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Hydrology & Hydraulics for Bridge Design. Bridge Hydraulics Overview Topics for this presentation: Item 1 – Design discharges (Hydrology) Item 2 – Channel.

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Presentation on theme: "Hydrology & Hydraulics for Bridge Design. Bridge Hydraulics Overview Topics for this presentation: Item 1 – Design discharges (Hydrology) Item 2 – Channel."— Presentation transcript:

1 Hydrology & Hydraulics for Bridge Design

2 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

3 Item 1: Hydrology Two primary methods used by ODOT to calculate flood discharges: USGS report (rural) USGS report (small urban)

4 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

5 USGS Report :

6 Drainage Area

7 Supplement to the Gazetteer Useful for calculating larger drainage areas Available from ODNR, listed as an out of print publication on website

8 Supplement to the Gazetteer

9 Main Channel Slope

10 Storage

11 Region for Drainage Area

12 Discharge Calculation for Ungaged Stream: The Region C multiple-regression equation for 100-year flood peak discharges is chosen: Q 100 = (RC)(CONTDA) (SLOPE) (STORAGE+1) 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: Q 100 = 236(0.290) (93.0) (0.0+1) Q 100 = 337 cubic feet per second

13 Confirm Suitability of Rural Equations Check basin characteristics with ranges for region Characteristics outside range occur infrequently

14 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

15 Peakflow Software Applies regression equations Performs gauging station adjustments Download from ODOT website

16 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)

17 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

18 044TOTAL 011 Curb & Gutter Streets 011Storm Drains 011 Channel Linings 011 Channel Improvements Lower 1/3Middle 1/3Upper 1/3 BDF=4+4+0=8 Basin Development Factor (BDF):

19 Confirm Suitability of Urban Equations 120BDF Precipitation Drainage Area MaximumMinimum Basin Characteristics

20 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)

21 ODOT Design Discharges Design Flood Frequency: Freeways/Controlled Access Facilities50 years Other Highways (2000 ADT)25 years Other Highways (<2000 ADT)10 years

22 Item 2: Channel & Bridge Characteristics Perform channel survey Data Requirements: –Cross section geometry –Roughness values –Bridge characteristics

23 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

24 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

25 Bridge Cross Section Requirements

26 Mannings Roughness Coefficients Various sources for n values Roughness varies with season (Use worst case)

27 FHWA-TS : Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains (

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

29 Item 3 – Hydraulic Analysis HEC-RAS Software – US Army Corps of Engineers (Hydraulic Engineering Center - River Analysis System).

30 Software and Users Manuals are downloadable for free from Corps of Engineers website ( 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

31 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 1.Steady flow 2.Flow type constant between sections 3.Normal depths considered vertical depths 4.Level water surface across channel 5.Sediment and air entrainment are negligible

32 Standard Step Method

33 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)

34 Cross Section Geometry

35 Bridge Geometry

36 Cross Section Layout

37 HEC-RAS Output


39 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

40 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)

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

42 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

43 Floodway Map

44 Flood Insurance Rate Map

45 Flood Insurance Study

46 NFIP Compliance ConditionRequirement 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

47 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

48 Ohios Conservancy Districts _Conservancy_District/Ohios_Conservancy_Districts.htm

49 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

50 Total Scour –three components: 1.Long term aggradation and degradation 2.Contraction scour 3.Local scour

51 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.

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

53 Contraction Scour


55 Local Scour at Piers Occurs due to the acceleration of flow around the pier and the formation of flow vortices.

56 Local Scour at Piers



59 Local Scour at Abutments




63 Scour with HEC-RAS


65 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

66 Rock Channel Protection at Bridges

67 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

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