Bridge Conceptual Design Guidelines - Alberta · 2020. 6. 8. · Bridge Conceptual Design...

Bridge Conceptual Design Guidelines 1

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Bridge Conceptual Design Guidelines

Version 3.0

© Copyright, January 2014 The Crown in right of the Province of Alberta, as represented by the Minister of Transportation Permission is given to reproduce all or part of this document without modification. If changes are made to any part, it should be made clear that that part has been modified.

Bridge Conceptual Design Guidelines

Version 3.0



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BRIDGE CONCEPTUAL DESIGN GUIDELINES

Volume 3.0

Technical Standards Branch

Alberta Transportation

May 2020

© Copyright May 2020

The Crown in right of the Province of Alberta, as represented by the Minister of Transportation

Permission is given to reproduce all or part of this document without modification. If changes are made to any part, it should be

made clear that that part has been modified.



Preface These guidelines cover all aspects of bridge conceptual design, also referred to as bridge planning, including bridge location, sizing, geometrics, hydrotechnical design, and river protection works. These guidelines apply to all Alberta Transportation projects involving bridge size structures, including all tasks identified as Bridge Planning as per the current version of the Engineering Consultant Guidelines, Vol. 1. Although this document is intended to be thorough, certain cases may arise where specific guidance is not provided or not applicable. Consultants working for Alberta Transportation must exercise technically sound and well-justified engineering judgment in the application of these guidelines. It is not the intent of the document to limit progress or discourage innovation. Consultants are encouraged to explore all engineering options they deem appropriate for a specific site. For situations where engineering analysis reveals that standards in this guideline are not appropriate for a specific project, the design exception process shall be followed, as per the Department’s Design Exceptions Guideline. Subject Matter Experts shall be informed for any proposed deviations from current standards or guidelines and will make a determination of whether a formal design exception submission is required. Documentation, either through a formal design exception submission or through content within the Conceptual Design Report, should include an appropriate level of engineering analysis, evaluation of alternatives (for example an option to meet the standard in comparison to an option to not meet the standard), risk assessments, mitigation strategies, and recommendations. The Subject Matter Expert shall determine if the design exception is approved, with final sign off required by the Executive Director of the Technical Standards Branch. Any project specific questions relating to these guidelines should be directed to the Project Manager. Any feedback or technical clarification requests relating to this document should be directed to the Bridge Planning Specialist, Bridge Engineering Section, Technical Standards Branch, Alberta Transportation. Approved: Caroline Watt, MEng. PEng. John Alexander, MSc. Peng. Bridge Planning Specialist Director Bridge Engineering Bridge Engineering Technical Standards Branch Technical Standards Branch Des Williamson, MSc. PEng. Executive Director Technical Standards Branch

http://www.transportation.alberta.ca/Content/docType253/Production/DesignExceptions.pdf



LIST OF CHANGES

The following page is reserved for documenting changes to this version of the Bridge Conceptual Design Guidelines. When changes are completed to the document, the following actions will be completed:

The version of the document will be updated;

A revision triangle will be placed next to the change in the document;

A basic description and the date of the change will be summarized below.

Document

Revision

Date Description



Table of Contents

1 Introduction .................................................................................................. 8

1.1 What is Bridge Conceptual Design? ..................................................................... 9 1.2 Why do Bridge Conceptual Design? ..................................................................... 9 1.3 Process Overview ............................................................................................... 10 1.4 Technical Considerations Overview ................................................................... 10

2 Data Collection Phase .................................................................................. 12

3 Site Inspection Phase .................................................................................. 13

4 Technical Input Phase .................................................................................. 15

5 Hydrotechnical Assessment Phase ............................................................... 16

5.1 Hydrotechnical Design Parameters .................................................................... 16 5.1.1 Channel Capacity......................................................................................... 17 5.1.2 Historic Highwater Observations ................................................................ 19 5.1.3 Basin Runoff Potential Analysis .................................................................. 20

5.2 Hydraulic Calculations ........................................................................................ 22 5.2.1 Bridge Hydraulics ........................................................................................ 22 5.2.2 Culvert Hydraulics ....................................................................................... 23

5.3 Navigation Protection Act Requirements .......................................................... 24 5.4 Fish Passage Requirements ................................................................................ 24 5.5 Deck Drainage Requirements ............................................................................. 26 5.6 Scour and Erosion Considerations ..................................................................... 30

5.6.1 Scour ........................................................................................................... 30 5.6.2 River Protections Works (RPW) Design ...................................................... 33 5.6.3 Degradation ................................................................................................ 35

5.7 Ice Considerations .............................................................................................. 35 5.8 Drift and Debris Considerations ......................................................................... 36 5.9 Channel Realignments ........................................................................................ 37

6 Geometric Assessment Phase ...................................................................... 38

6.1 Geometric Constraints ....................................................................................... 39 6.2 Structure Width .................................................................................................. 40 6.3 Railway Grade Separation Considerations ......................................................... 41

7 Conceptual Design Option Development ..................................................... 43

7.1 Horizontal Alignment ......................................................................................... 43 7.2 Vertical Profile .................................................................................................... 44 7.3 Bridge Opening ................................................................................................... 44

7.3.1 Wildlife Passage Considerations ................................................................. 46



7.3.2 Structural Considerations ........................................................................... 46 7.3.3 Stormwater Considerations ........................................................................ 47

7.4 Bridge Sized Culvert Requirements .................................................................... 47 8 Options Analysis.......................................................................................... 50

8.1 Evaluate Alternatives ......................................................................................... 50 8.2 Design Exceptions............................................................................................... 51

9 Reporting Requirements ............................................................................. 53

10 References .................................................................................................. 57

APPENDIX A: Sample Hydrotechnical Summary ......................................................... 60

APPENDIX B: Bridge Conceptual Design Summary Sheet ............................................ 62

APPENDIX C: Sample Report Sketches ....................................................................... 64

Sample River Crossing Bridge Conceptual Design Sketches ......................................... 65 Sample Grade Separation: Bridge Conceptual Design Sketches .................................. 67 Sample Design Data Drawings (historically used)......................................................... 69

APPENDIX D: Reference Documents .......................................................................... 71

Comparison of Velocity Distributions in Channels and Culverts .................................. 72 Estimation of Navigation Clearance Box Reference Water Level ................................. 83 Discussion on the Selection of the Recommended Fish Passage Design Discharge .... 91 Comparison of 3Q10 to Depth-Based Approach for Fish Passage Evaluation ........... 102



1 Introduction

The flowchart below illustrates the overall lifecycle approach for bridge structure management at Alberta Transportation:

The Department uses the following definitions to categorize bridge structures: Bridge Sized Culverts

Standard culverts are buried structures with diameters (or equivalent diameter based on

the sum of end areas) of greater than or equal to 1500mm and less than 4500mm.

Major culverts are buried structures with diameters (or summation of diameters) of

4500mm of greater, or structures of lesser diameter having complex site restraints or

specialized engineering requirements.

Standard Bridges

Bridge structures that are built using standard bridge design drawings. Typically standard

bridge construction comprises of standard precast girders with steel or concrete

substructure elements and supported on steel piles.

Major Bridges

Includes all other bridge structures including large or complex buried structures such as

open bottomed culverts. Major bridges are typically built from site-specific drawings but

can also be built from standard girder drawings with engineered modifications. Typically,

major bridges are river crossings, highway interchanges, or railways crossings.



The Bridge Conceptual Guidelines shall apply to the new design of bridge structures and river engineering protection works, and during portions of Functional Planning and Bridge Assessments studies. This document describes the processes and technical considerations to be used to arrive at the optimal bridge conceptual design. Depending on the project, bridge conceptual designers may play a leading or supporting role. To achieve the optimal design in these circumstances, significant communication and interaction is required. The Department has a number of other reference documents including, but not limited, to:

Highway Geometric Design Guidelines (HGDG) (Alberta Transportation, 2020)

Bridge Structures Design Criteria (BSDC) (Alberta Transportation, 2020)

Standard Specifications for Bridge Construction (SSBC) (Alberta Transportation, 2020);

Roadside Design Guide (RDG) (Alberta Transportation, 2020)

Engineering Drafting Guidelines for Highway and Bridge Projects (Alberta Transportation, 2020)

Engineering Consultant Guidelines for Highway, Bridge and Water Projects (ECG)

(Alberta Transportation , 2020)

In the event that discrepancies exist between this document and other references, the Bridge Conceptual Design Guidelines shall take precedence.

