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

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
The presentation will be divided into 6 general topics.

3. 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)

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 :
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.

6. 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

7. 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.

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:
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

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. 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

19. Confirm Suitability of Urban Equations12
BDF
41.2
31.5
Precipitation
4.09
0.026
Drainage Area
Maximum
Minimum
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 Facilities 50 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. Manning’s Roughness Coefficients
Various sources for “n” values
Roughness varies with season (Use worst case)

27. Guide for Selecting Manning's Roughness Coefficients
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. HEC-RAS Software
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

31. 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

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

38. 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 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

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. Ohio’s Conservancy Districts

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:
Long term aggradation and degradation
Contraction scour
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

54. 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

57. Local Scour at Piers

58. Local Scour at Piers

59. Local Scour at Abutments

60. Local Scour at Abutments

61. Local Scour at Abutments

62. Local Scour at Abutments

63. Scour with HEC-RAS

64. 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:
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