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Surface DrainageSurface DrainageSurface DrainageSurface Drainage

CE 453 Lecture 25CE 453 Lecture 25

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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• 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

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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 phases1. Estimate of the quantity of water to

reach the system2. Hydraulic design of system elements3. 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 or runoff

to rainfallI = 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 Coefficiento 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

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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

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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)

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

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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

ΣAi

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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) >= 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)

• 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

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Grass Surface, Mannings roughness coefficient = 0.4

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First guess I = 5 in/hr

knowns

Tc=18

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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• Q = CIA• Calculate 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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180

Qdesign = 180 x 1.0 x 0.6 = 108CFS

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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