REG 265 Surface DrainageSurface Drainage

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Objectives Identify rural drainage requirements and

design

Ref: AASHTO Highway Drainage Guidelines (1999), Guidelines for Road Drainage Design (Design Floods & Culvert Design – 2004))

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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 phases1. Estimate 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 of runoff to rainfallI = intensity of rainfall (in/hour or mm/hour)A = drainage area (acres or hectares)

The Rational Method

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Runoff Coefficient

o Coefficient that represents the fraction of rainfall that becomes runoff

o Depends on type of surface

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

The Rational Method

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Runoff Coefficient - urban

The Rational Method

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Runoff Coefficient For High Intensity Event (i.e. 100-

year storm)

The Rational Method

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Runoff Coefficient For High Intensity Event (i.e. 100-

year storm)

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 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

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)

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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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, this 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 (some sources 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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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