1.1 What is Bridge Conceptual Design?

The purpose of the bridge conceptual design phase is to determine and document the most suitable solution for a roadway to cross a stream, road, or other facility while considering relevant issues, risks, and constraints, and exploring all potential options. The results should:

Document data compiled, project constraints, design parameters, alternatives considered, and decisions made

Provide preliminary design information on the recommended concept to proceed to the Detailed Design phase.

The main difference between a design project and a functional planning study is the level of detail of data collection, analysis, and reporting. Refinement of bridge openings identified during high-level planning studies shall occur during the bridge design process, as additional information (survey, geotechnical, etc.) is gathered. Proceeding directly from functional planning level concepts to detailed design is strongly discouraged.

1.2 Why do Bridge Conceptual Design?

During the functional planning and conceptual design phases of a project, significant savings can result in comparison to the effort expended. Investing upfront effort into identification of constraints and exploration of options oftentimes results in savings during future life cycle phases. Additional benefits include better project scope definition, reduced project schedules, and simplified issue resolution. Bridges are typically replaced due to structural condition rather than functionality due to their high capital costs. With typical design lives of 75 years (50 years for culverts), bridges are the least flexible infrastructure component of the roadway network. Failure to consider future functional improvements can negatively affect safety, operations and economics of the roadway network. Therefore, it is essential to consider scenarios that may occur during the life span of a structure in order to develop an optimal lifecycle solution.

https://www.alberta.ca/highway-geometric-design-guide.aspxhttps://www.alberta.ca/new-design-detailed-engineering.aspxhttps://www.alberta.ca/bridges-and-structures-fabrication-and-construction.aspxhttps://www.alberta.ca/roadside-design-guide.aspxhttps://www.alberta.ca/new-design-detailed-engineering.aspxhttps://www.alberta.ca/engineering-consultant-guidelines-highway-bridge-water-vol-1-design-and-tender.aspx



1.3 Process Overview

The main steps in the process are:

1) Data collection 2) Site inspection 3) Arrange for technical input 4) Hydrotechnical assessment (as required): 5) Geometric assessment 6) Review technical inputs 7) Develop feasible options 8) Prepare draft Conceptual Design Report 9) Submit final Conceptual Design Report 10) Follow up in future project stages (as required)

1.4 Technical Considerations Overview

Technical considerations for any given project will vary depending on the specific project. In general, the list below shall be considered in developing an optimal bridge concept. Future Plans:

Life cycle analysis on bridge rehabilitation, culvert lining, traffic accommodation

Highway widening, twinning, minimize throw-away costs

Phased construction options

Net Present Value Hydrotechnical (stream crossings):

Design parameters for stream at crossing site

Structure impact on hydraulics – constriction, drift/ice handling

River issues – scour/erosion, bank stability, flow alignment, protection works Bridge/Highway Geometries:

Highway alignment – radius, superelevation, spiral, skew, safety, accesses, land severance

Gradeline – grade, K values, length of curve, bridge height, freeboard, cut/fill balance

Bridge geometrics - width, cross slope, sight distances, clear zone requirements

Roadside/median barrier requirements vs barrier free Structural:

Span arrangements (single vs. multiple spans), , pier location, skew

Deck drainage

Structure type, girder depths

Culvert vs. bridge, major bridge vs. standard bridge

Retaining walls vs open headslopes Geotechnical:

Slides, headslope ratios, remediation works

Pile depths, settlements

Retaining walls Environmental:

Regulatory requirements (Fisheries Act, Water Act, Navigation Protection Act, First Nations Consultation, etc.)



Environmental Evaluation report, QAES Report

Highway drainage, ECO plan

Sustainability and climate change considerations Construction:

Traffic accommodation (detour/staging)

Berm flow constriction/fish passage considerations

Method (culvert tunneling vs open cut, launching vs traditional girder erection, accelerated vs traditional construction)

Stakeholder:

Impacts to adjacent landowners

Impact on route length, safety, access relocation

Other stakeholders such as Municipalities

ROW concerns and purchase Other:

MSE walls, Utilities, Railways

Tunnel design (geometry, construction, dangerous goods impacts)

Bridge barriers, transitions

Pedestrian/cyclist requirements and warrants



2 Data Collection Phase

During the initial phase of a project, it is important to gather historical and current data related to the project site, and nearby sites that may contain relevant information. The amount and type of data available will vary by site but generally includes:

Bridge assessment, design, and construction reports and drawings

Bridge Inspection reports (Alberta Transportation , 2020) including Level 1 and Level 2 BIM Reports

Hydrotechnical reports including scour inspections, bank protection repairs, and highwater reports

Roadway reports

Functional planning studies

Geotechnical reports

Environmental reports

Railway crossing agreements or board orders

Topographic and other data is available through GIS data sets (Alberta Transportation, 2012). Data dates and sources should be noted, along with any changes from the date of acquisition

Aerial imagery (current and historical)

Site surveys for local streambed elevations, utilities, existing structure details, soil and waterside corrosion, etc.

Oftentimes, data will provided or identified as available in a project’s terms of reference. Requests for historical or corporate data (Alberta Transportation, 2020) from Alberta Transportation can be made through a project’s Project Manager. Note that historical data contained in the Regional offices may not be the same as date contained at the head office (Twin Atria Building). The Consultant is responsible for obtaining sufficient data and survey for each particular project.



3 Site Inspection Phase

For most projects, two site inspections are recommended. The purpose of the first inspection is to take photographs, record measurements such as stream parameters or clear roadway width, and identify any existing site issues or constraints such as debris, high load strikes, or tight construction areas. For some projects, it is valuable to conduct a site visit during different conditions such winter vs summer seasons, or high vs low traffic periods. Oftentimes, it is valuable to visit sites adjacent to the project site as well such as a river crossing upstream and downstream of the project site. All sites are unique, however, a sample inspection checklist is provided below:

Action Description

Determine channel dimensions

Identify portions of the channel that appear typical of the reach and represent the natural channel

Estimate bed width, top width, and bank height

Note highwater data Note type, location, elevation of any highwater/ice marks

Talk to landowners, local officials about history

Note any backwater impacts e.g. farmlands, buildings

Characterize stream and geomorphology

channel pattern – meandering or braided, incised channel or floodplain, differences between natural channel and in the vicinity of the structure

bank stability – slope, vegetation, material, height, erosion, rock outcrops, slides, springs

bed material – gravel, sand, silt, D50

bed forms – bars, islands

flow alignment – bends, skew

dimensions – water level and velocity

Notes signs of degradation/aggradation

Assess operation of existing crossing

general scour – bed lowering through the opening

local scour – holes near piers, protection works

condition of slope protection – cracks, loss of material

abrasion on pier nose plates

water or soil side corrosion

environmental sensitivities

drift accumulated at opening

high load strikes, deck drainage performance at overpasses

Identify hydraulic controls

rock ledges

changes in cross section geometry or slope

hydraulic structures like weirs, nearby bridges

lakes, beaver dams

Assess basin characteristics

terrain – flat, rolling, foothill etc.

land use – farming, forest, development

surface storage – lakes, slews

upstream controlling factors – outlets, weirs



Assess drift potential size/type of trees

active bank erosion, channel shifting

beaver activity

drift at bridge opening, on banks, on bars

Assess highway geometrics

Assess sight distance/visual problems on existing horizontal and vertical alignments

Note accesses/intersections near the bridge

Assess impacts of proposed changes to gradeline/alignment

Measure width of existing highway at either side of bridge, and depth of cover for culvert

Note if there will be significant ditch drainage toward stream

Note other features Identify utilities, structures, landowners that may be affected by bridge construction

Note any features of interest to be surveyed

Identify opportunities for a detour and detour structure, if needed

Identify access for survey and geotechnical testing

Locate nearby survey controls or benchmarks

Take photos/video for use in checking survey

Inspect nearby structures in the vicinity, as needed

Locate potential geotechnical testhole locations

Towards the end of the conceptual design phase it is useful to conduct a subsequent site visit, particularly for complex sites or major bridge projects, to envision or lay out options in the field. This helps to assess the feasibility of options in the field and allows an opportunity to identify any additional unforeseen issues or constraints, before finalizing the recommended conceptual design option. The Consultant is responsible for determining the number and scope of site visits required depending on the project and specific site needs, unless otherwise specified within a project’s terms of reference.



