Vegetated Swales in Residential Developments

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Vegetated Swales in Residential Developments

• Term Project Presentation
• CE 394K.2 Hydrology
• Presented by Chelsea Cohen
• Thursday, April 24, 2008

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What is a Vegetated Swale?

• Low Impact Development (LID) Technique
• Mimic natural landscape
• Open, shallow channels
• Slow Runoff
• Lower Velocities
• Promote Infiltration
• Filter Runoff
• Trapezoidal or Parabolic Shape
• Long, narrow, vegetated channels

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Objectives

• Model 25-yr and 100-yr design storms
• Flood Control
• Decentralized vs. Centralized System
• Vegetated Swale vs. Detention Pond
• Minimize Downstream Flow Off-Site
• Conveyance
• Contrast to Traditional Curb and Gutter
• Aesthetic Appeal in Community

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Design Criteria

• Seattle Street Edge Alternatives (SEA)
• Pilot project along residential street
• Wisconsin DOT Technical Standard
• California Storm Water BMP Handbook
• San Francisco Drainage Calculations
• City of Austin Drainage Criteria Manual

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Precipitation

• Synthetic Storm
• SCS Type III
• Rainfall Distribution
• 25-yr, 24-hr, 7.63in
• 100-yr, 24-hr, 10.18in

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Subcatchment Properties

• Six ¼ -acre lots in series
• 2 Slope
• Infiltration SCS Runoff Curve Number
• Assumed Type C Soil for moderately fine to fine
  soils
• Pre-development CN74
• Open space, Good Condition, Grass cover 75
• Post-development CN83
• Residential, ¼ acre, 38 impervious

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Vegetated Swale Geometry

• Longitudinal Slope Range 0.5-4
• 1 Vegetated Swale Slope
• Side Slope 3H1V or flatter
• Swale Bottom 2 to 8-ft
• 5-ft Bottom
• 25-ft Width
• 116-ft Length
• Dense Grass Cover
• n0.25

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3H1V
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Culvert Geometry

• Connect vegetated swales with culverts
• Sizes 6 and 10 PVC pipes
• 0.5 Pipe Slope

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Model Site Layout

• Safety Barrier between pedestrians and motorists

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SWMM Modeling Software

• EPAs Storm Water Management Model (SWMM)
• Rainfall-Runoff simulation model used for single
  event or long-term (continuous)
• Simulation of Runoff quantity and quality from
  primarily urban areas
• Iterative Process

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Animation 25-yr Model
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Animation 100-yr Model
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Compare Models

• Design Iteration between Swale Size, Pipe Size
  and Storm Capacity

25-yr
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Results

• Infiltration 13-17

• Post-Development Flow less than Pre-Development
  Flow
• Provide Detention Storage in Swales
• No increase in peak flow as a result of
  development

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Results (cont.)

• Low swale velocity to prevent scour and promote
  infiltration

• Swales do NOT over top for 25-yr or 100-yr storms

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Aesthetic Appeal

• Design the ROW to be beautiful as well as useful
• Neighborhood Enjoyment and Care
• Common destination for neighborhood residents
• Add native vegetation
• Seattle Street Edge Alternative (SEA) Project
• Reduced total volume of storm water flow leaving
  the street by 99

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Future Work

• Water Quality Improvement
• Cost Benefit
• Compare to Curb and Gutter System
• Application to Entire Neighborhoods

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Conclusion

• Management of Rainfall
• Filter and Slow the Flow of Storm Water
• Increase safety if convey 100-yr storm in swale
  and not roadway
• Cost Effective and Aesthetically Pleasing
• Responsible Design
• Environmental Benefits
• Methods to minimize developmental effects
• Attempt to mimic natural landscape prior to
  development

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Questions?