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Surface Drainage/RationalMethod
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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 hydrauliccapacity and lower ROW costs, but reducesafety and increase maintenance costs and
erosionThree phases
1. Estimate of the quantity of water to reach thesystem
2. 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
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 thatrepresents the
fraction of runoff torainfall
• Depends on type ofsurface
Iowa DOT Design Manual, Chapter 4, The Rational Method
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Runoff Coefficient
Iowa DOT Design Manual, Chapter 4, The Rational Met
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Runoff Coefficient
Iowa DOT Design Manual, Chapter 4, The Rational Method
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Runoff Coefficient
When a drainage area has distinctparts with different coefficients…
Use weighted average
C = C1A1 + C2A2 + ….. + CnAn
ΣAi
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Intensity
Average intensity for a selectedfrequency and duration
Based on “design” event (i.e. 50-yearstorm)
Overdesign is costly (what else?)
Underdesign may be inadequate
Duration
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Intensity
Based on values of Tc and T
Tc = time of concentration
T = recurrence interval or designfrequency
As a minimum equal to the time of
concentration, tc, (in/hr)
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Recurrence Interval (Design Event)
2-year interval -- Design of intakes andspread of water on pavement for primaryhighways and city streets
10-year interval -- Design of intakes andspread of water on pavement for freewaysand 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 fromhydraulically most distant point on the
watershed to the point of interest Assumes peak runoff occurs when I
lasts as long or longer than Tc
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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 orwhether some is channelized
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Time of Concentration (tc)
Ti = L
3600 V
where
Ti = travel time for section i in watershed(hr)
L = flow length (ft)
V = average velocity (ft/sec)
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T c: Equation from Iowa DOT Manual
(See nomograph)
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Nomograph Method
Trial and error
Estimate I
Determine Tc
Check I and Tc against values in Table5 (Iowa DOT, Chapter 4)
Repeat until I ~ Tc
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Example (Iowa DOT Method)
Iterative finding I and Tc
L = 150 feet
Average slope, S = 0.02 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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Try I = 5 in/hr
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T c = 18 min
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Example (continued)
Tc with first iteration is 18 min
Check against tables in DOT manual
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Keokuk is in SE, code = 9
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Convert intensity to inches/hour
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From previous chart:
6.32 inches occurs over5 days (120 hours) =
6.32 in/120 hours =0.05 in/hr
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From previous chart:
4.06 inches occurs over18 hours =
4.06 in/18 hours = 0.34in/hr
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From previous chart:
1.26 inches occurs over
15 min =
1.26 in/0.25 hours = 5.0
in/hr
For intensity of 5inch/hr, Duration is 15min
T c from nomograph was18 min
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Example (continued)
I < Tc
Next iteration, try intensity = 4.0 inch/hr
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Slope = 0.02
I = 4.25inches/hr
T c = 20 min
For second iteration, tc = 20 min, OK!
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Example (continued)
I < Tc
Next iteration,
try intensity =4.25 inch/hr
I = 4.25 inches/hour issomewhere between30 min and 15 min
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Example (continued)
I = 4 inches/hour issomewhere between30 min and 15 min
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Example (continued)
Interpolate
I at 20 min = 4.3
inches/hour
Close so I = 4.25
inches/hour
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Area
Area of watershed
Defined by topography
Use ArcView contours in lab
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Flow
Q = CIA
Calculate once C, I, and A have beenfound