Presentation on theme: "Hydrology & Hydraulics for Bridge Design"— Presentation transcript:
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 CharacteristicsItem 3 – Hydraulic Analysis using HEC-RASItem 4 – National Flood Insurance ProgramItem 5 – Scour Analysis & Channel ProtectionItem 6 – ODOT Submittal RequirementsThe presentation will be divided into 6 general topics.
3 Item 1: HydrologyTwo 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 OhioProvides multiple-regression equations to calculate discharges for gaged and ungaged streamsProvides a method to adjust discharges for gaged streamsContains data from streamflow gaging stations
5 USGS Report :This is a table showing the regression equations from reportThe variables in the equations include drainage area, channel slope, and storage.Interesting to note the standard error of the equations.
6 Drainage AreaThis 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 areasFor larger areas, additional tools are available
7 Supplement to the Gazetteer Useful for calculating larger drainage areasAvailable from ODNR, listed as an “out of print” publication on websiteThe 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 pointsSaves you from stringing multiple USGS quad maps.Available from ODNR, can be ordered from website.
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)-0.363Basic characteristics for the ungaged site are determined:CONTDA = square milesSLOPE = 93.0 feet per mileSTORAGE = 0.0 percentThese values are substituted into the Region C equation:Q100 = 236(0.290)0.756(93.0)0.285(0.0+1)-0.363Q100 = 337 cubic feet per second
13 Confirm Suitability of Rural Equations Check basin characteristics with ranges for regionCharacteristics outside range occur infrequently
14 Use of Gaging Station Data For ungaged sites on gaged streamsConfirm that drainage basin is rural and stream is unregulatedSite can be upstream or downstream of gauging stationResults of regression equations are adjusted to agree with data from nearby gaging stations
15 Peakflow Software Applies regression equations Performs gauging station adjustmentsDownload from ODOT website
16 USGS Report :Estimation of Peak-Frequency Relations, Flood Hydrographs, and Volume-Duration-Frequency Relations of Ungaged Small Urban Streams in OhioProcedure similar to that used for rural streamsEquations are not suitable for all urban streamsQ = f (Area, Slope, BDF)
17 Basin Development Factor (BDF): A measure of urban development within a drainage basin0 = No development12 = Maximum developmentDivide basin into three subdivisionsEstimate development in each subdivision
19 Confirm Suitability of Urban Equations 12BDF41.231.5Precipitation4.090.026Drainage AreaMaximumMinimumBasin Characteristics
20 Other Sources for Discharge Estimates HUD Flood Insurance StudiesU.S. Corps of Engineers Flood StudiesU.S. Soil Conservation StudiesAgencies responsible for flood control facilities (regulated streams)
23 Field Survey for Waterway Crossings Used to obtain channel cross-section data and establish roughness coefficients (“n” values)Photographs are requiredDetermine and document nature of upstream propertyAssess flood potential and Headwater controlsLook for evidence of scour
24 Channel Cross-Sections Number of sections depends on uniformity of channelLocate sections where bed profile, channel width or depth, or roughness change abruptlyOrientation perpendicular to direction of flow
26 Manning’s Roughness Coefficients Various sources for “n” valuesRoughness varies with season (Use worst case)
27 Guide for Selecting Manning's Roughness Coefficients FHWA-TS :Guide for Selecting Manning's Roughness Coefficientsfor 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)
29 Item 3 – Hydraulic Analysis HEC-RAS Software – US Army Corps of Engineers(Hydraulic Engineering Center - River Analysis System).
30 HEC-RAS SoftwareSoftware 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 informationHEC-RAS uses the Standard Step method to compute steady flow water surface profilesHEC-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 AssumptionsSteady flowFlow type constant between sectionsNormal depths considered vertical depthsLevel water surface across channelSediment and air entrainment are negligible
33 Defining flow data in HEC-RAS Required input for steady flow analysis:- Discharge at cross sections with a change in flow.- Boundary conditionDownstream Channel Slope (Used to calculate Normal Depth)Known value (If available)
39 Allowable BackwaterIn general, the bridge should be designed to clear the design frequency floodMeet NFIP (National Flood Insurance Program) requirementsMeet Conservancy District requirementsLimited 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 participateEach community adopts local ordinancesEnforced by local floodplain coordinator (see ODNR website for listing)
41 FloodwaysNo encroachment allowed in the designated floodway unless analysis shows no increase in flood levels
42 NFIP ComplianceObtain 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 coordinatorObtain local floodplain ordinances for community
46 NFIP Compliance Condition Requirement Construction in the floodway Analysis showing that proposed condition will not increase 100-year water surface elevationsConstruction in floodway fringeEmbankment is permitted in the floodway fringeConstruction in Flood Hazard Zone ASee local floodplain regulations for requirements
47 NFIP Compliance – HEC RAS Analysis Obtain original model used for FIS, if possibleIf original model cannot be obtained, use water surface elevations and flow rates from FIS to initiate analysisIf flow rates and water surface elevations are substantially different those based on the regression equations, include both on the structure site plan
49 Item 5 – Scour Analysis and Channel Protection Hydraulic Engineering Circular No. 18 (HEC-18):Evaluating Scour at BridgesPublished by FHWABest source of information on scour analysis & countermeasures
50 Total Scour –three components: Long term aggradation and degradationContraction scourLocal scour
51 Long-Term Aggradation and Degradation Not computed by HEC-RASWhat is the long-term trend?Trends can change due to natural or man-made causes.Evaluate using HEC-18 before performing analysisODOT District personnel and County Engineers are a good source of information.
52 Contraction ScourOccurs when the flow area of a stream is reduced by a natural contraction or a bridge restricting the flow
65 ODOT Scour Protection Requirements Deep foundations (piles or drilled shafts) or spread footings in rockSpill-through earth slopes armored with rock channel protectionMinimum size and thickness of RCP given in ODOT Bridge Design ManualIncrease thickness of RCP outside portion of curved channels or where ice flow is concern
67 Item 6 - ODOT Submittal Requirements: Include a “Hydraulic Report” with the Structure Type Study. This report should include:Computation of flood flowsHydraulic analysis of existing and proposed structure (include both hard copy and HEC-RAS files)Information on NFIP floodmaps and flood insurance studies referenced4. Scour analysis of proposed structure