REG 265
Surface Drainage
Objectives
 Identify rural drainage requirements and
design

Ref: AASHTO Highway Drainage
Guidelines (1999), Guidelines for Road
Drainage Design (Design Floods &
Culvert Design – 2004))
2
Surface Drainage
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
Surface water removed from pavement and
ROW
Redirects water into appropriately designed
channels
Eventually discharges into natural water
systems
Garber & Hoel, 2002
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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
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Inadequate Drainage

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Damage to highway structures
Loss of capacity
Visibility problems with spray and loss of
retroreflectivity
Safety problems, reduced friction and
hydroplaning
Garber & Hoel, 2002
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Drainage

Transverse slopes
–
–
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Longitudinal slopes
–
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Removes water from pavement surface
Facilitated by cross-section elements (cross-slope, shoulder
slope)
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
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Transverse slope
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Longitudinal slope
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Longitudinal channel
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Surface Drainage System Design
Tradeoffs: Steep slopes provide good hydraulic
capacity and lower ROW costs, but reduce
safety and increase erosion and maintenance
costs
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Surface Drainage System Design
Three phases
1.
2.
3.
Estimate quantity of water to reach the system
Hydraulic design of system elements
Comparison of different materials that serve same
purpose
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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)
The Rational Method
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Runoff Coefficient
o
o
Coefficient that
represents the fraction
of rainfall that becomes
runoff
Depends on type of
surface
The Rational Method
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Runoff Coefficient depends on:

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Character of surface and soil
Shape of drainage area
Antecedent moisture conditions
Slope of watershed
Amount of impervious soil
Land use
Duration
Intensity
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Runoff Coefficient - rural
The Rational Method
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Runoff Coefficient - urban
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The Rational Method
Runoff Coefficient For High Intensity Event (i.e. 100year storm)
The Rational Method
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Runoff Coefficient For High Intensity Event (i.e. 100year storm)
C = 0.16 for
low intensity
event for
cultivated
fields
C = 0.42 for
high intensity
event
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The Rational Method
Runoff Coefficient


When a drainage area has distinct parts with
different C values
Use the weighted average
C = C1A1 + C2A2 + ….. + CnAn
ΣAi
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Watershed Area


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For DOT method measured in acres
(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
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Watershed Area
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Topographic maps
Aerial photos
Digital elevation models
Drainage maps
Field reviews
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Intensity
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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
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Design Event Recurrence Interval

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
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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
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Time of Concentration (tc)

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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
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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
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Tc: Equation from Iowa DOT Manual
See nomograph, next page
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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
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Example (Iowa DOT Method)
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Iterate 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
28
Grass Surface,
Mannings
roughness
coefficient = 0.4
29
knowns
Tc=18
First guess I = 5 in/hr
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Example (continued)


Tc with first iteration is 18 min
Check against tables in DOT manual
Keokuk is in SE: code = 9
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Convert intensity to inches/hour …
32
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
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Slope = 0.02
I = 4.0
inches/hr
Tc = 20 min
For second iteration, tc = 20 min
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Example (continued)
I = 4.0 inches/hour is
somewhere between
30 min and 15 min,
Interpolate … OK!
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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
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Can also use equation, this example is
provided in Chapter 4-4 of the Iowa
DOT manual
37
Rational Method

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used for mostly urban applications
limited to about 10 acres in size (some sources suggest 200acre limit)
Q = CIA
Calculate Q once C, I, and A have been found
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Area

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Area of watershed
Defined by topography
Use GIS contours in lab
39
40
Lab-type Example
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60-acre watershed
50-year storm
Mixed cover
Rolling terrain
41
Qdesign = 180 x 1.0 x 0.6 = 108CFS
180
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What would the flow have been
had we used the rational
method?
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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
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