4 Technical Input Phase

After the historical data gathering and initial site inspection phases, additional information is often required to fully understand site constraints and issues. These tasks can range between projects but may involve:

Arranging for site survey to supplement data sets

Arranging for desktop or preliminary geotechnical investigation (oftentimes phased for major bridge or realignment projects, with boreholes drilled once an alignment is finalized)

Arranging for environmental data or regulatory inputs (refer to Environmental Regulations, (Alberta Transportation , 2020))

Arranging for subject matter expert input (roadway, structural, construction, environmental operations etc.), oftentimes in in the form of value engineering sessions

Completing structural assessments (Alberta Transportation , 2020) for rehabilitation options, if in scope

Once preliminary conceptual options are developed, obtaining additional technical input is recommended, particularly for major bridges or complex sites, including:

Checking the field survey and other data sets for completeness and accuracy

Obtaining opinions from others specialties on the feasibility of the option (roadway, geotechnical, environmental, etc.), and

Noting any new site constraints (e.g. ROW, site access, detour, environmental concerns, restricted activity periods, utility relocation, land purchase, budgets, schedule, stakeholders) or projects risks.



5 Hydrotechnical Assessment Phase

Hydrotechnical assessment and design parameters determination are required for stream crossings and should include sensitivity analyses/risk assessments. Obtaining design parameter confirmation with the Department is recommended prior to completing option analysis, to avoid rework.

5.1 Hydrotechnical Design Parameters

Design of stream crossings requires the flow depth (Y), mean channel velocity (V) and resultant flow (Q). Key principles in determination design parameters are:

Representative of the capacity of the channel to deliver flow from the upstream basin

Consistent with the historic high water observations

Consistent with existing hydraulic performance Flood frequency analysis has proven to be incapable of meeting these principles at most sites, as documented in Context of Extreme Floods in Alberta (Alberta Transportation, 2007) There are three main components to hydrotechnical design parameter determination:

Channel Capacity (CC) estimates the physical capacity of the stream to deliver flow to the crossing under flood conditions; governs for most sites

Historic Highwater (HW) Observations ensures parameters are representative of the largest observed historic events; can govern for some large crossing sites, confirms CC at others.

Basin Runoff Potential (BRP) Check checks to see if the basin can supply enough water to fill the channel; can govern for sites with very small drainage basins or down-cutting ravines.

Hydrotechnical summaries (accessible through the Department’s publically available Hydrotechnical Information System, HIS) record the process below and exist for over 1500 sites (see Appendix for sample). Existing summaries should be updated and new summaries developed for sites where such information is not available. Appendix A contains a sample hydrotechnical summary. The overall process to determine hydrotechnical design parameters for a site is:

Estimate typical natural channel parameters o B (bed width), T (top width), h (bank height), S (slope)

Calculate Channel Capacity (CC), using channel capacity calculator tool (CCCT) or other methods

o Determine Y, V, Q

Assemble Historic Highwater Data

Check if HW exceeds CC: If YHW > YCC, set Y = YHW in CCCT o Determine V,Q using CCCT

Calculate Basin Runoff Potential (BRP) o If drainage area < 100km2 , look up ‘q’, calculate QBRP o If drainage area > 100km2 , method does not apply

Check if BRP governs: If QBRP < Q, set Y = YBRP o Determine V,Q using CCCT

Recommend Q,Y,V values for Design

https://www.alberta.ca/new-design-bridge-conceptual-design.aspx



Additional data that is of interest in hydrotechnical design includes:

Drainage Area (DA) – topographic area potentially contributing flows and used in Basin Runoff Potential analysis. DA can be determined using DEM data and GIS tools, with some values noted in HIS.

Airphotos – can show changes in flow alignment over time, indicating lateral mobility of a stream. Bank erosion can be tracked using georeferenced airphotos with some banktracking summaries are available in HIS.

Scour surveys – can show vertical changes in streambed over time, including local and general scour and bed form movement. Some scour surveys are available in HIS.

AEP Flood Hazard Mapping Tool – areas may be subject to additional constraints.

AEP Fish and Wildlife Mapping Tool – provides information related to fish and wildlife data

AEP Code of Practice maps – these maps classify streams in terms of their importance in fisheries management, and note restricted activity period dates.

Local site hydraulic influences – such as other structures (weir, bridge, culvert, dam), sudden channel changes (slope, width), and confluences with other channels.

Site inspection observations – channel features, flow concentration and alignment, active bank erosion, and ice scars on trees.

Historical Reports (TRANS, AEP, Universities, Municipalities)

5.1.1 Channel Capacity

This technique estimates the capacity of the channel to deliver flows to a site at a defined depth above the bank height. The typical channel is a trapezoidal representation of the stream reach that the crossing is located on, as shown below:

Bed width (B) – width of the base Top width (T) – width of the top (at bankheight) Bank height (h) – height Slope (S) – hydraulic slope of channel (m/m) Sources of information for ‘B’ and ‘T’ include georeferenced airphotos, digital elevation models including LiDAR, site measurements. Many cross sections within the river’s reach should be used to determine an average natural channel section. The sections used should be on a natural, stable, and straight portion of the river within proximity of the project site. In many cases, channels in proximity to an existing crossing may have been modified during construction or influenced by the existing structure or adjacent land use, and may not represent the natural channel. Surveyed cross sections are usually too limited in number to enable estimation of the typical values. ‘B’ values can be estimated to the nearest meter from surface water width on airphotos at low water levels, with adjustments based on observations and survey data as appropriate. The bank height is the height at which the channel transitions to the floodplain, and there should be a sudden change (decrease) in the slope of the terrain perpendicular to the channel. Sources of information include surveyed cross sections, high resolution DEMs (e.g. LiDAR), photos with scalable objects, and site measurements. Other bank height definitions exist such as based on the line of permanent vegetation. This will typically be below the geometric definition of bank height used in bridge hydraulic analysis, and should not be used for this purpose. For high-level

Figure 1: Hydraulic Channel Definitions Sketch



assessments, an estimated bank height can be approximated assuming 2H: 1V channel side slopes between the channel bed (“B”) and top width (“T”). As the floodplain is activated at this level, the parameters are representative of a flood, and significant flow routing will occur. The flow depth assignment is: These values are based on the Context of Extreme Floods in Alberta (Alberta Transportation, 2007) analysis. Values that exceed the channel capacity are accounted for in historic highwater analyses. The equation for B>=10m is based on the Evaluation of Open Channel Flow Equations (Alberta Transportation, 2005) study. The Channel Capacity Calculator tool built by AT has all of these calculations built in. Hydraulic calculation of V is calculated based on bedwidth (B) as follows: Where: R = hydraulic radius = A/P A = typical cross section area of flow at YCC (m2) P = wetted perimeter of typical cross section at YCC n = Manning coefficient, with adjustment for Slope (S) as follows: The channel slope ‘S’ is typically a small number (= 10 14R0.6S0.4 NA

7 – 9 R0.67S0.5/n 0.040

4 – 6 R0.67S0.5/n 0.045

8) n = n – 0.005

0.005 – 0.015 n = n + 0.005

> 0.015 n = n + 0.01

http://www.transportation.alberta.ca/PlanningTools/Tools/Hydraulics/http://www.transportation.alberta.ca/PlanningTools/Tools/HIS/



5.1.2 Historic Highwater Observations

Key sources of historic highwater observations include: Hydrotechnical Information System (HIS):

Includes ~4000 flood/highwater records collected across the Province, dating from the early 1900s to the present,

Includes file histories and hydrotechnical summaries for bridge sites, along with basic structural and site inventory data.

Water Survey of Canada (Canada, 2020) (WSC):

Federal Government branch that measures, estimates, and publishes flow data at many sites across Canada. AEP is a partner of WSC.

