Hydrology & Hydraulics for Bridge Design
Bridge Design Objectives • Provide Provide for the the safe travel traveling ing of the the public across a waterway • Allowing Allowing for convey conveyance ance of storm discharges through the structure without adversely impacting the traveling public, damage to properties and the environment.
SIZING A BRIDGE WATERWAY OPENING • Determine how much water (Design Discharge for a given Storm Event) will be flowing through the Bridge Opening - HYDROLOGY • Determine the waterway area required to pass the design discharge without compromising the Bridge Design Criteria using a Standard Standard Step Backwater Backwater Method -- HYDRAULICS -- HYDRAULICS
Bridge Hydraulics Overview Objectives for this presentation: 1 – Design discharges (Hydrology) 2 – Channel & Bridge Characteris Characteristics tics – y rau c na ys y s s us ng ng -- A brief brief overview overview
BRIDGES OVER WATERWAYS
BRIDGE DESIGN CRITERIA: • Brid Bridge ges s should should be be able pass the 50 year storm with 2 feet of freeboard and some . • There should should be be no impact to the upstream water surface elevation and/or floodway and floodplain widths. • Br Brid idge ges s should should be be able to withstand scour during a 500 year storm event.
SIZING A BRIDGE WATERWAY OPENING • Deter Determin mine e Strea Stream m Width & Bed Elevation • Beam Elevation • De Dete term rmin ine e Roadway Elevation • Wa Water terwa way y Ar Area ea
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2 - Hydrology The primary methods used by NYSDOT to calculate flood discharges are: •
USGS Report 2006-5112 – Regression Equations
• NRCS TR 55
USGS Report 2006-5112: Techniques for Estimating FloodPeak Discharges of Rural, Unregulated Streams in New York State • Provides multiple-regression equations to calculate discharges for gaged and ungaged streams • Provides a method to adjust discharges for gaged streams • Contains data from streamflow gaging stations
NYS USGS Gaging Stations
NYS USGS Gaging Stations
http://waterdata.usgs.gov/nwis/rt
http://waterdata.usgs.gov/nwis/rt
USGS Report 2006-5112 : Regression Equations
USGS Report 2006-5112 : Hydrologic Regions
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USGS Report 2006-5112 :
USGS Report 2006-5112 :
Hydrologic Regions – Buffalo DOT Region 5
Hydrologic Regions – Buffalo NYSDOT Region 5
Hydrologic Regions 5 & 6 -- Variables A -- Drainage area, in square miles. L -- Main-channel stream length, in miles. SL -- Main-channel slope, in feet per mile. Difference n e evat on eet etween po nts percent an 85 percent of the distance along main stream channel BS -- Average basin slope, in feet per mile. Measured by contour-band method within contributing drainage area [ 3CL*CI/A ]
Hydrologic Regions 5 & 6 -- Variables SR -- Slope ratio. Ratio of main-channel slope to basin slope within the drainage basin, computed as SL / BS EL12 -- Percentage of drainage basin at or greater than 1,200 feet above sea level. ST -- Basin storage. Percentage of total drainage area shown as lakes, ponds, and swamps RUNF -- Mean annual runoff, in inches.
