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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 3
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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.
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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
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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
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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
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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
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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.
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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
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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.)
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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
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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.
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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
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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.
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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.
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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
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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
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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/
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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
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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
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Figure 2: Runoff Depth Map
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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).
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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).
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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.
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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.
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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
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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):
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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)
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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
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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))
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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.
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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.
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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.
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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
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Bridge Conceptual Design Guidelines 35
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(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
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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
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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.
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Bridge Conceptual Design Guidelines 38
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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.
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Bridge Conceptual Design Guidelines 39
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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