06 rural drainage

8/3/2019 06 Rural Drainage

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Surface Drainage/RationalMethod



2

Transverse slope



3

Longitudinal slope



4

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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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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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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4.06 inches occurs over18 hours =

4.06 in/18 hours = 0.34in/hr



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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

06 rural drainage

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