Introduction to Bridges
5.
BRIDGE HYDRAULICS AND GEOTECHNICAL
Objectives
 have an understanding of basic hydrology, hydraulics and soil
mechanics and how these impact the design and service life of bridges
Introduction
 Hydrology is the science of dealing with the occurrence, circulation
and distribution of water on the earth and atmosphere. Determines
stream properties and the amount of water expected at bridge site.
 Hydraulics is the science dealing with the laws governing liquids in
motion.
 Soil mechanics is the scientific approach to understanding the
behaviour and action of soil associated with engineering.
Overhead #2 - Hydrologic Cycle
 process consists primarily of four parts
1. Evaporation and Transpiration and vapour from oceans, plants, etc.
to produce clouds
2. Precipitation: rain, snow or hail
3. Surface stream flow: precipitation causes surface runoff which
forms lakes and streams
4. Ground water: water that penetrates and flows underground
Overhead #3 - Stream Gauging Records
Belke Consulting Ltd.
Overhead Notes
1 of 7
Introduction to Bridges
 Federal Government, water survey of Canada gauges on selected
streams for many years
 info includes daily discharges, maximums, minimum and averages
 North Saskatchewan River averages are 600 - 700 cm/s whereas
maximum about 5000 cm/s
Overhead #4 - Flood Frequency Analysis
 statistical analysis to estimate flood discharge
 statistical analysis value depends on number of years of data
 few records not reliable, may not include major event
 assigns probabilities to certain flow rates
 determines design flood, fish passage flood
 when streams aren’t gauged can use data from similar ones
Overhead #5 - Flood Design Frequency
Belke Consulting Ltd.
Primary Highway
-
100 years
Secondary Highway
-
50 years
Local Road
-
25 years
Risk or probability:
25 year
87%
50 year
64%
100 year
39%
Overhead Notes
2 of 7
Introduction to Bridges
 High water marks should be obtained after or during every flood to
verify designs and for future reference
Overhead #6 - Typical Streambed Profile
 flow from mountains and foothills to prairies
 slope changes as does the velocity
 erosion reach - steep slope = degradation
 transport reach - erosion = deposition (stable)
 deposition reach - flat slope, bed aggregation (deltas, alluvial fans)
Overhead #7 - Channel Types
 braided channels - steep, gravel bed, change quickly
 meandering - progress with cut-offs over time
 affect location of bridges
- braided need confining
- meander curves are unstable, moving
Overhead #8 - Continuity Equation
 basic hydraulic relationship
 Q=VA is continuity equation
 describes open channel flow
 used to determine size of bridge opening and size and type of bank
protection
Belke Consulting Ltd.
Overhead Notes
3 of 7
Introduction to Bridges
 with known Q, decreasing area increases velocity and scour
 water depth upstream (backwater) will also increase
 not economic to span whole channel
 desirable to constrict highwater with a shorter bridge
Overhead #9 - Typical Channel Before Constriction
 flowlines indicate discharge
 closer lines indicate higher velocity and/or deeper depths
 most flow in centre of channel
Overhead #10 - Channel Constriction Due to Bridge Structures
 effect of bridge structure on flow area
 V = Q\A where area is reduced the velocity must increase
 bridge opening can be further reduced by ice, drift, collapsed pipes,
struts, liners, etc.
 culverts more constricting at greater depths
 constrictions are more significant when flow over banks
Overhead #11 - Typical Channel After Constriction
 constriction at inlet
 expansion at outlet
Belke Consulting Ltd.
Overhead Notes
4 of 7
Introduction to Bridges
 higher velocities, to balance, energy loss and erosive forces
Overhead #12 - General Scour
 stream bed lowered during flood (more area)
 general scour with floods
 bridge constriction causes scour
 natural degradation also occurs
 infilling after flood recedes, rate depends on material
Overhead #13 - Local Scour
 caused by constriction or obstructions
 instream elements disturb flow patterns
 pier scour depends on pier shape, width and skew
 most bridges in Alberta have pile foundations, which tolerate some
local scour
Overhead #14 - Typical Riprap Bank Protection
 angular durable rock, graded
 maximum size depends on velocity
 launching apron - scour resistance plus bank stability
 filter fabric prevents loss of fines
Belke Consulting Ltd.
Overhead Notes
5 of 7
Introduction to Bridges
Overhead #15 - Structure Foundations
 all structures ultimately supported by soil or underlying bedrock
 structures are generally located to best suit users rather than soil
condition
 structure usually designed to suit
Overhead #16 - Soil Bearing Capacity
 capacity depends on soil characteristics (peat moss, clay, gravel till)
 soils in AB generally all preconsolidated by glaciers
 water content (saturated, pore pressures) affects capacity
 soil mechanical properties can be improved by compaction, drainage,
drying, etc.
Overhead #17 - Bridge Foundation Types
 spread footings used where good quality material found near surface,
0 - 5 ft.
 piles are used where good bearing available only at some depth
Overhead #18 - Foundation Loads
 abutments and piers designed to balance forces applied
 where forces aren’t balanced failure in form of settlement, leaning
(instability) will occur
 excessive horizontal or vertical movement undesirable
Belke Consulting Ltd.
Overhead Notes
6 of 7
Introduction to Bridges
 uneven movement destructive
Overhead #19 - Slides and Soil Creep
 evidence of soil movement
- trees with curved trunks
- displaced posts or poles
- roads moved out of alignment
- displaced retaining walls
- cracks in soil
Overhead #20 - Typical Slides
 usually caused by changes in moisture condition
 lubrication of underlying hard layers
 may occur over short period of time
Belke Consulting Ltd.
Overhead Notes
7 of 7