1. Surface Drainage
CE 453 Lecture 25

2. Objectives
Identify rural drainage requirements and design
Ref: AASHTO Highway Drainage Guidelines (1999), Iowa DOT Design Manual Chapter 4 and Model Drainage Manual (2005)

3. Surface Drainage
A means by which surface water is removed from pavement and ROW
Redirects water into appropriately designed channels
Eventually discharges into natural water systems
Garber & Hoel, 2002

4. Surface Drainage Two types of water Surface water – rain and snow
Ground water – can be a problem when a water table is near surface
Garber & Hoel, 2002

5. Inadequate Drainage Damage to highway structures Loss of capacity
Visibility problems with spray and loss of retroreflectivity
Safety problems, reduced friction and hydroplaning
Garber & Hoel, 2002

6. Drainage Transverse slopes Longitudinal slopes Longitudinal channels
Removes water from pavement surface
Facilitated by cross-section elements (cross-slope, shoulder slope)
Longitudinal slopes
Minimum gradient of alignment to maintain adequate slope in longitudinal channels
Longitudinal channels
Ditches along side of road to collect surface water after run-off

7. Transverse slope

8. Longitudinal slope

9. Longitudinal channel

10. Surface Drainage System Design
Tradeoffs: Steep slopes provide good hydraulic capacity and lower ROW costs, but reduce safety and increase erosion and maintenance costs

11. Surface Drainage System Design
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

12. Hydrologic Analysis: Rational Method
Useful for small, usually urban, watersheds
(<10acres, but DOT says <200acres)
Q = CIA (english) or Q = CIA (metric)
Q = runoff (ft3/sec) or (m3/sec)
C = coefficient representing ratio or 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

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

14. Runoff Coefficient depends on:
Character of soil
Shape of drainage area
Antecedent moisture conditions
Slope of watershed
Amount of impervious soil
Land use
Duration
Intensity

15. Runoff Coefficient - rural
Iowa DOT Design Manual, Chapter 4, The Rational Method

16. Runoff Coefficient - urban
Iowa DOT Design Manual, Chapter 4, The Rational Method

17. Runoff Coefficient For High Intensity Event (i.e. 100-year storm)
Iowa DOT Design Manual, Chapter 4, The Rational Method

18. Runoff Coefficient For High Intensity Event (i.e. 100-year storm)
C = 0.16 for low intensity event for cultivated fields
C = 0.42 for high intensity event
Iowa DOT Design Manual, Chapter 4, The Rational Method

19. Runoff Coefficient
When a drainage area has distinct parts with different C values
Use the weighted average
C = C1A1 + C2A2 + ….. + CnAn
ΣAi

20. Watershed Area For DOT method measured in hectares
Combined area of all surfaces that drain to a given intake or culvert inlet
Determine boundaries of area that drain to same location
i.e high points mark boundary
Natural or human-made barriers

21. Watershed Area Topographic maps Aerial photos Digital elevation models
Drainage maps
Field reviews

23. Intensity
Average intensity for a selected frequency and duration over drainage area for duration of storm
Based on “design” event (i.e. 50-year storm)
Overdesign is costly
Underdesign may be inadequate
Duration is important
Based on values of Tc and T
Tc = time of concentration
T = recurrence interval or design frequency

24. Design Event Recurrence Interval
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

25. Time of Concentration (tc)
Time for water to flow from hydraulically most distant point on the watershed to the point of interest
Rational method assumes peak run-off rate occurs when rainfall intensity (I) lasts (duration) >= Tc
Used as storm duration
Iowa DOT says don’t use Tc<5 minutes

26. 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

27. Tc: Equation from Iowa DOT Manual
See nomograph, next page

28. Nomograph Method Trial and error method:
Known: surface, size (length), slope
Look up “n”
Estimate I (intensity)
Determine Tc
Check I and Tc against values in Table 5 (Iowa DOT, Chapter 4)
Repeat until Tc (table) ~ Tc (nomograph)
Peak storm event occurs when duration at least = Tc

29. Example (Iowa DOT Method)
Iterative finding I and Tc
L = 150 feet
Average slope, S = 0.02 (2%)
Grass
Recurrence interval, T = 10 years
Location: Keokuk
Find I
From Iowa DOT Design Manual

30. Grass Surface, Mannings roughness coefficient = 0.4

31. knowns
Tc=18
First guess I = 5 in/hr

32. Example (continued) Tc with first iteration is 18 min
Check against tables in DOT manual
Keokuk is in SE: code = 9

33. Convert intensity to inches/hour …

34. For intensity of 5 inch/hr, Duration is 15 min
Tc from nomograph was 18 min ≠ 15 min
Tc ≠ Duration
Next iteration, try intensity = 4.0 inch/hr

35. Slope = 0.02
I = 4.0 inches/hr
Tc = 20 min
For second iteration, tc = 20 min

36. Example (continued)
I = 4.0 inches/hour is somewhere between 30 min and 15 min,
Interpolate … OK!

37. What does this mean?
It means that for a ten-year storm, the greatest intensity to be expected for a storm lasting at least the Tc (18 min.) is 4.0 inches per hour …
that is the design intensity

38. Can also use equation, an example is provided in Chapter 4-4 of the Iowa DOT manual

39. Rational method used for mostly urban applications
limited to about 10 acres in size
Q = CIA
Calculate once C, I, and A have been found

40. Area
Area of watershed
Defined by topography
Use GIS contours in lab

42. Lab-type Example 60-acre watershed 50-year storm Mixed cover
Rolling terrain

43. Qdesign = 180 x 1.0 x 0.6 = 108CFS
180

44. What would the flow have been had we used the rational method?
Q=CIA
Say, c = 0.2 (slightly pervious soils)
I=? Assume round watershed of 60 acres = 60/640 = sq mi … L=D≈1800’ , assume slope=4% (rolling?) … Tc for I=6in/h = 41 min vs. 60 min … I=4.8in/h = 45 min vs. 30 min … call it 5.5in/h
A=60 … Q=.2×5.5×60 = 66 CFS vs. 108 cfs