Hydrology & Hydraulics for Bridge Design

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

19. Basin Characteristics Minimum Maximum Drainage Area 0.026 4.09 Precipitation 31.5 41.2 BDF 0 12 Confirm Suitability of Urban Equations

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. 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

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 • 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 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

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: • Long term aggradation and degradation • Contraction scour • Local scour