This document provides an overview of hydrology and hydraulic considerations for bridge design. It discusses calculating design discharges using USGS reports, collecting channel and bridge characteristics through field surveys, performing hydraulic analysis using HEC-RAS software, meeting requirements of the National Flood Insurance Program, analyzing scour, and ODOT submittal requirements. The key steps involve estimating flood flows, modeling water surface profiles, evaluating flood risks and impacts, and designing protections against scour.

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 89-4126 (rural)
• USGS report 93-135 (small urban)

4. 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

5. USGS Report 89-4126:

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:
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

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 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)

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
41.231.5Precipitation
4.090.026Drainage 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 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. FHWA-TS-84-204:
Guide for Selecting Manning's
Roughness Coefficients
for Natural Channels and Flood
Plains
(http://www.fhwa.dot.gov/bridge/wsp2339.pdf)

28. 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

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 (
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

31. 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 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

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
1521.13)
• 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
http://www.miamiconservancy.org/Who_We_Are/What_Is_A
_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

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:
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