1 / 26

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

jabbott
Download Presentation

Basic Hydraulics: Culverts – I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related