AT Bridge and Culvert Hydraulics Guide - Alberta Overview •Hydraulic Modelling Approach •Open...

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AT Bridge and CulvertHydraulics Guide

Alberta Transportation, 2011

Overview

•Hydraulic Modelling Approach

•Open Channel Flow

•Bridge Constriction

•Culvert Hydraulics

•Fish Passage – Culverts

•Other Factors - Ice, Drift, Scour

•Reference Documents

Hydraulic ModellingApproach

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)

•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)

•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 ($)

•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

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?)

Boundary Conditions – Typ. Channel•Equivalent Trapezoid shape

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

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

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)

Stream Profile For - FISH CREEK

10000 15000 20000 25000 30000 35000 40000 45000 50000

Station (m)

Slope Of Line = 0.003

Open Channel Flow – Slope

Stream Profile for BF73713 on Fish Creek

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

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)

Boundary Conditions – Normal Flow

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

Boundary Conditions – Rating Curve(Channel Capacity Calculator)

0 10 20 30 40 50 60 70 80

Q (cms)

Rating Curve

Bank Height

Channel Capacity

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

020406080100120140160180200

Energy GradelineWater Surface EL.Normal DepthCritical DepthTop of BankBed

Bridge Constriction

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

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)

(V22 – V1

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;

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

Bridge Constriction - Input

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

Channel

Bridge

Bridge Constriction - Output

Bridge Constriction - Profile

020406080100120140160180200

Energy GradelineWater Surface EL.Normal DepthCritical DepthTop of BankBed

Bridge Constriction - Sensitivity

0.4 0.5 0.6 0.7 0.8 0.9 1

Bridge Bed Width Ratio

Depth Increase

V Ratio

* This plot is specific to the current scenario

Culvert Hydraulics

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)

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

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

Culvert Hydraulics – Flow Profile: Input

Culvert Hydraulics – Flow Profile: OutputResults Summary

Culvert Hydraulics – OutputDetailed Results

Culvert Hydraulics – Flow Profile: Output

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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

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

050100150200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

Fish Passage - Culverts

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)

1.041.267.41.21.00

0.760.974.51.00.75

0.490.672.30.80.50

VoutletVinletQVchannelY

020406080100120140160180200

Invert + Culvert

Energy Gradeline

Water Surface EL.

Top of Bank

Critical Depth

Normal Depth

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?

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

XS Station (m)

)Fish Passage - Culverts

Substrate

Increase Roughness with Rocky Substrate

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

⎟⎠⎞

⎜⎝⎛

Rn12ln5.2

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

Other Factors (Ice, Drift, Scour)

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

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

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)

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

Sit. ‘a’EL ~ 0.8 * Yt ~ 0.6m

Large Stream (B > 50m)

Small Stream (B < 50m)

Damage History

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 -$)

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

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

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)

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

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

Reference Documents

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