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Hydrology & Hydraulicsfor

Bridge Design

Bridge Hydraulics Overview

Objectives for this presentation:

1 – Design discharges (Hydrology)2 – Channel & Bridge Characteristics3 H d li A l i i HEC RAS3 – Hydraulic Analysis using HEC-RAS

-- A brief overview

Bridge Design Objectives

• Provide for the safe traveling of the public across a waterway

• Allowing for conveyance of storm discharges through the structure without adversely impacting the traveling public, damage to properties and the environment.

BRIDGES OVER WATERWAYS

• Bridges should be able pass the 50 year storm with 2 feet of freeboard and some lesser freeboard for the 100 year storm

BRIDGE DESIGN CRITERIA:

lesser freeboard for the 100 year storm. • There should be no impact to the

upstream water surface elevation and/or floodway and floodplain widths.

• Bridges should be able to withstand scour during a 500 year storm event.

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 Step Backwater Method -- HYDRAULICS

SIZING A BRIDGE WATERWAY OPENING

• Determine Stream Width & Bed Elevation

• Determine Low• Determine Low Beam Elevation

• Determine Roadway Elevation

• Waterway Area

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2 - HydrologyThe primary methods used by NYSDOT tocalculate flood discharges are:• USGS Report 2006-5112 – Regression Equations • NRCS TR 55

USGS Report 2006-5112:

Techniques for Estimating Flood-Peak 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

http://waterdata.usgs.gov/nwis/rt

NYS USGS Gaging Stations

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:Hydrologic Regions – Buffalo DOT Region 5

USGS Report 2006-5112:

Hydrologic Regions – Buffalo NYSDOT Region 5

Hydrologic Regions 5 & 6 -- VariablesA -- Drainage area, in square miles.

L -- Main-channel stream length, in miles.

SL -- Main-channel slope, in feet per mile. Difference in ele ation (feet) bet een points 10 percent andin elevation (feet) between points 10 percent and 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 level1,200 feet above sea level.

ST -- Basin storage. Percentage of total drainage area shown as lakes, ponds, and swamps

RUNF -- Mean annual runoff, in inches.

Am

ST

A

L85

Bridge Crossing

L10

Main Channel Slope

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Runoff (RUNF) Precipitation (P)

Watershed Plot

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• Agencies responsible for flood control facilitiesAgencies responsible 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

– Roughness values– Bridge characteristics

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 • Any signs of scour around abutments and piers• 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• Alignment of bridge abutments and piers to the

existing stream alignment

Perform field bridge and channel survey• Bridge skew with respect to stream channel

Perform field bridge and channel survey• High water marks

Perform field bridge and channel survey• Any signs of scour around abutments and piers

Perform field bridge and channel survey• Any signs of scour around abutments and piers

Perform field bridge and channel survey• 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:

• The width of each section should be 7 times the channel width extending in each direction 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

Channel and Floodplain Cross Sections:

• Cross Sections should be taken 2000 ft., 1000 ft., 500 ft., 400 ft., 300 ft., 200 ft., 100 ft., from the downstream fascia of the bridge.

C S i h ld b k b id l h l• 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.

Where are cross section required

Channel and Floodplain Cross Sections: • Any other structures located within the Downstream

and Upstream survey limits should be surveyed like the project structure

A h b d h d t i ifi t h i• Any sharp bends, head cut, or significant changes in 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 SectionsFrom an Existing FEMA Floodplain Map

Hydraulic Cross Sections Bridge Cross Section Requirements

Downstream toe of embankment slope

Upstream toe of embankment

slope

One bridge length upstream of Bridge

Downstream Fascia of Structure

Upstream Fascia of Structure

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Hydraulic Cross Sections200 ft 100+ ft100 ft

200+ ft

500

ft

1400 ft

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

U.S.G.S Water Supply Paper 1849(Available online, link found in HEC-RAS help menu)

3 – Hydraulic AnalysisHEC-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, boundary conditions

HEC-RAS Software

• 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, Momentum and Manning’s

Equation to evaluate points along the water surface profile.

Basic Assumptionsp1. Steady flow2. Flow type constant between sections3. Normal depths considered vertical depths4. The water level across a channel cross section is

uniform5. Sediment and air entrainment are negligible

Standard Step Method

Cross Section Geometry Cross Section Layout

Bridge Cross Section 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 requirementsLi i d 1 f i i 100 b k if id• 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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Any QuestionsAny Questions

Review

What equations are used to determinedesign discharges for bridge openings?

How wide should hydraulic cross sections be?What software is used for Bridge Hydraulics?What software is used for Bridge Hydraulics?What hydraulic design criteria should be used for bridge design?

Hydrology & Hydraulicsfor

Bridge Design (Part II)

Objectives for this presentation:

1. NATIONAL FLOOD INSURANCE PROGRAM – 30 min2. SCOUR ANALYSIS & CHANNEL PROTECTION – 30 min3. NYSDOT SUBMITTAL REQUIREMENTS – 30 min