Stage (depth above a datum) is measured continuously, with occasional flow measurements used to convert these stages to flows using a rating curve. Rating curves at higher values are often extrapolated. These should be used cautiously.

Actual flow measurement and peak stage data from the files of WSC is published within Alberta Transportation’s PeakFlow tool. This data can be used to assess published flow values.

Flow measurements in floods can be inaccurate (waves, turbulence, debris). Bridge Correspondence Files:

Twin Atria and Regional offices may not contain the same information. Bridge Design Drawings:

Often note highwater levels corresponding to floods, in addition to hydrotechnical design values. These should be confirmed by checking the original data.

Bridge Inspection Reports:

Sometimes, the observations carry over from previous inspections to the next, so it can be difficult to associate them with a specific event, unless noted.

Consultant access to inspection reports in TIMS can be submitted via the Bridge Management website

Site Inspection Observations:

Highwater marks may be present during a site inspection and can include deposits of silt and drift, grass and weeds on fences, and abrasion marks on piers. Debris blockages can sometimes influence highwater marks.

Local Sources:

Information on past floods may be available from landowners, municipal officials, newspaper records, social media, and maintenance contract inspectors.

Airphotos:

Photos at the peak of a flood will show the horizontal extent of flooding and enable estimation of high water levels. AEP maintains the provincial airphoto archives.

When evaluating highwater data, the flow depth should be for the channel. Any records caused by a constricted opening, superstructure in the water, or blockage due to drift should be accounted for. Data for structures located on the same stream and in proximity should be considered and judgment should be applied when considering data from sites with substantially different drainage areas or channel parameters. Measurements downstream of a bridge are more representative of the natural channel response under flood conditions. Measurements upstream and downstream will enable assessment of the bridge hydraulic performance. Timing of

http://www.transportation.alberta.ca/Content/docType30/Production/GDBrgPlTool.pdf



observations should be considered, as some may have been before the peak (no higher marks visible), and some may have been after (highwater marks visible). Some data may conflict with other data, or seem infeasible compared to physical parameters. The source of data should be considered when establishing validity.

5.1.3 Basin Runoff Potential Analysis

At some sites, there may not be enough supply of water from the basin to fill the channel to its physical capacity. This can occur when the drainage area (DA) is small (< 100km2) relative to the capacity of the channel. One case would be a small drainage basin (e.g. 5km2) that drains into a ravine that cuts through the valley wall of a larger river. The ravine may be steep and have high banks but the surrounding basin will not supply enough water to fill it. The Basin Runoff Potential Map (Figure 2) assigns the largest observed unit discharges (‘q’) to various hydrologic regions within Alberta. To estimate this upper bound QBRP from the basin, the selected unit discharge q and defined ‘DA’ are multiplied. Details are found in the Development of Runoff Depth Map for Alberta (Alberta Transportation, 2006). Known exceptions to the Basin Runoff Potential Map are the Cypress Hills and Swan Hills area, where higher unit discharges have been recorded due to the higher basin gradients and ‘q’ = 0.4cms/km2 is recommended. Some potential adjustments and limitations to QBRP include:

If a storage facility (dam/weir) is located upstream of the site, estimates for the downstream drainage area should be added to peak outflows to account for flow routing.

If there are significant amounts of poorly drained areas in the basin, these areas should be excluded from the value for DA used in the calculation.

If the basin covers multiple hydrologic regions, consider a weighted average for ‘q’.

If the basin contains an urban center, consider not using this technique as natural drainage patterns have been altered



Figure 2: Runoff Depth Map



5.2 Hydraulic Calculations

The hydrotechnical design values (along with fish passage and Q2, as required) will form the boundary conditions for calculations. For culverts and constrictive bridges, calculations involving gradually varied flow (such as backwater curves) and rapidly varied flow (abrupt energy losses over a short distance) are necessary. These calculations can be done by simple models using prismatic channels and one dimensional (section averaged) techniques, such as those facilitated by Alberta Transportations Flow Profile (Alberta Transportation, 2015). Advanced techniques, such as multi-section (e.g. HEC-RAS), two-dimensional, and unsteady flow calculations are not necessary and offer little value in bridge design. Some of the reasons to avoid using models that are more complex include:

Boundary conditions are one dimensional anyway

Natural rivers have mobile boundaries (scour, bed forms, lateral erosion)

Many natural factors cannot be modeled accurately – drift, ice, sediment transport

Data-sets don’t exist to support true calibration of complex models

Complicated outputs are difficult to interpret and assess

These models are expensive and require significant resources

Most of the output, accurate or not, is not needed to design a bridge Channel capacity method calculations do not account for flow adjacent to the channel in the floodplain. Hydraulic calculations suggest that the down-slope component of flow on the floodplain is a small portion of the channel flow (typically < 10%). Additional reasons include:

Relatively shallow Y and low V (high relative roughness)

Lack of a defined and continuous channel in the floodplain

Presence of many natural and man-made obstructions (trees, roads, development)

Most flow interacts laterally with the channel as levels change

With limitations in describing channel geometry, assumptions in hydraulic parameters such as roughness and loss coefficients, and naturally occurring features such as drift, ice and sediment, calculated precision should not be inferred as accuracy. In general, if confidence in Y is +/- 10% and V is +/- 20%, the parameters are acceptable. Sensitivity analysis should always be completed. For reporting, round Y and V to 10% (min. 0.1m for Y, min. 0.1m/s for V).

5.2.1 Bridge Hydraulics

Sizing a standard bridge, major bridge, or major buried structure involves placing the bridge fills and setting the roadway gradeline to provide the desired freeboard. A starting point is to place the fills parallel to the channel banks at the crossing location. From this point, a range of options can be considered in the optimization process. As the bridge opening decreases, the degree of constriction increases. This will result in increased velocities (V) and headloss (V2/2g) through the bridge opening. The hydraulic impacts of a constriction and increased velocity will result in increased size and quantity of riprap protection, bank erosion, increased water level and flood risk on adjacent developments, and reduced freeboard (possibly requiring a gradeline raise).



Hydraulic impacts are not sensitive to small changes in fill location, especially for low velocity crossings. Additional criteria for hydraulic modeling for bridges are as follows:

Hydraulic modeling is typically only required for constricted options (less flow area than the typical channel), for sites with high mean velocity (>3m/s), or for complex shapes such as an oversized open bottom buried structure.

The impact of cross-sectional flow area lost to protection works and piers should be considered for smaller crossings (B < 30m or lost area > 20%).

Head losses through a bridge opening should be based on the differential velocity head, with common coefficients (K) of 0.3 for contraction and 0.5 for expansion.

Bridge openings wider than the typical channel can provide advantages such as less protection works, if sufficient buffer is provided. In some cases, these benefits may counteract the expense of additional bridge length. Openings much larger than the typical channel can result in adverse impacts, such as sediment deposition and local flow realignment issues that can lead to increased bank erosion, and are generally more costly. Freeboard should be determined through optimization. The starting point for freeboard shall be 1.0m between design highwater elevation and the lowest point of the structure. Higher values are seldom justified hydraulically, but may result from gradeline optimization. Lower values should be considered if any the following conditions are met:

Reducing freeboard could result in a significant cost reduction (>15%)

There is a high degree of confidence in the design highwater level

There is limited potential and/or history for drift or ice accumulation at the site

The bridge is on a longitudinal grade, where most of the bridge has more than 1.0m freeboard

A single span bridge (no piers) is proposed, with less risk of blockage

The volume of traffic is low and detour length is short

A minimum freeboard amount of 0.3m is achieved

A shorter design life is desirable, such as for a temporary structure

5.2.2 Culvert Hydraulics

Culverts are available in a range of materials, shapes and sizes. Historically, the most cost effective and common solution is a single round culvert. In general, culvert shapes do not match the shape of a natural channel, resulting in flow contraction and expansion when water is entering and exiting a culvert. A useful starting point for sizing a culvert is the flow depth plus the burial depth (diameter/4) that results in an opening that would have close to no freeboard. From here, the culvert opening should be optimized for the site considering site-specific objectives and risks including AADT, height of fill, detour length, and structure design life. Hydraulic calculations are typically more complicated for culverts than for bridges, due to factors such as the different shape, burial depth, and the potential for full flow. Various combinations of rapidly varying flow, gradually varied flow, normal flow, and full flow are also possible. Supercritical flow may result in some cases, with the potential for hydraulic jumps. As such, hydraulic modeling is recommended to assess each option being considered. For the majority of highway sites, ponding or pooling of water above the culvert is not desirable, resulting in most culverts operating under subcritical flow conditions (flow depths controlled by downstream tailwater conditions).



