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A Builder’s Introduction To Low Impact Development

A Builder’s Introduction To Low Impact Development. How to Make LID Pencil Out For Your Project. Common Builder Questions. How do I know if my site is good for LID? Will LID be too expensive to consider? How many practices must I do to be considered a LID project?

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A Builder’s Introduction To Low Impact Development

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  1. A Builder’s Introduction ToLow Impact Development How to Make LID Pencil Out For Your Project

  2. Common Builder Questions • How do I know if my site is good for LID? • Will LID be too expensive to consider? • How many practices must I do to be considered a LID project? • Can I avoid some permits or reduce the size of my pond with LID? • Will the local government be on board? • Are there other ways to fund LID on my site?

  3. What is Low Impact Development? • Conserving vegetation & soils • Keeping natural flow paths • Increasing time on site LID mimics a site’s natural hydrology by: • Which means… • Less impervious surfaces • Disconnecting roofs, downspouts & parking areas • Disbursing small-scale controls vs. large detention pond Source: AHBL Engineering

  4. Hydrology 101 Native forest and soils intercept, store, and slowly convey precipitation. • 50% intercepted by leaves & evaporates • 30% stored by 2 – 4ft of organic & biologically active soil • Of interflow, 10 – 40% goes to groundwater (that acts as natural water storage and maintains instream flows) • Overland flow < 1% Source: LID Technical Guidance Manual, Fig 1.1

  5. Urbanization Changes Forest Function Traditional residential development removes almost all vegetation and topsoil. evapo-transpiration soil storage in groundwater rapid surface runoff pollutants to water channel instability Source: LID Technical Manual, Figure 1.3

  6. And We’re Growing! The Puget Sound is expected to have 1.4 million more residents by 2025. (Source: OFM) Between 1991 – 2001, impervious surface increased by 10.4% Source: Sightline Institute

  7. Traditional Stormwater Management Collecting & conveying to centralized ponds is reliable, predictable & simple to maintain BUT • 90% storage loss v. 25% storage gain • Heavy rains = excess to receiving waters • Flow/duration standards in 2005 Manual require larger ponds = costs and buildable land

  8. So…instead of focusing on structure, LID focuses on landscape. Disburse & infiltrate rather than capture and store.

  9. Benefits of LID • Smaller ponds = more land and less cost • Marketable as green neighborhood • Reduce stormwater utility or SEPA fees • Avoid/speed permits or other regulations • For local governments: • Prevents flooding • Maintains drinking water supplies & instream flows • Reduces maintenance costs • Less need for CSO storage & conveyance • Cost effective urban retrofit (especially with fewer streets, curbs & gutters)

  10. So if LID is so great, why aren’t we seeing it everywhere? • Still being tested • LID practices + native vegetation/open space + additional storage = $$$ • Expensive products and applications • Difficult to find the right soil • Few incentives & difficult to get flow credits • Local governments slow to allow • Conflict within government offices (planning v. public works v. fire department, etc.)

  11. SEA Streets, Seattle • Reduced impervious surfaces to 11% less than traditional street; vegetated swales; added over 100 evergreen trees & 1100 shrubs • Cost: $850K • Completed spring 2001 • Prevented all dry season runoff and 99% of wet season runoff 2000-03; no runoff since 2003 Source: Seattle Public Utilities

  12. High Point, West Seattle • 120 acres; Phase I 829 units finished 2006; Phase II 2009 • Mixed housing--all 3 star or more • ½ Seattle Housing Authority; ½ private • Vegetated swales w/ 3ft amended soils, porous walks, one porous street, football field detention pond w/ trail, overlook & artificial stream • Intended to process water like forest meadow • Monitoring started 1/07; data in 3 years Source: Seattle Public Utilities

  13. Fife Heights, Pierce County Source: AHBL Engineering & WSU, Pierce Co Extension Source: Pierce Co Public Works & Utilities • 8.9 acres; 35-lot residential subdivision • 48% + in open space; 150 ft. revegetated buffers • 20 ft. roadways, including sidewalks • Roadside bioswales instead of curb & gutter • Amended soils (low infiltrating glacial till) • Low Impact Foundation Technology (LIFT) on selected sites • Homes finished in 2006/07

  14. Sumner LID Pilot Project, Pierce County • 9 units on 2.7 acres • Completed in 2004 • Driveway pervious pavers; vegetated swales w/over-drains; amended soils; one sidewalk; curvilinear street design • Sumner monitoring—will implement LID ordinances if successful (open to LID now) Source: University of Puget Sound

  15. Cooper Crest, Olympia • First to build under mandatory LID ordinance • 140 units on 70 acres • 1 developer; 2 builders • 55% open space; 18-ft streets; amended yards; rain gardens; 20 ft areas to infiltrate roof run-off; some porous sidewalk • Complaints of poor soil, runoff, crawl space flooding, lawn erosion, and soil & rain gardens insufficient to handle stormwater

  16. Whidbey Island Projects • Pacific Crest Subdivision in Oak Harbor • Bioswales and narrow roads • Poor homeowner maintenance • The Highlands at Langley • No stormwater to city drains • Native vegetation; pervious pathways, bioswales; rain gardens • Under construction Photos courtesy of Stacy Smith, Whidbey Island Conservation Dist.

