inches of freeboard). Chesapeake Bay Stormwater Training Partnership 47

48 Sheet Flow Edge Chesapeake Bay Stormwater Training Partnership 48

49 Grass Channel No Edge of pavement drop w / flat pre-treat area Runoff must get into the channel! Chesapeake Bay Stormwater Training Partnership 49

50 Feedback No. 3 What are measureable criteria at the site that would trigger NSP maintenance or rehabilitation? Chesapeake Bay Stormwater Training Partnership 50

51 Take-Home Points: Start t with site design to reduce impervious i cover and volume associated with the Design Retention Volume Use disconnection and/or compost amendment techniques as part of a rooftop to stream design approach Consider conservation areas, stream buffers, filter strips as an alternative to structural practices Chesapeake Bay Stormwater Training Partnership 51

52 Chesapeake Bay Stormwater Training Partnership 52