In addition to passing the design flow, fish, and drift, the following issues should be considered:

Upstream flooding impacts - dependent on headloss and drift potential

Protection works – High velocity flows directed at unprotected banks downstream may result in increased erosion. Insufficient protection works at the downstream end may result in scour holes, which can impact the structure, adjacent banks, and fish passage

Uplift failure - ends should be checked against hydrostatic uplift pressure if design water levels upstream and downstream are higher than the crown of the culvert. Additional weight on the culvert ends, or installation of a cutoff wall may be required.

Embankment stability – excessive headloss can result in a large differential head across the culvert embankment, resulting in potential for piping failure. This can be mitigated with extension of clay seals, installation of an impermeable membrane, or the extension of concrete headwalls.

Road overtopping – excessive headloss can also result in the road being overtopped.

Future rehabilitation - for high fill (>6m) and high traffic (AADT >5000) crossings, consideration should be given to allow for future lining with minimal traffic interruption.

Sustainability and future climate change requirements

5.3 Navigation Protection Act Requirements

Transport Canada (TC) assesses navigation impact of a crossing structure based on the mean annual flood (Q2 or 1 in 2-year flood). Vessel clearance is measured from the Q2 elevation flood to the underside of the bridge. This applies to watercourses declared navigable under the Navigation Protection Act, along with sites determined by AT to be navigable. Further guidance is found on the Environmental Regulation webpage, including the Navigation Assessment form. For sites with nearby WSC gauges with long records, Q2 can be calculated as the average of the reported annual maximum mean daily flows. This analysis is documented in Estimation of Navigation Clearance Box Reference Water Level (Alberta Transportation, 2011) in Appendix D. For sites without data, the following method is proposed to estimate the equivalent to the Q2 water level:

1. Determine design flow (Q) as per Section 2.1 2. Calculate Q2 = Q/(4 + 600*S), where S is the channel slope in m/m 3. Calculate Y2 using channel capacity method 4. Add Y2 to streambed elevation to get the Q2 reference water level

5.4 Fish Passage Requirements

Alberta Transportation projects require adherence to Provincial and Federal legislation related to accommodation of fish passage through structures. The main principle of fish passage through culverts is to ensure the mean velocity throughout the structure is less than or equal to the mean velocity in the channel at QFPD. The reasoning behind the velocity comparison approach is that if the fish can adapt to comparable velocities in the stream, the culvert itself should not be a velocity barrier to them. This approach does not involve the use of fish swimming performance curves as these curves have often resulted in mean velocities that are a small fraction of the mean velocity in the channel, and cannot be met with a culvert or bridge crossing. Many of the studies used to develop such curves were completed in laboratories where natural stream variations such as pools and riffles, or vegetation were not present.



The Comparison of Velocity Distributions in Channels and Culverts (Alberta Transportation, 2010) study in Appendix D shows that there is significant variance in point velocities and areas of low velocities within culverts. In some cases, natural channels provide opportunities for rest in the form of riffles or reefs. However, many channels crossed by culverts have reaches with relatively uniform cross sections over the typical length of culverts. If the mean velocity for fish passage cannot be met, slight changes to the culvert configuration can be considered such as multiple culverts or a horizontal ellipse structure. A fish passage design flow, QFPD, is required for culverts on fish bearing streams as below:

1. Calculate YFPD = 0.8 – 34.3*S , where S is the channel slope in m/m 2. Minimum YFPD = 0.2 3. Calculate QFPD at YFPD

When comparing mean velocities, the precision should be extended to 0.01m/s due to the relatively low magnitude. This approach was developed with support from Alberta Environment and Fisheries and Oceans Canada and is documented in the Discussion on the Selection of the Recommended Fish Passage Design Discharge (Alberta Transportation, 2012), in Appendix C. This method ensures that fish passage is evaluated at a relatively high flow, while providing more consistent results than statistical estimates such as the 3Q10 flow. In general, increasing pipe diameter or burial depth are ineffective methods of reducing mean velocities at QFPD, as most of the additional area will be above the flow depth. Increasing the burial depth can also lead to sedimentation/maintenance issues, and construction challenges due to increased excavation depth and a more difficult (steep/long) upstream transition from the culvert to the channel for the fish to traverse. If a feasible configuration cannot be found, installation of substrate and holders inside the culvert should be considered. Substrate holders assist in retaining substrate material thereby increasing the effective roughness of the culvert and decreasing the mean velocity. For sites where substrate and holders are proposed, the following parameters are recommended:

Substrate holder should be made of steel and conform to the shape of the pipe up to the desired height.

Height of substrate holder should be 0.3m (0.2m if culvert diameter < 3m)

Spacing of holders should be based on height divided by culvert slope (minimum = 7m)

Substrate should be Class 1M or Class 1 rock, with an average thickness matching the height of the holder.

Substrate and holders are only required for portions of the pipe where the mean velocity exceeds the mean channel velocity (typically upstream 1/2 or 1/3).

Materials shall be as per the Standard Specifications for Bridge Construction (Alberta Transportation, 2020)

The hydraulic effect of substrate is assessed by blocking off the flow area filled by the substrate and increasing the effective Manning roughness coefficient “n”. The relative roughness depends on the substrate type and flow depth, as shown in Table 1 below. AT’s Flow Profile tool will block the substrate flow area and adjust the roughness parameters, if a substrate value is entered.



Table 1: Hydraulic Roughness Parameters

In February 2019, AT and AEP signed a Memorandum of Understanding stating that these Bridge Conceptual Design Guidelines shall be used for bridge and culvert design for highway crossings and that the Alberta Roadway Crossing Inspection Manual (Alberta Environment and Parks, 2015) shall be used to assess fish passage inspection compliance. For further guidance on legislative requirements, contact AT’s Environmental Regulation group.

5.5 Deck Drainage Requirements

The presence of barriers, curbs and raised medians impedes the ability of rainfall runoff to drain off bridges. Rainfall collects and is channeled along these barriers until it reaches a drainage point of sufficient capacity, or until the point of overtopping. Encroachment of water into driving lanes can result in a road safety hazard due to hydroplaning, driver avoidance (swerving to avoid ponding), and visibility (splashing on windshields). Local pooling of water for extended durations on the bridge deck can also result in an increased rate of deck deterioration due to sub-surface drainage. Historically, deck drains combined with optimized geometry is used to minimize lane encroachment and local pooling. Use of below deck drainage systems is generally avoided due to capital and maintenance costs, low reliability (durability, clogging, segments becoming separated), and safety concerns. Drainage issues should receive early attention at the planning stage, when there is opportunity to optimize bridge geometry. Optimization should include considerations to longitudinal grade, shoulder width, number of deck drains, amount of driving lane encroachment, roadway classification, safety concerns, risks, and costs. Detailed design of components, including deck drain and trough design, is further discussed in the Bridge Structures Design Criteria. The minimum desirable longitudinal gradient for bridges of 1% is specified in the Bridge Structures Design Criteria with deck drains as normal practice for river crossings. Bridge deck drainage analysis shall combine the Rational Method equation for runoff flow rate estimation and the

Flow Depth Y

(m)

