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Highway Drainage-Hydrology

Ch. 8

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Objectives

Know what design frequency means

Know how to delineate a drainage area ona topographic map

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Small Drainage Structures

Bridges: >=20 feet in span

Small Drainage Structures < 20 ft

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Roadside Surface Drainage System

Cross drainage structures

Roadside Surface Drainage System:

Gutters

DitchesClosed roadway system

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Legal Aspects-Protect:

The highway from rainfall and runoff

Adjacent land from discharge of artificially

collected and concentrated flow fromhighway channels

Floodplains

Water quality and natural resources

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Guidance for draining surface

waters: Keep existing drainage patterns (dont divert) Return drainage patterns to existing conditions

Establish natural patterns w/in ROW Take d/s PEs to the pt where velocitys have

returned to their natural state Take u/s PEs to provide storage for headwater

Improve d/s structures or create u/s storageareas Improve existing structures which become

inadequate by loss of u/s storage areas

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State & Fed Laws/Regs

Wetlands

Wild, Scenic & Recreational Rivers

Coastal Zone

Floodplains

Water quality Endangered Species

Fish & Wildlife

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Hydrology

Occurrence, distribution, and

movement of water

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Design Storm

Probability that a design storm will occur Q10 would occur theoretically once every 10

years (each year there would be a 10%probability that a storm will exceed the Q10flow).

Q50 would occur theoretically once every 50

years (each year there would be a 2%probability that a storm will exceed the Q50flow).

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Design Frequencies

Not economically feasible to design for allfloods for every structure for any type of

road

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Design Storm Frequencies

Highway class Culverts StormDrainage

Ditches

Interstates 50(check 100)

10(check 50 if sag)

25

Princ. Arterials 50(Check 100)

10(check 50 if sag)

25

Minor arterials& other

50(Check 100)

5(check 25 iif sag)

10

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Type of Project vs Extent of

Analysis Construction on new

Reconstruction on

existing 3R

Maintenance Culvert replacement

or relining

Complete analysis

Analysis for structures w/flooding history or servicelife < design life

Analysis for structures w/flooding history or inneed of replacement

None needed

Complete analysis

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Hydrologic Analysis

1. Preliminary Research

2. Field trip

3. Design Discharge (w or w/o hydrograph)

4. Calculate discharge-- methodology &frequency

5. Final field trip

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1. Preliminary Research

Obtain topo maps

Determine soils

Obtain flood insurance data Obtain existing data

Obtain aerial photos

Check field reconnaissance notes, historicalrecords, record plans

Calculate preliminary flow rates

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2. Initial Field Trip

a. Drainage patterns and areas

b. Land Use

c. Soil Types

d. Existing & previous floods

e.

Location of detention structures

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a. Drainage Patterns

Check that patterns coincide w/ what youhave determined preliminarily

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b. Land Use

Wooded, Crops, Pasture

Urban, Suburban or Rural

Undeveloped or Developed

If developed (% paved or roof area, %grass)

Potential for future development

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c. Soils

Confirm soil types

Any soils subject to erosion?

Any channel banks that may need to beprotected?

Rocks, clays, silts, wetlands, fragipan?

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d. Flood conditions

Determine HWE

Interview locals

Check for deposition & scour

Check debrisvelocity

Regional DEC office

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e. Detention Features

Wetlands

Ponding areas

Reservoirs or lakes

Flood control dams

Highway embankments Culvert locations

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Drainage design

Determine the maximum (peak) runoffvolume (in cfs) for a given design

frequency-------hydrology Size structure to carry the design flow -----

---- hydraulics

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Determining peak flows

Drainage Area

Infiltration

Time of concentration

Slopes

Rainfall Intensity

Storage

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Time of Concentration

Time required for water to flow from themost distant part of a drainage area to the

drainage structures (sheet flow, channelflow).