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Surface Drainage CE 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 Longitudinal slopes Longitudinal channels**

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 phases Estimate of the quantity of water to reach the system Hydraulic design of system elements 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 = CIA (metric) Q = runoff (ft3/sec) or (m3/sec) C = coefficient representing ratio or 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 that represents the fraction of rainfall that becomes runoff 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)**

C = 0.16 for low intensity event for cultivated fields C = 0.42 for high intensity event Iowa DOT Design Manual, Chapter 4, The Rational Method

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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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knowns Tc=18 First guess I = 5 in/hr

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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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Qdesign = 180 x 1.0 x 0.6 = 108CFS 180

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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 = 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

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