at bridge and culvert hydraulics guide - alberta overview •hydraulic modelling approach •open...

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AT Bridge and CulvertHydraulics Guide

Alberta Transportation, 2011

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Overview

•Hydraulic Modelling Approach

•Open Channel Flow

•Bridge Constriction

•Culvert Hydraulics

•Fish Passage – Culverts

•Other Factors - Ice, Drift, Scour

•Reference Documents

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Hydraulic ModellingApproach

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Hydraulic Modelling Approach

•Recommended Modelling Approach

•Section averaged (1D), based on typical channel section

•Neglect overbank d/s flow component

•Account for GVF, RVF where appropriate

•Roughness, Slope – use HDG approach

•Results – HW EL (freeboard), V (rock sizing)

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•Accuracy

•Don’t Confuse with Precision

•Limited by geometry, hydraulics (n, K), other (drift, ice, sediment)

•+/- 20% acceptable for Y, V (confidence in parameters)

•Consider sensitivity of design

•Round Y to 10% (min 0.1m)

•Round V to 10% (min 0.1m/s, 0.01m/s for fish passage)

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•Why not multi-section (HEC-RAS) or 2D?

•Boundary conditions – only 1D estimate anyway

•Mobile boundary – bedforms, scour, lateral erosion…

•Complex factors – drift, ice, sediment transport

•No ability to calibrate complex models

•Detailed output interpretation – lose impact

•No need for additional detail - accurate or not

•Unnecessary level of effort, resources ($)

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•Why neglect overbank d/s flow component?

•Small percentage (<10%) of channel flow

•Relatively shallow Y

•Low V (high relative roughness)

•Small downstream component in floodplain

•No defined, continuous channel in floodplain

•Natural obstructions – trees, topography variation

•Man-made obstructions – roads, development

•Backwater from channel – cuts across floodplain

•Most flow - lateral interaction with channel

•Consistent with flood observations

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Open Channel Flow

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Open Channel Flow

•Need Design Y,V,Q for channel

•Natural channel - no structure present

•Will form boundary conditions for structure hydraulics

•Defined by application of HDG

•Channel capacity, Historic HW, Runoff Potential

•Consistent with other sites on channel (HIS)

•Hydraulic Parameters

•Typical Channel (B, h, T, S, roughness?)

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Boundary Conditions – Typ. Channel•Equivalent Trapezoid shape

•B – Bed Width; h – Bank Height (rapid increase in surface width for Y > h); T – Top Width

B

h

T

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Typical :• Evaluate at many sections over nearby channel• Focus on relatively straight reaches• Avoid areas influenced by past construction• B, T – airphotos, survey, DEM• h – survey, DEM, site measurements, scale from photos• Many values published in HIS

Boundary Conditions – Typ. Channel

*Images from Google Earth

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Open Channel Flow – Slope•Rise / Run along channel

•Determine from DTM (HIS Tool)

•“Rise” must be clear (larger than bed irregularities)

•Typically requires longer “Run” than is practical to survey

•Channel survey expensive, awkward

•Structure may have influenced profile within survey

•Sites with slope break near crossing:

•Confirm based on channel changes e.g. planform

•HDG – focus on u/s channel (flow delivery)

•Hydraulics – focus on d/s channel (backwater effect)

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Stream Profile For - FISH CREEK

7352

3

7371

3

6885

1314

1000

1100

1200

1300

1400

1500

1600

10000 15000 20000 25000 30000 35000 40000 45000 50000

Station (m)

Elev

atio

n (m

)

Slope Of Line = 0.003

Open Channel Flow – Slope

Stream Profile for BF73713 on Fish Creek

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Boundary Conditions – Roughness

•B < 10m – Manning ‘n’ (per HDG, built into Channel Capacity Calculator tool)

•B >= 10m – Use AT Equation (see WWW page)

•Values consistent with observations

•Use results in consistent application across system

0.050 – 3m

0.047 – 9m0.0454 – 6m

‘n’B- 0.005< 0.0005 (B > 8m)

