# Hydrology & Hydraulics for Bridge Design

## Presentation on theme: "Hydrology & Hydraulics for Bridge Design"— Presentation transcript:

Hydrology & Hydraulics for Bridge Design
This presentation will cover Hydraulic Design of bridges. Culverts will be covered in a later section.

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 The presentation will be divided into 6 general topics.

Item 1: Hydrology Two primary methods used by ODOT to calculate flood discharges: USGS report (rural) USGS report (small urban) First topic is Hydrology, which is the determination of what volume of flow will pass through the bridge during storm events of various frequencies. This will be a main focus of this presentation, because there is a lot of trivial information to know, and a common area for mistakes in submittals. Two primary methods are used to calculate flood discharges: rural equations and urban equations. Rural is used for the vast majority of bridge projects. Urban equations are used only for urban sites with drainage areas less than 4 square miles. (Covered in BDM)

USGS Report : 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 : This is a table showing the regression equations from report The variables in the equations include drainage area, channel slope, and storage. Interesting to note the standard error of the equations.

Drainage Area This graphic shows one method that can be used to calculate drainage areas for use in the regression equations. Here, the drainage area is drawn on USGS quadrangle mapping, and the area is measured using a planimeter. There are also a number of computer programs available to measure drainage areas For larger areas, additional tools are available

Supplement to the Gazetteer
Useful for calculating larger drainage areas Available from ODNR, listed as an “out of print” publication on website The supplement to the gazetteer of Ohio streams can be used to help calculate drainage areas for bridge sites with large drainage areas. Drainage areas on larger streams are tabulated at various points Saves you from stringing multiple USGS quad maps. Available from ODNR, can be ordered from 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 = 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

USGS Report : 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

Basin Development Factor (BDF):
4 TOTAL 1 Curb & Gutter Streets Storm Drains Channel Linings Channel Improvements Lower 1/3 Middle 1/3 Upper 1/3 BDF=4+4+0=8

Confirm Suitability of Urban Equations
12 BDF 41.2 31.5 Precipitation 4.09 0.026 Drainage Area Maximum Minimum Basin 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)

Guide for Selecting Manning's Roughness Coefficients
FHWA-TS : Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains (http://www.fhwa.dot.gov/bridge/wsp2339.pdf)

U.S.G.S Water Supply Paper 1849

Item 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 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

Standard Step Method Basic Assumptions
Also known as the “Step Backwater Method” Uses the Energy Equation and Manning’s 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

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 ) 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

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:

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: 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