  17. LID & Government • EPA w/other clean water agencies signed letter of intent to formalize use of LID (April, 2007) • PSAT/PSP action agenda focuses on stormwater & LID • Ecology stormwater manual & permits • Localordinances • Seattle, Tacoma, King, Pierce, Snohomish & Clark – LID mandatory “where feasible” • Other jurisdictions must allow for LID and determine inconsistencies preventing LID

  18. State Stormwater Regulation Local ordinances must include “Provisions to allow non-structural preventive actions and source reduction approaches such as Low Impact Development Techniques (LID)…Provisions for LID should take into account site conditions, access and long term maintenance.”

  19. Local Ordinances Ordinances allow, encourage or require: • Pierce, King, Snohomish, Kitsap Counties • Bellingham, Bothell, Kirkland (clustering), Lacey, Langley, Marysville, Poulsbo, Port Angeles, Port Orchard, Redmond (manual), Sedro Woolley, Tumwater, Olympia, Seattle, SeaTac PSAT/PSP grants (ordinance changes under review): • Issaquah, Poulsbo, Woodinville, Edmonds, Normandy Park, Anacortes, Bremerton, Coupeville, Eatonville, Everett, Federal Way, Hamilton, Lake Forest Park, Lake Stevens, Mill Creek, Mukilteo, Oak Harbor, Clallam, Jefferson, Kitsap, Thurston, Whatcom, Mason, San Juan City of Seattle • 2/15/08 submitted final draft of stormwater code to DOE • Requires “green infrastructure technologies” of all single-family residential to “maximum extent possible”

  20. Built Green/LEED • NAHB model green home building guidelines (2006) • LID practices earn points toward green building designations • Preserve vegetation/trees, amend soil, limit impervious/add pervious, use drought tolerant plants, install cistern, etc. • LEED

  21. Types of LID Practices

  22. Types of LID Practices • Site planning & layout • Protecting vegetation/minimizing soil disturbance • Integrated Management Practices • Rain gardens • Amending soils • Permeable paving • Green roofs • Minimal excavation foundations • Roof rainwater collection systems

  23. Site Planning & Layout • Hydrology as part of initial site assessment and planning • Building sites, roads, and stormwater infrastructure designed around natural features and LID practices Source: WSU, Pierce County Extension (Kensington Estates)

  24. Minimize connected pervious area • Narrow streets/smaller turnarounds • Curvilinear design • Driveways (20%) – shared, porous, wheel strips, plastic grid • Sidewalks – 44-48”, one side, 2% slope to swale, porous • Fewer curbs & conveyances (that create flow) Source: Boston Metropolitan Area Planning Council (t); Puget Sound University (Sumner)

  25. Questions • Does the market want this type of lot design? • Will fire departments allow for narrow roads and limited access?

  26. Protecting vegetation/minimizing soil disturbance • LID Manual endorses: • 65% native cover = rural & large lots • 50% native cover = 4-6 du/acre • As much as possible = 6+ du/acre • Locate downslope • Protect tree root zones (1’ per 1” at chest) • Retain topsoil; use removed soil to amend Source: Timothy Lowry, Pierce Co Public Works & Utilities

  27. Questions • How do I keep subs, graders, and/or teenagers off the site (especially swales and gardens)? Are fences and signs really helpful? • Does less site disturbance really = less cost?

  28. Integrated Management Practices • Bioretention areas (rain gardens) • Amending soils • Permeable paving • Vegetated roofs • Minimal excavation foundations • Rainwater collection systems Source: Seattle Public Utilities

  29. Bioretention Areas (Rain Gardens) • Shallow landscaped depressions with designed soil mix & plants • Hold & treat water • Cells v. swales Source: (l) Seattle Public Utilities; (r) Olympia medical plaza

  30. Rain Garden Design • Flow entrance • Pre-treatment filter strip • Plants • Drought/saturation tolerant • No watering req’d after establishment • Soil zone • Loamy sand w/ compost amendment • 18” min soil depth • Mulch (not req’d) • Compost base/shredded hard or softwood elsewhere • Not grass clippings or bark • Filter fabric • Underdrain (if necessary) • Can be connected to another swale, cell or storm drain • 6” slotted, thick-walled plastic pipe • Max ponding depth 12” & 24 hr drawdown Source: MPCA Stormwater Manual, 2005

  31. Also… • Keep equipment out of rain gardens during construction! • General landscape maintenance – replace mulch every two years on residential lots. Source: North Carolina Office of Environmental Education

  32. Questions • Who is responsible for maintaining the garden(s)? Homeowner? Public works? Builder? Developer? • Does the rain garden have to be designed to handle construction run-off? • Is the rain garden finished before or after home construction (does the builder or developer finish the bioretention areas)?