+ 0.010> 0.015+ 0.0050.005 – 0.015

‘n’ adjS

V = 14 R0.67 S0.4

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Open Channel Flow – Type

•No Downstream (D/S) Hydraulic Influence

•Normal Flow (Sf = So)

•Tool – “Channel Capacity Calculator”

•D/S Hydraulic Influence

•Structure – e.g. weir, bridge, culvert, dam

•Channel change – slope, width

•Gradually Varied Flow (GVF) profile to crossing site

•Tool – “Flow Profile”

•U/S Hydraulic Influence – rare (steep, short impact)


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Boundary Conditions – Normal Flow

Blue – User Input ValuesGreen – Recommend n Value from AT HDGsRed – Calculated Results

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Boundary Conditions – Rating Curve(Channel Capacity Calculator)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 10 20 30 40 50 60 70 80

Q (cms)

Y (m

)

Rating Curve

Bank Height

Channel Capacity

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Open Channel Flow – GVF(Flow Profile – Backwater Curve from D/S Constriction)

98

99

100

101

102

103

104

105

020406080100120140160180200

Energy GradelineWater Surface EL.Normal DepthCritical DepthTop of BankBed

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Bridge Constriction

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Bridge Constriction•Bridge Size Optimization

•Starting Point – match typical channel

•Evaluate range of options – shorter and longer

•Constriction

•Bridge provides less flow area than typ. channel

•Shorter bridge but more protection works

•Will result in higher V (poss. larger rock)

•Will result in increased headloss (freeboard, u/s flooding)

•No constriction

•Bridge matches or exceeds flow area of typ. Channel

•No need for hydraulic modelling – use BC values

•Don’t exceed natural channel – lateral stability issues

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Bridge Constriction•Bridge Constriction Hydraulics – 3 sources of headloss

•Flow expansion at d/s side (RVF)•Higher V through constricted section•Flow constriction at u/s side (RVF)

Headloss = Ke + SfL + Kc(V2

2 – V12)

2g

(V22 – V1

2)

2g

Ke = Expansion Loss Coefficient (default = 0.5)Kc = Contraction Loss Coefficient (default = 0.3)V2 = Mean Velocity through Constriction (m/s)V1 = Mean Velocity through Channel (m/s)Sf = Friction slope (energy gradient) through constrictionL = Length of Constrictiong = acceleration due to gravity (m/s2)n = Manning Roughness coefficient

Sf = n2 V2 or Sf = V5/2

R4/3 733R5/3;

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Bridge Constriction - Calculations

•Calculation process (subcritical flow):

•Start with Boundary Condition D/S

•RVF for flow expansion

•GVF for constricted flow

•RVF for flow constriction

•GVF in U/S Channel

•Supercritical and/or combined profiles possible


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Bridge Constriction - Input

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Bridge Constriction - Input

99.5

100.0

100.5

101.0

101.5

102.0

102.5

103.0

103.5

-20 -15 -10 -5 0 5 10 15 20

Channel

Bridge

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Bridge Constriction - Output

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Bridge Constriction - Profile

100

100

101

101

102

102

103

103

104

104

105

020406080100120140160180200

Energy GradelineWater Surface EL.Normal DepthCritical DepthTop of BankBed

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Bridge Constriction - Sensitivity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.4 0.5 0.6 0.7 0.8 0.9 1

Bridge Bed Width Ratio

Dep

th In

crea

se (m

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

V R

atio

Depth Increase

V Ratio

* This plot is specific to the current scenario

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Culvert Hydraulics

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Culvert Hydraulics•Culvert Size Optimization

•Starting Point – Rise = burial + Y + headloss

•Evaluate range of sizes, shapes, barrels, profiles

•“Culvert Sizing Considerations” (AT webpage)

•Practical sizing – drift/ice, future lining (high fills, traffic vols)

•Hydraulics

•Always RVF (inlet, outlet) due to different shape

•Always GVF (burial provides tailwater)

•More profile type possibilities – hydraulic jumps, full flow

•Fish Passage evaluation - roughness

•AT Tools – “Flow Profile” (main), “HydroCulv” (multiple culverts)

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Culvert Hydraulics – Sizing Criteria•Upstream Flooding Impacts

