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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 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
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
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
Runoff Coefficient Iowa DOT Design Manual, Chapter 4, The Rational Method
Runoff Coefficient Iowa DOT Design Manual, Chapter 4, The Rational Method
Runoff Coefficient • When a drainage area has distinct parts with different coefficients… • Use weighted average C = C1A1 + C2A2 + ….. + CnAn ΣAi
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
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)
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
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
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
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)
Tc: Equation from Iowa DOT Manual (See nomograph)
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
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
Example (continued) • Tc with first iteration is 18 min • Check against tables in DOT manual
From previous chart: 6.32 inches occurs over 5 days (120 hours) = 6.32 in/120 hours = 0.05 in/hr
From previous chart: 4.06 inches occurs over 18 hours = 4.06 in/18 hours = 0.34 in/hr
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
Example (continued) I < Tc Next iteration, try intensity = 4.0 inch/hr
Slope = 0.02 I = 4.25 inches/hr Tc = 20 min For second iteration, tc = 20 min, OK!
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
Example (continued) I = 4 inches/hour is somewhere between 30 min and 15 min
Example (continued) Interpolate I at 20 min = 4.3 inches/hour Close so I = 4.25 inches/hour
Area • Area of watershed • Defined by topography • Use ArcView contours in lab
Flow • Q = CIA • Calculate once C, I, and A have been found