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Surface Drainage/Rational Method

Surface Drainage/Rational Method. Transverse slope. Longitudinal slope. Longitudinal channel. Surface Drainage System Design. Tradeoffs: Steep slopes provide good hydraulic capacity and lower ROW costs, but reduce safety and increase maintenance costs and erosion Three phases

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Surface Drainage/Rational Method

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  1. Surface Drainage/Rational Method

  2. Transverse slope

  3. Longitudinal slope

  4. Longitudinal channel

  5. Surface Drainage System Design Tradeoffs: Steep slopes provide good hydraulic capacity and lower ROW costs, but reduce safety and increase maintenance costs and erosion Three phases • Estimate of the quantity of water to reach the system • Hydraulic design of system elements • Comparison of different materials that serve same purpose

  6. Hydrologic Analysis: Rational Method Useful for small, usually urban, watersheds (<10acres, but DOT says <200acres) Q = CIA (english) or Q = 0.0028CIA (metric) Q = runoff (ft3/sec) or (m3/sec) C = coefficient representing ratio of runoff to rainfall I = intensity of rainfall (in/hour or mm/hour) A = drainage area (acres or hectares) Iowa DOT Design Manual, Chapter 4, The Rational Method

  7. Runoff Coefficient • Coefficient that represents the fraction of runoff to rainfall • Depends on type of surface Iowa DOT Design Manual, Chapter 4, The Rational Method

  8. Runoff Coefficient Iowa DOT Design Manual, Chapter 4, The Rational Method

  9. Runoff Coefficient Iowa DOT Design Manual, Chapter 4, The Rational Method

  10. Runoff Coefficient • When a drainage area has distinct parts with different coefficients… • Use weighted average C = C1A1 + C2A2 + ….. + CnAn ΣAi

  11. Intensity • Average intensity for a selected frequency and duration • Based on “design” event (i.e. 50-year storm) • Overdesign is costly (what else?) • Underdesign may be inadequate • Duration

  12. Intensity • Based on values of Tc and T • Tc = time of concentration • T = recurrence interval or design frequency • As a minimum equal to the time of concentration, tc, (in/hr)

  13. Recurrence Interval (Design Event) • 2-year interval -- Design of intakes and spread of water on pavement for primary highways and city streets • 10-year interval -- Design of intakes and spread of water on pavement for freeways and interstate highways • 50 - year -- Design of subways (underpasses) and sag vertical curves where storm sewer pipe is the only outlet • 100 – year interval -- Major storm check on all projects

  14. Time of Concentration (tc) • Time for water to flow from hydraulically most distant point on the watershed to the point of interest • Assumes peak runoff occurs when I lasts as long or longer than Tc

  15. Time of Concentration (tc) • Depends on: • Size and shape of drainage area • Type of surface • Slope of drainage area • Rainfall intensity • Whether flow is entirely overland or whether some is channelized

  16. Time of Concentration (tc) Ti = L 3600 V where Ti = travel time for section i in watershed (hr) L = flow length (ft) V = average velocity (ft/sec)

  17. Tc: Equation from Iowa DOT Manual (See nomograph)

  18. Nomograph Method • Trial and error • Estimate I • Determine Tc • Check I and Tc against values in Table 5 (Iowa DOT, Chapter 4) • Repeat until I~ Tc

  19. Example (Iowa DOT Method) • Iterative finding I and Tc • L = 150 feet • Average slope, S = 0.02 • Grass • Recurrence interval, T = 10 years • Location: Keokuk • Find I From Iowa DOT Design Manual

  20. Grass Surface, mannings roughness coefficient = 0.4

  21. Try I = 5 in/hr

  22. Tc = 18 min

  23. Example (continued) • Tc with first iteration is 18 min • Check against tables in DOT manual

  24. Keokuk is in SE, code = 9

  25. Convert intensity to inches/hour

  26. From previous chart: 6.32 inches occurs over 5 days (120 hours) = 6.32 in/120 hours = 0.05 in/hr

  27. From previous chart: 4.06 inches occurs over 18 hours = 4.06 in/18 hours = 0.34 in/hr

  28. From previous chart: 1.26 inches occurs over 15 min = 1.26 in/0.25 hours = 5.0 in/hr For intensity of 5 inch/hr, Duration is 15 min Tc from nomograph was 18 min

  29. Example (continued) I < Tc Next iteration, try intensity = 4.0 inch/hr

  30. Slope = 0.02 I = 4.25 inches/hr Tc = 20 min For second iteration, tc = 20 min, OK!

  31. Example (continued) I < Tc Next iteration, try intensity = 4.25 inch/hr I = 4.25 inches/hour is somewhere between 30 min and 15 min

  32. Example (continued) I = 4 inches/hour is somewhere between 30 min and 15 min

  33. Example (continued) Interpolate I at 20 min = 4.3 inches/hour Close so I = 4.25 inches/hour

  34. Area • Area of watershed • Defined by topography • Use ArcView contours in lab

  35. Flow • Q = CIA • Calculate once C, I, and A have been found

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