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CTC 261 Hydraulics Culvert Design

CTC 261 Hydraulics Culvert Design. Objectives. Know the factors associated with culvert design Know what a skewed culvert is. Culvert Design. Hydraulics Location Profile Material Type Strength Method of Construction Cost. Horizontal Alignment. Perpendicular to road Shorter

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CTC 261 Hydraulics Culvert Design

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  1. CTC 261 HydraulicsCulvert Design

  2. Objectives • Know the factors associated with culvert design • Know what a skewed culvert is

  3. Culvert Design • Hydraulics • Location • Profile • Material Type • Strength • Method of Construction • Cost

  4. Horizontal Alignment • Perpendicular to road • Shorter • Easy to build • May require channel relocation • Skewed • Longer • More complex to build • Channel relocation may not be needed

  5. Horizontal layout • Overhead • Location Plan (Figure 13-7, pg 333, 4thed) • Alternatives (Figure 13-10, pg 337, 4thed) • No skew (realign channel 160’ ds) • 45 deg skew (realign channel 60’ us) • 25 deg skew (realign channel 50’ us and long headwall downstream)

  6. Profile • Draw profile to help determine what size culvert will work • Show existing/proposed grade of roadway centerline • Show proposed culverts • Scale may be exaggerated in vertical direction

  7. Profile • Overhead (Figure 13-8; page 334, 4th ed.) • Shows twin 13’x 4’ (Trial 4) • Shows 12’x 6’ (Trial 5) • Note that profile would need to be modified

  8. Cross-Section • Draw cross-section (perpendicular to road) to help determine structure length

  9. Cross-Section • Overhead (Figure 13-2, page 336, 4th ed.)

  10. Culvert Replacement • Same location/new location • Same size/new size • Same material/different material • Maintenance of traffic during replacement

  11. Erosion at Inlet • Scour @ inlets • Prevention • Paving • Cutoff Walls • Headwalls • Wingwalls

  12. Erosion at Outlet • Scour @ outlets (very common) • Velocity higher than natl channel • Difficult to predict • Local scour (limited d/s distance) • General stream degradation • Prevention • Riprap • Energy Dissipators • Preformed scour holes

  13. Sedimentation • If culvert aligned w/ natl. channel-not usually a problem • Multiple barrels and culverts w/ depressions are susceptible

  14. Debris Control Routine maintenance may be adequate May need to Intercept/Deflect or Pass

  15. Economics • Service Life-Same as roadway • Comparison between bridge/culvert • Culverts less expensive • Culverts may cause more flooding • Bridge maintenance costs usually higher • Bridges usually more aesthetic • Comparison between materials & shapes • Risk Analysis

  16. Structural • Structural analysis • Embankments (dead loads) • Traffic load (live loads) • Hydrostatic/hydrodynamic forces • Bedding/Backfill is important • Flotation & Anchorage • Uplifting forces can cause damage (buoyancy) • Endwalls/Headwalls (retaining walls)

  17. Safety • Grates • Traversable for vehicles • Keep out children • Debris may be a problem • Inlets/Outlets • Locate outside clear zones • Guide railing may be needed

  18. Culvert Durability • Abrasion • High velocities carry rocks • Use concrete or lining • Oversize culvert & bury invert • Corrosion • Steel corrodes pH<6 or >10 • Metal susceptible in clay or organize mucks • Concrete susceptible to seawater, sulfates, Mg salts • Use Al in saltwater • Use linings • Vary concrete mix

  19. Environmental Considerations • Prevent sediments/oils, etc. from entering streams/wetlands during construction • Fish stream • Undisturbed streambed • Oversize structure and reproduce natural streambed • Multiple barrels to handle different flows

  20. http://keats.admin.virginia.edu/stormwater/fm/arena.html http://www.greeningaustralia.org.au/

  21. Culvert Example 13-2 (page 332; 4th ed) • Determine Design Flow • Frequency & Duration: Q100, 24-hr storm • Method: NRCS (SCS; TR-55) • (DA/Soils/Precipitation Data/Land Use/Cover, etc.) • Q100=100 cfs

  22. Culvert Example 13-2 (page 306) • Determine Allowable Headwater (AHW) and Tailwater (TW) • AHW by local regs is limited to the upstream culvert crown • Use Manning’s equation (Q100) & solve for normal depth via trial and error • TW=3.7 feet

  23. Culvert Example 13-2 (page 306) • Type of culvert: CBC w/ 45-deg wingwalls and a square edge entrance---trial and error

  24. Trial & Error (AHW=Inlet Crown)

  25. Culvert Example 13-2 (page 306) • Determine horizontal layout

  26. Culvert Example 13-2 (page 306) • Determine Velocity • Velocity = 5 ft/second • Determine need for stabilization • Appendix A-2; Use graded loam to gravel

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