Main Channel Slope A m ST
Bridge Crossing
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Runoff (RUNF)
Watershed Plot
Precipitation (P)
Basin Report
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Design Discharges
Other Sources for Discharge Estimates • FEMA Flood Insurance Studies • U.S. Corps of Engineers Flood Studies • U.S. Soil Conservation Studies • A encies res onsible for flood control facilities (regulated streams and dams)
2: Channel & Bridge Characteristics • Perform field bridge and channel survey • Data Requirements: – Cross section geometry -- Where are cross section required
Perform field bridge and channel survey • Alignment of bridge abutments and piers to the existing stream alignment • Bridge skew with respect to stream channel • High water marks
– Roughness values
• Any signs of scour around abutments and piers
– Bridge characteristics
• Stream bank and channel erosion, deposition, etc. • Any signs of ice and/or debris problems
Perform field bridge and channel survey • Alignment of bridge abutments and piers to the existing stream alignment
Perform field bridge and channel survey • Alignment of bridge abutments and piers to the existing stream alignment
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Perform field bridge and channel survey
Perform field bridge and channel survey
• Alignment of bridge abutments and piers to the existing stream alignment
• Bridge skew with respect to stream channel
Perform field bridge and channel survey
Perform field bridge and channel survey
• High water marks
• Any signs of scour around abutments and piers
Perform field bridge and channel survey
Perform field bridge and channel survey
• Any signs of scour around abutments and piers
• Stream bank and channel erosion, deposition, etc
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Perform field bridge and channel survey • Stream bank and channel erosion, deposition, etc
Perform field bridge and channel survey • Ice Problems
Perform field bridge and channel survey • Debris Problems
Where are cross section required • Upstream and Downstream Fascia of the Bridge -- Accurately locate streambed, low steel and roadway elevations of the existing structure • Centerline of Roadway across Bridge • At toe of roadway embankment slope upstream and downstream of existing bridge • At one bridge length upstream of the bridge • Channel and Floodplain Cross Sections
Bridge Cross Section Requirements
Bridge Cross Section Requirements
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Where are cross section required Channel and Floodplain Cross Sections:
Where are cross section required Channel and Floodplain Cross Sections: • Cross Sections should be taken 2000 ft., 1000 ft.,
• The width of each section should be 7 times the channel width extending in each di rection from the centerline of the channel, or the 100 year flood boundary, if a FEMA study is available. • Sections are to be taken perpendicular to flow direction within the channel and, overbank and floodplain areas
Where are cross section required
500 ft., 400 ft., 300 ft., 200 ft., 100 ft., from the downstream fascia of the bridge. • Cross Sections should be taken one bridge length plus 100 ft., 200 ft., and 500 ft., from the upstream fascia of the bridge • Cross Sections should not cross each other. • All offsets for each cross section should be taken from left to right, looking downstream.
Hydraulic Cross Sections
From an Existing FEMA Floodplain Map
Channel and Floodplain Cross Sections: • Any other structures located within the Downstream and Upstream survey limits should be surveyed like the project structure •
ny s arp en s, ea cut, or s gn cant c anges n the stream channel or floodplain within the survey limits should also be surveyed.
• Channels that are flatter than 0.0004 ft/ft requires an additional cross section at 4000 ft. downstream of the bridge
Hydraulic Cross Sections
Bridge Cross Section Requirements Downstream toe of embankment slope
Downstream Fascia of Structure
Upstream toe of embankment slope
One bridge length upstream of Bridge
Upstream Fascia of Structure
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Hydraulic Cross Sections 200 ft
100 ft
Manning’s Roughness Coefficients 100+ ft • Various sources for “n” values • Roughness varies with season (Use worst case)
200+ ft t f 0 0 5
1400 ft
U.S.G.S Water Supply Paper 1849 (Available online, link found in HEC-RAS help menu)
FHWA-TS-84-204: Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains
3 – Hydraulic Analysis HEC-RAS Software – US Army Corps of Engineers (Hydraulic Engineering Center - River Analysis System) .
HEC-RAS Software • Software and Users Manuals are downloadable for free from Corps of Engineers website (www.hec.usace.army.mil) • User inputs design flood flows, channel and structure survey information, boundary conditions • HEC-RAS uses the Standard Step method to compute steady flow water surface profiles • HEC-RAS is capable of modeling subcritical, supercritical, and mixed flow
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Standard Step Method
Standard Step Method
• Also known as the “Step Backwater Method” • Uses the Energy Equation, Momentum and Manning’s Equation to evaluate points along the water surface profile.
Basic Assum tions 1. 2. 3. 4. 5.
St eady flow Flow type constant between sections Normal depths considered vertical depths The water level across a channel cross section is uniform Sediment and air entrainment are negligible
Cross Section Geometry
Bridge Cross Section
Cross Section Layout
Bridge and Culvert Geometry
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HEC-RAS Output
HEC-RAS Output
HEC-RAS Output
HEC-RAS Output
HEC-RAS Output
Allowable Backwater • In general, the bridge should be designed to clear the design frequency flood • Meet NFIP (National Flood Insurance Program) requirements • Meet Conservancy District requirements • Limited to 1-foot raise in 100-year backwater if outside of NFIP jurisdiction • Backwater should not be allowed to flood “Unreasonably large areas of usable land” • Backwater should not be increased in urban areas
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Review
What equations are used to determine design discharges for bridge openings? How wide should hydraulic cross sections be?
What hydraulic design criteria should be used for bridge design?
Hydrology & Hydraulics for Bridge Design (Part II) Objectives for this presentation:
1. NATIONAL FLOOD INSURANCE PROGRAM – 30 min 2. SCOUR ANALYSIS & CHANNEL PROTECTION – 30 min 3. NYSDOT SUBMITTAL REQUIREMENTS – 30 min
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