Adjusted Manning’s n Class 1M Riprap

Adjusted Manning’s n Class 1 Riprap

0.1 0.161 ---

0.2 0.079 0.141

0.3 0.064 0.095

0.4 0.057 0.079

0.5 0.053 0.071

0.6 0.050 0.065

0.7 0.048 0.062

0.8 0.047 0.059

0.9 0.046 0.057

1.0 0.045 0.055

1.1 0.044 0.054

1.2 0.044 0.053

1.3 0.043 0.052

1.4 0.043 0.051

1.5 0.042 0.050



Manning equation for calculation of the resulting flow depth adjacent to the barrier (bridge rail or raised median). The equations are based on Design of Bridge Deck Drainage, Hydraulic Engineering Circular No.2 (Federal Highways Administration, 1993). Combining these equations and accounting for cumulative deck drain discharge at key locations along the deck facilitates the calculation of encroachment of runoff into lanes. For safety reasons, encroachment should be minimized with a desirable maximum encroachment of 0m into the driving lane for divided highways and 1.0m for undivided highways. For all cases, a minimum lane width of 2.5m shall be maintained and the maximum water depth within a travel lane shall be 25mm. The following design parameters shall be used: i = 75 mm/hr, C = 0.9, n = 0.016, as further discussed below. Reasons for selection of 75 mm/hr as the design rainfall intensity:

Based on a factor of safety of 1.25 provided on a 60mm/hr rainfall intensity

Allows for potential future climates change. Increased magnitude of short duration, high intensity storms have been identified as a potential risk for infrastructure management by Environment Canada

Comparable to the City of Edmonton design rainfall intensity (76mm/hr) (City of Edmonton, 2015)

Based on a threshold for driver visibility and probability of occurrence for Alberta

60mm/hr is the average annual, maximum 5-minute rainfall intensity across Alberta, based on Intensity Duration Frequency (IDF) data published by Environment Canada. Twenty-nine IDF Curves from across the Province were analyzed, with an average period of record of 28 years of data and maximum period of record of 59 years. The earliest gauge data dates back to 1914

Rainfall intensity exceeding this value would be expected about 40 times during a bridge structure’s 75-year design life. Probabilities of occurrence for other rainfall intensities are summarized in the table below:

5 minutes is the shortest intensity rainfall measurement recorded by Environment Canada. Lesser duration storms are considered to have minimal impact on traffic due to the very short duration. Longer duration storms are likely to exceed the time of concentration of rainfall on most bridges. As an example, the time of concentration of rainfall with a 60mm/hr intensity on a 100m long bridge, assuming a 1% grade, is about 0.85 minutes (Federal Highway Administration, 2009).

A 60mm/hr rainfall intensity results in a significant reduction in visibility (25% of clear day visibility). Little incremental visibility loss is expected to occur for higher intensities as shown in the table below, adapted from (Texas Transportation Institute, 1977):



Reasons for selection of 0m lane encroachment for divided highways:

These bridges typically carry higher volumes of traffic and are often located in urban areas.

These roadways are typically designed to a higher standard (130km/hr, wider shoulders) resulting in higher travel speeds, and drivers not expecting to slow down.

There is a greater probability of a vehicle in the adjacent lane, travelling at different speeds, which may impede the ability to see or react to a hazard such as an encroachment.

Reasons for selection of 1.0m lane encroachment for undivided highways:

These bridges typically carry lower volumes of traffic and are typically in rural areas.

These roadways are typically designed to a lower standard (110km/hr), resulting in lower travel speeds and a reduced expectation of service.

There is a low probability that encroachment will occur on both sides of a bridge structure, at the same time as when two vehicles are passing by each other during a rainfall event.

A typical design vehicle width of 2.6m and lane width of 3.7m (HGDG) allows a driver to stay within their lane even after moving over to avoid the 1.0m encroachment.

Rational Method Equation: This equates the rate of rain falling on the bridge to the volume of runoff, and the equation is:

Q = CiAd / 3600 Where: Q= runoff rate (L/s) C= runoff coefficient (0.9, representative of pavement, is to be used for bridge decks) i = rainfall intensity (mm/hr) (75mm/hr recommended unless site specific data is available) Ad= contributing deck area (m2) to point of analysis Manning Equation: The Manning equation relates the depth of flow to the runoff rate as follows:

Q = 1000AfR2/3 S1/2 / n Where: Q= runoff rate (L/s) Af= flow area (m2) P= wetted perimeter (m) R= hydraulic radius (m) S= longitudinal slope of deck (m/m) at point of analysis n= roughness coefficient (use n = 0.016 for bridge decks)



The typical bridge deck runoff channel will have the following shape:

Where: Y= depth of flow (m) e= superelevation or crown rate T= top width of flow (m) Therefore:

Solution: • For specified T (shoulder width for no encroachment), calculate longitudinal flow capacity (Q, Manning eqn.). • Use Q with the Rational Method equation to calculate length to first deck drain • Calculate drain spacing using deck drain flow (at specified T) with Rational Method equation • Use spacing as approximate guide to optimally locate deck drains on structure. • Iterative solution may be required for variable grade/width bridges decks. For detailed deck drainage design, including analysis and sizing of deck drains and trough drains, Hydraulic Engineering Circular 21: Design of Bridge Deck Drainage (Federal Highways Administration, 1993) and Hydraulic Engineering Circular 22: Urban Drainage Design Manual (Federal Highway Administration, 2009) (Federal Highway Administration, 2009) shall be referred to, in conjunction with the latest version of the Bridge Structure’s Design Criteria. Specific considerations at the Conceptual Design Phase include:

Minimizing the number of deck joints and deck drains

Deck drains shall not discharge onto underpassing traffic lanes or pedestrian facilities

Drainage shall not be discharged onto any exposed substructure concrete surfaces

Shoulder use by bicyclists; eliminate snag hazards and minimize dips/elevation changes with deck drains in the travel paths if bicycle traffic is expected

Locating and designing future drainage considerations for projects where deck widening will occur in the future

Accommodation of hazardous materials or deleterious substances for environmentally sensitive sites such as streams with critical fish habitat or adjacent water intake facilities

Directing drainage away from MSE wall structures and major buried bridge structures, through grading and the use of membranes (refer to Bridge Structures Design Criteria)

Erosion control at discharge locations



5.6 Scour and Erosion Considerations

Natural channels have mobile boundaries, both vertically and horizontally. Lateral movement of stream banks is referred to as erosion (typically addressed with river protection works), while vertical changes are referred to as scour. Scour and erosion susceptible structures have features located within the active floodplain. Examples includes culvert soil back-fill envelopes, roadway embankments, open bottomed culverts, and shallow bridge foundations as loss of soil support for these structures could result in sudden and complete failure of the structure. If such structures are proposed, mitigation measures are required to reduce the risk of vulnerability and increase resiliency. Potential mitigation solutions may include

Increasing structure size or shape

placing foundations below scour depths

placing rock riprap or river protection works (see Section 5.6.1)

the use of cutoff walls

the use of clay seals

low permeability end treatments

encapsulating backfill with geotextile

the use of sheet piles to protect a footing foundation

establishing a monitoring program. Historically, due to costs required for mitigation measures, open bottom structures and MSE walls located within floodplains have had limited use for Provincial highway projects. Some situations where they may prove to be cost effective include uses as temporary or detour structures, on Local Road projects where additional risk may be acceptable, or at sites where erosion and scour are not concerns such as wildlife passage structures or railways. For more guidance on culverts and buried structures design, refer to Section 7.4.

5.6.1 Scour

The two main types of scour relevant to bridge design are:

General/constriction scour - streambed lowered throughout the opening

Local/pier scour – hydraulic conditions around pier shafts may cause scour holes to form

Figure 3: Scour Definitions (Adapted from Guide to Bridge Hydraulics (TAC, 2008))



In the early 1990s, Alberta Transportation developed a formalized pier scour monitoring program. This involved collecting baseline site parameters, creating a criterion for prioritization and scour susceptibility, determining site survey requirements, and developing scheduling requirements. Through this process, about 200 scour susceptible bridge sites were identified and are now monitored as part of the Level 2 BIM Scour Survey program. As part of a research project, data collected during this program was compared to the industry standard modified Melville approach recommended in the Guide to Bridge Hydraulics (Transportation Association of Canada, 2004). In general, it was found that the modified Melville approach over predicted the local scour when compared to design conditions. It is worth noting that all of AT’s field data was collected after flood conditions have passed due to safety concerns, and that some infilling may have occurred. For some sites, it is likely that scour deeper than that measured occurred during high flow events. AT intends to update these results over time to account for the collection of additional data for this program. Figure 5 below shows the results of this research. The modified Melville approach predicted local pier scour depths between 1.8m to 13.2m, with an average of 4.8m. The observed scour depth measured through the Scour Survey program ranged from -0.3m (increase in streambed elevation) to 4.0m, with an average of 1.5m. The difference between predicted and measured scour ranged from -0.2m (underestimation) to 10.8m, with an average over estimation of 3.3m. There also appeared to be no noticeable trend that related observed scour depth to flow, average sediment size, or pier width, although these factors are used in the modified Melville approach to determine the theoretical scour depth.



