Post on 27-Mar-2015
Hydrology & Hydraulicsfor
Bridge Design
Bridge Hydraulics Overview
Topics for this presentation:Item 1 – Design discharges (Hydrology)Item 2 – Channel & Bridge CharacteristicsItem 3 – Hydraulic Analysis using HEC-RASItem 4 – National Flood Insurance ProgramItem 5 – Scour Analysis & Channel ProtectionItem 6 – ODOT Submittal Requirements
Item 1: Hydrology
Two primary methods used by ODOT to calculate flood discharges:
• USGS report 89-4126 (rural)• USGS report 93-135 (small urban)
USGS Report 89-4126:
Techniques for Estimating Flood-Peak Discharges of Rural, Unregulated Streams in Ohio
• 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
USGS Report 89-4126:
Drainage Area
Supplement to the Gazetteer
• Useful for calculating larger drainage areas
• Available from ODNR, listed as an “out of print” publication on website
Supplement to the Gazetteer
Main Channel Slope
Storage
Region for Drainage Area
Discharge Calculation for Ungaged Stream:
The Region C multiple-regression equation for 100-year flood peak discharges is chosen:
Q100 = (RC)(CONTDA)0.756(SLOPE)0.285(STORAGE+1)-0.363
Basic characteristics for the ungaged site are determined:CONTDA = 0.290 square miles
SLOPE = 93.0 feet per mile
STORAGE = 0.0 percent
These values are substituted into the Region C equation:
Q100 = 236(0.290)0.756(93.0)0.285(0.0+1)-0.363
Q100 = 337 cubic feet per second
Confirm Suitability of Rural Equations• Check basin characteristics with ranges for region
• Characteristics outside range occur infrequently
Use of Gaging Station Data• For ungaged sites on gaged streams• Confirm that drainage basin is rural and stream is
unregulated• Site can be upstream or downstream of gauging station• Results of regression equations are adjusted to agree with
data from nearby gaging stations
Peakflow Software• Applies regression equations• Performs gauging station adjustments• Download from ODOT website
USGS Report 93-135:Estimation of Peak-Frequency Relations, Flood Hydrographs, and Volume-Duration-Frequency Relations of Ungaged Small Urban Streams in Ohio
• Procedure similar to that used for rural streams• Equations are not suitable for all urban streams• Q = f (Area, Slope, BDF)
Basin Development Factor (BDF):
• A measure of urban development within a drainage basin
0 = No development
12 = Maximum development• Divide basin into three subdivisions• Estimate development in each subdivision
044TOTAL
011Curb & Gutter Streets
011Storm Drains
011Channel Linings
011Channel Improvements
Lower 1/3Middle 1/3Upper 1/3
BDF=4+4+0=8
Basin Development Factor (BDF):
Confirm Suitability of Urban Equations
120BDF
41.231.5Precipitation
4.090.026Drainage Area
MaximumMinimumBasin Characteristics
Other Sources for Discharge Estimates
• HUD Flood Insurance Studies• U.S. Corps of Engineers Flood Studies• U.S. Soil Conservation Studies• Agencies responsible for flood control facilities
(regulated streams)
ODOT Design Discharges
Design Flood Frequency:
Freeways/Controlled Access Facilities 50 years
Other Highways (≥2000 ADT) 25 years
Other Highways (<2000 ADT) 10 years
Item 2: Channel & Bridge Characteristics• Perform channel survey• Data Requirements:
– Cross section geometry– Roughness values– Bridge characteristics
Field Survey for Waterway Crossings• Used to obtain channel cross-section data and establish
roughness coefficients (“n” values)• Photographs are required• Determine and document nature of upstream property• Assess flood potential and Headwater controls• Look for evidence of scour
Channel Cross-Sections
• Number of sections depends on uniformity of channel• Locate sections where bed profile, channel width or
depth, or roughness change abruptly• Orientation perpendicular to direction of flow
Bridge Cross Section Requirements
Manning’s Roughness Coefficients
• Various sources for “n” values• Roughness varies with season (Use worst case)
FHWA-TS-84-204:Guide for Selecting Manning's Roughness Coefficientsfor Natural Channels and Flood Plains
(http://www.fhwa.dot.gov/bridge/wsp2339.pdf)
U.S.G.S Water Supply Paper 1849(Available online, link found in HEC-RAS help menu)
http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indirects/nvalues/index.htm
Item 3 – Hydraulic Analysis
HEC-RAS Software – US Army Corps of Engineers(Hydraulic Engineering Center - River Analysis System).