  33. Amending Soils • Stripped, compacted = impervious • Compost amended soils ↑ retention and ↓ irrigation needs Glacial till w/high runoff, poor turf quality • Ways to enhance/amend: • Avoid • Amend existing disturbed soils • Stockpile on-site & replace (test and amend w/ organic to 8” depth) • Import topsoil Same soil with 30% compost Source: Soils for Salmon, UW Soil Study

  34. Amended Soil Design • Landscaped areas = 10% organic content • Turf areas = 5% organic content • 2:1 ratio of loose soil to loose compost landscaped (4:1 turf areas) • Depth of 12” (8” amended & 4” scarified) • Mulch planting beds w/ 2 – 3” (max) organic material • Adds .36¢/sq ft (1997 City of Redmond study) Source: www.BuildingSoil.org

  35. Questions • What is the most cost efficient way to amend soil (amend existing, stockpile, or import)? • Will amending soils change traditional spec landscaping (using evergreens, rhodies, etc.)?

  36. Permeable Paving • The good news: initial research shows that properly designed and maintained permeable pavement can almost eliminate surface flows for traditional Puget Sound rain events. • The bad news: technical design standards, few credits, requires precision installation and long term maintenance.

  37. Permeable Paving Cross Section • Surface layer • Aggregate base (two sizes of stone) • Geotext fabric (?) • Treatment layer (sand or amended soil) uncompacted • Underdrain (if necessary) Source: A. Richard Miller

  38. Permeable Asphalt & Concrete • Less sand + admixes = 14-18% void • 100-200+ in water/hr • Access roads, driveways, parking lots • Asphalt cost neutral but few local projects & contractors • Concrete 3X more expensive; placing & finishing almost same Source: TecEco Porecocrete, Australia

  39. AsphaltSedro-Woolley Photos courtesy of Pete Haase

  40. Concrete • StoneyCrete – TX (1/4 – 3/8”) • PercoCrete – Kenmore (sandy) • Do not overwork mix, over-handle during finish, and allow 7 covered cure days • Bunyan Power Screed • Use National Ready Mix Concrete Association (NRMCA) certified pervious concrete placer • $3 – 5 sq ft for surface material & install; base aggregate and maintenance add’l Source: Chris Webb & Associates, Bellingham

  41. Top: Stratford Place, 20-lot LID development in Sultan (2002) Pervious Concrete, Inc., Snohomish WAwww.perviouscrete.com Bottom: PercoCrete installation on 15’ road Sammamish www.percocrete.com

  42. Interlocking Concrete Pavers • Cast-in-place or pre-cast • SF-RIMA, Turfstone, or EcoStone • Residential access roads, driveways, walkways • Subgrade compaction (for stability) makes for impermeable soils (no flow credits) unless high sand & gravel content • $2.50 - $4.50/sq ft for pavers & install; base aggregate separate Source: (t) Kitsap HBA – SF-RIMA, (m) Mutual Materials - EcoStone, (b) MM - Turfstone

  43. Plastic Grid System • Lightweight grid of plastic rings; geotext fabric fused to bottom • GravelPave 2 or GrassPave2 • Alleys, driveways, utility access, overflow parking Source: Invisible Structures, Inc.

  44. Keep in Mind… • Should not receive water from adjacent areas (during and after construction) • Maintenance required 1 – 2X annually (vacuuming, replacement) • Homeowners can’t seal driveways or walks • Not allowed for slopes exceeding 5 – 10%

  45. Questions • Which method is best for residential applications? • Who provides maintenance (vacuuming) services in the Puget Sound area? • Will local governments allow pervious asphalt roads in subdivisions?

  46. Green Roofs • Extensive v. intensive • $10 – 15 sq ft installed (w/o plants) • 12 – 15 lbs/sq ft w/ 4” soil mix Source: Going Green at the Beach

  47. Minimal Excavation Foundations • Driven piles and connection component at or above grade • Rick Gagliano has patent on pre-cast w/ driven piles; homes on Gig Harbor and Bainbridge Source: PinFoundations, Gig Harbor

  48. Roof Rainwater Collection • Rainwater for non-potable uses (landscaping, toilets) • Ecology & water rights? Sources: (l) Phinney Neighborhood Assoc, (r) Construction Resources, (ur) City of Portland

  49. Flow Credits • Projects have to provide flow control (per muni permits) • Discharge = pre-developed forested discharge • Manual Vol I, Table 2.2 shows whether you need a pond • LID may get you under impervious threshold that triggers flow control req’mt OR you can reduce facility size

  50. Chapter 7 of LID Technical Manual Flow Credits

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