Figure 4: Theoretical vs Measured Scour Depths for Alberta Bridges

Although this research project focused on a small sample of bridge sites across the Province, there was an observed data trend towards a maximum prolonged scour value of less than 3m. Measured scour values at two bridge sites exceeded this value: BF78104 and BF73949. For BF78104 (Highway 32 over the Macleod River), the 4.0m measured depth of scour is thought to be due to the 45 degree flow alignment with the center pier and thalweg location. For BF73949 (Highway 2 over the Peace River), the 3.8m measured depth of scour is thought to be due to the accumulation of drift and thalweg location. Modern bridge construction equipment and materials has made piled in-stream foundations cost-effective. In general, as long as the foundation penetrates >5m into the streambed, pier scour should not be an issue. In the rare case of spread footings or short pile foundations, the bottom of the foundation should be lower than the estimated pier scour depth. The design pier scour depth shall be estimated as 2 times the effective pier width to a maximum of 3.0m below streambed unless site-specific data is available to suggest deeper parameters are warranted. To minimize impacts to navigation, the top of pile cap shall be beneath streambed elevation, to a maximum practical limit of 2m below streambed. Foundation design should consider the impact of loss of material up to the design scour depth. Geotechnical recommendations, structural design implications, and constructability should also be considered in the foundation design, as further described in the Bridge Structures Design Criteria.



5.6.2 River Protections Works (RPW) Design

Fills placed in the active waterway generally require protection to prevent erosion. The major types of protection works systems include headslope protection, guidebanks, and spurs. Rock riprap is the preferred material for protection of bridge headslopes, culvert ends, and river protection works. Reasons include over 60 years of proven performance history; systems that resist drift, abrasion and ice forces with the flexibility to accommodate settlement and launching; proven velocity based criteria for selection of rock protection systems, within many publications and studies; relatively low cost and generally readily available sources of rock riprap; relatively easy monitoring, maintenance and repair procedures; and laterally mobile streams require a “hard” solution to maintain flow alignment. Bioengineering options, such as willow staking within a rock riprap protection system, may compromise the function of the geotextile and impact hydraulic capacity of the bridge opening on smaller channels. These options may be considered for projects beyond the extent of the crossing, such as fisheries compensation in a channel, and erosion protection within ditches. Selection of the appropriate class of rock riprap is based on mean velocity (V) at the design flow with materials as per the Standard Specifications for Bridge Construction. Rock class shall be:

Rock Class V (m/s)

Evaluate no rock 1.0

1 2.5

2 3.2

3 4.0

Class 1M riprap is seldom used on bridge projects, with either Class 1 or no protection options typically assessed for very low velocity sites. Rock gradation is important to ensure interlocking of the rock. In some very high velocity cases, a modified gradation has been used for aprons, with smaller sizes excluded from the mix. The angularity of the rock (less rounded) becomes more important at high V sites as the rock is more likely to interlock when it is angular. For sites where V exceeds 4.0m/s, addition of H or sheet piles in the apron may be considered to enhance protection. Rock larger than Class 3 is usually not considered due to limited availability, cost, and difficulty in transportation and placement. The typical rock protection detail (Figure 3) involves:

Lining the bank with a single rock thickness (‘t’, equal to the maximum rock diameter)

Double thickness launching apron at the toe, with half-buried below streambed to accommodate rock launching into future scour holes.

Typical apron length is 4-5 times the maximum thickness of the rock.

The sloping portion is to be at a maximum slope of 2:1 (H: V). Trimming of the natural bank may be required for extended bank protection options (no fill placed).

The protection should extend to the design highwater/high ice elevation.

Place non-woven geotextile filter fabric to prevent the loss of fines under the rock, as detailed in the “Standard Specifications for Bridge Construction”. The key-in involves 0.3m of filter fabric being trenched vertically into the fill.

For protection placed on earthen material, extending the earthen slope vertically by 1m above the top of rock will provide a suitable working base for placing protection works.



An additional berm width (typically 3-4m wide, and about 1m about top of work elevation) can address rock placement, geotechnical stability and/or wildlife passage.

The protection works system should extend well into a stable bank or naturally protected feature. This will minimize the risk of the system being outflanked/eroded from behind. On streams with high lateral mobility, historical river banktracking will help to determine the extent of protection works required. Stable natural features, such as rock outcrops, should be utilized as appropriate. Guidebank and spur configurations are often best developed in plan view on top of airphotos. Alberta Transportation has developed a Spur Planning Geometry tool to support this.

Guidebanks are protected fills built parallel to the flow that extend beyond the bridge. They improve and maintain flow alignment through the openings on laterally mobile streams. Parallel flow alignment is preferred to skew to reduce the structure’s size and minimize erosive forces on banks/protection works. Guidebanks typically extend from the headslope, and flare towards the bank in an elliptical shape with a 2:1 ratio of distance along the stream to perpendicular to the stream. The transition from the channel into the bridge opening should be smooth. Spurs are fills that project perpendicular into the river, with protection works on the ends. They deflect flow from a bank or align flow. They are typically used in groups with other spurs or in addition to guidebanks, and can be more cost-effective that continuous protection. Spurs with significant projection may cause a contraction of flow, be difficult to construct/maintain, and may require extensive environmental approvals. Principles for spur design are:

Spacing = 4 times the projected length of spur into the flow at highwater (each spur assumed to protect the bank for 2 times the projected length upstream and downstream).

Spacing should typically not exceed the bankfull channel width (minimize risk of channel relocating between spurs

For spurs with short shanks (relatively small projection into flow), spacing = 4 to 6 times the effective protected width of the spur nose

Adjustments to spur spacing may be necessary for river changes (e.g. bends). Additional references for river protection design include Hydraulic Engineering Circular 23: Bridge Scour and Stream Instability Countermeasures Experience, Selection, and Design Guidance

m

1

t

Top of Rock EL

(Des. HW)

t

t

Bed EL

(Theor.)

Bottom of

Rock EL

Apron

Length

Berm

Width

Berm EL

RPW Definition Sketch

Headslope

Ratio

m

1

t

Top of Rock EL

(Des. HW)

t

t

Bed EL

(Theor.)

Bottom of

Rock EL

Apron

Length

Berm

Width

Berm EL

RPW Definition SketchRPW Definition Sketch

Headslope

Ratio

Figure 5: River Protection Works Definition Sketch



(Federal Highway Administration, 2009) and Hydraulic Design Series 6: River Engineering for Highway Encroachments (Federal Highway Administration, 2001).

5.6.3 Degradation

Degradation is the long term lowering of a channel elevation over a significant distance, in comparison to localized streambed scour. It can occur naturally or because of manmade activities, such as channel straightening. Degradation can result in unstable banks and exposed structural elements. It is important to differentiate degradation from scour as solutions and mitigation measures may be different. Signs that degradation may be an issue at a site include:

changes in historical streambed surveys

changes in historic airphotos, showing progressive slumping, channel deepening over a significant length, or vertical banks

history of hydraulic structures and channel modifications/shortening on the stream

ongoing maintenance concerns

ravine like section approaching a confluence If degradation has occurred, some judgment will be required to determine if further degradation may occur. This can be based on changes in rates of progress over time, and whether the degradation was caused by man-made (reaction of one-time intervention) or natural (potentially ongoing) activities. Additional information is contained in the Steam Degradation Technical Note (Alberta Transportation, 2013).