• 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
• 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
HEC-RAS Software
Standard Step Method• Also known as the “Step Backwater Method”• Uses the Energy Equation and Manning’s Equation to
evaluate points along the water surface profile.
Basic Assumptions1. Steady flow2. Flow type constant between sections3. Normal depths considered vertical depths4. Level water surface across channel5. Sediment and air entrainment are negligible
Standard Step Method
Defining flow data in HEC-RAS
Required input for steady flow analysis:
- Discharge at cross sections with a change in flow.
- Boundary condition
• Downstream Channel Slope (Used to calculate Normal Depth)
• Known value (If available)
Cross Section Geometry
Bridge Geometry
Cross Section Layout
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 (Ohio Revised Code, section 1521.13)
• Backwater should not be allowed to flood “Unreasonably large areas of usable land”
• Backwater should not be increased in urban areas
Item 4 - National Flood Insurance Program (NFIP)
• Most Ohio communities participate• Each community adopts local ordinances• Enforced by local floodplain coordinator
(see ODNR website for listing)
Floodways
No encroachment allowed in the designated floodway unless analysis shows no increase in flood levels
NFIP Compliance
• Obtain floodway map, flood insurance rate map, and flood insurance study for site. (All available on FEMA website)
• If the site falls within a special flood hazard area, any construction must be approved by local floodplain coordinator
• Obtain local floodplain ordinances for community
Floodway Map
Flood Insurance Rate Map
Flood Insurance Study
NFIP Compliance
Condition Requirement
Construction in the floodway
Analysis showing that proposed condition will not increase 100-year water surface elevations
Construction in floodway fringe
Embankment is permitted in the floodway fringe
Construction in Flood Hazard Zone A
See local floodplain regulations for requirements
NFIP Compliance – HEC RAS Analysis
• Obtain original model used for FIS, if possible• If original model cannot be obtained, use water
surface elevations and flow rates from FIS to initiate analysis
• If flow rates and water surface elevations are substantially different those based on the regression equations, include both on the structure site plan
Ohio’s Conservancy Districts
http://www.miamiconservancy.org/Who_We_Are/What_Is_A_Conservancy_District/Ohios_Conservancy_Districts.htm
Item 5 – Scour Analysis and Channel Protection
Hydraulic Engineering Circular No. 18 (HEC-18):Evaluating Scour at Bridges
Published by FHWA
Best source of information on scour analysis & countermeasures
Total Scour –three components:1. Long term aggradation and degradation
2. Contraction scour
3. Local scour
Long-Term Aggradation and Degradation
• Not computed by HEC-RAS
• What is the long-term trend?
• Trends can change due to natural or man-made causes.
• Evaluate using HEC-18 before performing analysis
• ODOT District personnel and County Engineers are a good source of information.
Contraction Scour
• Occurs when the flow area of a stream is reduced by a natural contraction or a bridge restricting the flow
Contraction Scour
Contraction Scour
Local Scour at Piers• Occurs due to the acceleration of flow around the pier and
the formation of flow vortices.
Local Scour at Piers
Local Scour at Piers
Local Scour at Piers
Local Scour at Abutments
Local Scour at Abutments
Local Scour at Abutments
Local Scour at Abutments
Scour with HEC-RAS
Scour with HEC-RAS
ODOT Scour Protection Requirements
• Deep foundations (piles or drilled shafts) or spread footings in rock
• Spill-through earth slopes armored with rock channel protection
– Minimum size and thickness of RCP given in ODOT Bridge Design Manual
– Increase thickness of RCP outside portion of curved channels or where ice flow is concern
Rock Channel Protection at Bridges
Item 6 - ODOT Submittal Requirements:
Include a “Hydraulic Report” with the Structure Type Study. This report should include:
1. Computation of flood flows
2. Hydraulic analysis of existing and proposed structure (include both hard copy and HEC-RAS files)
3. Information on NFIP floodmaps and flood insurance studies referenced
4. Scour analysis of proposed structure