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Drainage Problem: Designing a Ditch for a 25-Year Flood

This exam question focuses on designing a ditch for a 25-year flood in order to manage runoff from a field. The problem includes calculations for flow depth, velocity, and freeboard, and requires the use of the Iowa Runoff Chart.

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Drainage Problem: Designing a Ditch for a 25-Year Flood

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  1. EXAM 2 REVIEW

  2. 1. Drainage problem (25 pts) • Below you see a cross-section of a ditch. It runs parallel to a 200-acre field consisting of permanent pasture and rolling terrain. Assume that all of the runoff goes to the ditch. The slope of the ditch is .02 feet per foot and the Manning’s number is .03. Use a trapezoidal channel with a five foot base and a 2H:1V side slope and the attached Iowa Runoff Chart. Assume you need 2 feet of freeboard (vertical) in the ditch to avoid inundating the base material. Show your solution lines on the graphs. • How deep should you design the ditch if the design storm is a 25 year flood?

  3. Qdesign = Q * FF * LF • Qdesign = 430 * 0.4 * 0.8 = 137.6

  4. Therefore the flow depth is about 1.8 feet. • Adding two feet of freeboard gives us a total depth of about 3.8 feet. • The velocity is about 9 fps, so unless the channel is paved erosion is likely to be a problem.

  5. 2. Vertical and Horizontal Alignment problem (25 pts) • A two-lane rural road (12-foot paved lanes, 2% normal crown section, ignore shoulder) has a design speed of 60 mph and a maximum (full) superelevation rate of 6 percent. There is a horizontal curve in the road in the vicinity of a vertical curve. Assume a spiral curve is used to attain superelevation. What is the elevation of the outside edge of the pavement at the station half way along the spiral? • Station SC (spiral to curve) = Station VPI • VPI elevation = 700.00 feet • g1 = +2 g2 = -3

  6. Remembering that the normal crown is taken out in the tangent runoff, we know that the cross-slope transitions from 0% to full super uniformly in the spiral.

  7. Using the maximum slope ratio from the table we calculate the length of the spiral: • ls = 6% * 12’ * 222 = 159.84 say 160’

  8. We now calculate the parameters for the vertical curve: A = g2 – g1 = -3 – 2 = -5 L = KA = 151 * 5 = 755 (say 760) r = A/100L = 0.0000658 e = AL/800 = 4.75

  9. Assume a station for the VPI of 100+00 (= sta SC) Elev VPC = elev VPI – 0.02 (L/2) = 700 – 7.6 = 692.40 Sta VPC = 100+00 – 380 = 96+20

  10. Elev VPI (curve) = 700- e = 695.25 • Sta x = sta SC - ls/2 = 100+00 – 80 = 99+20

  11. 4. Answer the following questions (each is worth 3 points). • Discuss the tradeoffs (safety, cost, or whatever) between flat 6:1 and maximum 4:1 median cross slope. • ROW greater for 6:1, so cost is higher • 6:1 is safer than 4:1

  12. 4. Answer the following questions (each is worth 3 points). • Name the three median types. Depressed, flush, raised • In general, slopes above what ratio require a barrier? • 1:3 • Describe the traffic conditions that you would expect at a Level-of-Service F. Break down, no ability to maneuver, high workload

  13. The coefficient “C” in the Rational Method equation to calculate discharge represents what ratio? Runoff/rainfall • Which is a better way to deal with an object in the clear zone, removal or a guardrail? Why? Removal. Guardrail is an obstacle. • The “I” in the Rational Method equation is a function of what two factors? • Sag and crest vertical curves have maximum lengths for drainage function. Is this more likely to be true in rural or urban areas? Why?

  14. The “I” in the Rational Method equation is a function of what two factors? Time of concentration and return period • Sag and crest vertical curves have maximum lengths for drainage function. Is this more likely to be true in rural or urban areas? Why? Urban. In rural areas cross-slope carries the water off the pavement, curbs in urban areas prevent this.

  15. Name three of the five possible actions to deal with an object in the clear zone. Remove, relocate, reduce severity, redirect, delineate

  16. At what level of service would you expect to see breakdown? F

  17. There are at least four characteristics that should be considered in the design (i.e., sizing, shape, etc.) of a drainage channel (for example - ease of maintenance). Name three others. Capacity, minimum hazard to traffic, hydraulic efficiency

  18. “C” is a function of several factors and/or surface characteristics. Name two. slope, material.

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