Surface Drainage

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Surface Drainage

• CE 453 Lecture 24

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

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Surface Drainage

• 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

• 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
• 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

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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
• Estimate of the 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
• (lt10acres, but DOT says lt200acres)
• 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

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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
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Runoff Coefficient depends on

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

• When a drainage area has distinct parts with
  different C values
• Use the weighted average
• C C1A1 C2A2 .. CnAn
• SAi

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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

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Watershed Area

• Topographic maps
• Aerial photos
• Digital elevation models
• Drainage maps
• Field reviews

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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

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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

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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) gt
  Tc
• Used as storm duration
• Iowa DOT says dont use Tclt5 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)

• 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

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Grass Surface, Mannings roughness coefficient
0.4
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knowns
Tc18
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
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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, an example is provided in
Chapter 4-4 of the Iowa DOT manual
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Rational Method

• used for mostly urban applications
• limited to about 10 acres in size (Garber and
  Hoel suggest 200-acre limit)
• Q CIA
• Calculate Q once C, I, and A have been found

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Area

• Area of watershed
• Defined by topography
• Use GIS contours in lab

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Lab-type Example

• 60-acre watershed
• 50-year storm
• Mixed cover
• Rolling terrain

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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?

• QCIA
• Say, c 0.2 (slightly pervious soils)
• I? Assume round watershed of 60 acres 60/640
  0.093 sq mi LD1800 , assume slope4
  (rolling?) Tc for I6in/h 41 min vs. 60 min
  I4.8in/h 45 min vs. 30 min call it 5.5in/h
• A60 Q.25.560 66 CFS vs. 108 cfs