Hydrology

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Hydrology HydraulicsforBridge Design
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Bridge Hydraulics Overview

• Topics for this presentation
• Item 1 Design discharges (Hydrology)
• Item 2 Channel Bridge Characteristics
• Item 3 Hydraulic Analysis using HEC-RAS
• Item 4 National Flood Insurance Program
• Item 5 Scour Analysis Channel Protection
• Item 6 ODOT Submittal Requirements

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Item 1 Hydrology

• Two primary methods used by ODOT to calculate
  flood discharges
• USGS report 89-4126 (rural)
• USGS report 93-135 (small urban)

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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

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USGS Report 89-4126
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Drainage Area
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Supplement to the Gazetteer

• Useful for calculating larger drainage areas
• Available from ODNR, listed as an out of print
  publication on website

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Supplement to the Gazetteer
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Main Channel Slope
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Storage
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Region for Drainage Area
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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(STORAGE1)-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(0.01)-0.363 Q1
00 337 cubic feet per second
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Confirm Suitability of Rural Equations

• Check basin characteristics with ranges for
  region
• Characteristics outside range occur infrequently

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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

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Peakflow Software

• Applies regression equations
• Performs gauging station adjustments
• Download from ODOT website

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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)

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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

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Basin Development Factor (BDF)
BDF4408
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Confirm Suitability of Urban Equations
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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)

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ODOT Design Discharges

• Design Flood Frequency
• Freeways/Controlled Access Facilities 50 years
• Other Highways (2000 ADT) 25 years
• Other Highways (lt2000 ADT) 10 years

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Item 2 Channel Bridge Characteristics

• Perform channel survey
• Data Requirements
• Cross section geometry
• Roughness values
• Bridge characteristics

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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

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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

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Bridge Cross Section Requirements
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Mannings Roughness Coefficients

• Various sources for n values
• Roughness varies with season (Use worst case)

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FHWA-TS-84-204 Guide for Selecting Manning's
Roughness Coefficients for Natural Channels and
Flood Plains
(http//www.fhwa.dot.gov/bridge/wsp2339.pdf)
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U.S.G.S Water Supply Paper 1849 (Available
online, link found in HEC-RAS help menu)
http//wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indi
rects/nvalues/index.htm
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Item 3 Hydraulic Analysis

• HEC-RAS Software US Army Corps of Engineers
• (Hydraulic Engineering Center - River Analysis
  System).

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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
• 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

• Also known as the Step Backwater Method
• Uses the Energy Equation and Mannings Equation
  to evaluate points along the water surface
  profile.

Basic Assumptions

• Steady flow
• Flow type constant between sections
• Normal depths considered vertical depths
• Level water surface across channel
• Sediment and air entrainment are negligible

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Standard Step Method
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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)

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Cross Section Geometry
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Bridge Geometry
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Cross Section Layout
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HEC-RAS Output
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HEC-RAS Output
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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

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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)

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Floodways

• No encroachment allowed in the designated
  floodway unless analysis shows no increase in
  flood levels

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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

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Floodway Map
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Flood Insurance Rate Map
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Flood Insurance Study
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NFIP Compliance
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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

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Ohios Conservancy Districts

• http//www.miamiconservancy.org/Who_We_Are/What_Is
  _A_Conservancy_District/Ohios_Conservancy_District
  s.htm

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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
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Total Scour three components

• Long term aggradation and degradation
• Contraction scour
• Local scour

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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.

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Contraction Scour

• Occurs when the flow area of a stream is reduced
  by a natural contraction or a bridge restricting
  the flow

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Contraction Scour
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Contraction Scour
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Local Scour at Piers

• Occurs due to the acceleration of flow around the
  pier and the formation of flow vortices.

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Local Scour at Piers
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Local Scour at Piers
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Local Scour at Piers
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Local Scour at Abutments
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Local Scour at Abutments
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Local Scour at Abutments
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Local Scour at Abutments
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Scour with HEC-RAS
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Scour with HEC-RAS
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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

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Rock Channel Protection at Bridges
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Item 6 - ODOT Submittal Requirements

• Include a Hydraulic Report with the Structure
  Type Study. This report should include
• Computation of flood flows
• Hydraulic analysis of existing and proposed
  structure (include both hard copy and HEC-RAS
  files)
• Information on NFIP floodmaps and flood insurance
  studies referenced
• 4. Scour analysis of proposed structure