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Basic Hydraulics: Culverts – I

Basic Hydraulics: Culverts – I. Concepts. A culvert conveys surface water through a roadway embankment or away from the highway right-of-way (ROW) or into a channel along the ROW

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Basic Hydraulics: Culverts – I

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  1. Basic Hydraulics: Culverts – I

  2. Concepts • A culvert conveys surface water through a roadway embankment or away from the highway right-of-way (ROW) or into a channel along the ROW • In addition to the hydraulic function, the culvert must also support construction and highway traffic and earth loads; therefore, culvert design involves both hydraulic and structural design • Culverts are considered minor structures, but they are of great importance to adequate drainage and the integrity of the facility.

  3. Definitions • Culvert = relatively short length of conduit used to transport water through an embankment • Components • Outlet • Barrel(s) • Inlet

  4. Shapes • Typically, several shapes provide hydraulically adequate design alternatives:

  5. Multiple barrel culverts

  6. Materials • Commonly used culvert materials include concrete (reinforced and non-reinforced), steel (smooth and corrugated), aluminum (smooth and corrugated), and plastic (smooth and corrugated) • The selection of material for a culvert depends on: • structure strength, • hydraulic efficiency, • installation, local construction practices, • durability, • cost.

  7. Pertinent dimensions Circular culvert – diameter Box culvert – rise & span

  8. Terminology HW = headwater TW = tailwater

  9. Headwater • Headwater is the depth of water on the entrance or upstream side of the culvert as measured from the inlet invert • The Tailwater is the depth of water on the exit or downstream side of the culvert, as measured from the downstream invert

  10. Culvert hydraulics • What we need to know: • For given discharge, what size culvert is required to carry the flow without overtopping embankment? • For given culvert size, will specified discharge overtop? • For given culvert size, what is capacity without overtopping? • To answer these, must compute headwater depth, using principles of hydraulics

  11. What’s in control? • “Control” of culvert flow may be: • At inlet (inlet control) • At outlet (outlet control). • Analysis of a culvert requires us to: • Assume inlet control, calculate headwater depth. • Assume outlet control, calculate headwater depth for control at outlet. • Pick higher of two values as appropriate headwater value.

  12. Data needed for culvert analysis

  13. Inlet control • Occurs when flow capacity of entrance is less than flow capacity of barrel. • Control section is located just inside culvert entrance. • Water surface passes through critical depth. • Inlet capacity depends on entrance geometry.

  14. Computing inletcontrol headwater • Culvert headwater depth for inlet control depends on the inlet shape and efficiency • Federal Highway Administration (FHWA) developed a series of nomographs for standard culvert shapes that compute headwater depth for inlet control

  15. Using FHWA nomographs • Identify the culvert type (concrete pipe, box, CMP, etc.). • Find correct nomograph. • Start on the pipe diameter scale line • Draw a straight line through the discharge scale line • Read the value of HW/D from the appropriate HW/D scale for the entrance type for your culvert. • Usually two or three “scales” on nomograph that represent the type of inlet. • For example, on next page, scales for (1) square edge with head wall; (2) groove end with headwall; (3) groove end projecting

  16. Example of FHWA nomograph Example: D = 42 in. Q = 120 cfs

  17. Entrance treatment Source: HEC-2 Users Manual

  18. Entrance efficiency

  19. Outlet control • Occurs when flow capacity is limited by downstream conditions (high-tailwater) or by capacity of the barrel

  20. Outlet control examples

  21. Outlet control computations • To analyze, use energy equation:where Zup = upstream invert elevation; HW = depth at inlet; Vup = average velocity upstream; g = acceleration of gravity; Zdown = downstream invert elevation; TW = depth at outlet; Vdown = average velocity downstream; hL= total energy loss through culvert. • Since Vup Vdown we can simplify the equation

  22. Energy loss equations • Energy loss isin which hL= total head loss; he= entrance loss; hf = friction loss; ho= outlet (exit) loss • Friction loss estimated with Manning’s equationin which L = culvert length (feet); Q = flow rate in the culvert (cfs); n = Manning's roughness coefficient; A = area of flow (square feet); R = hydraulic radius (feet)

  23. Entrance loss coefficients(For outlet control only)

  24. Weir flow • Flow over the roadway can be computed as weir flow. • Check the the headwater elevation to see if weir flow occurs. • If headwater elevation is higher than the roadway, use iterative procedure, balancing weir and culvert flow. • Solution found when weir flow and culvert flow produce same headwater elevations.Qtotal = Qweir + Qculvert = Qgiven

  25. Flow Rate (cfs) Flow analysis for culverts Outlet Control Culvert Plus Roadway Overtopping Roadway Crest Headwater Elevation (ft) Inlet Control Top of Culvert

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