5.7 Ice Considerations

Ice impacts include design forces on piers, vertical clearance for ice jams, and structure blockage due to icing (aufeis). Where historic observations of ice impacts are available, these shall be considered in determining design parameters. Ice jams form when pieces of broken ice form a partial blockage of the channel. The constricted opening may result in headloss, and the accumulation of broken ice upstream of the toe may result in sustained high water levels for long distances. Highwater levels are the result of the increased wetted perimeter of the floating ice combined with the high effective roughness of the broken ice, and the submerged thickness of the ice itself. Broken pieces of ice may also project above highwater. Jams can form during freeze-up or break-up. Break-up jams form due to a weakened ice cover or increased runoff flows physically breaking competent ice. In general, the more competent the ice, the more severe the ice jam. Ice jam elevations can be several meters higher than highwater from summer flood events and can form/release very quickly. Some principles to consider in assessing ice jam potential are:

Check available records. Note location of the toe of jam, the maximum depth or elevation, ice thickness, and competency of the ice.

Ice jam risk is high where there is potential for upstream portions of the basin to have runoff in spring while there is still ice downstream (rivers flowing from south to north)

The maximum height of a jam is generally upstream of the toe and is a function of the ice thickness and roughness, and the depth of water below.

If a jam forms, it requires lateral support to remain. The maximum elevation is restricted to the range of bank height plus the thickness of the ice floes.

http://www.transportation.alberta.ca/Content/docType30/Production/StreamDegradationTechnicalNote.pdf



Trees along the channel banks can develop ice scars, typically on the trunks facing the stream (bark removed by ice abrasion).

Ice jams tend to form at locations where there are significant changes to channel slope, width or plan. Confluences of streams with tributaries are also susceptible.

When ice breaks up or ice jams release, floating ice moves downstream and potentially impacts piers. This can result in significant loading on piers. As defined in the Canadian Highway Bridge Design Code, the three components of loads are ice strength (classified into situations, a – d), elevation, and ice thickness. Historic records should be reviewed for information that may help quantify these three parameters such as abrasion on pier nose plates. When historic observations do not exist, the following parameters are recommended, based on a system wide review of historical observations across Alberta. In Table 2 below, ‘Y’ is the design flow depth and ‘t’ is the design ice thickness. In some cases, two combinations of parameters may be appropriate, such as weaker load at higher elevation and stronger load at lower elevation. Structural analysis can determine which set of parameters may govern pier design.

Table 2: Recommended Ice Design Parameters

Icing (aufeis) occurs when a structure is blocked by solid ice. The ice can form due to repeated freezing of water supplied to the site, such as an upstream spring. During spring runoff, a buried structure may be blocked/iced due to not being exposed to the sun. Icing reports are often noted in maintenance records. Remediation options include a larger opening or installation of an additional culvert above the main structure. Maintenance such as removal of the icing before spring runoff by thermal or physical means may be required for some sites. Deicing lines have been used historically but are not recommended due to maintenance and environmental concerns.

5.8 Drift and Debris Considerations

Flood events are frequently accompanied by drift that can impede flow, or change flow alignments. Factors indicating potential drift at a site include significant amount of trees near the channel and its tributaries in the drainage basin, laterally mobile streams with active bank erosion, historic records noting issues, accumulations at piers/on point bars/or at a culvert opening, and presence of beaver dams. Debris is often a controlling factor in design for mountainous streams where the sediment capacity need is greater than the hydraulic capacity need. Culverts are more prone to drift/debris concerns than bridges, as the surface width provided by a culvert decreases at higher stages. Some mitigation options for culverts include:

Increase in size/change in shape - consider cost-effectiveness.

Flared inlets with raised crown elevations to maintain flow in case of a drift blockage.

Damage History Small Stream (B < 50m) Large Stream (B > 50m)

Minor Sit. ‘a’ EL ~ 0.8 * Y t ~ 0.6m

Sit. ‘b’ EL ~ 0.6 * Y t ~ 0.8m

Major Sit. ‘b’ EL ~ 0.6 * Y t ~ 0.8m

Sit. ‘c’ EL – observations. t ~ 1.0m



Drift alignment piles that may require expensive modeling to correctly configure the piles. Success is not guaranteed in the event of channel changes.

A single pipe is preferable to multiple smaller pipes for handling drift.

Seek geotechnical advice regarding debris volume and design considerations

Drift/debris collectors, such as racks placed upstream, have proven to be problematic. Failure due to outflanking is common, releasing an accumulation of material towards the crossing and creating lateral stream instability concerns.

If a bridge is not a practical option, a culvert designed with flood resiliency features to withstand a potential blockage and requiring maintenance after a runoff event, may be the most cost-effective option.

For bridges, mitigation options include:

Reducing the number of piers for drift to accumulate on

Locating piers outside of the main flow

Increasing span length over the main flow

Providing additional freeboard to minimize risk to superstructure

Consider guidebanks that may help align drift through the opening

Remove drift from piers at existing bridges with pier scour vulnerability

5.9 Channel Realignments

Channel realignments can result in cost-effective, sustainable, and optimal solutions. Many projects, such as twinned highway structures, high fill culverts, buried structure bridges, and bridges on highly mobile streams require some form of channel work. The main benefit channel realignment is reduced skews, resulting in simpler designs. Flow alignment and fish passage may be improved with realignments. The main principle in designing channel realignments is to mimic a stable section of natural channel in plan-form, cross section, and profile. A man-made channel should be designed with similar B, h, T, and S values as the natural channel. This should result in a stable, low-maintenance, and low environmental impact solution. A larger opening or milder slope has the potential to result in aggradation (potential sediment accumulation) while a smaller opening or steeper slope has the potential to result in degradation. It is important to communicate this to regulators in the approval process.



6 Geometric Assessment Phase

Highway geometric design shall follow AT’s Highway Geometric Design Guide, along with any relevant Design Bulletins. However, constraints due to bridges can have a significant impact on road geometry. Identification of potential bridge constraints and accounting for them during geometric layout of the road is often the most cost effective method of optimizing the overall project. The roadway design and/or functional planning teams typically lead these projects. Integration of bridge planning expertise will ensure bridge issues are identified during the preliminary stages and that the project as a whole is optimized. Existing data is required to assess the functionality and safety of the existing highway and bridge. A new design should address and remediate existing concerns at a bridge site including high collision rates, substandard geometrics, poor sight distances, access management, insufficient clear zones, insufficient freeboard, high structure skew, or bridge grade. Future parameters are required to ensure lifecycle performance. Existing parameters required include:

Horizontal alignment (curve radius, crown, super-elevation, clearance to barriers, clear zones, shy distances, sight distances)

Vertical profile (grades, K values for sags and crests, grade on bridge, sight distances)

Existing bridge geometry (width, spans, height, skew)

Other (traffic volumes, highway classification, detour length, collisions,drainage concerns)

New design parameters should consider:

Potential to upgrade in the future (future classification), existing functional plans, horizontal alignment, vertical alignment, roadside design parameters.

Existing and future performance throughout the lifespan of the bridge.

Geotechnical and environmental constraints

Roadway constraints that may limit the use of a certain structure type such as limited height of cover over a culvert, sag curves on bridges, super elevation, drainage, or preferential icing.

For bridge projects with highway deficiencies, bridge impacts on future highway improvements should be considered. A minimal option for “spot” bridge projects involves replacing the bridge at its current location with similar geometry. The upper bound option would meet all conditions of the design roadway designation. Additional options between these extremes may include improvements to geometric deficiencies through modified alignments and/or gradelines. Adjacent roadway alignment deficiencies away from the bridge, such as horizontal or vertical curves can sometimes be addressed through a separate project and/or at a later date.

Twinned structure locations can be limited by the existing structure. Generally, a similar roadway profile should be used when structures are in close proximity to minimize retaining wall/grading needs and maintain driver expectations. An absolute minimum separation distance of 3m is required for twinned highway structures to limit likelihood of pedestrians jumping between structures and allow for an adequate construction work zone. Headlight glare, signage, drainage, and impacts on existing structure foundations should also be considered when locating a twinned structure. In the case of developer driven adjacent structures, a minimum separation distance of 10m is required to minimize risk to the existing structure.



6.1 Geometric Constraints

Horizontal clearances for bridge structures shall be as per the Roadside Design Guide. If lower values are proposed, the design exception process shall be followed, with evaluations based on level of risk, length of impact, economics, and past precedence. Bridges shall be on tangent horizontal alignments as curved bridges require extra design and detailing, and cost more for construction and mainte

Bridge Conceptual Design Guidelines - Alberta · 2020. 6. 8. · Bridge Conceptual Design...

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