•Fish Passage

•Drift

•Icing

•End Protection Works

•Uplift Failure

•Embankment Stability

•Road Overtopping

•Blockage

•Future Rehabilitation

•Others… site specific

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Culvert Hydraulics - Tool Comparison

1Up to 5No. Barrels

ManualYes – Q,DSensitivity

Full ContextTW OnlyChannel

MultipleOne SlopeProfile

K * (V2-V1)2/2gK * V2/2gRVF

2009~ 1991First Year

Flow ProfileHydroCulv

**

** Very conservative (punitive) for well sized culvert

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Culvert Hydraulics – Flow Profile: Input

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Culvert Hydraulics – Flow Profile: OutputResults Summary

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Culvert Hydraulics – OutputDetailed Results

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Culvert Hydraulics – Flow Profile: Output

98.0

99.0

100.0

101.0

102.0

103.0

104.0

105.0

106.0

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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Sensitivity Analysis to Determine Culvert Sizing-Try round pipe, avoid “Full Flow”

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98.0

99.0

100.0

101.0

102.0

103.0

104.0

105.0

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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Sensitivity Analysis, Continued.. -Try elliptical pipe, avoid “Full Flow”

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98.0

99.0

100.0

101.0

102.0

103.0

104.0

105.0

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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Culvert Hydraulics – Flow Profle: Input(Multi-sloped Culvert)

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Culvert Hydraulics – Flow Profile: Output (Multi-slope)

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Culvert Hydraulics – Flow Profile: Output (Multi-slope)

98.0

99.0

100.0

101.0

102.0

103.0

104.0

105.0

106.0

050100150200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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Fish Passage - Culverts

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Fish Passage - Culverts•Principle

•Make culvert NOT a velocity barrier to fish

•Compare to typical natural channel hydraulics

•Use V (section average) as indicator

•Design Flow Parameters

•Evaluate at Q > normal, Q < flood

•Evaluate over range - sensitivity

•Calc Q in channel at typically Y = 0.5 to 1.0m (less than bank height)

•Hydraulics

•Burial results in increased flow area, decreased velocity

•GVF in barrel (lose burial TW with length – backwater effect)

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46


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1.041.267.41.21.00

0.760.974.51.00.75

0.490.672.30.80.50

VoutletVinletQVchannelY

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98.0

99.0

100.0

101.0

102.0

103.0

104.0

105.0

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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Fish Passage - Culverts•Velocity Reduction Options - Effective

•Increase Pipe Roughness – long culverts, steep grades (normal flow)

•Use Multiple Pipes – wide, shallow channels; consider drift blockage

•Use Wider Shape (Box, Ellipse) – cost vs bridge

•Velocity Reduction Options – NOT Effective

•Increase Pipe Diameter - mostly air space

•Increase Burial – ineffective >1m, ponding, u/s barrier, excavation?

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98

99

100

101

102

103

104

-5 -4 -3 -2 -1 0 1 2 3 4 5

XS Station (m)

Elev

atio

n (m

)Fish Passage - Culverts

Substrate

Increase Roughness with Rocky Substrate

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Fish Passage - Culverts•Increase Roughness

•Effective – long, steep pipes (Burial TW lost, normal flow)

•Install 0.2m – 0.3m thickness rock (e.g. class 1M, 1)

•Install metal weirs at regular spacing to retain substrate

•Substrate may also act as mitigation measure (DFO)

•Estimate ‘n’

•Based on roughness height of substrate (k ~ 3.5D84)

•Equate Manning and Chezy equations

•Assume roughness applies to entire ‘P’ (low flow)

•Sensitive to flow depth (R), iterative calculations

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⎟⎠⎞

⎜⎝⎛

=

kRg

Rn12ln5.2

6/1

1.20.35Class 1

0.70.2Class 1M

k(m)D84(m)Rock

n = Manning roughness coefficientR = Hydraulic Radius (A/P)g = acceleration due to gravity (9.806m/s2)k = roughness height (m)D84 – bed particle size (m), 84% smaller

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Other Factors (Ice, Drift, Scour)

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Ice•Potential Impacts on Structure

•High Ice (Ice Jams) may govern Min. Btm. Flg.

