REG 265 Surface Drainage Surface Drainage
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