2. 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. 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. 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. 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. 6
Drainage
• Transverse slopes
– 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
11. 11
Surface Drainage
System Design
Three phases
1. Estimate of the quantity of water to
reach the system
2. Hydraulic design of system elements
3. Comparison of different materials that
serve same purpose
12. 12
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 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. 13
Runoff Coefficient
o Coefficient that
represents the
fraction of rainfall
that becomes
runoff
o Depends on type of
surface
Iowa DOT Design Manual, Chapter 4, The Rational Method
14. 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
17. 17
Runoff Coefficient For High
Intensity Event (i.e. 100-year
storm)
Iowa DOT Design Manual, Chapter 4, The Rational Method
18. 18
Runoff Coefficient For High
Intensity Event (i.e. 100-year
storm)
Iowa DOT Design Manual, Chapter 4, The Rational Method
C = 0.16 for
low intensity
event for
cultivated
fields
C = 0.42 for
high intensity
event
19. 19
Runoff Coefficient
• When a drainage area has distinct
parts with different C values
• Use the weighted average
C = C1A1 + C2A2 + ….. + CnAn
ΣAi
20. 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
23. 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. 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. 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. 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
28. 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. 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
37. 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. 38
Can also use equation, an example is
provided in Chapter 4-4 of the Iowa
DOT manual
39. 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. 40
Area
• Area of watershed
• Defined by topography
• Use GIS contours in lab
44. 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 = 0.093 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
