Vegetated Swales in Residential Developments

1. Term Project Presentation CE 394K.2 Hydrology Presented by Chelsea Cohen Thursday, April 24, 2008 Vegetated Swales in Residential Developments

2. 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

3. 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

4. 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

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

6. 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: CN=74 • Open space, Good Condition, Grass cover >75% • Post-development: CN=83 • Residential, ¼ acre, 38% impervious

7. Vegetated Swale Geometry • Longitudinal Slope Range: 0.5%-4% • 1% Vegetated Swale Slope • Side Slope 3H:1V or flatter • Swale Bottom 2 to 8-ft • 5-ft Bottom • 25-ft Width • 116-ft Length • Dense Grass Cover • n=0.25 25’ 3H:1V 5’

8. Culvert Geometry • Connect vegetated swales with culverts • Sizes: 6” and 10” PVC pipes • 0.5% Pipe Slope

9. Model Site Layout • Safety Barrier between pedestrians and motorists

10. SWMM Modeling Software • EPA’s 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

11. Animation: 25-yr Model

12. Animation: 100-yr Model

13. Compare Models • Design Iteration between Swale Size, Pipe Size and Storm Capacity 25-yr 100-yr

14. 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

15. Results (cont.) • Low swale velocity to prevent scour and promote infiltration • Swales do NOT over top for 25-yr or 100-yr storms

16. 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%

17. Future Work • Water Quality Improvement • Cost Benefit • Compare to Curb and Gutter System • Application to Entire Neighborhoods

18. 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

19. Questions?