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