•Ice Loads on Piers (CAN/CSA-S6-S06, Section 3.12)

•Strength (situation)

•Elevation

•Thickness

•Icing (Aufeis) may affect culvert operation/design

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Ice – Ice Jams•High Ice (Ice Jams)

•May govern on some large rivers

•Difficult to calculate analytically

•Consider in developing opening, span configuration

•Rely on observations

•Historic (on file, dwgs)

•Site (ice scars on trees, abrasion on substructure)

•Consider u/s and d/s sites, similar sites in the area

•Look for Potential Ice Jam Triggers

•Change in profile

•Major tributary

•Natural or man-made constriction

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Ice – Design Pier Loads•Consider sensitivity of structure to design loading

•Base design on observations

•Review past designs on stream

•Review historic records, site observations

•Ice scars on trees

•pier nose abrasion, broken piles

•U/S winter ice cover

•Timing of annual breakup

•If little data, consider ‘typical’ values (next page)

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Ice – Design Pier Loads•Typical values (based on common past practice):

Sit. ‘c’EL – observ.t ~ 1.0m

Sit. ‘b’EL ~ 0.6 * Yt ~ 0.8m

Major

Sit. ‘b’EL ~ 0.6 * Yt ~ 0.8m

Sit. ‘a’EL ~ 0.8 * Yt ~ 0.6m

Minor

Large Stream (B > 50m)

Small Stream (B < 50m)

Damage History

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Ice - Icing•Icing (Aufeis)

•Opening partially blocked by solid ice

•Water freezing in place (u/s spring, culvert - burial)

•Capacity not there during spring runoff

•Prediction – site observations, flood history, MCI

•Mitigation

•Bridge

•Raise gradeline (upsize)

•2nd culvert (higher)

•Maintenance (deicing line -$)

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Drift•Potential Impact on Structure

•Opening partially blocked, reduced capacity

•Culvert – overtopping, u/s flooding, uplift failure

•Bridge – damage, pier scour, flow deflection against banks

•Prediction

•Historic observations, MCI – flood conditions

•Tree density adjacent to stream and tributaries

•Low bank stability – provide large trees to stream

•Beaver dams

•Tree size – largest tree can start accumulation

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Drift - Mitigation•Culvert:

•Consider Bridge

•Larger Size (likely marginal impact)

•Flared inlet (maintain flow with blockage)

•Flow alignment piles

•Bridge

•Increase minimum centre Span

•Maintenance

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Scour•Lowering of streambed

•Types:

•Natural (passing of bed forms)

•Constriction (across channel, increased V)

•Bend (outside, secondary currents)

•Pier (local, obstruction to flow)

•Impact:

•Pier foundation design

•RPW design – headslope protection, launching apron

•Difficult to calculate, use practical design (long piles)

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Scour – Estimation Difficulties•Changes in flow alignment (lateral mobility)

•Passing bedforms

•Variable foundation materials

•Weathering of exposed rock

•Formation of natural armour layers

•Infilling during flood recession

•Compounding different scour types

•Time dependency

•Theoretical equations vs practical observations

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Scour - Mitigation•Use Deep Piled Foundations (BPG No. 7)

•River Protection Works (BPG No. 9)

•Protect headslopes

•Maintain flow alignment – guidebanks, spurs

•Practical design of launching apron length (~ 5*Dmx)

•Pier Scour Inspection Program (BIM - existing structures)

•Pier Scour Rehabilitation:

•RPW – control flow alignment

•Structural underpinning

•Bed armouring – can exacerbate problem

•Accelerated replacement

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Reference Documents

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Reference Documents•Hydrotechnical Design Guidelines for Stream Crossings

•Culvert Sizing Considerations

•Guide to Bridge Planning Tools

•BPG Tool Application Guide

•AT “Flow Profile” Tool documentation

•HIS Tool Overview

•Evaluation of Open Channel Flow Equations

•BPG 7 – Spread Footings

•BPG 9 – Rock Protection for Stream Related Infrastructure

•BPG 13 – Freeboard at Bridges

http://www.transportation.alberta.ca/565.htm